GeoGuide 3 Rock and Soil Descriptions Hong Kong

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GEOGUIDE 3

GUIDE TO

ROCK AND SOIL

DESCRIPTIONS

GEOTECHNICAL ENGINEERING OFFICE

Civil Engineering Department

The Government of the Hong Kong

Special Administrative Region



GEOGUIDE 3

GUIDE TO

ROCK AND SOIL

DESCRIPTIONS

GEOTECHNICAL ENGINEERING OFFICE

Civil Engineering Department

The Government of the Hong Kong

Special Administrative Region



2

© The Government of the Hong Kong Special Administrative Region

First published, July 1988

Reprinted, April 1991

Reprinted, May 1994

Reprinted, January 1997

Reprinted, November 2000

Prepared by:

Geotechnical Engineering Office,

Civil Engineering Department, Civil Engineering Building, 101 Princess Margaret Road,

Homantin, Kowloon,

Hong Kong.

This publication is available from:

Government Publications Centre, Ground Floor, Low Block, Queensway Government Offices,

66 Queensway, Hong Kong.

Overseas orders should be placed with:

Publications Sales Section,

Information Services Department, Room 402, 4th Floor, Murray Building,

Garden Road, Central, Hong Kong.

Price in Hong Kong: HK$68

Price overseas: US$14 (including surface postage)

An additional bank charge of HK$50 or US$6.50 is required per cheque made in currencies

other than Hong Kong dollars.

Cheques, bank drafts or money orders must be made payable to

The Government of the Hong Kong Special Administrative Region.

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FOREWOR

This Geoguide pre se nt s a recommended sta nd ar d of good practice f or th e

description of Hong Kong r oc ks and soils for engin eeri ng pu rpos es. This need

was recog nize d in July 1983, when a Subcommittee of t h e Building Auth orit y

Working Pa rt y on Geotechnical Regulations reviewed t h e application in Hong

Kong of t h e Brit ish S ta nd ar d BS 5930 1981, Code of Pra cti ce f or Site

Inve stig atio ns. With re g a rd t o Section 8 of BS 5930 (Des crip tion s of Soils an d

Rocks), t h e Subcommittee concluded t ha t i t sh ould n ot be recommended fo r

general use in Hong Kong, because parts of the sect ion were not relevant to

local conditions or conflicted with cu r r en t Hong Kong practice.

This Geoguide should be u sed in conjunct ion with th e companion

document, Guide to Site Investigation (Geoguide 2 . which covers the topics

dea lt with in Sect ion s 1 to 7 of BS 5930. Tog eth er, th e se two Geoguides

expand upon, and largely replace, Chapter 2 of the Geotechnical Manual for

Slopes (1984).

A s with other Geoguides, this document gives guidance on good practice,

and its recommendations a r e not mandatory. Considerable deba te has always

existed over th e use of 's ta ndar dise d ' methods and terminology f or th e

des crip tion s of roc ks and soils. Many diffe rent schemes exist , an d i t is

recognized th at t h e pract i t ioner may wish to us e al ternat ive methods to thos e

recommended herein.

The Geoguide was pr ep ar ed in t h e Geotechnical Control Office

G C O )

under the general direct ion of

M r

J.B. Massey. It was dr af te d by

D r

R.P.

Martin, with assistance from

Dr

R L Langford. who wrote most of t h e material

for Appendix A an d provided many of t h e photogr aphi c il lustration s. The final

product ion was supervised by D r P.L.R. Pan g. The Geoguide inc or por ate s

background mater ia l and par t s of ear l ie r dra f t s prepare d by D r R . Shaw of t h e

G C O and by D r S.R. Hencher, a former member of the G C O s ta f f . D r T.Y.

Ir fa n , toge ther with many oth er G C O staff members, made valuable comments

on ear l ie r vers ions .

To ens ur e tha t th e Geoguide would be conside red a con sens us document

by t he vari ous inte rest ed part ies in Hong Kong. a draf t version was circulated

widely fo r comment in earl y 1987 t o cont racto rs, consul t ing e ngin eers .

academic inst ituti ons and Government Departments. Many organization s and

individuals made ve ry helpful comments, and the ir contr ib ut ions a r e grateful ly

acknowledged.

Prac t i t ioners

ar e encouraged to comment a t any t ime to t h e

G C O o n t h e

cont ents of thi s Geoguide, s o th at improvements can be made to f ut ur e

edi t ions.

E W Brand

Principal Government Geotechnical Engineer

July 1988

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1

5

CONTENTS

P a g e

NO.

T I T L E P GE

FOREWOR

CONTENTS

1

INTRODUCTION

1 1 PURPOSE AND SCOPE

1 2

GENERAL GUIDANCE

1 2 1

Definitions of Rock and Soil

1 2 2

The Hong Kong Geological Su r v e y

1 2 3

Mater ia l and

ass

C h a r a c t e r i s t i c s

1 2 4

Descr ip t ion in Dif ferent Locat ions

2

DESCRIPTION OF ROCKS

2 1

GENERAL

2 2 PURPOSE AND SCOPE OF R O K DESCRIPTION

2 3 DESCRIPTION OF

R O K

MATERIALS

2 3 1

S t r e n g t h

2 3 2 Colour

2 3 3 T e x t u r e a n d F a b r i c

2 3 4 Weather ing a n d Al tera t ion

2 3 5 Rock Name (In cl ud in g Grain Si ze)

2 3 6 Addit ional Information

2 3 7 Examples

2 4

DESCRIPTION OF

R O K

MASSES

2 4 1

General

2 4 2

S t r u c t u r e

2 4 3

Discont inui t ies

2 4 4

Rock

ass

Weather ing

2 4 5

Addi t ional Informat ion

2 4 6

Examples

2 5

ADDITIONAL GEOLOGICAL INFORMATION

3 D E S C R I P T I O N O F S O I L S

3 1

GENERAL

3 2

PURPOSE AND SCOPE OF SOIL DESCRIPTION



Page

NO.

3.3 MATERIAL CHARACTERISTICS OF SOILS

3 6

3 3 1

S t r e n g t h

3 6

3 3 2 Colour

37

3 3 3 Parti cle Sh ap e an d Composition

37

3 3 4 Soil Name

3 8

3 3 5 Additional Information

4 1

3.4 MASS CHARACTERISTICS OF SOILS

4 1

3 4 1 S t r u c t u r e

4 1

3 4 2 Discontinuities

4 2

3 4 3 St at e of Weathering

43

3 4 4 Additional Information

44

3.5

SOILS DERIVED FROM INSITU ROCK WEATHERING 44

3.6

COLLUVIUM

46

3.7

FILL

47

3.8

ADDITIONAL GEOLOGICAL INFORMATION

48

3.9 EXAMPLES 48

E N G I N E E R I N G C L A S S I F I C A T I O N S O F R O C K S A N D S O I L S

53

4 1 GENERAL

53

4 2 GEOMECHANICAL CLASSIFICATION OF

ROCK

MASSES 53

4 3 SOIL CLASSIFICATION FOR ENGINEERING PURPOSES

54

5 , LEGEND FOR MAPS PLA NS AND DIA GR AM S

57

5 1

SYMBOLS FOR ROCKS AND SOILS

57

5 2 OTHER SYMBOLS

57

5 2 1 Symbols for Borehole Records

57

5 2 2 Symbols for Geological Structures and Boundaries

57

REFERENCES

59

T A B L E S

6 9

LIST OF TABLES

7 1

TABLES

73



P a g e

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F I GU R E S

L I S T O F F I G U R E S

F I G U R E S

P L TE S

L I S T O F P L A T E S

P L A T E S

PPENDIX

NATURE AND OCCURRENCE OF HONG KONG ROCKS AND

S U P E R F I C I A L D E P O S I T S

GLOSS RY



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9

INTRODU TION

1.1 PURPOSE

AND

SCOPE

The purp os e of t his Geoguide is to p re se nt a recommended st an da rd of

good practic e for th e descri ptio n of Hong Kong ro cks a nd soils for enginee ring

pur pos es . I t is a companion document to Geoguide 2 Guide to Site

Invest igat ion GCO , 1987a).

This Geoguide is aimed primarily a t t h e pr actis ing civil or geotechnical

engineer , b ut i s a l so in tended for use by geologis ts , engineer ing geologist s and

othe r profess ionals working in th e ear th sc iences . I t has been prepared on th e

assumption t ha t th e us er may not have any special ist knowledge of descript iv e

systems or methods.

The overall s t r uc tu re a nd many individual text sect ions of this Geoguide

a r e ba se d on Section 8 of BS 5930 1981, Code of Practice for Site

Investigat ions (BSI, 1981). This British St and ard (BS) has been selected a s t h e

basic reference document for both this Geoguide and Geoguide 2, in the belief

t h at many of i t s sec tions a r e applicable to Hong Kong conditions with out th e

need fo r major modification. However. th e layo ut of thi s document differ s

cons ider ably from th a t of Section 8 of t h e BS, and a nu mber of new te xt

sect ions

have been added. to geth er with many more tables, f igu res a nd plates.

These chan ges reflect not only diff erences of emphasis with r eg ar d t o local

geological conditions, b ut al so th e need fo r more il lustration an d explanation

of geological term s fo r th e non-specialist us er.

The following section of this

chapter contains some general guidance on

descript ive methods and terms. The two major chap ter s of t he Geoguide

(Chapters 2 a n d 3 ar e devoted t o methods of descript ion fo r th e two main

gr oup s of engine erin g materials (i.e. rock s and soils). Following th es e is a

s h o r t e r c h a p te r conc erne d with engi neer ing classifications of roc ks an d soils

(Chapter 4 . The f i f th chapter presents recommended rock and soi l symbols to

be used for maps, plans and diagrams. Appendix A , which contains a

geological summary of t h e na tu re an d oc cur ren ce of Hong Kong roc ks a nd

soils, is intend ed t o fulfil a similar role to th at of Appendix in th e BS.

This is followed by a glo ss ary of term s. Also incl uded se pa ra te ly i s a

checklist for f ie ld or laboratory use.

This Geoguide provide s guid ance for good p ractice in th e us e of one

scheme of rock a nd soil descript ion. I t is recognized th at pract i t ioners may

wish to continue to use oth er des cript ive methods and terminology. Whatever

scheme is employed, the impor tant principle is th at a ll descript ive t erms should

be defined clearly and used consistent ly.

1.2 GENERAL GUIDANCE

1.2.1 Definitions of Rock an d Soil

The engi neer ing usa ge of rock and soil diff ers from t h e geological

usa ge of rock an d superficial deposits in cu r r en t Hong Kong practice. The

two schemes can be d is t inguished for most prac t ica l purposes by using t he

following simple definitions . In engi neer ing ter ms , a soil is an y natu rally

formed ea rt h material or fil l which can be br oken down by h and in to it s

const i tuen t gr ains ; conversely , a rock cann ot be broken down, o r may only be

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part ial ly broken down by hand, depending on i ts weathered condit ion. In

geological term s, superficial deposit cove rs any geologically recent.

unlithified, transported material of sedimentary origin (Bennett, 1984a); rock

re fe rs t o an y l i thif ied, sol id material of i gneous, sedimentary , pyroclastic o r

metamorphic orig in. Practically all of Hong Kong s sup erfic ial depos its wer e

formed within th e Qu at er na ry period (i .e. within t h e last two million ye ar s) ,

whereas most of th e rocks ar e much older than th is (see Appendix

A .

The

simplest way of relat ing th e two schemes is to consider engine ering soils as

comprising all superficial deposits and fil l thos e ro cks which have

weathered insi tu to th e condit ion of a soil in engineerin g terms. There ar e

one or two except ions to th is genera l d ist inct ion, suc h as rece nt h ard beach

rock a nd cor al (both of which would b e mapped by geologists as sup erficial

depo si ts ) , bu t s uch materials ar e of ve ry rest r ict ed exte nt in Hong Kong.

The above engineerin g defini t ions of rock an d soi l ar e used th rou gh out

this Geoguide, except in Appendix

A

and in a few other cases where the

geological definitions a r e clearly implied by t h e text. I t should be emphasised .

however , th a t th er e a re no hard and fa st ru les; i t may well be appropr ia te to

us e different definit ions. depending on th e part icular requir ements of th e

engineer ing projec t .

1.2.2 The Hong Kong Geological Survey

This Geoguide us es t he classification system and nomenclature fo r ro cks

and superficial d epos its developed by t h e Geological Sur vey Section of t h e

Planning Division. Geotechnical Control Office (h er ea ft er re fe rr ed to a s th e

Hong Kong Geological Survey

(HKGS)

.

The description and classification of

rock s used by geologists for mapping pur pos es re qui res a detai led considerat ion

of mineralogy an d p etr og rap hy, which may be of in te re st to eng ine ers only in

special circumstances. However, th e value of an engineer ing rock o r soi l

descript ion is often increased if th e materials encount ered a r e placed in th e

context of th e geological s t ru c t ur e of t he area around t he s i te . In th is

resp ect , t h e engi neer should cons ult th e geological maps and memoirs p roduced

by th e HKGS. In cases of difficulty when identifying rock typ es , o r

int erp ret ing geological maps, th e engin eer should consul t a geologist fo r

assis tance .

A

new programme of detailed sys tema tic geological mapping b y th e

HKGS. a t a scale of 1:20 000, ha s bee n u nd er wa y in t h e Geotechnical Control

Office si nc e 1983. As of t h e en d of 1988, six maps a nd th r e e memoirs will be

available (Addison, 1986; G C O 1986a; 1986b; 1987b; 1988a; 198813; 1988c;

Langford e t al, 1988; St ra ng e Shaw. 1986). The full mapping programme,

which i s summar ised in Geoguide 2 (GCO, 1987a). will cov er t h e whole of t h e

Territory. both onshore and offshore, and will eventually comprise fifteen maps

an d six memoirs (s ee pp 188-189). On completion, th is mapping will su pe rs ed e

th e earl ier geological s ur ve y work carr ied o ut b y Allen

Stephens (1971).

1.2.3 Material and Mass Char acte rist ics

Complete rock and soil descriptions should include information on both

material and mass characterist ics.

For rocks, the dist inct ion between material and mass characterist ics

depe nds on th e size of th e sample in relat ion t o the typical spacin g of

discontinuities and oth er aspects of rock str uc tu re . Rock material ref ers to

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th e relatively s tr on g cohesive assemblage of mineral par ticles t ha t form th e

int ac t rock blocks between discontinuities in t h e rock mass. Therefore, rock

material character ist i cs can be observed and described most easily in hand

samples of rock a nd dril lcore. Rock mass re fe rs to a la rg er volume of rock

th at contains discontinuities such as joints , faults and bedding planes; such

fea tur es a re b est described in f ield exposures, although some indication of mass

characteristics may be obtained from boreholes.

For soils , the distinction between material and mass c hara cter ist ics also

depends on th e size of th e sample being describe d. In practice, thi s

distinction may not be a s cle ar as for roc ks bec ause some of t h e

characte r ist ics , a t both th e mater ial an d mass scales, may be d estr oyed o r

altered signif icantly if t he soil has been d ist urb ed or remoulded in relation to

it s or iginal undis turb ed condition. Therefore particula r attenti on should be

paid to t h e deg ree of sample distu rba nce when making soil desc ript ions . Mass

character ist ics in soils can only be described satisfactorily in undisturbed

samples or exposure s. Weathering proces ses a r e of part icul ar importance in

Hong Kong, and many of th e soils enco unte red in engine erin g works a r e th ose

deri ved from insi tu weathering of rocks. Where the y retain t he original rock

mater ia l te x ture a nd fabr ic , thes e engineer ing so ils repr esen t a special case for

description, because they can be describe d both as rocks and as soils. This is

considered in gr ea te r detail in t he main text.

1 2 4

Description in Different Locations

Rock and soil descriptions for engineering applications in Hong Kong are

typically carr ied ou t in t hr ee main locations, viz

(a) in th e f ield , a t a natural or man-made exposure,

(b ) in t he f ield, on core obtained from a groun d

investigation dril l ing r ig . and

(c) in th e laboratory, on pieces of core o r ot he r small hand

samples and various types of confined samples.

General guidance on description in each of these locations is given in

Table

1.

I t i s emphasised tha t t he scope of th e descr ip t ion , and t he degr ee of

emphasis given to particul ar descripti ve i tems, may need to be varied to sui t

th e partic ular application (e.g . projects involving slopes, tunn els , foundations,

e t c) . I t i s the responsib i l ity of t he pro je ct engineer or engineer ing geologis t

to decide on the appropr ia te scope and deta i l required .

With r ega rd to th e description of core samples in th e f ield and in t he

labo rato ry, two common aspe cts of poor practic e should be mentioned. Fi rst ,

descriptio ns should re fer only to th e specif ic locations from which t he samples

have been taken. Descriptions of small dis tur bed samples obtained from drill

b i t cu t t ings are of ten used wrongly to character ise t he n at ure of t he mater ia l

throughout t he complete core r u n or ad jacent core run s . second re la ted

point is th at only a small percent age of soil samples reco vere d by tr ipl e-t ube

core-bar re l s a re ev er sp l i t open and descr ibed . Most samples ar e usually

reserved for laboratory tes t ing . s ign if ican t proport ion ar e of ten d iscarded

a t the e nd of th e pro ject without being opened fo r e i t her descr ip tion o r

testing. Even if samples a r e opened for t h e purpo se of selecting te st

specimens, t he remainder of t he co re is of ten discarded without being

desc ribe d. Adequate description of all borehole samples reco vere d is esse ntia l


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DE S RIP TIO N OF RO KS

2.1

G E N E R L

The recommendations i n thi s c ha pt er a re ge nerally applicable t o all local

rock types. For some of t he important descriptive character ist ic s , such as

st at e of we athe ring, specific recommendations a r e given only for some of t h e

common rocks. This disproportiona te trea tme nt ref lects th e gre at er

engineering experience gained in cer tain rock types (mainly granite,

granodiorite and some tuffs) .

2.2 PURPOSE

ND

SCOPE OF

R O K

DESCRIPTION

The main p urpos e of a rock description for engineering purpo ses i s to

give an indication of t h e likely enginee ring prope rtie s of t h e rock .

complete description should comprise a simple rock name, qualified by selected

descr ip t ive terms for s t ren gth , co lour , tex t ure or s t ru ct ure , g ra in s ize , s ta t e of

weathering and alteration, discontinuities and other chara cte r ist ics as

appropr ia te .

Geological classification of roc ks is nece ssar y to in te rp re t th e geological

st ru ct ur e of an a rea , and t o establish good geological correlation between

boreholes; i t can also be important when rock i s requi red for const ruction

purposes. for example as building stone, concret e aggr egat e or roadstone.

s

with ot he r geological cl assificat ions, t h e HKGS roc k classification scheme does

not include engi neer ing prope rtie s of rock. In practice, however. engi neer ing

properties a r e of ten closely related to geological character ist ics , a nd engi nee rs

with local exper ienc e may in t e rp re t th e HKGS rock classifi cation t o some

extent in terms of likely engineering characte r ist ics . This is considered

f u r th e r i n Section

2 3 5

In t h e following sections, mater ial character i st ics ref ers to essentially

uniform pieces of rock and drillcore; discontinuities and othe r s t ru ct ur al

fea tur es will not normally b e considered in th e material description unless the y

occu r as an in tri nsi c chara ct eri sti c with a spac ing of less t ha n abo ut 200 mm

(e.g. s latey cleavage) . This is th e scale a t which detailed description and

logging of samples, and most engineerin g te st s , a re unde rtak en. Mass

character is t ics ref ers to larger vo lumes of rock t hat incorporate t he usual

s t ruc tur al features ; they can be fu lly apprecia ted only t hroug h carefu l f ie ld

description. This is th e scale which is usually most rele vant t o engineering

design and construction. Mass engineering properties a re generally much more

difficult to determine than material properties , because of t he gre at er inf luence

of s tru ctu ral de fects and th e ir r egu la r distr ibution of different component

materials within the mass.

In most Hong Kong ro cks , t he pres ence of discontinuit ies and t h e effect s

of weat heri ng will have a gr ea t influence on engi neer ing behaviour. Hence,

t he des criptive methods recommended in this cha pte r place emphasis on su ch

fea tu res .

In a rock desc ription, the main cha ract er is t ic s should preferably be given

in t he following o rd e r (ba sed on Hawkins.

1984)

(a) s t ren gt h (materia l) ,


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(b ) colour (material) ,

(c ) textu re and fabr ic (mater ia l) ; s t ru c t ur e (mass) ,

( d ) st a t e of weathering and al terat ion (material and mass) ,

(e ) rock name (in capita ls, e.g. GRANITE), inclu ding gr ai n

size (material),

(f) discontinui t ies (mass) , and

( g ) additional geological information.

In t he following sect ions, each of t he charac terist ics in this l ist is

discu ssed in detail fo r both rock material and rock mass, as appro priat e.

The

scope of th is scheme is similar to th e schemes recommended by BSI (1981),

Geological Society (1972, 1977) a nd IAEG (1981). The di ff er en ce s in t h e

pre sen t scheme ar e ones of detai l, mainly with re spe ct to th e descript io n of

th e weathere d s ta te of t h e rock material and rock mass, an d an expansion of

th e descript ion of discontinui t ies and oth er aspec ts of rock mass str uc tu re .

In addit ion to descript ion, in some circumstances i t may be usefu l t o

make an overall classification of rock masses fo r engin eerin g pur pos es.

Guidance on rock mass classification is given in Chapter

4

2.3 DESCRIPTION

OF

R O K MATERIALS

2.3.1 Strength

A

recommended scale of s tr en gt h, based on uniaxial compressive st re ng th

test i ng (UCS), is given in Table 2. This scale is similar to t ha t us ed in BSI

(1981). bu t has been extended a t th e weaker end in ord er to cover th e

extremely weak decomposed rock materials t h a t ar e commonly enc oun ter ed in

Hong Kong. Simple field identification te s ts have also been added s o th at th e

st r eng th te rms may be es t imated pr ior to any labora tory tes t ing; thes e a re

bas ed on t h e classifications giv en by t h e Geological Society (1977) an d Miller

et a1 (1986).

The st re ng th of rock material determined in th e uniaxial compression

te s t i s dependent on th e moisture content of t he spec imen, anisot ropy and t he

te s t p rocedure adop ted . A review of compressive strength test pract ice in

Hong Kong ha s be en made by Gamon Sz eto (1984).

The point load t e s t (PLS) is a usefu l index te st f or est imating th e

compressive s tr en gt h of moderately weak t o extremely s tro ng r ock s (ISRM,

1985). Approximate PLS values fo r th e gra nitic an d volcanic roc ks in Hong

Kong a r e included in Table 2 These values a re deriv ed b y applying a

correlation factor of 24 t o UCS values. They a r e base d on th e work repo rte d

by Lumb (1983) an d Gamon (1984). an d on a revie w of existing li te ra tu re fo r

isotropic igneous and volcanic rocks carried out within the Geotechnical

Control Office. The corr elatio n fa ct or of 24 i s gener ally les s relia ble fo r

rocks a t the weaker end of t he PLS te s t scale.

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2.3.2 colour

Colour may be expressed quantitatively in

terms

of th re e parame ters

hue , chroma an d value (Munsell, 1941). Hue is a basic colour o r a

mixture of basic colours, chroma is th e bril l iance or intens ity of t h e colour,

and value is th e lightn ess of th e colour. The complete Munsell syste m

contains a large number of examples of different hues. chromas and values,

and is too detailed for general engine ering use.

much simpler qualitative

system is recommended, as s et ou t in Table 3. In cases where the colour

distribution is non-uniform, an additional descriptor should be used in

conjunct ion with the three basic terms (Table 3).

For a more detailed des criptio n of colour, t h e Rock-Color Cha rt

(Geological Society of America, 1963) i s recommended. This c h a r t is a

simplification of t h e Munsell system. b u t it con tai ns many more hue s, chromas

and values than Table 3.

I t should be noted th at wetting a rock sample decreas es th e value (i .e.

makes t he sample dar ke r) , but does not chan ge th e hue o r chroma.

Therefore,

a good rock colour descript ion should s tat e whether t he sample was wet or dr y

when described. In wri t ten repor ts, descript ions should ideal ly be suppl e-

mented by colour pho tog rap hs, which shou ld always include a stan da rd colour

chart for reference (e.g. Kodak Color Control Patches).

2.3.3 Tex tur e and Fab ric

Texture is a broad

term

th a t re fer s t o th e genera l physica l appearance

of a rock. I t covers th e geometric aspect s, such as size and sh ape of th e

component grain s o r cry sta ls, and th e relat ionships between the se asp ects (e.g.

distr ibut ion of variou s grain sizes and crystal l ini ty, or th e deg ree to which

cry sta ls have developed in the rock). The term is usually applied to th e

small-scale fea tu re s visible in hand specimens. If the rock i s composed of

ve ry small gra ins , t h e description of t ex tu re may not be possible without th e

us e of a hand le ns o r a microscopic examination of a thin slice of t h e rock.

The te rm s t ruc tu re i s used for the larger -scal e physical feat ure s of a rock

and i s considered in Section 2.4.2. However, terms describing st ru ct ur al

features with a spacing of less than about 2 mm may als o be ap pli cab le to

rock material descriptions.

The most common tex tu ra l terms applicable to Hong Kong ro ck s a r e

i l lustrated in Plate 1 and a r e defined in th e Glossary. The use of the se terms

is general ly re str ic te d to th e crystal l ine igneous and non-fol ia ted metamorphic

rocks (Table 5; see also Appendix A.2.5) , the gr ain s o r cryst als of which have

usually formed in close mutual association (e.g. du ri ng solidification of an

igneous rock from a magma).

The main a spe cts of rock tex tur e il lustrated b y Plate 1 a re re la t ive gra in

size (e.g. equ igr anu lar , megacr ystic) an d cryst allini ty. Some methods of

textur al descri ption, e.g. IAEG (1981), also cover abs olute gra in size, or t he

ave rag e dimension of th e mineral or rock fragme nts which make up t h e rock.

However, i t is common practi ce in Hong Kong to link g ra in size terms directl y

to th e rock name. Therefore, grain size is consi dered in Section 2.3.5. Grain

shape is another aspect sometimes taken into account in the description of

sedimentary rock t ex tu re s, bu t s ha pe terms a r e more commonly used in soil

descript ions when individual intact grains can be easi ly separated (see Section

3.3.3).


T a b l e of C

on t en t s






Fabr ic re f ers spec if ica l ly to the a r rangeme nt of t he const i tuent gra ins

o r c rysta ls in a rock. Preferred or ienta tion of thes e const i tuents is of ten the

most noticeable as pe ct of t h e rock fa bric .

In igneous and o th e r c ryst a l line rocks, t he fabr i c i s t h e pa t t e rn

produced by th e var ious shap es and or ienta t ions of t he c rysta l l ine and non-

crysta l l ine pa r t s of th e rock. I t i s dependent on th e re la t ive s izes and sha pes

of th ese par t s and t he i r posit ions wi th res pec t t o one another and to the

groundmass , where present . In sedimentary rocks , preferr ed or ienta tion , where

pre sen t , of t he individual grains a nd the ir posi tion in relat ion to any

cementing material a r e usual ly th e most important aspec ts of t h e fabric .

Fabrics in f ine sandstones and mudstones cannot be described sat isfactori ly

without t h e use of a hand len s or microscope.

The orientat ion of grai ns an d cry stal s may b e described in qual i ta t ive

terms o r , a l te rnat ively, may be quantif ied by measurement with a compass-

cl inometer if t he fabric direct ions ar e clear to t h e naked eye. For rock s

insi tu, fabr ic orientat ions may be s tated ei the r direct ly as a compass bearing

and dip angle , or indi rec t ly in re la tion to o th er pa r t s of t he fabr ic , o r to

st ru ct ur al planes suc h as joints a nd bedding (e .g. e longate part ic les in a

sedimentary rock would often be described as having a preferred orientat ion

paral le l t o th e bedding planes) . In recovered samples, fabr ic orientat ions can

only be des cribed indirect ly, u nless t h e configurat ion of t he whole sample in

th e gro und is recorded accurately at th e time of sampling.

Fabric also includes an y small discontinuit ies o r planes of s eparat ion

thr ou gh or between gr ains or crystals. These a r e commonly termed micro-

fr ac tu re s and may be caused by mechanical weathering , tectonic act ivi ty,

stress-re l ief or oth er factors. Microfractures can have a signif icant effect on

engineer ing proper t ies and ar e par t icular ly common f ea tu res in the coars er-

grai ned gra niti c rock s. No specific term s ar e recommended for t h e description

of microfrac tures , bu t as a genera l ru le the i r in tensi ty , spac ing, cont inui ty an d

any preferred orientat ion should be noted. Plate 2 shows four examples of

microfractu res, for which ap prop riate descript io ns would be a s follows

Plate 2A. Randomly-oriented mic rofr actu res inte rsec ting

a n iron-stained joint surf ace in highly decomposed,

co ar se -g ra in ed GRANITE. Spaci ng var iab le. 2-20 mm.

Some have open aper tu res u p to 1

mm

wide. Rock eas ily

crumbled to f ine grave l and f iner-s ized f ragments due . to

microfracturing.

Plate 2B. Continuous s ubve rt ic al microfractu res paral lel

to tension cr ac ks in sa nd y SILT (Residual Soil). Spacing

5-20

mm Soil loose an d ve ry fr iable a s a r es ul t of

microfracturing.

Plate 2C. Continuous curved rnicrofractures following the

bo und ary sh ap e of a larg e core ston e of moderately

dec ompos ed, med ium -gr ain ed GRANITE. Spaci ng 5-10

mm

Effect of microfractures is to form thin curved shells of

rock.

Plate 2D. Intense random microfracturing between and

th ro ugh qua r t z and fe ldspa r g ra ins in s l ight ly

decomposed , medi um-g rain ed GRANITE. Spac ing gen era lly

1 mm

T a b l e of C on

t en t s


T a b l e

of C on t en t s

T





T h e d e s c r i p t i o n o f t e x t u r e an d f a b r i c s h o u ld i n c l u d e a n y o t h e r n o ta b le

f e a t u r e s o f t h e s m a ll -s ca le p h y si ca l a p p e a r a n c e o f t h e r o c k .

For exam ple , i n

ce r t a i n vo l can i c l ava s , sm all pore s o r vo i d s ( t e rm ed ve s i c l e s ) a re som e ti m es

v i s i b l e . The s i z e , sha pe , spac ing and o r i en t a ti on o f vo i d s shou ld be de sc r i be d

w h e r e a p p r o p r i a t e .

T e x t u r e a n d f a b r i c a re p r ob a bl y t h e m o s t d i f f i c u l t a s p e c t s o f a r o ck

d e s c r i p t i o n f o r t h e n o n - s p e c i a l i s t .

t

i s n o t e a s y t o g i v e s im p le p r ec is e

g u id a nc e o n t h e u s e o f a pp r o p r ia t e te r m s .

f p ro ble m s a r e e n c o u n t e r e d , t h e

eng i n eer shou ld cons u l t a geo l ogi s t f o r a s s i s t an ce .

2.3 .4 Weather ing and Al t era t ion

( 1 ) G en er al A s p e c t s o f W e a t h e r i n g . W e a t h e r in g h a s a v e r y s i g n i f i c a n t

e f f e c t o n t h e e n g i n ee r i n g p r o pe r ti es o f r o c k . M ost e n g in e e r in g p r o je c ts i n

H ong K ong en cou n t e r sub s t an t ia l t h i ck ne s s e s o f w ea t hered ro ck , w h i ch m ay

v a r y s i g n i f ic a n t l y i n d e g r e e o f w e a th e ri n g o v e r r e l a ti v e ly s h o r t d i s t a n c e s .

T h e r e f o r e , c a r e f u l de s cr ip t i o n an d a s s e s sm e n t o f t h e s t a t e o f w e a th e r in g o f t h e

rock m ateria l i s o f par t i cu lar im por tan ce .

Th e t w o m ai n com ponen t s o f w ea t her i ng a re m echan ica l d i s i n t eg ra t i on

and chemical decompo s i t ion . Genera l l y , bo th mechanical and chemical e f f e c t s

a c t t o g e t h e r a nd a r e i n t e r d e p e n d e n t , b u t , d e p e n d i n g o n t h e pa st a nd p r e s e n t

c lim a tic a n d h y d ro lo g ic a l r e g i m e s . o n e o r t h e o t h e r o f t h e s e a s p e c t s m a y b e

dominant .

Mechan ical w ea t her i ng o r d i s i n t egra t i on i s caused b y phys ica l p roces se s

s u c h a s f r o s t a c t io n , a b s o r p t i o n an d r e l ea s e o f w a t e r , a nd c h a n g e s i n

t e m p e r a t u r e an d s t r e s s a t o r n ea r t h e e x p os e d r o c k s u r f a c e . t r e s u l t s i n t h e

o p e n i n g o f d i s c o n t in u i t i e s , t h e f o r m a ti on o f n e w d i s c o n t i n u i t ie s b y r o c k

f r a c t u r e , t h e o p e ni n g o f g ra in b o u n d a r ie s , and t h e f r a c t u r e o r c le a v ag e o f

ind iv idu al minera l gra ins .

Di s in t egra t ion o f rock mater ia l can a lso be caused

or speeded up b y b i olog ica l f ac t o r s suc h a s t r e e roo t g row t h i n j o i n t s.

Decomposit ion o r chemica l weather in g i s t h e process b y which chemical

r e a c t i o n s , s u c h a s h y d r a t i o n , ox i d a ti o n , i on e x c h a n g e a nd s o l u ti o n , t r a n s f o r m

roc ks and m i nera l s i n t o new chem ica l com b i na ti ons t ha t a re s t ab l e u nd er

prevai l ing env i ron men ta l con di t ion s . Decomposit ion cau ses some s i li ca te

m i n er a ls s u c h a s f e l d s p a r s t o c h a n g e t o c l a y m i n e ra l s , b u t o t h e r s , n o t a b ly

q u a r t z , r e s i s t t h i s ac ti on a nd m ay s u r v i v e l a r g e l y u n c h a n g e d . S o lu ti o n i s a

par ti cu la r l y i m po r t an t a spec t o f chem ical w ea t her i ng i n carb ona t e roc ks s uch

as li m es t ones . C hemical w ea t her i ng a lso r e su l t s i n d i s co loura ti on o f t h e rock

w h e n c om pa re d w i t h i t s c o lo u r i n t h e f r e s h ( u n w e a t h e r e d ) s t a t e .

D ecom pos iti on i s t h e dom i nan t a sp ec t o f w ea t her i ng i n Hong K ong ro ck s .

Rock material we athe ring in Hong Kong rare ly pro duces a hom ogen eous

w ea t hered rock m ass w h ere a ll rock m a t e ri al i s w ea t hered t o t h e same d eg re e ,

o r e v e n a s im p le w e a t h e r e d p r o f il e w h e r e t h e d e g r e e o f w e a t h e ri n g d e c r e a s e s

p r o g r e s s i v e l y w i t h d e p t h . C om ple x v a r ia t io n o f w e a t h e ri n g t h r o u g h o u t t h e r o c k

m a ss i s m o re o f t e n t h e r u l e . T h i s r e f l e c t s s u c h v a r ia b l e f a c t o r s a s o r i e nt a ti o n

a nd s p a ci ng o f d i s c o n t i n u i ti e s in t h e r o c k . g r o u n d w a t e r f lo w p a th s a nd t h e

r em o v al o f o v e r ly i n g w e a t h e r ed m a te ria l b y e r o s i o n . T o a p p r e c ia t e f u l l y t h e

p a t te r n o f w e a t h e ri n g i n a ro c k m a s s , i t i s n e c e s s a r y t o m a k e c a r e f u l

o b s e r v a t i o n s o f t h e d e g r e e o f m a te ria l w e a t h e ri n g i n n a t u ra l e x p o s u r e s .

e x c a v a t i o n s ,

bor eho le samples and t r ial pi t s .

T a b l e of C on

t en t s


T a b l e

of C on t en t s

T





Weathering i s a gra dua l, continuous process.

~

is difficult to describe

it s effect quantitatively as a precis e degr ee of chan ge from an original

unweathered condit ion. For general descript ive purpo ses, i t is convenient to

classify th e weather ing s equ enc e into a number of easily recognizable gr ade s.

In view of the dominance of chemical decomposition in Hong Kong rocks,

material weathering gr ad es hav e been traditionally classified using th e term

decomposed ra th er than th e more genera l term weathered . This practice is

retained here. The term weathered is used in the rock mass weathering

classification in Section 2.4.4.

(2) Classification of Dec om~ osi tio n Grades . Recommendations fo r t h e

des cri pti on of decomposition gra de s of rock material ar e given in Table

4.

The

left-hand side of th e table gives a summary of th e general characterist ics th at

can be use d t o as se ss decomposition g ra d e in most Hong Kong rock s. This is

general ly applicable to all the igneous and pyroclast ic rock s, b ut i t can also be

used fo r ot he r rocks which have st re ng th s in th e fresh condit ion similar to

those of th e grani tes and tuf fs , i.e. in the s t rong to extremely s t rong r ang e in

Table 2 (Lumb. 1983). However. not all t h e ge ne ra l cha rac ter ist ics lis ted in

Table 4 a r e applicable to some of t h e weaker s edimen tary a nd metamorphic

rocks. I t may be more convenient to use a different c lassif icat ion for thes e

materials (e.g. Beggs Tonks, 1985, used fo ur class es fo r descri ption of

sedime ntary and metamorphic rocks in t h e Yuen Long are a). If a dif fer ent

number of c lasses is used, and t he class boundaries cannot be equated

conveniently with an y of th os e in Table

4

i t is recommended that alternative

ter ms and symbols a r e adopt ed and defined clearly t o avoid confusion.

The g ra de classification is of ve ry limited u se f or de scrip tion of

carbon ate rocks suc h as l imestone and marble . Since pure carbonate material

dissolves completely in co ntact with weakly acidic groun dwa ter , th er e is no

gra dua l transi tion from fr es h rock to resid ual soil. However, most car bon ate

roc ks contain a small percentage of non-soluble impurities (e.g. qua rtz , iron

oxides, clay minerals) which ca n accumulate in th e form of resid ual deb ris as

th e su rr ou nd in g car bon ate material is removed in solution. The only

sig nif ica nt ca rb on at e rock i n Hong Kong is th e marble of t h e Yuen Long

Formation, which gener ally has a ver y low pe rc en tag e of impuri ties (Pascall.

1987: Langford e t al. 1988). Hence t h e amount of res idu al d eb ri s pro duc ed b y

wea the rin g of t h e marble itself is usually negligible, and is of little import ance

for engineering. I t should be noted th at this de bri s should not be classified as

a t r ue insi tu resid ual soil since th e solution of th e carbonate material res ul t s

in a complete collapse of t h e original rock fab ric .

The general cha racter ist ics shown in th e left -hand side of Table 4 a r e

based on ob serva tion o r simple tes ts th at r eq ui re a minimum of field or

lab ora tor y equipme nt (e.g. use of a geological hammer, break age of lumps by

hand, visual eviden ce of discolouration). water sup ply an d small contai ner

ar e needed to car ry o ut th e slake tes t , which is used to dist inguish between

completely an d highly decomposed rock. The re su lt s of th e slake t es t may be

affected by differ ences in th e initial sample moisture content. Generally, i t is

a fair ly rel iable indicator when used in part ly sat ura ted soi ls , b u t i t is diff icul t

to apply in fully s at ur at ed soils sampled from below t he wat er tabl e (Howat.

1986).

Discolouration may not be an easy indica tor t o use, becaus e it relies on

knowledge of t h e colour of t h e fr es h r ock , which is often not exposed.

Generally, i t is a good indicator of t h e differen ces between fre sh , sligh tly,

moderately and highly decomposed rock. Slightly decomposed material can

usually be distin guish ed from fre sh rock by sta ining in th e vicinity of rock


T a b l e of C

on t en t s


T a





joints often a brown staining ca use d by t h e formation of iron oxides).

Moderately decomposed rock i s usually stained th rou gh out , while highly and

completely deco mposed rock commonly show a complete colo ur ch a n g e when

compared with t he f re sh rock.

Very few of t he gen eral charact eris tics in Table

4

are def in i t ive for

ass essi ng th e decomposition gr ade s. I t is recommended th a t a number of

di f fe rent tes t s and observa t ions should b e carr ied ou t wherever possible before

t h e assignmen t of t h e decomposition gr ade .

The remaining columns of Table

4

give typica l charac ter i s t ics for the

f o u r most common rock ty p es i n Hong Kong.

These can be used in addit ion to

th e genera l indica tors d iscussed above , providing th e rock t yp e can be

identif ied. The cha racte rist ics include typical se quences of colour change s.

decomposi tion of cert ain minerals, and t h e res ul t s of o the r simple str en gt h

index tes ts .

A t t h e less decomposed end of t h e scale, t h e Schmidt hammer is a rapi d

an d simple field te st . However, as disc uss ed in Geoguide 2 GCO, 1987a), c a r e

is required when using the hammer on weak, cracked or f issured rocks, o r on

any rou gh rock su rfac e. Notes on the us e of t he hammer ar e given in Table

4

A t

th e more decomposed end of t h e scale, a stan dar d hand penetromete r

can be used to give an approximate indicat ion of undrained sh ea r st r en gt h

Table

4 .

A s with th e s lake tes t , th e resul t s may be af fected by changes in

th e sample moisture content , b ut i t is general ly rel iable for dis t inguishin g

between highly an d completely decomposed rock i n gran itic materials. In th e

medium- and coa rse-gra in ed igneous rocks , ano ther useful tes t i s to ass ess th e

deg ree of a l terat ion of t he fe ldsp ar grain s by probing with a knife o r pin, o r

by crumbling between th e fin ger s. Different ty pe s of feld sp ars may decompose

a t d i f fe rent ra tes : th e engineer should consul t a geologis t for ass is tance in

ident if icat ion and descript ion where appropriate .

The six decomposition gr ad es in t h e most common Hong Kong r oc ks a r e

i l lustrated in Plate 3 In general , decomposi t ion effects a re most obvious in

th e igneous and highly metamorphosed roc ks, part icularly coarse-graine d t yp es

which posse ss lar ge decomposable minerals. In sedimentary and less highly

metamorphosed ro cks , th e effe ct of decomposition on cha nge s in colour and

mechanical pro pert ies i s less marked; i t may not be eas y to assig n

decomposition grades in these rocks.

Assessm ent of decomposition gr ad e us in g Table

4

i s adequate for genera l

des crip tion s, but subdivisi on of t he g ra de s may be justified if a more detailed

descriptio n is r eq ui re d; for example when making detailed correlations between

laboratory t es t resu l ts for engineering design and de gree of decomposi tion.

For th is ty pe of desc ript ion, more detailed obs erva tion s of t he rock

text ure/ fabr ic should be made and individual index te s t resu l ts on specif ic

samples should be quoted. Other, more prec ise, labo rato ry and field index

te s ts should also be considered e .g. quick absorpt io n, densi ty, slake durabi l i ty

and point load s tr en gt h te sts in gr ade s 1-111 materials; SPT, dr y de nsi t y and

part ic le size distr ibut i on te st s in gra des IV VI). Fu rt he r guidance on rock

and soil index te st s is given by Brown 1981) an d BSI 1975) resp ectiv ely. A

review of th e use of index tes ts for engineering ass essment of weathered roc ks

ha s been made by Martin 1986).

The most detailed method of d es cri bin g d eg re e of decomposition i s to

use a wholly quantitative index. A n example is the X index for grani te

prop osed by Lumb 1962), which is bas ed on a comparison of t h e weig ht ratio s

T a b l e of C on

t en t s


T a b l e

of C on t en t s

T





of qua rtz to felds par in th e fre sh and decomposed rock. A number of other

quantitative indices have also been defined on the basis of mineralogical

examination (I rf an Dearman.

1978).

However, t h e calculation of t he se

variou s indices involves th e use of detailed petro graphi cal analysis, which is

general ly inappropriate for rout ine descript ions.

The distinction between completely decomposed rocks and residual soils

(i.e. Tablerades V and VI in is important fo r full description of th es e

materials.

Grade VI residual soils have lost all evidence of the original rock

texture .

Therefo re, a full descr iption of t he se materials can only be made in

soil terms (see Chapter 3 . Since grade V materials retain th e original rock

texture , i t is recommend ed that complete descriptions should be made in rock

terms (see Sections 2.3.1 t o 2.3.6), supplemented where necessary by additional

soil terms to cover compactness/consistency and particle size distr ibutio n

applicable to t h e remoulded condition. Fu rt he r guidance is given in Section

3.5.

13) Sta te of Disintegration. The ass ess men t of decomposition gr ad e

should be supplemented

by descript ion of t h e sta te of disintegrat ion of t he

rock material. This can be import ant in term s of t h e likely eng ine eri ng

behaviour of th e rock. For example, an intens ely disin teg rat ed, friable,

moderately decomposed rock may well show th e engin eering proper ties to be

expecte d of highly o r completely decomposed material. I t shoul d be noted th a t

small-scale cracki ng an d fr ac tu ri ng of rock can be caused by facto r s o the r

than disintegration (mechanical weathering), see for example Plates 28 and 2D.

In many rocks , i t is not easy to separ ate the effects of t he different factors.

If th er e is doubt on th e origin of c rac ks and microfractures, they may be

described und er th e general non-genet ic heading of rock fabric (se e Section

2.3.3).

A

fu rt he r difficulty with th e description of disintegration is th at,

unlike decomposition, often i t is not possible to di sting uish a pr ogr ess ive

seq uen ce of i ncreas ing disintegration o ve r th e complete material weathering

scale. Once clay minerals s t a r t to form in th e weathe ring proce ss, cr ack s can

be closed or healed a s th e original rock fabr ic begins to be destro yed,

leading to an app are nt reduct ion in t he deg ree of disintegrat ion with

increasing weathering.

4 ) St at e of Alteration. Rocks may be al ter ed by circulat ion of hot

ga se s an d fluids associated with lat er st ag e intru sion. Common alteration

terms ar e kaolinized an d mineralized (s ee Glossary). The terms used f or

des cri pti on of decomposition gr ad es of rock material may be us ed wh er e

ap pr op ri at e (e.g. a kaolin depo sit may be des cri bed a s completely decomposed),

because in many c ases t he effects of a l terat ion ar e not easily dis t inguished

from tho se b rou ght a bou t by decomposition. In Hong Kong roc ks, alteration is

often visible in the coarse-grained grani t ic rocks, part icularly around q ua rt z

veins . The most common effec ts ar e fel dsp ars alte red to soft white kaolin in

relatively undecomposed material , and a n overall reduction in t he material

grain size. Also, th e qua rtz content may app ear lower than in th e surr oun din g

unalte red material , due to solution of original qu ar tz by hydrothermal activity.

2.3.5 Rock Name (I nc lu din g Grain Size)

Recommended r ock names ar e given i n Table 5 They should be written

in capital le tt er s. This tab le follows t h e sys tem of rock classification an d

nomenclature u sed by t h e Hong Kong Geological Sur ve y, bu t it is inte nde d only

as a genera l guide fo r engin eers. Geological tra ini ng is req uir ed for

satis factor y identification of rocks . The eng ine er need not be overl y


T a b l e of C

on t en t s


T a





concerned about t h e large number of igneous rock t yp es shown in Table

5,

o r

t he app ar en t complexity of th ei r classification. Outcrops of most of t he basic

an d intermedi ate typ es a r e only found in small ar ea s of Hong Kong. Granite,

granodiori te and rhyoli te a re t he most common igneous rock s in t h e Terri tory.

Grain size terms, which re fe r to th e aver age dimension of t h e mineral o r

rock fragments comprising th e rock, ar e included in Table 5, either implicitly

in the rock name o r as a specific qualifying term.

I t should be no ted th a t t he

gra in s ize descr ip tors for grani te , ash tuff and superf icia l deposit s (i.e. coa rse,

medium, fine) have

diffe rent limiting dimensions. For thi s reason , i t may not

be possib le to use t he cor rec t te rm for gra in s ize unless th e rock t yp e can be

identified accurately. In cases where the rock name is not known, i t is

recommended th at t he grain size should be wri t ten quanti ta t ively as pa rt of

th e description, toget her with an appr opria te textural term (e .g. megacryst ic

rock with large grains

10

to 25 mm s e t in a g rou ndm ass of smaller gra in s 2 to

4

mm ). St ra ng e

(1984)

has given a clear explanation of th e syste m of textura l

and grain size terms used for t he grani tes in Hong Kong.

The smallest grain size visible to t he naked ey e is ab out 0.06

mm

Ident ificat ion of gr ains smaller than this req uir es th e use of a hand lens o r a

microscope.

With experience, Table 5 can be interpreted to some extent in terms of

broad engineering characterist ics . For example, avera ge joint spacing in th e

igneous rocks t en ds t o increase with increasing grain size; compressive

st ren gth of f res h igneous and pyroclast ic rocks ten ds to decrease with

increasing g rain size. However, i t is emphasised th at Table

5

cannot be used

for detailed int erp ret ati on of engineer ing chara cteri stics . Complete engin eerin g

desc ripti ons of roc ks should include information on th e oth er i tems covered

elsewhere in thi s ch ap te r, as well as t h e rock name.

If th er e is dou bt about th e corr ect rock name, this may b e indicated in

th e description by us e of a suit able qualifying term (e.g. prob ably ) o r a

ques tion mark. F ur th er gu idanc e and explanation of th e HKGS syste m of rock

naming and classification is given in Appendix A

2.3.6 Addit ional Information

Any additional fe at ur es which could be of im portance in asses sing t h e

nat ure and engineering propert ies of th e material should be described aft er th e

rock name.

It is recommended th at th e resul t s of any quanti ta t ive index te st s (e.g.

point load st re ng th , Schmidt hammer r ebo und ) should be recorded a t this point ,

even if the y h ave been inter pre ted in defining ot he r descript ive characte rist i cs

such as st re ng th or sta te of weathering. These resu l ts may be useful for the

in terpre ta t ion of o t her tes t s

carried out on specific samples.

2.3.7 Examples

The following examples of roc k material de scr ipt ion s ar e given for

guidance in the use of a ppro priat e descript ive terms. Samples corresponding t o

thes e descr ip tions a re i l lus t rated in Plate 4

T a b l e of C on

t en t s

T a b l e of C

on t en t s

T a b l e

of C on t en t s





Plate 4A. Igneous Rock). Very stro ng , dr y, brownish

gr ey spo tted with single black bioti te crys tals a nd

occasional clusters of small biotite flakes, inequigranular,

sli gh tly decompose d, co ar se -g ra in ed GRANITE. Poin t load

s t r e n g t h

6.5

MPa. Schm idt hammer re bo un d val ue

5 5

measured on si te) .

Plate 4B. Pyroclast ic Rock). Weak, dr y , l ig ht yellowish

brown to pinkish br own, highly decomposed, coar se as h

TUFF with some small < 100 mm2 isolated areas of

randomly-oriented microfractur es, aver age spacing 2

mm,

located close to th e ed ge s of some of th e la rg er

int act mineral grains.

Plate 4C . Metamorphic Rock). Moderately weak, d r y ,

l ight grey mott led and streaked with orangish brown,

v er y narrowly cleaved, moderately decomposed,

PHYLLITE. Prominent ora ng ish o r re dd is h brown mineral

coating iron oxides?) visible on joint su rfa ce in pa rt of

sample. Exposed small ar ea s of clea vage planes a r e

undula t ing and shiny.

Plate 4D . Sedimentar y Rock). Moderately str on g, d r y ,

l igh t brownish g rey s tr i pe d with da rk brown and black,

thinly-laminated. fine SANDSTONE and MUDSTONE.

Mudstone forms the darker laminat ions: these are

0 1

2 mm in thic kne ss , mostly con tinuous bu t occasionally

impersist ent with convoluted o r branching en ds. Rock

not signifi cantly affected by we athe ring , i .e. rock colour

and s t re ngt h not s igni ficant ly d i f fe rent f rom fres h

material from field evid ence ).

2.4 DESCRIPTION OF ROCK MASSES

2.4.1 General

Rock masses should be described by f i r st considering t he material

characteris t ics of t h e rock, th en adding information about mass-scale

characteris t ics. With referen ce to th e l ist in Section 2.2, a mass des cription

would normally include a sta tement of st re ng th , colour, st r uc tu re , sta te of

mass weathering and alteration, rock name, discontinuities and additional

geological information. More detailed information abo ut t h e

tex tu re / fabr i c and

st at e of weat heri ng/a ltera tion of di ffer ent materials within the mass can be

adde d if necessary , b u t this may not be requ ired ,

depending on the nature of

th e projec t and the s tage a t which th e descr ip t ive information i s used see

Section 1.2.4).

Ini tia l ly, th e mass should be divided into sui table descript ive uni ts . This

of ten pre sen ts th e b iggest problem for engineer ing assessment . I t i s not

possible to give specific recommendations, as t h e requir emen ts may d iffe r from

one projec t to another , b ut th e genera l aim should be to d iv ide th e mass in to

geotechnical unit s, each of which has re asonably uniform charac teri stic s with

re ga rd to overal l engineerin g behaviour. In Hong Kong rock s, variat ions in

rock ty pe , degr ee/ex tent of weathering and natu re/e xten t of discontinui ties ar e

general ly t he most importan t characterist ics to conside r in th e selection of

geotechnical uni t boundaries.

T a b l e of C on

t en t s


T a b l e

of C on t en t s

T





In a relatively homogeneous rock mass in a sing le rock typ e, t he r e may

be no need for thi s subdivision and th e description of mass aspects should be

quit e str aig htf orw ard . Conversely, in a complex expo sure comprising two o r

more rock t yp es , each of which may var y significan tly with re ga rd t o

weathe ring and discontinuities, t he initial subdivision of t h e mass will b e of

gr ea t importance for good descript ion. In part icular , interbe dded sedimentary

and mixed

pyroclast ic/sedirnentary

rock se quen ces often pre sen t problems for

mass description. For example, the presence of one rock which is particularly

susceptible to weathering may affect th e weathering of ad jacen t rocks, an d th e

overall engi neeri ng pr opert ies of th e int erb edd ed rock mass may depe nd more

on th e one rock ty pe than t h e other s. Granitic rock s, and thick accumulat ions

of single types of pyroclast ic rock (tu ff) , ar e usually easier to subdivide, with

variat ion in weathering often being t he most important aspect .

Once the rock mass has been divided into appropriate geotechnical units,

mass characteristics should be assessed in detail and combined with material

chara cteri st ics (as noted above) to form t h e complete mass des cript ion for eac h

unit. The information on mass chara cteri stics should include

(a ) Section 2.4.2),escript ion of geological st ru ct ur e (se e

(b ) th e na ture , or ienta tion, spacing, persis tence , roughness ,

a pe r ture, infill ing and s eepag e aspects of discontinuit ies

(see Section 2.4.3), and

(c )

Section 2.4.4).etails of th e mass weathering profile (s ee

These th re e aspect s may have t o be considered separately fo r each rock

ty pe if a rock mass uni t contains more than one rock type . Reference may

also need to be made to major geological s tr uc tu re s such a s faults and folds

and di f ferent types of igneous int rusions (see Appendix and Bennett. 1984b.

for further information).

The term ' str uct ure ' is commonly used in different ways and requ ire s

fu r t he r explanat ion.

In th e broad est geological sense, st ru ct ur e includes two

main g rou ps of fea tur es , i .e . fract ure s (or discontinuit ies) , and folds (s ee

Appendix

A . 7 .

The size of the se fe atu res can v ar y widely, both in areal

extent and cross-sect ion. Large-scale as pects of regional rock str uc tu re , suc h

as major faul ts and folds , a re of ten not re levant a t th e sca le of an

engineering si te . The engin eer is usually concerned more with th e smaller-

scale str uct ura l featu res; for example, individual joint systems, l ineat ion and

foliat ion. However, such fe ature s ar e ult imately related to regional str uc tu re

and canno t be appreciated fully without some unde rstan ding of th e regional

s t r u c t u r e .

With r efe ren ce t o t he li st of desc ripti ve items in Section 2.2, fea tures

su ch as foliat ion and l ineation a re included un de r th e i tem 'str uct ure ' in the

following section, whilst 'discontinuities ' a r e consi dered s epara tely in Section

2.4.3.

Section 2.4.2

hus.

' s t ruc ture ' i n is used in a narrow sense. f or want of

a sui table a l te rnat ive heading. In th e broad sense , s t ru c t ur e includes

discontinuities, as noted above.

Another term in fairly common use by engineering geologists is

's tr uc tu ra l domain' (s ee Glossary). This term should only be applied to

chang es in th e discontinuity pa t te rn in th e rock mass, and not to chang es in

rock typ e o r weathering. I t would not be correc t to use the term for th e

geotechnical units created by subdividing a rock mass for descript ion as

T a b l e of C on

t en t s

T a b l e of C

on t en t s

T a b l e

of C on t en t s





discussed above, unless this subdivision were made solely on the basis of

discontinuity variation.

2.4.2

St r u c t u r e

The s t ru c t ur e of th e rock mass i s concerned wi th th e la rger-scale in t e r -

relat ionship of textural features (see Section

2.3.3 .

Common terms used to

des cri be sedimentary ro cks include bedd ed , laminated or massive ; igneo us

an d pyroclastic rocks may be massive o r flow-banded ; metamorphic roc ks

may be foliated . band ed o r cleaved . Eutaxitic is a term often applied to

welded tu ff s containing flatt ened len ses of pumice or oth er material which g ive

th e rock a d is t inc t ive s t reaked appearance . These te rms ar e def ined in th e

Glossary and ar e i l lustrated in Plate 5. Additional information on bedding is

given in Section

3.4.1.

Recommended descr ip t ive te rms for the spac ing of p lanar s t r uc t ure s a r e

given in Table 6

For sedimentary rocks , s t ruc tures such as bedding may be

described as thick beds o r thickly-bedded ; for example, a thickly-bedded

sandstone . For igneous and metamorphic rocks , th e appropr iate descript ive

terms f or th e s tr uc tu re should be us ed; for example. medium fol iated schis t ,

ver y narrowly cleaved phyllite , ver y thickly flow-banded rhyolite .

There is some overlap between textural characteristics of rock materials

and s t ru c tu ra l charac ter i s t ics of rock masses . St ru c tura l fea tures wi th a

spacing of less than abou t 200 mm may also be applica ble to t h e des cri pti on of

rock material (see Section

2.3.3 .

2.4.3 Discontinuities

1) Nature and Descriptive Method.

A

discontinuity i s a f ra c tu re or

plane of weakness in th e rock mass acr oss which t h e rock material i s

stru ctura l ly discontinuous and has zero, or a relat ively low, tensi le st r en gt h.

Discontinui ty is a col lect ive term and includes joints, f issu res, faul t s , she ar

planes, c leavages, schistoci ty, bedding planes and othe r planes of weakness. I t

is important th at discontinui ties ar e described careful ly and precisely, because

the y control th e engineering behaviour of most rock masses.

Complete des cri pti ons of dis conti nuiti es shoul d incl ude information on

the i r locat ion and or ienta t ion , spac ing, pers i s tence , rough ness , ape r tu re ,

infil l ing and see pag e chara cteri stics . This l ist is based on th e recommenda-

tions given by ISRM

1978 ) .

which should b e consulted fo r more detailed

information on all these aspects.

Some discontinuities, su ch a s tecto nic joints, usually occu r in more th an

one direction in a rock mass and often form a number of disti nct s ets . A

gener al description of a discontinuity se t can often be made by combining

chara cteri st ic values, o r small rang es of values, for each of th e aspects

i n t h e

above l ist . Alternatively, sep ara te ful l descript ion of individual discontinuit ies

may be requ ired if t hey ar e of part icular importance to the engineering

projec t. Other typ es of d iscont inui ty , such as faul t s , tend to occur as unique

featu res an d should be described individual ly if the y ar e relev ant to the

project .

Where possible, i t is desi rable t o different iate between th e origins of

th e various ty pe s of discontinui ty, because the ir engineering prop ert ies may be

related t o the ir g enesis (e .g. discontinui t ies formed by tensi le forces, such as

stress-rel ief joints, may behave different ly t o discontinui t ies formed by sh ear ,

T a b l e of C on

t en t s

T a b l e of C

on t en t s

T a b l e

of C on t en t s





su ch as slip su rf ac es and faults ). General information on variou s typ es of

dis con tin uity in Hong Kong has been given by Bu rn et t Lai 1984). Gamon

Finn 1984a) an d Nau 1984).

There ar e two levels a t which a discontinui ty su rv ey may be car rie d out ,

depend ing on t h e amount of detail req uir ed.

In a subjec t ive b iased) sur vey ,

only those d iscont inui t ies tha t appear to be impor tant to the projec t a re

described. In an object ive random) su rv ey , a ll discontinui t ies tha t int ers ect a

fixed line, o r ar e located within a demarcated a re a of t h e rock face, ar e

described. The main disadvantage with th e object ive approach is th at i t is

time-consuming an d tedious. Sub seq uen t da ta analys is may requ ir e some form

of automatic da ta proce ssi ng

to make th e analysis efficient. However, if t he r e

is any doubt about the nature of the discontinui ty pat tern, and i ts re lat ion to

the proposed engineering works, an object ive survey should be carried out .

Borehole cores provide essentially one-dimensional dat a on discontin-

ui ties. These data may be seriously biased if joint se ts a r e oriented such th at

unidirectional boreholes ten d to miss them e.g. sub -ve rti cal joints missed by

vert ical boreholes) . These er ro r s can be reduce d by dril l ing in different

directions e . g . inclined/horizontal holes) and b y checking regional joint

pat ter ns be fore commencing gro und investigation. Even if borehole da ta a r e

not serious ly biased, cor es rar ely provi de good information on pers ist ence ,

infil ling and s eepa ge charac terist ics. Good field exposures ar e needed for full

descri ption of discon tinuit ies.

It i s common p ractice to supplement th e description of discontinuities in

rock core wi th severa l quant i ta t ive indices re la t ing to th e f rac tur e s ta te of th e

rock mass se e item

9 )

in this sect ion).

useful aid fo r th e systematic record ing of discontinuity da ta is a

st an da rd d ata sheet. An example is shown in Figure 1.

2) Location an d Orientation. I t is import ant to rec ord th e location of

each individual discontinui ty described. This is often sta ted a s relat ive

position along a fixed datum line, or g rou nd co -ordi nates plus elevation in an

expos ure. Information shoul d preferably be record ed on a map o r plan.

The orientation of a discontinuity is described by th e dip direction, th e

compass b eari ng of t h e maximum inclination measu red clockwise from t r u e

nor th, a nd by t h e dip, t he maximum inclination of th e discontinuity m easur ed

from horizontal. Dip direct ions and dips ar e normally measur ed with a

compass and cl inometer, and should be expressed to the near est degree. In

order to different iate clearly

between dip direction and dip, t h e dip direction

value should always be given with th re e digi ts an d th e dip with two digi ts

e.g. dip direction/dip 025/60).

Orientation da ta can b e obtain ed in va ri ou s ways. The most common

method is to measure t h e dip direction an d dip of discontinuities which

int ers ect a l ine drawn ac ross an exposed rock face. Data may also be obtained

from orient ed rock cor e o r by means of a downhole ins tru men t suc h as th e

impression packer

GCO ,

1987a).

The fo ur main methods of pre sen tin g orientation dat a ar e by map

symbol, per spe cti ve diagram, joint ro se tt e an d spher ical projection. Map

symbols a r e shown in Table

23.

Persp ect ive diagrams ar e part icularly helpful

for un dergr ound work, because they can depic t th e rela tionship between t he

proposed engineer ing s t ru c t ur e and th e rock mass s t ru c tu re . Jo int rose t tes and


T a b l e of C

on t en t s


T a





spherical projections are commonly used for the quantitative presentation and

analysis o f orientation data. Detailed discussion of all th es e methods is

beyond the scope o f th is Geoguide. The S R M (1978) report should be r efer red

to for f urt her guidance.

A

clear introduction to the use of spherical

projections has be en given b y Hoek Bray 1981).

Although stereographic projection analys is i s a popular and powerful

tech niq ue, it can easily be misused i f i ts limitations are not fu lly appreciated.

(Brand et al.

1983 ;

Hencher. 1 9 8 5 ) . The project engineer should be aware of

th is when making discon tinuit y descriptions. Wher ever possible,

a

further

inspection of th e rock exposure should be made af te r th e analysis is complete

to check tha t th e results are valid.

3 ) Spacing. Recommended te rms for th e description of discon tinui ty

spacing are given in Table 7 . These terms can be applied to both rock core

and rock face exposures . They may be used t o describe th e spacing o f

discontinuities in a single set or for th e average spacing o f all discontinuities

measured along a tra ve rs e line.

The description of discontin uity spacing can be supplemented by

referen ce t o th e shape of th e rock blocks bounded by th e discontinuities.

Common terms are bl oc ky , tabular , columnar and polyhedral . Thes e are

defined in th e Glossary and are illustrated in Figure 2. The use of such terms

requires an understanding o f th e distribution of discontinuities i n three

dimensions; th er ef or e, th ey cannot be used in th e description o f drillcore.

4 ) Persistence. Persistence re fe rs t o th e areal extent or size o f a

discontinuity within a plane.

t

is one of t h e most important items in

discontinuity description; unfor tunate ly, however, it is dif ficu lt to quant ify

accurately because it is rarely possible t o see th e three-dimensional exte nt o f

a discontinuit y. For most practical purposes, persis tence can only be assessed

ve ry approximately b y measuring th e discontinuity trace length on th e su rfaces

o f rock exposures.

A discontinuity set of te n ten ds to have a characteristic

range of persistence which di ff er s from that o f other sets within t he same

rock mass.

For the description of individual discontinuities, it is recommended that

the measured maximum persistence dimension should always be used where

possible. The description should also state whether the discon tinui ty exte nds

outside th e exposure, terminates against solid ro ck, or terminates against other

discontinuities. In th e case o f general descriptions o f di f fe re n t discontinuity

se ts , relative terms should be used . For example, in a rock mass with thr ee

discontinuity set s, t he most persistent set could be described as per sis ten t ,

th e intermediate set as sub-pe rsiste nt and th e least persistent set as

non-

persistent .

5 ) Roughness. The roughness o f a discontinuity is made up of two

components large-scale waviness and small-scale unevenness (Figure

3 ) .

Waviness re fe rs to undulations o f th e surface of the discontinuity over

distances of typically ten s of metres. Unevenness re fe rs to th e bumps,

asperities and small r idges on th e sur fac e of the d iscontinuity over distances

o f typical ly one centimetre to a few metr es. Other general terms which are

used quite commonly are fi rs t-order rou ghness for waviness and

second-

order rou ghness for th e smaller-scale superimposed unevenness .

Roughness may be measured quantitatively by using linear profiling, a

compass and disc-clinometer or a photogrammetric method. A clear


T a b l e of C

on t en t s


T a





introduction to th es e th re e methods has been given by ISRM (1978).

The most

commonly-used is t h e compass and disc-clinometer, which involves measuring

discontinui ty dip direct ion and dip angles on a ser ie s of circ ular plates of

different diameter G C O , 1987a). The res ult s ar e usually pres ente d and

analysed s tereographical ly .

For general descriptive purpo ses, waviness should be assesse d by

estimating dimensions of wave length and wave amplitude (Figure

3 .

These

could b e single values for a s ingle discontinuity o r char act er is t ic values for a

d iscont inui ty se t . Unevenness should be descr ibed us ing two terms, th e f i r s t

refe r r in g t o lengths of several cen t imetres and t he second to leng ths of u p to

seve ral metr es. Nine classes of une ven nes s ar e formed by combinations of

thes e two terms, as i l lus t ra ted and def ined in Table 8.

The term slickensided

should only be used if th er e is clear evidence of previous s he ar displacement

along the discontinuity , such as str iations in the direction of inferred

movement.

The main reason for des cribing discontinuity r oug hne ss is to ass is t in

es t imat ing d iscont inui ty she ar s t r eng th GCO . 1987a; Hoek Bray. 1981; ISRM.

1978). Hencher Rich ards (1982) an d Richa rds Cowland (1982) ha ve

described in some detail th e effect of r oughn ess on th e field s he ar s tr en gt h of

gra nit e shee ting joints in Hong Kong. If quant ita tive measuremen ts a r e not

made, the descr ip t ive terms in Table 8, in conjunction with the estimation of

waviness, can b e used to make comparative asse ssme nts of t h e contrib utio n of

roughness to shear strength. as discussed by ISRM (1978).

6 ) Aperture. Aperture is t he perpendicula r distance between adja cent

walls of an open discontinuity , in which th e interv enin g sp ace is f i lled by ai r

o r water. I t should be distinguis hed from t he width of an infilled

discontinui ty (s ee item (7) below). Ape rtu res a r e caus ed by a numbe r of

factors ,

suc h a s tensile o peni ng, washing o ut of infilling materials, solution, o r

sh ea r displacement of discontinuiti es with signifi cant rou ghn es s. Description of

ape r tu re s ize is impor tan t because i t has a marked ef fect on the sh ea r

st re ng th and hydrauli c conductivity of a discontinuity.

Aper ture s ize should be descr ibed us ing t he terms g iven in Table 9. If

th e d iscont inui ty is c losed , with zero aper tur e , i t should be descr ibed as

tight . The us e of t he se terms may not provide a reliable indication of t he

hydraulic prop erti es of discontinuities , par ticular ly where th e discontinuities

have been distu rbed by blasting or surf ac e weathering. The inf luence of

ape r tu re on the hydraul ic proper t ies of th e rock mass is bes t ass essed by

insitu permeabili ty testing

GCO , 1987a).

(7) Inf ill ing. Inf ill ing is th e term for t he mater ial th at se par at es th e

adj ac ent rock walls of a discontinuit y. This term is pre fer red to filling ,

which is normally used to des cri be t h e placement of fill material s (see Section

3.7). I t should b e noted, however, th at not all inf il l materials ar e necessar ily

t r an s p o r t ed in to t h e

discontinuity a t a l ater s tag e; some can form insitu , e .g .

by t he action of i nte nse weatheri ng along a joint.

Inf ill materials are usually weaker than t he pa rent rock. This is of ten

t he most important engineering charact er ist ic.

Typical infill materials are soil.

decomposed o r disintegrat ed rock, minerals su ch as qua rtz o r calcite (of ten

termed veins ) , manganese o r kaolin, or , in th e case of faults or sh ea r zones

along which signif ica nt displacement has occu rred, fault gouge or breccia ( see

Appendix A.7 and the Glossary) .


T a b l e of C

on t en t s


T a





No specific te rms ar e recommended fo r th e descr ipti on of infill

mater ials. If th e mater ials a r e decomposed/disintegrated rocks or soi ls , they

should be d escr ibed i n accordance with Section 2.3 o r 3.3 respectiv ely. If they

ar e specific minerals , th e mineral t ype , par t ic le s ize and s t r en gt h

compactness/consistency)

should be descr ibe d where possible. Whatever th e

t y p e of material, d escr ipti ons of infilling shou ld always include some

information on their width (ideally maximum, minimum and average widths in

mm) and seepage asp ects (e .g. a r e the mater ials dr y, damp/wet, do they show

permanent seepage?)

8 )

Seepage. Seepage along discontinuit ies is of ten of g re at engineer ing

importance an d dese rve s ver y careful assessment in a comprehens ive rock mass

descr iption. Seepage aspe cts of unfi l led discontinuit ies should be descr ibed

us ing one of th re e bas ic terms, viz dry , damp/wet l (b ut with no f re e water )

an d seepage present . For th e las t ca tegor y, th e quant i ty of water flowing a t

t h e point of obse rvation should be noted in l i tres/secon d o r l i tres/mi nute,

ei th er by estimation or approximate measurement. Unless t h e rock mass is

completely d ry , i t is often diff icult t o select char acte r is t ic val ues of seepage

fo r discontinuity set s , in which case supplemen tary descr iption of see page

variabil i ty within th e mass should be given. The date of observat ion should

a lways be noted when seepage i s desc r ibed , so th a t t he seepage amount can be

re la ted to th e wet and dr y seasons . If poss ible, subse que nt observat ions

should be made a t th e height of t he wet season and a t th e end of t he dr y

seas on in or de r to give a n indicatio n of maximum a nd minimum seepag es.

In most unweathered rocks and par t ial ly weathered rocks in the

PW90/100 zone (Table 10). t h e flow of wa ter t ak es place mainly t hr ou gh

discontinuit ies . Some sedimentary rocks may be exceptions to thi s rule,

because a s ignif icant proportion of th e flow can occu r thro ug h th e int act rock

material. In more intense ly w eath ere d rock masses (t he PW50/90 to PW0/30

zones in Table l o , i t is much more difficult to give a gene ral indication of

typical gro undw ater movement. because t h e rock mater ial weathered t o a soil

may be a t leas t a s permeable a s th e discont inui ty sys tem. Careful observat ion

of seepage sources in natu ral exposure s and excavations can provide valuable

information on th e hydrogeology of th e rock mass, par t icular ly when related to

other data sources such as piezometr ic levels measured in boreholes (GCO,

1982).

(9) Frac ture Sta te. number of indices can be used for quant i t a t ive

descr ipti on of t he f ra ct ur e s ta te of th e rock mass as determined from borehole

cor es. Thes e a r e Total Core Recovery , Solid Core Recovery , Rock Qual ity

Designation and Fra ctu re Index. These indices should be used whenever

possible to supplement the descr iption of discontinuit ies in rock core.

Only natura l geologica l f r ac t ur es should be tak en in to account for th e

descr ipt ion of f r ac tu re s tate . Artif icial f r ac tu re s produce d, for example. by

dril ling o r blast ing should be excluded from t h e assessment, al though precise

inte rpr etat ion of f r ac tu re or igin may be diff icult .

r ough sur f ace with f r e sh

cleavage planes in individual rock minerals usually indicates an artif icial

f ra c tur e . genera l ly smooth or weathered sur face , or a sur f ace coated with

inf i l l mater ials such as calcite or kaolin, clear ly indicates a natural

discontinuity. Addit ional guidanc e notes on th e inte rpr etat ion of f r ac tu re

or ig in a r e given by ISRM (1978).

In cases of doubt . i t i s cus tomary to reg ar d

the discont inui ty as na tura l .

Previous inconsis tenc y in th e use of f ra ct ur e index def init ions h as led to

some diff iculty in measurement and interpretat ion. The def initions an d term s


T a b l e of C

on t en t s

T a b l e o

f C on t en t s

T a b

l e of C on t en t s




given below a r e base d on t h e recommendations made by Norbury e t a1 (1984).

Sol id core is th e key term to be defined in the assessment of f r act ur e

s ta te ; i t i s reg arde d a s core with a t lea s t one ful l d iameter (b ut not

necessar i ly a full circumference) measured along t he c ore axis between two

na tura l f r ac tures . O n th e basis of th is def init ion, th e four quan ti tat i ve

fra ct ure indices ar e i l lust rated schematically in F igure 4 and a re def ined as

follows

Total Core Recovery, TCR

( ),

i s th e percentage ra t io of

core reco vered (whe the r solid, int act with no full

diameter , o r non -intact ) to th e total length of c.ore ru n.

Solid Core Recovery, SCR ( ) , is the percenta ge ratio of

solid core recovered to the total length of core run.

Rock Qua lity Designa tion. R O ( ) , i s t he tota l length of

solid core pieces, each gre at er th an 100 mm between

natural f r act ures , expressed as a percentage of the tota l

length of core run.

Fra ct ur e Index, FI (No./m r u n ) , is t he number of clear ly

ident if iable f ract ures pe r metre ru n of intact core pieces ,

measured o ver core leng ths of reasonably uniform

char acte r . This index does not necessar i ly apply to

whole core run s . If th er e is a marked change in

f r ac tu r e f r equency du r ing a co r e r un , t he f r a c tu r e i ndex

should be calcula ted f or each pa r t of t he r un separate ly .

The term non-intact (NI) should be used when the core

is frag ment ed. Additional detail can be given by quoting

t h e maximum, mean a nd minimum l en gt h of co re pieces

recovere d for any co re length of reasonably uniform

charac te r .

I t i s impor tant to note tha t measures of f rac tu re spacing suc h

as

R Q

and Fr ac tu re Index may b e biased, depend ing on th e or ientation of th e

borehole in relation to th e dominant discontinuity sets . This problem is

discussed in some detail by Beggs & McNicholl (1986) in r elat ion t o s it e

formation work s a t Ap Lei Chau, Hong Kong.

2.4.4 Rock Mass Wea thering

A

section th rou gh a weathered rock mass of ten shows a ran ge of rock

mater ial at var ious st ag es of decomposition an d disintegration. Although the

proportion of t he more intense ly weathered rock is general ly g reat er c lose to

the ground sur face , i t i s unusual t o encounter a weather ing profi le which

shows an orderly progression of successively less weathered layers, f rom a

res idua l soi l a t th e sur face to an unweathered rock mass a t dep th .

To acc oun t for complex weatherin g profiles, desc ript ive schemes for rock

mass weatheri ng should b e flexible an d simple to apply in t h e field. For

engin eering p urp oses , th e usual method of description is to identi fy pre-defin ed

weatheri ng zones within t h e rock mass. Different zonal classification schemes

may b e appropr ia te , depending on the n atu re of th e engineering p roject te.g.

tunnell ing, foundation design , s lope stabil i ty assessmen t) . The scheme

recommended below may r eq ui re modification (e.g. by subdiv ision o r


T a b l e of C

on t en t s


T a





amalgamation of zone s), o r replacement by an alt erna tive scheme. to s ui t

part icular si tuat ions.

Rock mass wea ther ing classifications a r e usually est ablis hed on the basis

of d iffering prop orti ons of rock and soil , t h e pres enc e or abs ence of mass

st ruc tur e. and th e deg ree of discolourat ion of discontinui ty surf aces . simple

general scheme based on these characterist ics is given in Table 10.

I t should

be noted that this zonal c lassif icat ion differs substant ial ly from that

recommended in BS 5930 (BSI, 1981). In or d er t o avoid confusion between t h e

two. new self-explanatory zone descript ions an d symbols ar e used.

The scheme in Table 10 is based on the four-zone scheme original ly

prop osed b y Ruxton Ber ry (1957). a modified form of which is given in t h e

Geotechnical Manual for Slopes G C O , 1984). The important diff eren ces

between t h e two schem es may be summarised a s follows

( a ) Table 10)he present scheme ( is expanded to s ix zones

because the re ap pear s to be a broad consens us of opinion

th a t a four-zone scheme is not adequ ate for engineer ing

purposes . The ext r a two zones a re c rea ted by th e

addition of an 'unweather ed' zone, comprising 100% rock

(which, in fac t, is implicit a s a f ifth zone in t he Ruxton

Berry scheme), and by th e introduct i on of a 30% rock

boundary .

(b ) Whereas th e Ruxton Berry scheme re fe rs only to

geological characte rist ics, some general ised engineerin g

charac ter i s t ics a re inc luded in Table 10. However, i t is

emphasised tha t these charac ter i s t ics a re only in tended

as a ver y approximate guide to th e engineer ing behaviour

of the different zones.

(c ) Unlike th e Ruxton Berr y scheme, th e pr ese nt scheme

i s ot in tended to represent an idea l i sed weather ing

prof i le . Rather , i t i s in tended t ha t the scheme should be

applied in a f lexible se ns e to su i t th e actual dis tr ibut ion

of weat heri ng zones in th e rock mass. This point is

i l lus t ra ted in Figure 5 and explained further below.

The rock percentages in Table 10 a r e notionally by volume. In most

case s, however. information on th e three-dimens ional ex ten t of t h e mass is

limited, an d it is usually only possible to make a ro ug h estimate of t he se

percentages .

One of th e most st r ikin g f eat ure s of mass weathering in certain ro cks i s

t h e development of core ston es (Ruxton

Berry. 1957; see also Plate

6 .

In

general , th e coarser -gr aine d, more widely-jointed Hong Kong rock s s uch a s

gran i te a nd lapi ll i tuff ten d t o weather with t he development of corestones.

whereas the f iner-g rain ed, more closely-jointed rock s do not. In principle,

Table 10 is applicable to al l rock t ype s, b ut in pract ice i t is much easier to

appl y in corestone-forming rocks. be cause th e differ ent proport ions of rock

and soi l in the part ia l ly weathered zones can be recognized more readi ly in the

f ie ld . For th e non-corestone-forming rocks , i t i s necessary t o make a careful

ass essm ent of t h e dif fer en t gr ad es of rock material decomposition be fore

dividing th e mass in to weather ed zones usi ng th is scheme. Sometimes, i t may

be found th a t th e rock weathers so uniformly t ha t i t i s impossible to ident i fy

th e intermediate zones given in Table 10, in pa rti cul ar t h e PW50/90 an d

T a b l e of C on

t en t s


T a b l e

of C on t en t s

T





PW30/50 zones. In s uc h cases, i t may be ap pro pri ate to use a smaller numbe r

of classes by combining cer tai n zones. Altern atively , us e of a di ff er en t zonal

classification should be considered.

I t i s of ten found th a t some weather ing zones a re absen t , or a r e pre sent

only to a ve ry small extent . The distri butio n of weatheri ng zones can be

determined by mapping natur al exposures and excavat ions, b ut t hes e may not

be repr ese nta tiv e of t h e whole mass. Figure illustrates an idealised

wea the red profile, an example of a complex b u t more re alistic profile, an d a

section thr ou gh a corestone-forming ro ck mass showing th e subdivision of t h e

mass into weather ed zones us ing t h e scheme given in Table 10. Examples of

complex weathered rock mass exposures are shown in Plate

7.

In car bon ate rocks , only small amounts of soil a r e produced du ri ng

weathering unless th e rock contains a high perce nta ge of impurities see

Section 2.3.4 2)). The partially weath ere d PW50/90 to PW0/30 zones ar e ra re ly

developed to any significant thickness . Typically, weathered profiles show a

relat ively thin lay er of residual debri s overlying an i rre gul ar s urf ace of

unweathered o r partially weathe red PW90/100 rock. The contac t between th e

rock and soil is usually ve ry s ha rp . Karst fea tu re s formed by solution along

discontinui t ies ar e th e most d ist inct ive aspec ts of mass weathering in carbona te

rocks and ar e often of g re at engineering signif icance and concern. Useful

information on t h e description and en gineer ing as sess ment of weath ering

eff ects in car bo na te ro ck s is given by Dearman 1981) and Fookes Hawkins

1988). The occ urr ence of cave rnous gro und in th e buried marble a t Yuen

Long s de sc ri be d by Pascal1 1987).

When co re s in decomposed rock a r e logged , th e decomposition gr ad es of

th e rock material should be included in th e descript ion, bu t not th e rock mass

weath ering zones. Zonal int erp ret ati on should not be done as p ar t of rou tin e

core description. borehole is essentially a l ine sample th ro ug h t h e rock

mass. and it may not be repre sen tati ve of t h e overall pat ter n of mass

weathering. I t is part icularly difficul t to con str uct a rel iable weathering zone

model in corestone-forming ro ck s from borehole eviden ce alone.

For examples of t h e use of specif ic wea the rin g zone classifications fo r

eng ine eri ng pr oj ec ts in Hong Kong, re fe re nc e shoul d be made to Gamon Finn

1984b) fo r ass ess men t of lar ge excavations in gra nit e a t Kornhill , Ir fan

Powell 1985a) for foun dation as ses sme nt in grano diorit e a t Tai Po, an d

Watkins 1979) fo r tunnelling a nd dam foundation st udi es in various ign eous

and volcanic roc ks in th e ea st er n New Terri tories .

2.4.5 Additional Infor mation

Any additional information th at will as si st th e eng ine er in under sta ndi ng

t h e natu re of th e rock mass should be record ed. An example is th e possible

occ urr enc e of voids in car bon ate rocks s uc h as limestone an d marble. If

discovered. t h e geometry of any voids should b e described wh ere possible , as

well as th eir re lat ionship to discontinuit ies in the sur rou ndi ng rock mass and

any s igns of groundwater o r seepage.

Special note should b e made if a ny of th e mass chara cte ris tic s descr ibed

ar e considered to be unusual in relat ion t o th e re s t of th e mass descript ion.

I t is part icularly important t o indicate whet her t he sample of th e rock mass

descri bed is cons idered to be rep res ent ati ve of th e whole mass which is

rel eva nt to th e engin eering project. The limitations in he re nt in making mass


T a b l e of C

on t en t s


T a





descr iptions f rom small isolated exposures, or f rom borehole evidence alone,

should always be kept in mind. A considerable d eg re e of professional

judgement and commonsense is requ ired . The engi neer should assess t h e

validity of t h e geotechnical model used in th e design as engin eer i ng works

proceed and f ur th er exposu res become avai lable . If var ia ble ground condi t ions

a r e encounte red , rock mass desc r ip t ions should be r ev i sed dur ing cons t ruc t ion

where necessary.

2.4.6 Examples

Two examples of rock mass des cr ip tion s ar e given for g uida nce in th e

use of ap propr ia te desc r ip t ive terms. The rocks cor r esponding to these

desc r ip t ions a r e i l lus t r a ted in Plate 8.

( a ) Plate 8A. (Py roc las tic Rock Mass). The

mass

i s sp l i t

in to two bas ic uni ts for descr ipt ion

( i ) Uni t 1 Very s t rong . g reen ish g r ey ,

massive, partially weathered PW90/100,

coar se ash TUFF, with t h re e major joint

sets (a ) 010/87, medium-spaced.

pe r s i s ten t , smooth an d s t eppe d , t igh t .

d ry ; (b ) 120 /35, ve ry c lose ly - spaced, su b-

persis tent , smooth and planar , extremely

nar row, genera lly d r y b u t wi th severa l

minor see pag e poin ts of 1 litre/rnin in

western lower half of face; (c) 345/60,

closely-spaced, non-persis tent , smooth

a nd p l a na r , t i gh t , d r y .

( i i) Unit 2 Weak, red di sh brown. par t ial ly

weathered PW0/30, coarse ash TUFF.

Unit 2 over l ies Unit 1. T he bounda r y i s s ha r p a nd d ip s

a t approximate ly 30•‹ to th e wes t across t h e excavat ion

face.

( b ) Plate 89. (Ig neo us Rock Mass). The len gth of core from

23.73 t o 27.05 m i s spl i t in to two uni ts on t he bas is of

d i f f e r i ng g r a in s i z e a nd f r a c tu r e f r e que nc y

(i) Unit

1

(23.73 to 26.26 m Very s t rong ,

grey mottled with pink and dark brown.

slightly decomposed, medium-grained

GRANITE. with widely-spaced, rough and

undula t ing , brown-s ta ined jo ints d ipping

0 t o 10". TCR 100%. SCR 100%. R Q

100%. FI 1.2.

(ii) Unit 2 (26.26 t o 27.05 rn) Very st ro ng .

l ight greyish pink, s l ightly decomposed.

f ine -gra ined GRANITE, with closely- to

medium-spaced, genera l ly rough and

s tep ped bu t a l so smooth an d p lanar (one

subv er t i ca l jo int ) , brown-s ta ined jo ints

dipping 0 to l o o , 40" and 85".

T a b l e of C on

t en t s


T a b l e

of C on t en t s

T





TCR 100 . SCR 55 . R Q 4 4 . FI 7.6.

The core was wet when described.

(Note Since only a ve ry small portion of t h e mass is

exposed in th e core, t he descript ion is made essent ial ly

in terms of rock material characteristics, plus

information on discontinuities.)

2.5 ADDITIONAL GEOLOGICAL INFORMATION

Once th e material and mass ch aracte ristic s of t he rock hav e been

described, the final item in a complete rock

description should be t he name of

t h e geological formation from which t h e sample rock material o r mass ha s been

selected. gui de to t h e name of t h e geological formation is given in t h e

maps and memoirs pro du ce d by t h e Hong Kong Geological Su rv ey . The name

sho uld b e writt en with capital initial le tt er s (e.g . Ap Lei Chau Formation).

The geological formation should be named w her e thi s can be d one with

confidence, bu t i t is often diff icul t to ident ify

a

formation name from a small

sample, o r to locate formation bou ndarie s in a borehole or expos ure; con ject ure

should be avoided.

The principal rock t yp es associated with a specific formation ar e often

indicated on t he geological map, b ut i t should be remembered th at , a t a

particular location or horizon, the actual rock type may be completely

diffe rent from th at indicated un de r th e heading of principal rock type .

Geological formations may be quite variable in their range of rock types, and a

knowledge of t h e formation will often indic ate th e possible ra ng e of r oc ks t o

be exp ecte d. For example, th e Shing Mun Formation of th e Repulse Bay

Volcanic Group is a complex formation that contains lapill i , coarse ash and

fine ash tuf fs, tuf f i tes and a ra ng e of sedimentary rocks from conglomerate to

mudstone. The en gi ne er should re fe r to t h e HKGS maps an d memoirs fo r

guidance, or consul t

a

geologist for assistance where necessary.


T a b l e of C

on t en t s


T a





[BL NK P GE]

T a b l e of C on

t en t s


T a b l e

of C on t en t s

T





3 5

DES RIPTION OF SOILS

3.1 GENERAL

T h e re c o m m e n da t io n s i n t h i s c h a p t e r a r e g e n e r al ly a p p li c ab l e t o all t y p e s

o f s u p e r f i c i a l d e p o s i t s . in c l u d in g f i ll . So me o f t h e r e co m m e n d a ti o ns a r e a l so

a p pl ic ab le t o so ils d e r i v ed f r o m t h e i n s i t u w e a t h e ri n g o f r o c k s . I n v i e w o f

t h e i r imp o r ta n c e f o r e n g in e e r in g i n Hong Ko n g , s oi ls d e r i v e d f r o m in s i t u r o c k

w e a th e r in g , c o l l u v iu m , a nd f il l a r e c o n s id e r e d i n g r e a t e r d e ta i l u n d e r s e p a r a t e

s e c t i o n s ( s e e S e c t io n s 3.5

t o

3 . 7 ) .

T h e s e s e c t i o n s f o l l o w th e r e c o mme n d a t io n s

fo r a g e n e r a l d e s c r ip t i v e me tho d fo r s oi ls g i v e n i n S e c t i o n s

3 .2

t o 3.4.

3.2

P U R P O S E

AND

S C O P E

O F S O I L D E S C R I P T I O N

T h e m a in p u r p o s e o f a so il d e s c r i p t i o n f o r e n g i n e e r i n g p u r p o s e s i s t o

g i v e a n i n d i ca t i o n o f t h e l i k e l y e n g i n e e r i n g p r o p e r ti e s o f t h e so il. I n t h i s

s e n s e , so il d e s c r ip t i o n s i n p r in c ip l e c a n b e ma de u s in g t h e s ame a p p r o a c h

a d o p te d f o r ro c k d e s c r i p t i o n s i n C h a p t e r

2

b u t t h e r e i s o n e i m p o r t a n t

d i f f e r e n c e . U n l i k e r o c k s , m o s t so ils c a n b e e a s il y d i s t u r b e d d u r i n g e x c a v a t i o n ,

s am p li ng o r t e s t i n g , and t h i s m ay h a v e a m a rk ed e f f e c t o n e n g i n e e r i n g

p r o p e r t i e s . So il d e s c r ip t i o n s s h o u ld i n c lu d e a n o t e o n t h e d e g r e e o f s amp le

d i s t u r b a n c e , w h e r e t h i s i s c o n si d er e d t o b e i m p o r t a n t. T h e d e g r e e o f

d i s t u r b a n c e r a n g e s f r o m t h e c o m pl et el y u n d i s t u r b e d , i n s i t u f ie ld c o n di ti o n t o

t h e f u l ly

d i s t u r b e d , r e m o u l de d c o n d i ti o n o f a sa m p le t h a t h a s b e e n c o m p l e t e l y

b r o k e n d o w n i n t o i t s c o n s t i t u e n t g r a in s .

F u r t h e r i n f o r m a t i o n a nd g u i d a n c e o n

s a mp lin g m e th o d s i n r e la ti o n t o soil d i s t u r b a n c e i s g i v e n i n Ge o g u id e 2 Guide

t o S i t e I n v e s t i g a t i o n G C O , 1987a).

S oil d e s c r ip t i o n s c a n b e ma d e d i r e c t l y f r o m f ie ld e x p o s u r e s a nd

e x c a v a t i o n s o r f r o m s am p le s r e c o v e re d f r o m b o r e h o le s o r e x c a v a t i o n s . I n t h e

f ol lo w in g s e c t i o n s , m a t e ri a l c h a r a c t e r i s t i c s o f s oils r e f e r s t o t h o s e

c h a r a c t e r i s t i c s t h a t c a n b e d e s c r i b e d f r o m v i s u a l a nd m a nu al e x a m in a ti o n o f

r e l a t i v e l y sm all v o l u m e s o f so il i n e i t h e r d i s t u r b e d o r u n d i s t u r b e d s a m p le s .

M a ss c h a r a c te r i st i c s o f soils r e f e r s t o c h a r a c te r i s ti c s t h a t c a n o n l y b e

d e s c r ib e d s a t i s f a c t o r i ly i f t h e or ig in a l s oil s t r u c t u r e r e m a i n s i n t a c t , i .e . t h e y

c a n b e o b s e r v e d o n l y i n t h e f ie ld o r t o som e e x t e n t i n a l a r g e u n d i s t u r b e d

sample . Ad d i ti o na l g eo lo g ic a l i n fo r m a t io n , s u c h a s t h e g e o lo g ic al f o r ma t io n .

a g e and t y p e o f d e p o s i t , s ho u ld a ls o b e in c lu d e d i n t h e d e s c r i p t io n w h e n e v e r

p o s si b le , b u t t h e s e a s p e c t s m a y n o t b e r e a d i ly d e t e r m i n ed w i t h o u t a d et ai le d

g eo lo gi ca l s t u d y o f t h e a re a a ro u n d a s i t e .

I n a soil d e s c r i p t i o n , t h e m ain c h a r a c t e r i s t i c s s h o u ld b e g i v e n i n t h e

fo l l o win g o r d e r

( a ) s t r e n g t h , i .e . c o m p a ct n es s o r c o n s i s t e n c y ( m a t e r i a l ) ,

( b ) c olo ur ( m a t e r ia l ) ,

( c ) p a r ti c le s h a p e a nd c o mp o s i t io n (m a te r ia l ) ,

( d l s t r u c t u r e ( m a s s ) .

( e ) s t at e o f w e a th e ri ng ( m a s s ) ,

T a b l e of C on

t en t s


T a b l e

of C on t en t s

T





(f ) soi l name (in capitals, e .g. SAND), grading

and plast ic i ty (materia l) ,

(g ) d iscontinuit ies (mass) , and

( h ) addit ional geological information.

I n t h e following se ct ions, t h e mat eria l c hara ct e ri s ti c s i n t h i s s e quenc e

a re cons ide red in Section 3.3 and th e mass cha rac te r i s t ic s in Section 3.4. The

scopeof th is scheme is s imilar to th e schemes recommended by BSI (1981)an d

IAEG (1981), b u t t h e l ay ou t of t h es e two s ec ti on s, a nd t h e o r d e r i n which

i nd iv idua l c hara ct e ri s ti c s a r e c ons i de red , have bee n modified t o conform a s

c losely a s possi bl e t o t he schem e used fo r r oc k descr ip ti on i n Chapter 2.

I t should be noted t h a t t h e term ' st ru ct ur e' a s used in t hi s c ha pt er

r e f e rs t o m r n s t r u c t u r e , i .e. s t r u ct u r a l f e at u re s of a soil mass which c an b e

identif ied by t h e naked eye. The descript ion of soil -structure is

considered in Section 3.4.4.

The recommended scheme for th e rapid identi f ica tion and descr iption of

soi ls i s summarised in Table 11. Each main item i n t h i s Table i s d i sc usse d i n

fu r the r detai l in th e following t ex t sect ions.

In a dd it io n t o d es cr ip ti on , i t may be u se fu l in some c ir cu ms ta nc es t o

make an overa ll c la ss if icat ion of soi ls fo r eng inee ring purposes . Guidance on

soi l c lassif ica t ion i s g iven in Cha pt er

4.

3.3 MATERIAL CHARACTERISTICS OF SOILS

3.3.1 S t re n gt h

T he s t r e n g t h of a soil may b e a lt er ed s ig ni fi ca nt ly b y d i st u rb a nc e o r

remoulding dur ing sampling and tes t ing . S tr en gth should th ere fo re be

desc ribed in the undi s tu r bed fie ld condi tion whenever possible; a l t erna tive ly ,

t he h i ghe st -qua li t y, l e as t d i s tu rbe d sa mple shoul d be u sed .

The s t r en gt h of cohesive soils i s noticeably affected by moisture

content . In Hong Kong, th is i s part icularly t he case fo r soils in t he

u n sa t ur a te d zone a bo ve t h e w at er tab le , w he re s ign if ica nt s ho rt -t er m a n d

seasona l f luc tua tions in mois ture conten t can occur . S t rength desc rip t ions of

cohesive soi ls should th eref ore inc lude an indica tion of the moisture condi tion.

For example. possible moisture condi tion c lasses could be 'dr y ' , 'moist' , 'wet' ,

e tc . Any c la sses u sed shoul d be def ined in t erms of simple f ield r ec ogni ti on

c r it e ri a f or p ar ti cu la r s oi ls , a nd s ho ul d be r el at ed t o l ab or at or y- me as ur ed

moisture cont ents where possible . A s a genera l ru le , soil s t re ng th in th e f ie ld

s ho ul d b e d es cr ib ed a t t h e n a tu r al field m oi st ur e c on te nt , a n d a n y s am pl es

t aken should be kep t a t tha t samemois ture conten t . Guidance on the handl ing

and s to rage of samples i s g iven in Geoguide 2

G CO ,

1987a).

The recommended quali ta tive scales for s t re ng th assessment are given in

Table 11. Thes t ren gth of coarse andver ycoarse soil s (san ds, gravels , cobbles

a nd boul der s ) i s de sc r ibe d in t erms of c om pa ct ne ss o r r e la ti ve de ns i ty

(e.g.

'loose' o r 'dense ' ) . The s tr en gt h of fine soils i s described in terms of

c onsi st ency ( e. g. ' so ft ', ' firm' , ' st if f' , e t c ) . E qu ival en t quan t it at i ve sca le s of

s t r eng t h for these two groups of soi ls a r e g iven in Table 12. Compactness and

c ons i st enc y t erms c a nno t be a pp li ed e as il y t o o rga ni c so il s, w hich shoul d be

T a b l e of C on

t en t s


T a b l e

of C on t en t s

T





described as compact , spongy o r plast ic (Table 11).

The compactness te rms for sand s and grave ls in Table 12 a r e based on

values measured in boreholes by t he S tan da rd Penetrat ion Test

BS I ,

1975).

This scale is recommended for us e only in tr an sp or te d soils. Ther e is no

generally accepted classification of N values and densi ty terms for soi ls

deri ved from insi tu rock w eathering in Hong Kong; fo r desc ript ive p urpo ses,

an y measured values in th es e soils should be reco rded directly. When used

fo r design purp ose s, a correct ion facto r is often applied to values to

account for overburden pressure , energy diss ipa t ion in the dr i l l rods , and the

effe ct of low permeabi lity in fine s an ds a nd s ilt y sa nd s (Rodin e t al, 1974;

Skempton, 1986). If th e descript ive terms a r e based on corr ected values.

this should be noted.

The consistency terms for f ine soi ls in Table 12 are based on values of

undra ined s he ar s t r eng th . For descr ip t ive purposes , a rapid approximate va lue

of undrained sh ea r st r en gt h can be obtained by using a small laboratory sh ea r

van e o r han d penetrometer (Head, 1980).

The pr ese nce of a mineral cement in t h e soil may have a sign ifica nt

effec t on the soil s t ren gth . Very few s tudies have been carr ied out on

cementation in Hong Kong soils, bu t i t ap pe ar s th a t colluvial matrix material

(Ruxton, 1986) an d fi ne -g ra in ed marin e soils (Howat. 1984; Tovey. 1986a; Yim

Li, 1983) can have relat ively high s tr en gt h and s t i ffness du e to th e pre senc e

of iron oxide, and possibly othe r, cementing age nts . The pre sen ce of iron

oxides in th e soil is often indicated b y a dist in ct ive brown o r reddi sh brown

colouration. If a mineral cement app ear s to be pr esen t , i t is useful to note

whe ther slaking oc cur s on immersion of a n on-s atur ated sample in water.

Non-cemented soils usually sla ke in wate r.

3.3.2 Colour

It is recommended t ha t soi l colours should be described according to th e

scheme given for rocks in Table 3 . This scheme is also summarised in Table

11. s with r ocks , i t should be sta ted if t h e soi l was wet o r d r y when

described , becaus e this can have a marked influence on th e colour descript ion.

(Note that the moisture condition may a l ready have been recorded in the

descript ion of st re ng th ; see Section 3.3.1) .

Sample disturbance or remoulding may

de str oy some of t h e original soil

colouring. Therefore, soi l colours should be described in th e und istu rbed f ie ld

condit ion whenever possible. Bedding planes, re l ic t joints and other s t ru ctu ral

fea tur es often show a dist inct ive colour chan ge when compared with the

surrounding soi l matrix, and this should be noted where applicable .

3.3.3 Pa rti cle Sh ap e an d Composition

Part ic le sh ap e may be d escribed by re fere nce to t h e three-dimensional

form of t h e part ic les, thei r angulari t y (which indicates the d egre e of rounding

at edge s and co rne rs ) and the i r sur face texture . In genera l , s imple v isual

assessment of thes e charac ter i s t i cs i s adequate for rout ine descr ip t ions . bu t

more precise measurements may be required in some cases (e.g. assessment of

granular soi ls as potent ial sources of aggregate , detai led assessment of

sedimentary texture and fabr ic , e tc ) .

T a b l e of C on

t en t s


T a b l e

of C on t en t s

T





Common terms fo r simple description of form and angu larit y a r e i l lus-

t r a t ed in Tables 13 an d 14.

For a more r igor ous descript ion of sha pe charac t-

erist ics, form and angu lari ty may be quantif ied by refe renc e to th e axial ra t ios

of t he part ic le diameters and th e radi i of cur va tu re of t he part ic le co rne rs in

the projected plane. qua nti tat ive classification of partic le form is shown in

Figure 6. quanti ta t ive measure of part ic le rou ndne ss is given by

where i s th e radius of cur va t ure of a corner of th e par tic le surface , i s th e

radi us of th e maximum inscribed circle in th e projected plane and is th e

number of cor ners .

s roundness inc reases , r approaches and

P approaches

one.

Common terms us ed to de scribe th e sur fac e tex tur e of part ic les ar e

smooth , rough , glas sy , honeycombed , pitted an d stri ated (s ee Glossary).

The composition of particles visible to th e naked e ye or u nd er a hand

lens may also be described. Gravel and larg er part ic les are usual ly rock

fragments

(e .g . grani te , tuf f , schis t ) . Sand and f iner par t ic les a re genera lly

individual mineral grai ns (e .g. quartz , mica, fe ld spar ) . Gravel and sa nd

particles may be coated with specific minerals, suc h a s l imonite and o th er iron

oxides, manganese o r calcite. Soils containing an appreci able propo rtion of

shel ls may also be described a s shelly (see also Section 3.4.1).

3.3.4 Soil Name

(1) General Aspects . The basic soil typ es and the i r sub-divis ions a r e

shown in Table 11. The soil name is based on particle size dist ribu tion and

plast ici ty propert ies. These characteris t ics are used because th ey can be

est imated with suff ic ient accuracy fo r descr ip t ive purposes , and can be

meas ured with reasonable precision if req uir ed. They give a gene ral indication

of t he probable engineering characteris t ics of t he soi l a t an y part icu lar

moisture content. Table 11 provides guidance essentia l ly for th e rapid

ident if icat ion of t h e soi l ty pe by hand a nd ey e in th e f ie ld or in th e

laboratory . If nec ess ary, t h e soi l ty pe can be confirmed by determining th e

part ic le size distr ibut ion and plast ici ty pr opert ies from laboratory tes ts (BSI,

1975).

Because of t h e subje ct iv e natu re of t he rapid ident ification proced ure, i t

is often found t ha t t he init ia l descript ion of soi l ty pe is not co nsis tent with

th e resul t s of labora tory grad ing an d plast ici ty tes t s . In such cases , i t i s good

pract ice to rev ise t he soi l name in l ine with th e labora tory te s t da ta, b ut t he

original descript ion should st i l l be pre serv ed as a record of t he obs er ver s

opinion. note sho uld always be add ed to t h e description if th e soil name

has been modified on th e basis of labor atory te st s.

Table 11 is a slightly modified an d re ar ra ng ed form of t h e vers ion given

by BSI (1981) This method of naming and ident ifyin g basic soil ty p e s has

been crit icis ed in some detail by Child (1984) and N orbu ry e t a1 (1984). They

proposed an alt ern ati ve method which is based more on th e probable mass

engineering behaviour of the soil (following the scheme used in

CP 2001 (BSI,

1957)) , ra th er t han on st r i c t g rading l imits as in the pr ese nt scheme.

However, th e us e of thi s a l ternat ive method de pend s more on th e judgement of

T a b l e of C on

t en t s


T a b l e

of C on t en t s

T





t h e individual who makes th e description. The scheme outlined in Table 11 is

considered to be easie r f or the non-spec ia l i st to apply a nd i s there fore

recommended.

In addition t o identification of t h e soil name, gr adi ng an d plasticity

characteris t ics can also be used to make an engineer ing classif icat ion of soils .

This i s considered fur th er in Chapter 4

2 ) Table 1arti cle Size Distributio n. From 1, i t can be seen tha t ,

wher e a soil (excluding an y boulde rs o r cobbles) contains abou t 35 o r more of

fine material, i t is des crib ed a s a 'si lt ' o r 'clay' (fin e soil). With les s th an

35 of f ine material, i t is described as a 'gravel ' o r 'sand' (coarse soi l) . In

th e f ie ld, o r when laboratory descript io ns a r e made in adva nce of grad ing

te st s, the se percen tage s can only be est imated. If accu rate determinat ion is

requi red , labora tory te s t s a re necessary . The grave l , sand and s i l t par tic le

s ize rang es can be f ur th er subdivid ed into coarse, medium an d f ine part ic les

(Table 11). The gr adi ng of sa nd s and gr ave ls may be qualified a s 'well-grad ed'

o r 'poorly-grad ed' ; poorly-g raded materials may be divided fu rt he r into

'uniform' and 'gap-gra ded' as defined in Table 11. Terms su c h a s well- o r

poor ly-graded ar e used here in th e engineer ing sen se and ar e th e reve rs e of

th e sort ing terms used in t h e geological sen se . For example, a soil t h at

comprises a wide ra ng e of particle sizes is a well-graded soil to an en gin eer ,

b u t a poorly-so rted soil to a geologist.

The broad basis fo r th e desc ription of composite soils (i.e. mixtures of

basic soi l types) is a lso given in Table 11.

The terms ar e defined according to

th e percentage of s econ dary cons t i tu ents and a r e prefixed to th e main soil

name. These ter ms re fe r only to mixtures of two basic soil ty pe s (e.g. a sil t

or clay with a g r a v e l o r s a n d ) .

Since th e coar se fraction in a composite soil can itself be d ivided in to

approximate proport ions of san d and grav el by est imation, i t is possible to

desc ribe more complex soil mixtures in terms of t h re e basic soil typ es. o r more

tha n t hr ee if th e soi l a lso contains ver y c oarse material (cobbles and boulders) .

The full explanat ion of t h e use of seco ndar y const i tuen ts for d escribing

composite soils is given in Tables 15an d 16. Both tables include examples to

i l lus t ra te th e appropr ia te use and sequence of te rms.

I t should be noted th at no guidance is given for t he simultaneous

descript ion of si l t and clay where both ar e pr es ent in a f ine soil or in a

composite soil. Terms such a s 'si l ty clay' o r 'v ery clayey sil t' can only be

used sat isfactorily a fte r determinat ion of grading and plast ici ty p ropert ies by

labora tory tes t s

(Norbur y e t al. 1984). For rapi d descrip t ions, one of t he

names should be used if th e f ine fract ion app ear s to be dominated by ei t her

silt or clay. Where no distinction can be made between sil t an d clay, both

names should be used, s epar ated by a st ro ke. This method can be applied to

th e f ine f rac t ion as e i ther th e pr inc ipa l o r secondary const i tuent (e .g . 'very

si lt y/ cl ay ey SAND'. 's lig htl y san dy SILT/CLAY'). The te rm 'FINE SOIL' may

also be us ed as t h e name of th e principal const i tuen t when i t is not possible

to dist inguish between si l t and clay.

The recommended method of naming ve ry coar se (bou lder an d cobble

size) soi ls, and soils comprising mixtures of ve ry coarse and f iner (g rav el to

clay size) material. is given in Table 16. Usually, th es e soils can only be

described sat isfactori ly in excavat ions o r exposures . I t is often useful to

record t he rock ty pe of th e const i tuent boulders o r cobbles (Table 5) , because

th is may help in con st ru ct in g a geological model of th e site.

Fur the r gu idance

T a b l e of C on

t en t s


T a b l e

of C on t en t s

T





on the description of colluvial soils containing v er y coars e material i s given in

Section 3.6.

Organic soils may often be recognized by t h e pr ese nce of plant remains.

Soils that consist predominantly of plant remains, either fibrous, or pseudo-

fibro us o r amorphous, may be descr ibed a s peat .

rapid assessment of particle size distribution has to be made on the

bas is of th e ap pe ar an ce an d feel of t h e soil sample.

I t is re lat ively easy to

dist inguish between gravel s and san ds, or between gravel ly and s an dy f ine

soi ls , because th e part ic le size which separa tes gravel and sand (2 mm) is

easily visible. Partic les of 2 mm size a r e abo ut t h e la rg es t th at will cling

to ge th er when moist owing to th e capillary attraction of water. Well-graded

and poorly-graded materials can also be distinguished by visual inspection,

al though this is more difficul t for sand than fo r gravel . I t is less easy to

dis tin gui sh between san d and s ilt. Material of 0.06

mm

s ize fee ls harsh but

not gr i t ty when r ubb ed between the f ingers , a nd t he part icles a r e a t th e limit

of visibili ty with th e naked eye. Finer pu re sil t-sized material feels smooth to

t h e touch. The feel of a fine soil can also be used to make a ve ry

approximate distinction between sil t an d clay. Silt feels slightly g ri t t y when

ru bb ed on th e teeth whereas clay feels gr eas y ( t his simple te st should only be

carrie d ou t on non-organic samples, for health reason s) . rapid assessment of

plasticity usually provid es a more reliable means of dis tin gui shi ng silt from

clay (s ee item (3) below).

(3) Table 11,lastici ty. As shown in clay and sil t , both alone and in

mixtures with coarser material , may be classified for descriptive purposes into

th r ee gro up s, viz non-plastic o r low plasticity (generally sil ts), intermediate

plasticity (lean clays) , an d high plasticity (f at clays ). For rapid des cripti on in

the f ie ld or in the laboratory, these classes may be est imated on the basis of

visua l identification an d hand t es ts , which ar e summarised in Table 11 a nd a r e

dis cus sed in more detail below. more ac cu ra te des cri pti on of plasti city can

also be made on th e bas is of lab ora tor y determination of t h e liquid limit (BSI.

1975) an d t h e extende d classification scale given in Table 17.

The rapid method is based on the general assessment of cohesion and

plasticity in coarse soils which contain a signi ficant fine fracti on. In fine

soils , th e method is based on th e assessment of d r y

s t ren g th , t oughness and

dilatancy.

In o rd er to examine coar se soils fo r cohesion and plasticity, a sample

should f i r st be loosened if ne cessa ry, for example by crush ing with th e foot or

a hammer. handfu l of th e material should th en be moulded an d pre sse d

togeth er in th e hands. I t may be necessary to add water and to pick out th e

la rg er pieces of gravel. soil shows cohesion when, a t a suita ble moistu re

con ten t, i t s particles stick tog eth er to give a relatively firm mass. soil

shows plasticity when, at a suitable moisture cont ent , i t can be deformed

withou t ru pt u re , i.e. witho ut losing cohesion. firm cohesi ve piece of soil

which deforms readily without r u pt ur e will generally contain a significan t

propo rtion of clay. Conver sely, a soil which loses cohesion quickl y and

crumbl es quickly when deformed will ten d to have a high pr oporti on of s ilt in

the f ine fract ion.

Notes on d r y st re ng th of s i l ts and clays ar e included in Table 11.

Toughness of a fine soil re fe rs t o t h e ch ar act er of a thr ea d of moist soil

rolled on t he palm of th e hand , moulded t oge the r, a nd rolled again until i t has

dri ed sufficiently t o break a t a diameter of abou t 3 mm, as in t he plastic limit


T a b l e of C

on t en t s


T a





tes t BSI . 1975). In thi s condition, inor gani c clays of high plasticity ar e fairly

stiff a nd toug h. th os e of low plasticity a r e so fte r an d more crumbly.

Inorganic s il t s g ive a weak and of ten sof t thre ad t ha t bre aks up, c rumbles

readily, and may be difficult to form.

In t h e dilatancy t es t , a small piece of soil moistened t o be so ft , bu t not

stic ky, is held on t h e open horizontal palm of t h e hand.

The side of t h e han d

is then ja r red aga i nst th e o the r hand sev era l t imes. Dilatancy i s shown by the

appe aran ce of a sh iny film of water on t he su rf ac e of t h e soil; when t h e soil

i s squeezed or pre ssed wi th th e f ingers . t he s urf ace dul ls as th e soil s t if fens

an d f inal ly crumbles. These react ions a r e marked only for predominantly si l t -

sized material and v er y f ine san d, and will general ly indicate th e pre senc e of

thes e mater ia ls .

Fu rt he r usefu l guidance on the rap id descript ion of plast ic ity with

refe renc e to dr y st re ng th , toug hnes s and di la tancy is given by ASTM 1985a).

In or ganic soi ls . small quanti t ies of d ispe rsed organic matter can pr oduce

a d is t inc t ive odour and a da rk g rey , dark brown o r dark b lu ish g rey colour .

With la rg er quanti t ies of orga nic matter , f ine soi ls usual ly h ave high, ve ry high

o r extremely high l iquid l imits, sometimes extending up t o se vera l hun dr ed pe r

cent , but th ese values may dr op signif icant ly if th e soil is a ir-dried. Close to

th e plast ic l imit , org anic soils and pea t hav e a ver y weak, spon gy o r f ibrous

thr ead , which may be difficult t o form a t al l, and the ir lumps crumble readi ly.

3.3.5 Additional Info rmat ion

Any additional information on the strength. colour, composition, grading

an d plastic i ty of t h e soi l th at would be of value in assess ing i ts n at ur e and

engineering p rop ert ie s should be recorded. Special note should be made if any

of th e descr ip t ive chara c ter i s t ics of th e mater ial a r e considered to be unusual

in relat ion to th e r e s t of i ts descript ion. I t should be indicated if the re is

doubt as t o whether t he sample descr ibed i s represen ta t ive of th e mater ial

from which i t was sampled, due, for instance. to t he f ra ct ur e of part ic les or

loss of f ines dur ing sampling, o r to th e sample size o r borehole diameter being

too small in relation to t h e grad ing of t h e material being sampled. Some

information sho uld also be give n on t h e de gr ee of di stu rb anc e of th e sample if

this is considered to be important e .g. in relat ion to descript ion of s t r en gt h

and colour) .

3.4 MASS CHARACTERISTICS OF SOILS

3.4.1 Structure

The impor tant typ es of s t ru c t ur e in soi l s a r e bedding in superf icia l

deposi t s , and re l ic t rock s t r uc t ure s see Plate 5 and discontinui t ies in soi ls

derived from insitu rock weathering.

Genera l charac ter i s t ics tha t should be considered in the descr ip t ion of

bedding inc lude th e ty pe of bedding, a r rangem ent of th e beds , and th e spac ing

between bedding planes. Other characterist ics t ha t a r e rele vant specif ical ly to

individual bedding

planes , such as or ienta tion , surf ace texture , opennes s , e tc ,

a re considered in Section 3.4.2.

Common ty pe s of b eddi ng a r e i l lus trat ed i n Figure 7 and a re def ined in

T a b l e of C on

t en t s


T a b l e

of C on t en t s

T





t h e Glossary.

In Hong Kong soils, t h e occur ren ce of bedding

is usually limited

to al luvial and marine deposi ts .

The arra nge ment of beds may be described b y re fere nce to th e deg re e of

s t rat i f ica t ion and t h e spac ing of the s t ra ta . In te rs t ra t if ied deposi t s a r e those

in which t he re a r e layer s of diff eren t typ es of material . which may b e of

cons tant thic knes s, o r may thin o ut locally or occur as lenses. If b eds of

al ternat ing soi l ty pe s a re too thin to b e described individual ly, t h e soil may b e

desc ribe d a s int erb edd ed (e.g. SAND with int erb edd ed CLAY ), t he fi rs t soil

t y p e mentioned being dominant; o r as interlaminated (e.g. thinly in te r-

laminated SILT and

C L A Y ) .

Par t ings a r e bedd ing su r faces tha t sepa ra te

easily. e.g. a thin lay er of sil t in more cohesive material. The na tu re of a ny

part ing material should be noted. Where two o r more soi l ty pe s ar e pres ent in

a deposit , a r rang ed in an i r regu lar manner , th e soil s may be descr ibed as

inter mixe d (e.g . SAND inter mixe d with CLAY ). Thick be ds which co ns is t

essentially of one soil type and show no significant variation in material can

be described as homogeneous .

Apart from variat ion in basic soi l typ es, bedding f eat ur es can also be

identi fied by o ther sedimentary s t r uc tur es , such a s she l l bands and root holes.

Knowledge of shell ty pe s a nd

densi ty within a bed may as si st in st rat ig rap hic

correlat ions. Dominant shell typ es should be noted (b y cor rec t scient if ic

name) , ske tched o r photographed (St r ange

Sh aw, 1986; Wang Yim, 1985;

Whiteside. 1983).

Multiple seq uen ces of depo sition involving combinations of mari ne an d

alluvial so ils ha ve bee n id en ti fi ed in Hong Kong (Liu Gammon, 1983; Yim,

1983). Abr upt chang es in bedding characterist ics may occur in suc h sequences.

Fu rt he r general information on bedding ch aracterist ics and th e deposi tional

envi ronm ent of local alluvial a nd marine soils i s given by Dutton (1984), Holt

(1962) a n d Lumb (1977).

Two other general structural terms commonly applied to sedimentary

soils a r e f issured , i f th e soi l is cracked or fragmented, and intact if no

f iss ures a re prese nt . Fissure s a r e most common in f ine-gra ined mar ine and

alluvial soi ls , part icularly where th es e soi ls have been exposed t o air dryi ng.

Organic soils a r e commonly descr ibed as fibrous o r amorphous (Table 11).

The spacing of bed ding planes, f issu res, shel l bands and ot he r

sedimentary s t ru c t ur es should be descr ibed using t he te rms given for p lanar

s t ruc tures and discont inui t ies in Tables 6 an d 7, as summarised in Table 11.

Apart from sedimentary soils , planar st ru ct ur es may also be visible in

soils derived from insi tu rock weathering. They should be described in

accordance with th e terms defined in Section 2.4.2 and Table 6.

3.4.2 Discontinuities

As dis cussed f or rocks in Section 2.4.1, detailed description of soil

s t ru c t ur e ( in i t s br oadest sens e) should inc lude a fu l l account of individual

discontinui ties. in addit ion to th e gene ral descript ion of plana r s t r uc tu re s

outl ined in th e previous sect ion. The discussion in Section 2.4.1 of the

variable use of th e term str uct ure for rock descript io ns applies equal ly well

to soils.

Soil discontinu ities a r e individual bedd ing planes, lamination planes and

T a b l e of C on

t en t s


T a b l e

of C on t en t s

T





f iss ures in t ran spo rte d soi ls , and rel ic t joints in soi ls derived from insi tu rock

weathering. Faults and sh ea r planes may also occur in both ty pe s of soil b u t

are generally much less common.

If

a

full descr iptio n of discontinuities in

a

soi l mass is requir e d , t h e

same proce dures and terms given for rock discontinui ty descript ion in Section

2.4.3 should be used . However, with regar d to st re ng th , deformation,

permeabil i ty an d ot he r engineering ch aract erist ics, the influence of discon-

tinuities on mass behaviour i s generally much le ss marked in a soil mass th an

in a rock mass. The ref ore , a full des cri pti on of soil discont inuit ies may only

be req uir ed in parti cular circumstances e.g. discontinuities which control slope

stabili ty .

Fu rt he r information on th e descript ion and en gineering as sess ment of

dis con tin uit ies in some Hong Kong soils is give n by Harris 1985). Hunt 1982)

and Koo 1982a, 1982b).

3.4.3 St at e of Weath ering

clear distinctio n must be made between th e weathering of superficial

deposi ts i .e . t ra ns por ted soi ls) and th e weathering of rocks insi tu which has

led to t h e formation of eng ine eri ng soils se e Sections 2.2.4 a nd 2.3.4).

Description of soils deriv ed from insi tu rock weather ing is cons idere d fu rt he r

in Section 3.5. This section is concerned only with the description of

weathering in t ransported soi ls .

I t is highly l ikely th at t he t ran spo rte d soils in Hong Kong a re general ly

much you nge r tha n t h e soils deriv ed from insit u rock weathering Bennett ,

1984a).

Also, the t ransported soi ls , unl ike the igneous and pyroclast ic rocks,

have not formed un de r condit ions of high tempera ture and

pressure , which

means th at t hei r suscept ibi l i ty to weathering processes in gener al is much

lower. Therefore, th e degree and extent of weathering in th e t ra nsp ort ed soils

is generally much less marked th an in th e thick zones of inten sely weat hered

rock found ove r much of th e Terri tory. Nevertheless, th e chang es bro ugh t

about by weathering can st i l l have a signif icant effect on th e engineering

propert ies of t ransported soi ls .

The occurrence of weathered transported soils in Hong Kong is generally

limited t o t h e older colluvial and alluvial deposits. Most of t h e marine

deposits show no obvious si gn s of we ather ing, b ut some weathered marine soils

can be found in are as where they have been exposed previously du ring periods

of lower sea-level.

In fine soils, where individual mineral and rock fragments cannot be

identified by eye, t h e most distinctive as pec t of weat herin g is discolouration

cau sed by decomposition of t h e soil parti cles a nd precipitation of var iou s

oxides. Discolouration is most noticeable in alluvial sedim ents.

non-uniform

colour distribution Table 3). often comprising mottled yellow, r e d an d brown

colours, can be a dis tincti ve fe at ur e in t he se soils Shaw e t al , 1986; Willis

Shirlaw. 1983). In offsho re sedimentary sequ enc es, th er e is often a marked

cont rast between mott led, w eathered alluvial sediments and th e overlying

unweathered marine muds Plate 9) .

In coarse soils, or composite soils containing coarse fragments, the

weathered s tat e of individual gravel and larger-sized rock

fragments can also

be descr ibed. in addition to discolouration of th e whole soil . Since th es e

T a b l e of C on

t en t s

T a b l e of C

on t en t s

T a b l e

of C on t en t s





fragme nts a r e pieces of rock material , t h e terms an d methods given in Section

2.3.4 can be used to descri be th eir weathe red state. Common weathe ring

fea tur es ar e decomposition of individual mineral grai ns o r whole rock

fragments, and crackin g o r disintegrat ion, which may show up a s concentric

layering approximately paral le l to th e fragment boundary. Fu rt he r guidance on

weathering description in colluvium is given in Section 3.6.

Weathering fea tur es in soi ls may be destr oyed by sample dis tur ban ce o r

remoulding. These fea tur es should the ref ore be described in th e field when-

ever possible; a l ternat ively, the highest-qual i ty undisturbed sample should be

used.

3.4.4 Additional Information

Because of sample dist urb anc e o r inad equate sample size, i t is often

difficult to make a full description of th e mass cha rac ter ist ics of soils.

Even

in a field exposure, ve ry car efu l and detailed inspection may be nec ess ary fo r

acc ura te identification of st ru ct ur al fea tur es. Additional information should be

added to the descript ion i f the sample is not considered to be representat ive

of th e soil mass, o r if i t shows si gns of significant dis turban ce.

One ot he r gro up of fe at ur es which should always be describ ed if pr es en t

in t h e soil is voids. The most important typ es of void ar e pipes and tunne ls

caused by subs urface e rosion

Nash Dale. 1983), bu t ot he r fea tur es su ch as

animal burro ws an d ro ot holes should also be noted

if they ar e likely t o have

a significant

effect on t he mass prope rties of t h e soil. Soil pipes have been

recorded in Hong Kong in both colluvium and soils derived from weathered

rocks. In some cases they hav e been observe d within landsl ide sc ar s Nash

Chang, 1987), and ha ve the re fo re been co nsider ed a s a possible cau se of slope

instabili ty. The geometry an d see pag e asp ects of voids should be rec ord ed

where possible.

The recommendations in Sections 3.4.1 and 3.4.2 do not cover the

descri ption of soil micros tructu re. This can be import ant fo r engin eering

purpo ses, e .g. soils with pronounced small-scale fabric s, such a s ver y th in

laminat ions in marine clays, microfractures t hr ou gh and ar ound mineral gra ins

in soils derived from insit u rock weathering , etc. Partial asses sment of thes e

fea tur es by e ye may be possible , depending on thei r spacing and continui ty.

but , if th e soi l microfabric is of part icular importance to t h e engineerin g

projec t, t h e use of a microscope is recommended. gener al introd uction t o

t h e s t ud y of soil microfabric. us ing optical microscopy, electron microscopy an d

X-ray diffraction tech niq ues , is given by Tovey 1986b3. Fu rt he r information

on microfabric description in grani tic soils, and its relationship to engin eering

pro per ties . i s given by Baynes Dearman 1978a. 1978b).

t

a s l ight ly la rger

scale, McGown e t a1 1980) pro vid e usef ul gui dan ce on th e classification and

descript ion of fa bric fe atu res visible to th e naked eye or un de r a hand lens in

sediments.

3.5 SOILS DERIVED FROM INSITU R O K WEATHERING

Soils der ived from ins itu rock w eat her ing o ccu r ove r much of Hong

Kong. both on land an d offshore. Assessment of t h e engin eering behavi our of

th es e soils is of gr ea t importance for many as pec ts of t h e design a nd

construct ion of si te formations and foundat ions. The sta rt i ng point for such

assessments is a good engineering description.


T a b l e of C

on t en t s


T a





These soils can be divided into two main ty pe s, viz

( a )

soi ls tha t r e tain t he o r iginal t ex ture , f abr ic and s t r uc tu re

of t he p are nt rock, also known a s 'saproli tes' , and

( b ) soils in which the rock texture fabric structure has been

dest roy ed, properly descr ibed as ' residual soils '.

Both thes e soils ar e shown in th e context of weather ing of t he par en t

rock in Tables 4 a nd 10. A t th e materia l scale , th e f i r s t of the se soil typ es

(saproli te) cor resp onds to completely decomposed rock (g ra de V i n Table 4.

b u t may also include less decomposed i nten sely disi nteg rate d material (e.g.

gr ad e IV) th at can be completely broke n down to a soil. The second ty p e i s

t h e gr ad e VI resi dual soil.

A t

th e mass scale, saproli te forms th e non-rock material in the par t ial ly

we ath ere d (PW90/100 t o PW0/30) rock mass z one s in Table 10. The second soil

ty pe ( th e s t ru ctu rel ess resid ual soil) comprises th e RS zone in Table 10; th is i s

identical to t he r esid ual soil (g ra de VI) in Table 4 but s imply refers to a

la rg er volume of material.

Different appr oach es a r e recommended fo r making full descr ipti ons of

these two main soil types.

Saprol i tes (i .e . soi ls tha t re ta in t he rock texture , fabr ic and s t r uc tu re )

a r e a special ca se fo r descr ipt ion, because they can be descr ibed e i th er in rock

( see Chapter 2) o r soi l ( see Chapter 3) te rms , o r bo th. The recommended

scheme is to u se t he rock terms given in Chapter 2, together with the soil

s t r e ng th ( s e e Section 3.3.1) and soil name (see Section 3.3.4) appl icable to th e

remoulded condition ad ded in br ack ets . For example, t h e desc rip tion of a hand

sample might be 'extremely weak, dr y, light yellowish bro wn, equi gr an ula r.

completely decompose d, co ars e-g rai ne d GRANITE (d en se , slig htly si lt y grav el ly

SAND)'. The exception to th is recommendation comes when th e r e is do ub t

abo ut t h e origin of t h e soil, e.g. a ve ry small sample might not contain

suff icient evidence of or iginal rock t extu re f or th e or igin to be determined.

In such case s, th e sample should be d escr ibed by means of th e soil term s given

in Chapter

3, fo llowed by a n in terpre ta t ion of t he pare nt rock and weathered

st at e in brack ets , e .g. ' s t if f , moist , grey ish brown, s l ightly gravelly san dy

SILT/CLAY (completely decomposed coarse ash TUFF?)'.

A t

the mass scale, a

full description of a sapr olit e should include a detailed description of

discontinuit ies su ch as rel ict joints (see Section 2.4.3).

Full descri ptio ns of r es idual , s t ruc t ure les s soils should b e made by means

of t h e soil term s defi ned in Sections 3.2 a nd 3.3. If t he re is suff icient f ield

evidence in th e weather ing prof i le to identify t he par ent rock from which th e

resi dual soil has formed, th is should be add ed in bracke ts , e .g. ' firm, d r y ,

brown, s ligh tly sa nd y SILT/CLAY (Residual soil der iv ed from fine as h TUFF)'.

Residual soils may be difficult to d isti ngui sh from oth er soils su ch a s colluvium

and fill. This is con side red in Section 3.6.

True res idual soi ls a re rare l y developed to a ny s ignif icant th ickne ss in

Hong Kong (usually less tha n 3 m). Generally, th ey a r e much less impo rtan t

to engineer ing than the saprol i tes .

Fu r t her informat ion on th e nature , descr ipt ion and engineer ing prop er t ies

of Hong Kong soils derived from insitu rock weathering is given by Bennett

(1984a). G O (1982), Henc her Martin (1982), Lumb (1965) an d Ruxton


T a b l e of C

on t en t s

T a b l e o

f C on t en t s

T a b





Berr y (1957). Some illu stra ted examples of complete desc ript ions ar e given in

Section 3.9.

3.6 COLLUVIUM

Colluvial soils o r mass wasting depos its (s e e Appendix A.6.2) ar e formed

by e ar th materials slipping , flowing o r rolling down slopes u nd er t he action of

grav ity. Typical colluvial depo sits in Hong Kong ar e st ru ct ur el es s mixed

accumulations of soil an d rock frag ment s deposited on an d a t t h e base of

nat ur al slopes. The yo ung er colluvium is often loose, whe reas some of t he

older colluvium may be partially or wholly cemented.

The deposi ts often have

a dist in ct ive lobe- o r fan-sh aped su rfa ce form an d may be interlayered with

alluvial fan de posi ts formed by th e action of run ni ng water. Colluvium is

widely dis tri bu ted thr ou gh ou t t h e hilly te rr ai n of Hong Kong. It occ urs mostly

in t he form of sc at te re d, relatively small accumulations on t h e lower pa rt s of

t h e st ee p major slopes . The maximum th ic kn es s of colluvium rec ord ed in Hong

Kong is abou t 35 m, bu t t hi s is exceptional; most colluvium de posi ts a r e less

t h a n 10 m thick.

Detailed field s tud ies of th e locations, sh ap es a nd compositions of

colluvial depo sits by t h e Hong Kong Geological S ur ve y h as enabl ed a

classification of t h e de pos its t o be made according to th ei r mode of formation

b y d if f er en t mass movement pro ce ss es . On t h e 1:20 000 HKGS Maps (e. g.

GCO

1986a), mass wast ing deposi ts ar e sub divided int o d ebri s f low deposi ts , ta lu s

(rockfall) deposits, mixed deb ris flow an d talu s dep osit s, an d slide deposits.

Fu rt he r information on t he nat ure of t h e material in each of t hes e classe s is

given by Addison (1986) an d Benn ett (1984a).

For rout ine purposes , t he desc ription of colluvium should follow the

recommendations g ive n in Sections 3.2 t o 3.4. Since most colluvial deposits

conta in ve ry coarse (cobble- and boulder-s ize) f ragments , th e d iscussion of

composite soils in Section 3.3.4 (2 ) an d Table 16 is of pa rt icular relevance for

descript ion. Although col luvial deposi ts ar e usual ly described a s st ru ctur eles s ,

Ruxton (1984)

sug gest ed tha t a var iety of s t ru c t ur es exis t in the col luvium in

t h e Mid-levels ar ea of Hong Kong Isla nd.

If p res ent , s t ru c tu ra l typ es can only

be ident if ied and descr ibed adequate ly in la rge exposures .

Use of th e proc edur es an d terms in Sections 3.3 a n d 3.4 should give

a

sati sfac tory basic descrip tion . However, if th e colluvium is of par ticul ar

importance to th e eng inee ring project , i t is recommended t ha t th e fol lowing

addit ional aspects should be described

th e proport ion of ve ry co arse fragments (cobbles and

b o u l de r s ) t o t h e n e a re s t l o ,

any preferent ial d is t r ibut ion of t he ver y

coarse f ragments ,

the angular i ty , s t rength and decomposi t ion grade of the

v e r y c o a rse fragment s , us ing t he terms given in Table 14,

Section 2.3.1 a n d Section 2.3.4, a n d

th e thic knes s of any clearly ident if iable weathering r in ds

developed on th e ver y coarse f ragments (measured in

mm), together with any other notable features of this

rin d (e.g. colour, decomposition grade. de gr ee of

T a b l e of C on

t en t s


T a b l e

of C on t en t s

T





f rac tur i ng) re la t ive to th e remainder of t he cobble o r

boulder.

The importance of th es e additional observations has been demonstra ted

by detailed inves tiga tion s carr ied o ut in t h e Mid-levels ar ea of Hong Kong.

These invest igat ions have indicated t ha t th re e se par ate classes of colluvium

may be recognized on th e basis of th e colour and st i ffness of t h e matrix, th e

rat io of ve ry coarse fragm ents t o matrix material , and t h e de gr ee of

decompositi on of t h e v e r y co ar se fr ag me nt s (GCO, 1982; Lai Taylor, 1983).

These classes probably reflect differen t ages of deposi tion. Where differe nt

classes can be obser ved clearly in th e f ield, i t is useful to note this in th e

descript ion. Recognit ion of different classes may assist in int er pre t in g th e

geological st ru ct ur e of a si te , and in asses sing laborator y test re su l t s on

matrix materials.

Colluvium may be diff icult to dis tin gu ish from ot he r typ es of soil,

part icul arly soils deri ved from ins itu rock weather ing and fill. Distinctive

fea tures t ha t can help to dis t inguish between thes e soi l typ es a re shown in

Table 18. One part icular problem th at is often encountered in t h e descript ion

of drillcore and small excavations (e.g. trial p its ) i s t h e definition of th e

boundary between t he base of t he colluvium an d th e underlying decomposed

rock. The two most rel iable dist inguish ing charac teris t ics ar e usually a chan ge

in colour and t h e abse nce of small-scale rock te xt ur e in th e colluvial matrix

material. Examples a r e shown in Plate

1 0

An example of t h e des crip tion of colluvium is g iven in Section 3.9.

3.7 FILL

Fill o r made ground is a ve ry common t y p e of soil foun d in th e

developed are as of Hong Kong. The ext ent and t hic kn ess of fill soils var y

widely, rang ing from relatively small fill platforms use d fo r building

developments on st ee p slop es, to lar ge a re as of coa stal reclamation.

Characteristics of f i lls such as colour, compactness /consiste ncy and g rain size

can var y ov er a ve ry wide range , dependent mainly on t he or igin of th e

material, and th e methods of placement an d compaction.

Good engin eeri ng desc ript ions a r e of gr ea t importance in fill materials,

which may be difficult t o sample and t es t sat isfactori ly if t he y a r e

heterogene ous, o r if they contain large fragments of foreign materials.

The descr iptio n of fill should follow t h e recommendations g ive n in

Sections 3.2 t o 3.4. However, this t yp e of r outi ne descript ion should be

accompanied by addit ional information, where considere d rel evan t to t h e

engineerin g project , on th e following asp ects

(a) origin of th e fi ll material, whet her natu ral ea rt h material

o r otherwise (e .g. domestic ref use, pulverized fuel as h,

e tc) .

(b ) presence of la rge fore ign objec ts , suc h as pieces

concrete, masonry. brick. wood, metal or plastic,

of

(c ) pre sen ce of voids o r collapsible hollow obje cts.

( d ) pres ence of chemical waste, part icularly if i t app ear s t o

T a b l e of C on

t en t s


T a b l e

of C on t en t s

T





conta in dangerous o r poisonous substan ces,

(e) organic mat ter content and any s t ron g smel l,

( f ) s t r ik ing co lours , and

(g ) da tes readable on bur ied newspapers , e tc .

With r eg ar d t o item (a ) in th is l ist . information abo ut th e origin of t h e

fil l is often useful in major earthw orks , e .g. in th e assessmen t of labo ratory

t es t res ul ts an d field measurements of compaction performance.

Concerning th e s t ru c t ur e of th e soi l (see Section 3.4.1). i t i s important

to describe any layering tha t may be pres ent . On slopes, layering

approximately parallel to t he original slope surf ace indicates th at t h e f il l has

probably bee n end -ti ppe d an d is l ikely to be in a loose condition. The

boundari es between dif ferent f il l laye rs, or between fil l and t he un derlying

natura l soil , a re of ten marked b y ab ru pt chang es in root content of th e soil

a n d t h e

presence of old er, buried topsoils r ich in orga nic matter .

Fill th at does n ot contain obvio us inclusions of fo reig n materials may be

difficult to dist ingu ish from ot he r ty pe s of soil . The penultimate pa ra gr ap h of

Section 3.6 and Table 18 should be noted.

An example of th e

description of fill is given in

Section 3.9.

3.8

ADDITIONAL

GEOLOG I CAL

INFORMATION

Once th e mater ial an d mass charac ter i s t ics have been descr ibed , the

fina l item in a complete soil desc ript ion sho uld be a geological name which

indicates t h e geological origin o r soil ty pe. Many of t he app ro pri ate names

a r e shown i n th e lege nd s on t h e geological maps p roduced by t h e HKGS.

The

name shoul d be writ ten with cap ital initial le tt er s (e.g. Alluvium, Fill, Marine

mud, Colluvium).

A s

a general rule, a geological name should only be added to

a descript ion wh ere t he origin of t he material is reasonably c ertain; conject ure

should be avoided. However, if th e ob ser ver wishes to record do ubtful

in terpre ta tion. an acceptable a l te rnative i s to indica te t he uncer ta inty b y use

of a suita ble qualifying term o r a question mark (e.g. 'probably Colluvium'.

'Residual soil?')

3.9 EXAMPLES

Seven different soi ls a r e i l lustrated in Plate 11. The f i r s t four a r e hand

samples a nd t he las t th re e a re mass exposures .

The ful l descript ions of these

soils ar e l isted below for guidance in th e us e of appr opr iate terms.

( a ) Plate 11A (Residual Soil). Loose, dr y. yellowish brown.

with occasional rounded quartz grains 2 t o 4 mm size,

si lt y gr av el ly SAND (Residual soil, from field evi de nc e

prob ably deri ved from coars e ash o r lapilli TUFF).

Slakes readily in water.

( b ) Plate 1 B (Completely Decomposed Grani te) . Extremely

weak, dr y, l ight yel lowish brown spo tted with gr ey, da rk

brown and white, completely decomposed, medium-

T a b l e of C on

t en t s


T a b l e

of C on t en t s

T





grained GRANITE, with occasional, discontinuous.

r andomly-or iented mic ro f ractures . of ave rage spac ing 2

t o 5 mm, v is ib le in s ev er al small a r ea s < 50 m m 2 on

su r f a c e of sample, g en er al ly s e pa r at i ng i n ta c t f e ld sp ar

gra ins from su r rou nd ing matrix. One prominent

m icr of ra ctu re , a p e r t ur e 1 mm l en gt h 20 mm, c r os s es

c e n t r e of sample. Slakes readily in water. Hand

penetrometer s he ar s t re n gt h index 180 kPa. (Loose,

sl ightly si l ty/clayey, gravelly SAND). (Note

material is

described as a decomposed rock, except fo r t h e

compactness and par ti cl e s ize d i s tr ibu t ion app li cable to

th eremoulded condition,which ar eaddedi npare nth ese s).

( c ) Plate 11C. (Marine Mud). S ti ff , moist, d a r k b ro wn ish

g r e y , s li gh tl y s an d y SILT/CLAY (Marine mud). S he ar

vane s t ren g th index 120 kPa . Con ta ins occas ional small

f ragments of whi te she ll s .

( d ) Plate 11D. (Marine San d) . Loose, moist, l ig h t b ro wn ,

s li gh tl y g ra ve ll y f in e t o c oa rs e SAND (Marine s a nd ).

Contains some a n gu l ar a n d su b an g u la r sh el l f ra gm en ts

and whole she ll s up to 30 mm i n l e ng th . SPT N value of

8 recorded in borehole A1 a t th e level of th is sample .

(e l Plate 11E. (Colluvium). For d escr ip ti ve p u r po se s, t h e

col luvia l depos i t i s d iv ided ( top downwards) in to th r ee

layers , mainly on t he basis of differing size an d

p ro po rt io n of t h e v e r y c oa rse f ra gm en ts a n d d e gr e e of

c em en ta ti on of t h e m atr ix. L ay er i s a bo u t 2

m

thick

a n d f or ms approximately t h e u p p e r half of t h e d ep os it .

Layers 2 and 3

a r e b o th a bo ut 1

m

thick.

( i ) Layer 1. Dense. d r y , yellowish bro wn

( l ar ge cobbles a nd bo ulder s a r e l ig ht

g r e y ) , b ou ld er y COBBLES with much

f ine r mater ia l ( s light ly g ravel ly , sandy

sil t /clay). (Colluvium). Very coars e

f ra ct io n c om pr ises mostly a n gu l ar a n d

s ub an gu lar cobbles of v er y s tr on g to

moderatelystrong,slightlytomoderately

decomposed, f ine as h tuff ; a lso contains

someangular andsubangular bouldersup

to0.8 m diameter and severa l de tached,

p ar tl y f ra gm en te d b lo ck s of r oc k ( fi ne

a s h t u f f) u p t o 1.2 m diameter showing

closely-spaced joints. Thef iner material

is part ial ly cemented; removed easi ly by

hammer bu t crumbled by hand only with

difficulty.

(ii) Layer

2.

Very s ti ff , d r y . yellowish

brown, slightly gravelly, sandy

SILT/CLAY with many (approx ima te ly

30%)subangular to subrounded cobbles

a n d small b ou ld er s of s t r on g t o mod-

era te ly s t rong, moderately decomposed.

f ine ash tuff (Colluvium). The matrix i s


T a b l e of C

on t en t s


T a





part i al ly cemented (as in l aye r 1 .

(iii) Layer 3 . Very s t if f , moist , da rk b rown

(boulde rs a re l igh t g rey) , s l igh t ly sandy

gravel ly SILT/CLAY with many (approx-

imately

4 0% )

s u ba n gu l ar t o s u br o un d ed

cobbles andoccasional rou nded boulders

of v er y s tr on g t o moderately s tro ng .

s l ight ly to modera tely decomposed, f ine

ash tuff (Colluvium). The matrix i s

wholly cemented; re qui res f irm blows of

hammer t o remove a nd cannot be

crumbled completely by hand.

Based on th e degree of cementa tion of th e matrix, layers

1 a n d 2 a r e p ro ba bl y much y o u n g er t h a n l a ye r

3 .

( f ) Plate 11F. (Al luvium). This small exposure i s descr ibed

a s an in te rbedded deposi t because t he scale of t he

individual l ay er s is too small t o w ar ra nt a s ep ar at e

desc rip t ion fo r each l ayer .

Loose , moist , l ight brown, s l ight ly s i l ty /c layey, gravel ly

SAND with i nt er be dd ed s of t, moist. g rey is h brown.

s l ight ly sand y SILT/CLAY (Al luvium). Thickness of sand

beds in the ra nge 80 to 200

mm

clay beds 20 to 60

mm.

Inte rbed boundar ies genera l ly planar and sub-horizonta l .

occas iona lly h ighly i r regu la r and show s lump s t ru c tur e .

Some s an d b ed s h ave a thickly-laminated s tr uc tu re .

o th ers ar e homogeneous; c lay beds are th inly- laminated.

( g ) P late 11G. (Fil l). F ou r d i st inc t l a ye r s a r e v is ib le i n t he

t r i a l p i t and a re desc r ibed f rom th e top downwards . The

th i rd , da rk-co loured l aye r i s much th inn er than the o the r

t h r e e l a ye r s .

( i ) Layer 1. Sof t, d r y , l i gh t yellowish

brown. s an dy SILT/CLAY with many

a ngu la r c obbl es a nd small boul der s of

m oderat el y s t r ong t o m oderat el y wea k,

mo der atel y t o h ig hly decomposed, a s h

tuff and occasional pieces of brick (Fil l) .

Boulders concent ra ted a t th e base of the

laye rwithoccasional discontinuousvoids

u p t o 100

mm

diameter. Roots u p t o

5

mm diameter a r e s c at te r ed t h ro u gh o ut

t he l a ye r .

(ii) Layer 2. Soft, moist, brownish r ed ,

s li gh tl y s an dy SILT/CLAY with some

ro otle ts a nd small a n gu l ar c ob bles of

m oderat el y s t r ong t o m oderat el y weak,

m od er at el y t o hi gh ly decomposed. a s h

tuff (Fi l l) .

T a b l e of C on

t en t s


T a b l e

of C on t en t s

T





( i i i ) Layer 3 . S o f t to f i r m , w e t , d a r k g r e y i s h

b r o w n , s l i g h t l y g r a v e l l y s a n d y

SILT CLAY (F i l l ) .

( i v ) L a y e r

4

Firm, wet . brown, s l i gh t ly

s a n d y SILTICLAY with occasional smal l

suban gu lar co bb le s o f moderate ly weak .

h i g h l y d e c o m p o s ed . a s h tu f f (F i l l ) .

T a b l e of C on

t en t s


T a b l e

of C on t en t s

T





[BL NK

P GE]

T a b l e of C on

t en t s


T a b l e

of C on t en t s

T





ENGINEERING CL SSIFIC TIONS OF ROCKS ND SOILS

4.1 GENERAL

A

complete rock o r soil description gives information on t h e

cha ract eris tic s of a specific sample, whet her th is is a hand-sized piece of

material, a len gth of drillcore o r a mass exp osu re. Few samples have identic al

descrip tions. Engineering classif ication, on th e ot he r hand, involves placing

t h e rock o r soil in to a limited n umber of broad gr ou ps , each of which can b e

expected to show reasonab ly dist inctive engin eering behaviour . I t is empha-

s ised th at c lass i ficat ion in thi s se nse re fe rs to the overall grouping of rocks o r

soi ls with rega rd to general engineer ing behaviour . I t should be dis t inguished

from th e HKGS geoloqical classification of roc ks an d sup erfi cia l dep osi ts Table

5 see also Appendix

A

and f rom s ~ e c i f i c c lass if ica t ions of individual

charact er is t ics , su ch as th e classification of rock mater ial s t re ng th Table o r

decomposition grade Table 4 .

The main v alu e of broad eng ine erin g classifications i s to give a simple

general guide to th e behaviour of t he rock o r soil dur ing construct ion.

Because of th e emphasis on constru ction a spec ts , the se classif ications ar e

usually establis hed mainly with refe renc e to mass behaviour . There a re various

methods of classify ing th e mechanical behavi our of rock masses. These a r e

considered in Section 4.2. For soils, eng ine erin g classifications ba sed on

gra din g and plastici ty ca n gi ve a good gui de a s to how t h e remoulded soil will

behav e when used as a constru ction material. This is consid ered in Section

4.3.

Engineering classifications of rocks and soils have not been applied

extensively in Hong Kong. The text in th e next two sections is the refo re

deliberately short . The aim is to provide brief ge neral guidance, and to quote

key re fer en ce s an d examples th at can be followed u p in g re at er det ail if

r equi red .

4.2 GEOMECHANICAL CLASSIFICATION OF

R O C K

MASSES

Geomechanical classifica tions of ro ck m asse s ar e based on combining

sev eral cha ract er is t ic s of t he rock mass and rock mater ial into groups which

can be used to ass ess th e overal l engineer ing behaviour of th e rock mass.

A

gen eral introd uct ion to th is topic is given by Bieniawski 1976).

The best -kno wn examples of th es e classificatio ns come from t h e field of

tunnell ing an d und erg rou nd excavations, in parti cular t h e Norwegian NGI)

sy ste m Bart on e t al, 1974) an d th e South African CSIR) Rock Mass Rating

syst em Bieniawski. 1974). These syst ems were se t up by combining su ch

charact er is t ics a s dr i l lcore quali ty R Q D ) , compressive strength of rock

material , spacing an d condition of discontinuit ies , and gr oun dwa ter conditions,

to give an overal l ra t ing o r rock mass qual i ty in rela t ion to th e requirements

for tunn el su pp or t. For example, t h e CSIR syste m has five rock mass classe s

ran gin g from ve ry good rock to very poor rock.

ot he r wel l-known class if icat ion schemes a re t he f rac tu re spacing/point

load s t ren gt h c lassi f ica tion cha r t s used to ass ess r ippabi l i ty or ea se of

excavation Franklin e t al , 1971). and th e empir ical s t re ng th cr i te r ia developed

b y Hoek Brown 1980) for a rock mass classif ication based on rock type,

joint sp acin g and d eg ree of joint weat heri ng. Rock mass weat heri ng zones


T a b l e of C

on t en t s


T a





( see Section 2.4.4.). when used in conjunct ion with ot he r mass an d material

propert ies (e .g. discontinui t ies and s tr en gt h) , can also form th e basis of a rock

mass eng ine eri ng classification. For example, Dearman e t a1 (1978) us ed t h e

BSI (1981) mass w ea th er in g sch eme t o make a six-fold classification of

weat here d g ran ite s and gnei sses from t h e point of view of eas e of excavation,

tunnel sup por t , foundation sui tabil i ty, dr i ll ing r a te s and othe r fac tors . A

use ful summary of seve n d iffer ent rock mass classifications developed for

various engineerin g works in Jap an has been compiled by th e Japan Society of

Engineering Geology (1987).

I n Hong Kong. rock mass classif ications hav e been appl ied to a limited

ext ent in t he field of tunnelling a nd la rg e rock excavations. McFeat-Smith e t

a1 (1985) us ed a simple five-fold classification ba sed on mass weather ing an d

joint spac ing for the assessment of temporary tunnel su pp or t and fo r

cont ractu al ten der ing an d payment purp oses . Beggs McNicholl (1986)

examined t h e use of a simplified CSIR s yste m dur ing t h e investigation an d

des ign s ta ge of l ar ge rock excavation s for hou sing development a t Ap Lei

Chau. Hong Kong Isla nd. Six-fold wea the rin g-b ase d classifications were us ed

by Watkins (1979) fo r cons truc t ion -sta ge mapping of foundat ions and tu nnel s

fo r t h e High Island water scheme, an d by Powell Ir fan (1986) for preliminary

assessm ent of fai lure modes and design of remedial measures a t th re e landsl ide

si tes . Whiteside Brac egir dle (1984) developed a five-fold classification ,

similar t o th e NGI system, for ass ess ing rock mass looseness and t he

requi rements for u nder grou nd su ppo r t in small excavat ions in severa l d i f fe rent

Hong Kong rocks.

The Hoek Brown (1980) rock mass s tr en g th classification has also been

applied in Hong Kong in s eve ral cases of slope stabili ty desi gn in dist ur bed .

closely jointed volcanic rocks (unpublished work by the G C O . Hoek (1983,

1986) has commented on the practical application of this approach to mass

s t reng th a ssessment .

4.3 SOIL CLASSIFICATION FOR ENGINEERING PURPOSES

The aim of most engineering

soil classifications is to place a soil into a

limited number of gr ou ps on th e basis of t h e grad in g and plasticity of a

dis t urb ed sample . These charac ter i s t ics a r e independen t of the par ticular

condit ion in which a soi l occu rs, and the y d isr ega rd th e influence of t he

st ru ct ur e of th e soi l mass. Therefore, th e value of thi s typ e of classif icat ion

is t ha t i t g ives a guide as t o how th e d is turb ed soil will behave , a t d i f fe rent

moisture cont ents , when used as a construct ion material. I t does not provide

an y gu idance as to how th e und istu rbed soi l mass will perform du ring

const ruc t ion

(e.g. in relat ion to set t lement under foundat ion loading or

stabili ty of excavation faces).

A well-known example of a soil classification sy ste m is t h e British Soil

Classification System (BSCS), which is desc rib ed b y BSI (1981). This sy ste m,

sl ightly modified in accordance with Table 15, is summarised in Tables 19 a n d

20, and in Figure 8. The pr inc ipa l so il g rou ps a r e th e same as those shown in

Table 11, bu t th e sub gro ups a r e d iv ided fu r t he r on th e bas i s of l aborato ry

t e s t s .

This classification is ca rr ie d ou t on material nominally fi ne r th an 60

mm

Coarser material (boulders and cobbles) should be removed an d i ts proport ion

of t h e whole soil should be est imated and recorded as cobbles and /or

boulders (see Section 3.3.4(2)) . The grading of th e gravel and f iner material,

T a b l e of C on

t en t s


T a b l e

of C on t en t s

T





an d th e plasticity of th at frac tion of t h e material passing a 425 pm sieve, is

then determined from laboratory tests.

Grading and plast ic ity characterist i cs a r e divided into a number of

clearly defined ranges, each of which may be re fer red to by a descript ive name

and a descr ip t ive le t te r , as shown in Table 19. The soi l grou ps in th e

classifica tion a r e formed from combinations of t h e ra ng es of chara cter isti cs.

Table 20 gives th e names of t he grou ps an d th e symbols th a t should be used.

The let ter d escribing t he dominant size fract ion should be placed f i rs t in th e

symbol (e.g. CS, sandy CLAY ; SC, ve r y cl ayey SAND; S-C, clayey SAND). Any

gr oup may be qualified as 'organic ' if or gan ic matt er is a significant

const i tuen t , in which case th e le t ter '0' is suffixed to t h e gro up symbol (e .g.

CHO organ ic CLAY of high plasticity; CHSO, or ga ni c sa nd y CLAY of high

plasticity). However, t h e most important gro up of organ ic soils is tha t which

plots below t h e A-line, MO (Figure 8) , an d which comp rise s most M-soils of

high liquid limit and above.

Part ic le size distr ibut ion can be plotted on a grading cha rt , as shown in

Figure 9. This ass is ts in designat ing th e soils as 'wel l-graded' o r 'poorly-

graded' . and , if poorly-graded, whether 'uniform' or 'gap-grad ed' . Typical

examples of t he gr ading curv es of thes e and ot her materia ls a re shown in

Figure 9. Many Hong Kong soils. par tic ula rly colluvium an d soils de ri ve d from

insi tu weather ing of coarse-grained igneous rocks , a r e charac ter i sed by a ver y

wide ra ng e of g rai n sizes from gra vel to clay (excluding ve ry coar se material)

and can be described as 'widely-graded' (F igure 9).

Soil classification systems

have no t been widely us ed in Hong Kong, bu t

they may have applicat ion in p roje cts involving major earthfi l l ing works o r th e

us e of n atur al o r screen ed soils as road construct io n materials. However,

caut ion is needed in th e use of t he se sy stems fo r soi ls derived from insi tu

rock weathering.

The grad ing a nd plast ic ity cha racterist ics of saprol i tes and resi dual soils

may be affected b y pretreatmen t methods or variat ions in moisture content

(e.g. wh eth er tes te d in an air-dr ied or nat ur al condition). BS 1377 (BSI, 1975)

draws at tent ion to t h e difficulty of test ing certain t ropical soils and highly

aggregated soi ls , with regard to th e use of d ispers ing agen ts and pre t rea tment

methods in grading tes ts . and air-dried o r natu ral moisture condit ion samples

in Atter ber g limit te st s. However, no explicit recommendations a r e given in

BS 1377 fo r dealing with th es e problematical soils. Very littl e work h as been

done on this topic in Hong Kong. Useful backgr ound information and d ata for

saprol i tes and residual soi ls

in oth er pa rt s of th e world ar e given by Mitchel l

Si ta r (1982) an d th e Committee on Tr opica l Soils of t h e ISSMFE (1985). In

addition to normal grading and plasticity tests, dispersion tests (ASTM, 1985b;

Decker

Dunnigan, 1977; Flanagan Holmgren, 1977; Sh er ar d e t al, 1976;

S ta nd ar ds Association of Australia, 1980, 1984) may p rov e us eful in t h e

inte rpre tat io n of th e l ikely engineering behaviour of thes e soils . I t is

recommended th a t t he use of any pre t rea tment methods o r d is pers ants for

grading and plast ic i ty tests should always be recorded in ful l on laboratory

te s t r e su l t s shee t s and in repor t s .

Apart from the general difficulties which may be met in the classifica-

tion of soils der ived from weathered ro cks , t h e us e of th e BSCS in part icul ar

has been cri t ic ised when applied to coarse-gr ained grani t ic soils . The

dist incti on between coa rs e a nd fine soils in t h e BSCS (i.e. 35% pass ing a 63 pm

siev e), means th at a sig nificant proportion of decomposed gran itic soils would

be classif ied as f ine soil , according to th e aver age grad ings given b y Lumb

T a b l e of C on

t en t s


T a b l e

of C on t en t s

T





1962).

Granitic soils tend to be regarded as coarse soils in current Hong

Kong practice as far as their general engineering behaviour is concerned.

Therefore, other classification systems, such as the American

UC S

system

ASTM , 1985c), may be more appropriate for this type of soil.

T a b l e of C on

t en t s


T a b l e

of C on t en t s

T





LEGEND FOR MAPS PLANS AND DIAG RAM S

5.1 SYMBOLS FOR ROCKS AND SOILS

Recommended symbols a r e listed i n Table 21 for the pr inc ipa l rock and

soi l ty pe s th at a re l ikely to be encountered in Hong Kong. The symbols ar e

simple an d dist inct ive , and they combine easily int o symbols for composi te

ty pes of rock s an d soils. The symbols a r e based upon thos e given by th e

Geological Society 1972), with some alt era tio ns ; additional symbo ls ar e also

given in the same publication.

5.2 OTHER SYMBOLS

5.2.1 Symbols f o r Borehole Records

Recommended symbols for borehole records are given in Table 22.

Examples of completed borehole logs a r e give n in Geoguide Guide to Site

Invest igat ion G C O , 1987a).

5.2.2 Symbols fo r Geological St ru ct ur es an d Bou ndar ies

Recommended symbols for ge nera l planar s t r uc tu re s a re given in Table

23. For each s t ruc tur a l typ e , the long ba r of th e symbol indica tes th e s t r ike

di rec t ion , and th e s ho r t ba r indica tes the d ip amount in de gree s measured f rom

th e horizontal . Formerly, th e dip arrow was used exclusively to indicate t h e

direction and amount of dip of bed ding planes. It is sti l l use d occasionally

and provides an acceptable a l te rna t ive to the bar symbol.

Bedding, foliation, bandin g and cleavage in sedimen tary a nd metamorphic

rocks may be cor ruga ted o r undula t ing , a l though th e genera l d isposit ion may be

horizontal, inclined o r vertical. These conditions may be indicated by si nuou s

s t r i k e b a r s .

Recommended symbols for o ther geological st ructures and geological

boundar ies a r e g iven in Table 24. dist incti on i s made on t he Hong Kong

Geological Sur ve y maps between bo und aries of superficial depo sits an d

bou ndar ies of solid rock . Some indication is usually given of th e accu rac y of

boundari es, bro ken l ines denoting unce rtai nty in t he posi t ions of solid rock

boundarie s and faul ts . This principle may be applied to th e t r en d and, where

appropr ia te , to th e posit ion of t he t races of o th er p lanar s t ruc tur es .

On larg e-sc ale eng inee ring geological plans, fau lts and f ault zones d o not

call for dist inct ive str uct ura l symbols. They ar e usually mapped as zones. of

which t he margins a r e p lot ted and fo r which t he in te rn a l s t ru c t ur es and f il ling

materials a r e mapped in detail. The symbol on Table 22 may be used to

indicate th e margins of t h e faul t , and th e same principle may be applied to th e

detai ls included in the borehole record.

T a b l e of C on

t en t s


T a b l e

of C on t en t s

T





[BL NKP GE]


T a b l e of C

on t en t s

T a b l e o

f C on t en t s


T




REFEREN ES

Addison, R . (19 86) . Geology of S ha Tin, 1:20 000 She e t 7.

Hong Kong

Geoloqical Su r v e v Memoir No. 1. Geo techn ical C ont rol Office. Honq

Kong. 85 p.

Allen, P.M.

S tephens , E.A. (1971). Rep ort on th e Geological Sur ve v of Hong

Konq, 1967 1969. Hong Kong Gove rnment P r e s s , 116 p. pl us 2 maps .

ASTM (1985a). St an da rd practice for descri ption a nd identification of soils

(visu al-ma nual) pro ced ure . Te st Designation D2488-84. 1985 Annual

Book of ASTM S ta nd ar ds . American Society f or Tes ting an d Materials,

Philadelphia. vol. 04.08. pp 409-423.

ASTM (1985b). S tan dard te s t method fo r d ispers ive character i s t ic s of c lay soi l

by double hydro mete r. Test Designation D4221-83a. 1985 Ann ua l Book

of ASTM S ta n da rd s , American Soc iety for Test ing a nd Materials,

Phi lad elp hia , vol. 04.08, p p

733-735.

ASTM (1985~).

Sta nda rd t e s t method fo r classif ication of soils for engineer ing

pu rpo ses . Tes t Desig nat ion D2487-83. 1985 Ann ual Book of ASTM

St an da rd s. American Society for Testing a nd Materials, Philadelphia, vol.

04.08, pp 395-408.

Barton. N . Lien,

R

Lunde. J. (1974). Engin eerin g classification of rock

masses for t h e design of tun nel su pp or t . Rock Mechanics, vol . 6

pp 189-236.

Bayn es. F.J. Dearman.

W . R .

(1978a). The relat ionsh ip between t h e micro-

fab r i c and th e eng ineer ing p rope r t i es of wea thered g ra n i t e .

Bulletin of

t h e Inte rn at io na l Association of E ngi nee rin g Geoloqy, no. 18, pp 191-197.

Bayn es, F.J. Dearman, W . R . (1978b). The micro fabr ic of a chemically

wea there d grani te. Bulletin of t h e Int ern ati ona l Association of

ngineering Geology, no. 18. pp 91-100.

Be gg s, C.J. McNicholl. D.P. (1986). Formation of a hi gh ro ck sl op e

at

Ap

Lei Chau. Hong Kong. Pro cee din qs of th e Confe renc e on Rock

Engin eeri ng a nd Excavation in an Urba n En vironment, Hong Kong, pp 1-

14. (Disc ussio n. p p 480-481).

Beggs. C.J. Tonks , D.M. (1985). Eng ine eri ng geology of t h e Yuen Long

bas in. Honq Konq Engi nee r. vol. 13. no. 3. pp 33-41.

Be nn et t, J.D. (1984a). Review of S upe rfi cia l Deposits and Weathering in Honq

K

GCO Publi catio n No. 4/84, Geotechnical Contr ol Office, Hong

Kong. 51 p

Bennett. J.D. (1984b). Review of Tectonic History. St ru c tu re an d

Metamorphism of Hong Kong.

GCO

Publication No. 6/84, Geotechnical

Control Office, Hong Kong. 63 p.

Bieniawski, Z.T. (1974). Geomechanics clas sific ation of rock mas ses an d its

application in tunnel l ing. Proceedinqs of th e Third Inter nat io nal

Congr ess of t h e Intern ation al Society for Rock Mechanics, Den ver,

vol. 2A, pp 27-32.


T a b l e of C

on t en t s


T a





Bie niawski, Z.T. 1976). Rock mass classif icat ions in rock engine ering .

Proce edinq s of t h e Symposium on Exploration fo r Rock Engine ering .

Johannesburg, pp 97-106.

Brand ,

E.W.

1988). Bibli ograp hy on

the

Geology and Geotechnical Engineering

of Honq Konq t o December 1987. G C O Pub licati on No. 1/88.

Geotechnical Con tro l Office, Hong Kong. 150 p .

Brand , E.W. He nc he r, S.R. Youdan.

D.G.

1983). Rock slope eng ine eri ng in

Hong Kong. Proc eedin gs of t h e Fif th Inter nat io nal Congress of t h e

In te rn at io na l Societv f o r Rock Mechanics, Melbourne, vol. 1, p p C17-C24.

Discussion, vol. 3, pp G126).

Brown. E.T. Ed it or ) 1981). Rock Cha rac ter iza tion Te sti ng a nd Monitoring

ISRM Sug ge st ed Methods. Pergamon Pr es s, Oxford, 211 p.

BSI 1957).

Si te Inves t igat ions CP 2001 1957). Bri t ish St an da rd s Inst i tut ion.

London. 123 p.

BSI 1975). Methods of

Test

for Soil for Civi l Engineering Purposes

BS1377 1975). British St an da rd s Ins tit uti on. London. 144 p.

BSI 1981). Code of Pra cti ce fo r Site In ve st ig at io ns BS 5930 1981). Britis h

Standards Inst i tut ion, London, 148 p.

Burne t t ,

A.D.

Lai, K.W. 1984).

A

revie w of t h e photo-geological l inea ments

an d fau lt sys tem of Hong Kong. Pro cee din qs of t h e Conference on

Geological

As pec ts of Site Inve st ig at io n, Hong Kong, pp 113-131.

Discussion, p p 135). Publi shed

as

Geological Societ y of Hong Kon g,

Bulletin no. 2, edited by I. McFeat-Smith, 1985).

Child. G H 1984). Soil desc ript io ns quo vadis ? Proceedings of t h e 20th

Regional Meeting of t h e Eng ine eri ng Group of t h e Geological Soc iety,

Guildford, UK. pp 73-81. Discussion, pp 82-84). Publis hed as Site

Inv est iga tio n Pra cti ce As ses sin g BS 5930, ed it ed b y A.B. Hawkins.

Geological Societv . Enqin ee rin g Geoloqy Spec ial Publica tion no. 2, 1986).

Also publi shed in p re pr in t vol. 2, pp 131-150).

Committee on Tro pic al Soils of t h e ISSMFE 1985). Pec ulia rit ies of

Geotechnical Beh aviou r of Tropical Laterit ic and Saprolitic Soils

Pr og re ss Report 1982-1985). Brazilian Society fo r Soil Mechanics. Sao

Paulo, 449 p.

Dearman, W R 1981). General Report. Sess ion 1 Engineer ing prope r t ies of

carb onate rocks . Proceedings of th e Symposium on Engin eer in g

Geological Problems of C ons tru cti on on Soluble Rocks, Ista nb ul .

Published in Bullet in of t h e Interna t iona l Association of Eng ine eri ng

Geology. no. 24. pp 3-17).

Dearman, W.R., Ba yn es , F.J. Ir fan. T.Y. 1978). En gi ne er in g gr adi ng of

we ath ere d gra ni te . Engin eeri ng Geology, vol. 12. pp 345-374.

Dec ker, R.S. Dunni gan, L.P. 1977). Development a n d us e of t h e Soil

Conservation S erv ice Dispersion Test . Disp ersi ve Clays, Related Piping,

an d Erosion in Geotechnical Proje cts . American Society for Test ing an d

Materials. Spec ial Technical P ublication no. 623. p p 94-109.


T a b l e of C

on t en t s


T a





Dut ton, C. 1984).

~e ol og ic al nvest igat ions for new town p lanning a t Tin Shui

Wai, New Te rr ito ri es , Hong Kong. Pro cee din gs of th e Confe rence on

Geological As pe ct s of Sit e Inve st iga tio n. Hong Kong. p p 189-202.

Pu bli sh ed a s Geoloqical Society of Hong Kong, Bulletin no. 2, ed it ed by

I. McFeat-Smith, 1985).

Fla nagan. C.P. Holmgren, G.G.S. 19771. Field me th od s f o r de te rminat ion of

so luble sa l t s and perce nt sod ium from ext rac t fo r iden t ify ing d isper s ive

clay soils. Dis per siv e Clays. Related Pipinq, an d Erosion in Geotechnical

Pro jec ts. American Society fo r Testing an d Materials. Special Technical

publication no. 623, pp 121-134.

Fookes, P.G. A.B. Hawkins 1988). Limestone wea the ring it s en gi ne er in g

sign if icance and

a

propose d classification scheme. Qu ar te rly Jo ur na l of

Engineering Geology, vol. 21. pp 7-31.

Franklin, J.A., Broch,

E.

Walton, G 1971). Logging t h e mechanical

ch ar ac te r of rock. Transact ions of th e Inst i t ut ion of Mining an d

Metallurgy,

vol. 80. pp A1-A9.

Gamon, T.I. 1984). The us e of t h e poin t load test for the determinat ion of

s t r e n g t h of wea th er ed ro ck s in Hong Kong. Geological Society of Hong

Konq Newsletter, vol. 2, no. 4, pp 9-16.

Gamon. T.I. Finn, R.P. 1984a). The s t r u c t u r e of t h e Hong Kong gr an it e a

pre limi nar y ap pr ai sa l. Geoloqical Societ y of Hong Konq Newsl ette r,

vol. 2, no. 2, pp 5-10.

Gamon, T.I. Fi nn, R.P. 1984b). Simplified de sc ri pt iv e sch eme an d

classif icat ion system for th e logging of c u t slope faces. Proce edings of

t h e 20th Reqional Meeting of t h e E ngi nee rin q Group of t h e Geoloqical

Society. Guildford. UK pp 253-260. Publis hed a s Site Inves tigatio n

Pr ac tic e As se ss in g BS 5930, edi te d by A.B. Hawkins. Geological Soc iet y,

Eng ine eri ng Geology Speci al Publication no. 2, 1986). Also pub lis hed in

preprint vol. 1, 1984. pp 238-248).

Gamon, T.I. Sz et o, P.L. 1984). The det erm ina tio n of t h e uniaxial

compressive st re ng th of rock material a review of cu r re nt pract ice in

Hong Kong. Pro cee ding s of th e Confer ence on Geoloqical Asp ect s of

Site Inv est iga tio n, Hong Kong. pp 9-20. Discussion. pp 69-72).

Published as Geoloqical Society of Hong Kong, Bullet in no. 2, edi te d by

I. McFeat-Smith, 1985).

G C O 1982). Mid-levels S tu dy Rep ort on Geoloqv, Hydrology an d Soil

P r o ~ e r t i e s . Geotechnical Control Office, Hong Kong. 2 vols, 266 p, pl us

54 drgs.

G C O 1984). Geotechnical Manual fo r Slopes. Second edi tio n). Geotechnical

Control Office. Hong Kong, 295 p.

GCO

1986a). Sh a Tin Solid an d Su pe rfi ci al Geology 1:20 000 map). Hong

Kong Geological Su r v ey Map Se ri es HGM 20. Sh ee t 7. Geotechnical

Control Office, Hong Kong, 1 map.

G C O 1986b). Hong Konq a nd Kowloon Solid an d Su pe rf ic ia l Geology

1:20 000

ma^ .

Hong Kong Geological S u r v e y Map Se ri es HGM 20. Sheet

11. Geotechn ical Con trol Office. Hong Kong, 1 map.


T a b l e of C

on t en t s

T a b l e o

f C on t en t s

T a b





GCO

1987a). Guide t o Site Inve stiga tion Geoguide 2). Geotechnical Control

Office. Hong Kong. 362 p.

GCO

1987b). Hong Kong Sou th an d Lamma Isl and Solid an d Supe rfici al

Geology 1:20 000 map). Hong Kong Geological S u r v e y Map Se ri es

HGM

20, Shee t 15. Geotechnical Control Office, Hong Kong, 1 map.

G C O

1988a). San Tin Solid an d Su pe rf ic ia l Geology 1:20 000 map ). Hong

Kong Geological S u r v ey Map S er ie s

HGM

20, Sh ee t 2. Geotechnical

Contr ol Office, Hong Kong, 1 map.

GCO 1988b). Tsing Shan Castle Pea k) Solid an d Supe rfic ial Geologv

1:20 000 map) . Hong Kong Geological S u r v e y Map Se ri es HGM 20, Sheet

5. Geotechnical Cont rol Office, Hong Kong, 1 map.

GCO

1 9 8 8 ~ ) . Yuen Long Solid an d Sup erf ici al Geology 1:20 000 map) . Hong

Kong Geological S ur ve y Map Se ri es H G M 20, Sh ee t 6. Geotechnical

Contr ol Office. Hong Kong, 1 map.

Geological Socie ty of America 1963). Rock Color Ch ar t. Geological Soc iet y

of America, Boulder. Colorado.

Geological Society 1972). The pre pa rat ion of maps and plans in terms of

eng ine eri ng geology. Geological Society Engi neeri ng Group Working

Pa rt y Repo rt. Qu ar te rl y Jou rn al of Eng ineer ing Geology, vol. 5, pp 295-

381.

Geological Society 1977). The descr iptio n of rock masses fo r eng ine eri ng

pu rpo ses . Geological Society Engi neer ing Group Working Pa rt y Report,

Q ua rt er ly Jo ur na l of Engi neer ing Geology, vol. 10, pp 355-388.

Harr is , R 1985) . Mass o r massive a tech niqu e fo r jointed soils . Ground

Engineering, vol. 18, no. 5, pp 24-31.

Hawkins, A.B. 1984). Rock de sc ri pt io ns . Pro cee din gs of the 20th Regional

Meeting of t h e Eng ine eri ng Group of t h e Geological Society. Guildford,

U K

pp 59-66. Discussion. pp 66-72). Published as Site Inve stiga tion

Practice As se ss in g BS 5930, ed it ed b y A.B. Hawkins. Geological Soc iet y,

En gin eer ing Geology Special Publication no. 2, 1986). Also pub lis hed in

p r ep r in t vol. 2, pp 107-130).

Head. K H 1980). Manual of Soil La bo ra to ry Tes ti ng , Volume 1

oil

Classification an d Compaction Tests . Pent ech Pr es s, Plymouth, UK .

339 p.

Hen cher , S.R. 1985). Limitations of st er eo -g ra ph ic projecti ons fo r rock slope

sta bil ity ana lys is. Hong Kong Eng ine er, vol. 13, no. 7, p p 37-41.

Henc her, S.R. Martin, R.P. 1982). The des cri pti on an d classification of

weat here d roc ks in Hong Kong for engi neer ing purp oses. Proc eedi ngs of

t h e Se ven th S ou the ast Asian Geotechnical Conference, Hong Kong.

vol. 1, p p 125-142. Di scu ssion, vol. 2, p p 167-168).

Hen cher , S.R. Rich ards , L.R. 1982) . The basi c f r ict ional res is t anc e of

sh ee ti ng join ts in Hong Kong gra nit e. Hong Kong Engi neer , vol. 11,

no. 2, pp 21-25.

T a b l e of C on

t en t s


T a b l e

of C on t en t s

T





Hoek, E (1983). S t r en gth of jo inted rock masses . ~Q ot ec hn iq ue , o l. 33,

pp 187-223.

Hoek,

E

(1986). Practical rock mechanics developments ov er t h e pa st 25

year s . Proceedings of th e Conference on Rock Engineer ing an d

Excavation in an Urban Environment. Hong Kong,

pp ix-xvi.

Hoek, E Brown. E.T. (1980). Empirical s tr en g th cr ite rio n fo r rock masses.

Jo ur na l of Geotechnical Enqin eer inq , American Society of Civil

En gi ne er s, vol. 106, p p 1013-1035.

Hoek, E. Br ay, J.W. (1981). Rock S l o ~ eEnqineer ing. (Third edi t ion) .

In st it ut io n of Mining an d Metallur gy, London, 358 p.

Holt, J.K. (1962 ). The soi ls of Hong Kong's c oa st al waters. Proceedings of

t h e Symposium on Honq Konq Soils. Hong Kong. p p 33-51. (Di scu ssion.

pp 141-148).

Howat, M D (1984). Marine o r te rr es tr ia l? The soils of Tai Tam Bay.

Proc eedi nqs of t h e Con ference on Geoloqical Aspe cts of Site

In ves tiq ati on, Hong Kong, p p 151-165. (Pu bli she d a s Geoloqical Socie ty

of Honq Konq. Bulletin no. 2, ed it ed b y I. McFeat-Smith, 1985).

Howat, M D (1986). Reply t o di sc us sion by T.I. Gamon on "Completely

wea the red gran i te soi l o r rock?" by M D Howat.

Qua r ter ly Journa l of

Enq ine eri nq Geoloqy. vol. 18, pp 199-206. (Dis cussi on, vol. 19, p p 433-

437). (Reply t o discu ssi on . vol. 19, p p 437).

Hunt.

T (1982). S l o ~ e ailures in colluvium overlying weak residual soils in

Hong Kong. Pro cee din gs of

t h e ASCE Specialty Conference on

Engineer inq and Construct ion in

Tropical and Residual Soils. Honolulu,

Hu nt le y, S.L. Randall. P.A. (19811. Recognit ion of colluvium in Hong Kong.

Honq Konq En qi ne er , vol. 9. no. 12, p p 13-18.

IAEG (1981). Rock an d soil descr iptio n and classification fo r eng ine eri ng

geological mapp ing. Bulletin of t h e In ter nat io nal Association of

En gi ne er in g Geoloqy. no. 24. pp 235-274.

I r f a n . T Y Dearman, W R (1978). The engi neer ing pe tr og ra ph y of a

weathered gra ni t e in Cornwall . England. Qu ar t er l y Jour nal of

Engineering Geoloqy, vol. 11, pp 233-244.

I r f an , T Y Powell, G E (1985a). Verification of fo und ing dep th of la rg e

diam eter piles on gra ni ti c roc k. Honq Konq Eng ine er, vol. 13, no. 8,

p p 11-17. (Dis cussi on, vol. 13, no. 12, pp 7-8).

I r fa n, T.Y. Powell. G E (1985b) . Engineer ing geological inve stig ation s fo r

pile foundations on

a

deeply we athe red gran it ic roc k in Hong Kong.

Bulletin of t h e Int er na ti on al Association of E ngi nee rin g Geoloqy, no. 32.

pp 67-80.

ISRM (1978). Sug ges ted methods fo r t h e quan ti tat ive desc r ipt ion of

discont inuit ies in rock masses . In ternat ion al Journal of Rock Mechanics

a n d Mining Scien ces Geomechanics Ab st ra ct s, vol. 15, p p 319-368.

T a b l e of C on

t en t s


T a b l e

of C on t en t s

T





ISRM 1985). Sug ges ted methods for determining point load st re ng th .

In ternat ional

Jou rna l of Rock Mechanics a n d Mininq Sciences

Geomechanics A bs tr ac ts . vol. 22. p p 51 -60.

Ja pa n Socie ty of Engin ee rin g Geology 1987). Rock Classification in Ja pa n.

The Jap ane se Group of t h e Int ern ati ona l Association of Eng inee ring

Geology, Japan Society of Engineering Geology, Tokyo, 37 p.

Koo.

Y C

1982a). Relict join ts in completely decomposed volcanics i n Hong

Kong. Cana dian Geotechnical Jo ur na l. vol. 19, pp 117-123.

Koo, Y C 1982b). The influence of rel ict jo ints on slopes in wea there d,

water-laid volcanics in Hong Kong. Proc eedin qs of t h e Se ve nt h

So ut he as t Asian Geotechnical Conference. Hong Kong, vol. 1, pp 23-30.

Lai,

K.W.

Tayl or, B.W. 1983). The cla ssi fic ati on of colluvium in Hong Kong.

Proce edinq s of t h e Meetinq on t h e Geology of Sur ficial Deposits in Hong

Kong, Hong Kong. p p 75-85. Pu bl is he d as Geological Society of Hong

Konq, Bull etin no . 1 , ed i ted by W.W.S. Yim, 1984) .

Langford, R.L. Lai, K.W., A r thu r ton , R.S. Shaw, R 1988). Geoloqv of t h e

Western New Territories. 1 20 000 Sh ee t s 2. 5 a n d 6. Hong Kong

Geological S u r v e y Memoir No. 3, Geotechnical Con tro l Office. Hong

Kong, in press.

Liu, K H Gammon, J.R.A. 19831. Q ua te rn a ry geology, we at he ri ng a nd

geomorphology of Hong Kong. Pro cee din qs of t h e Meetinq on t h e

Geology of Su rf ic ia l Depos its i n Hong Kong. Hong Kong. p p 49-59.

Published as GeoIosical Society of Hong Kong, Bulletin no. 1, ed it ed b y

W.W.S. Yim, 1984).

Lumb, P. 1962). The pr op er ti es of decomposed gr an it e. GBotechnique, vol. 12,

pp 226-243, plus 2 plates.

Lumb, P. 1965). The re si du a l soi ls of Hong Kong. GBotechniaue, vol. 15,

p p 180-194. Di scu ssi on, vol. 16, 1966, p p 78-81 an d 359-360).

Lumb, P. 1977). The mar ine soils of Hong Kong a n d Macau. Pr oc ee di ng s of

th e Int erna t iona l Symposium on Soft C l a y Bangkok, pp 45-48.

Published u nd er th e t i t le Geotechnical Aspects of Soft Clays, edi ted by

R.P. Brenner E.W. Bran d, Asian In st it ut e of Technology, Bangkok,

1977).

Lumb, P. 1983). Engin eeri ng prope rti es of fr e s h an d decomposed ign eou s

ro ck s from Hong Kong. Eng ine eri ng Geoloqy,

vol. 19, pp 81-94.

McFeat-Smith, I.. Tu rn er , V.D. Brac egird le.

D.R .

1985). Tunnelling

con dit ion s in Hong Kong. Hong Konq En gi ne er , vol. 13, no. 6, p p 13-

25. Dis cus sio n, vol. 13, no. 9, p

5 .

McGown. A.. Marsland. A. Radwan, M. Ga br , A.W.A. 1980). Rec ord ing a n d

in te rp re ti ng soil ma cro fab ric data . GBotechniaue, vol. 30, pp 417-447.

Martin, R.P. 1986). Use of index tests fo r eng ineer ing assessment of

weathered rocks . Proceedings of th e Fi fth In ternat ional Congress of the

In te rn at io na l Association of En gin ee rin g Geology, Buenos Aires, vol. 2.

pp 433-450.


T a b l e of C

on t en t s


T a





Miller,

D R

Ba rr et t. J.R. Bamford, W.E. (1986). A field classif icati on f o r

uniax ia l s t re ng t h of d iamond dr i l l cor e us ing s imple f ield index tests

Au st ra li an Geomechanics News, no.11, Ju n e , p p 14-18.

Mitchell, J.K. Si ta r,

N

(1982). Eng inee r ing p rope r t i e s of t rop ica l r e s idu a l

so ils . Proceed ings of t h e ASCE Spec ia l ty Conference on Engineer i ng an d

Construction in Tropical and Residual Soils , Honolulu,

pp 30-57.

Moye, D G (1955). Enginee r ing geology for t h e Snowy Mounta ins scheme.

Jo ur na l of t h e Ins t i t ut i on of Engi nee rs . Austra l ia , vol . 27, pp 281-299.

Munsell , A.H. (1941). A Color Notation Mun sell Soil Color Ch ar ts . Muns ell

Color Co. Inc., Baltimore, USA.

Nash, J M Dale, M.J. (1983). Geology a n d hydr ogeo log y of na tu r al tu nn el

e ros i on in superf ic ia l depos i t s in Hong Kong. Proceed inqs of t h e

Meeting on t h e Geology of Sur fic ial Deposits in Honq Kong, Hong Kong,

pp 61-72 . (Publ ished as Geological So ci et y of Hong Kong. Bull eti n no. 1,

edited by W.W.S. Yim, 1984).

Nash, J.M. Ch ang .

D C H

(1987). A propos ed e r os ion model for c u t s lopes in

Hong Kong. Pro ceed ing s of th e Eighth Asian Reqional Confer ence o n

Soil Mechanics and Foundation Enqineerinq. Kyoto. vol.

1

pp 477-482.

Nau. P.S. (1984). Joi nt sys te m of Hong Kong gr ani te . Pro ceed ing s of t h e

Confer ence on Geological Aspects of Si te Inve sti gat ion , Hong Kong, pp

89-98. (Published

as

Geological Society of Hong Kong. Bulletin no.

2.

edited by I. McFeat-Smith. 1985).

Norbu ry , D R Child.

G H

Spi nk, T.W. (1984). cri tic al rev iew of Sectio n 8

(BS 5930)

Soil an d Rock Description. Pro ceed ing s of t h e 20th Regional

Meeting of t h e Engineering

Group of t h e Geological Societ y.

Guildford,

UK, pp 331-342. (P ub li she d

as

Si te Inves t iga t i on Prac t ice Assess i ng BS

5930

edited by A.B. Hawkins. Geological Society, Engineering Group

Special Publication no. 2 , 1986). (Also pub lis hed in pr ep r i nt vol. 2,

pp 353-369).

Pascall, D (1987). Cave rno us gr ou nd in Yuen Long, Hong Kong. Geotechnical

Engineering, vol. 18, pp 205-221.

Powell. G.E. Ir fa n, T.Y. (1986). Slope remedia l wo rk s i n we at her ed r oc ks f or

d i f fe r ing r i sk s . P roceed ings of the Confe rence on Rock Eng inee r ing and

Excavation in a n Urb an Enviro nment, Hong Kong, pp 347-355. (Di scuss-

ion, pp 466 and 469-470).

Ri cha rd s, L.R. Cowland, J W (1982) . The e f fec t of s u r f ace rou ghn es s on th e

f ield sh ea r s t r en g t h of shee t i ng jo in t s in Hong Kong g ran i t e .

Kong Engineer, vol. 10, no. 10, pp 39-43.

Rodin, S., Co rb et t, B.O., Sher woo d, D.E. Th or bu r n. S. (1974). Pen et ra ti on

te s t i ng in t h e Un i ted Kingdom S ta te - o f - t he -a r t r ep o r t . P roceed ings of

the European Symposium on Penetra t ion Testing, Stockholm, pp 140-146.

Ruxton, B.P. (1984). The s t r u c t u r e of some de br is f lows in Hong Kong.

Proceedings of the Conference on Geologica l Aspec ts

of

Si te

Inv est iga ti on, Hong Kong. pp 105- 11 1. (Pu bli shed

as

Geological Society

of Hong Kong, Bullet in no. 2. ed it ed b y I McFeat-Smith. 1985).

T a b l e of C on

t en t s


T a b l e

of C on t en t s

T





Ruxton, B.P. 1986). Ir on cementation in bou lde r colluvium matrix u n d e r Hong

Kong city. Pro cee din gs of

the

S y m ~ o s i u mon t h e Role of Geology in

Ur ba n Development in So ut he as t Asia Lan dp lan 111). Hong Kong.

pp 359-372. Publ is he d as Geological Soci ety of Hong Kong, Bulletin no.

3, edited by P.G.D. Whiteside, 1987).

Ruxton, B.P. Be rr y, L. 1957) . Weather ing of gr an it e an d associated

eros iona l fe at ur es in Hong Kong. Bulletin

of

the Geological Society of

America. vol. 68, pp 1263-1291. plu s 1 pla te.

Shaw, R., Zhou, K. Gervais, E. Allen, L.O. 1986). Re su lt s of a

pala eonto logi cal in ve st ig at io n of Chek Lap Kok bo reh ole B13/B13A)

Nor th Lant au. Geological Socie ty of Honq Konq Newslette r, vol. 4,

no. 2, pp 1-12.

S h e r a r d , J .L. Dunnigan.

L P

Decker. R.S. St ee le . E.F. 1976). Pinhol e

test

fo r identifying dis per siv e soils . Jour nal of t h e Geotechnical Engi neer ing

Division, American Socie ty of Civil En gi ne er s. vol. 102. pp 69-85.

Skempton, A.W. 1986) . S ta nd ard penetra t ion test pr oc e du r e s a nd t h e e f f e ct s

in s ands of ove rbu rden pres sure , r e la t ive dens i ty , pa r ti c le s ize , age ing

a n d over cons olid ation . Geotechnique , vol. 36, pp 425-447.

S ta nd a rd s Association of Aust ralia 1980). Determination of Emerson Class

Number of a soil. Aust ralia n St an da rd Methods of Testin q Soils fo r

Eng ine eri ng Pu rp os es , no. AS 1289. C8.1-1980, S t an d ar d s Association of

Australia , Sydney. 3 p.

St an da rd s Association of Aust ralia 1984). Determination of pinhole dis per sio n

classification

of

a soil. Australian St an da rd Methods of Testing Soils fo r

Eng ine eri ng P ur po se s. no. AS1289, C8.3-1984, S ta n d a rd s Association of

Australia , Sydney, 5 p.

St ra ng e, P.J. 1984). Towards a simpler classification of t h e Hong Kong

gr an it es . Proc eedin gs of th e Conference on Geological Aspe cts of Site

In ves tig ati on, Hong Kong, p p 99-103. Pub lis hed a s Geological Society of

Honq Konq, Bulletin no. 2, edited by I. McFeat-Smith, 1985).

S trange, P . J . Shaw,

R

1986). Geology of Hong Kong Is land a nd Kowloon,

1:20 000 She e t s 11 a n d 15. Hong Kong Geological Su r vey Memoir No. 2,

Geotechnical Control Office, Hong Kong, 134 p.

Tovey. N.K. 1986a). Micr oana lyses of a Hong Kong mari ne clay. Geotechnical

Engineering. vol. 17. pp 167-210.

Tovey, N.K. l986 b). Microfabric, chemical an d mineralogical stu di es of soils

tec hn iqu es. Geotechnical Engi neer ing, vol. 17, pp 131-166.

Wang, P. Yim, W.W.S. 1985). Pre lim ina ry in ve st ig at io n on t h e occ ur r en ce of

marine microfossils in an offshore drillhole from Lei Yue Mun Bay.

Geological Soc iety of Hong Kong Ne ws le tter , vol. 2, no. 1,

pp 1-5.

Watkins, M.D. 1979). Engi neer ing geological mapping fo r t h e High Islan d

wa ter scheme in Hong Kong. Proc eedi ngs of t h e Symposium on

Engineer ing Geologica l Map~ing,Newcastle-upon-Tyne. Published in

Bulletin of t h e In te rn at io na l Association of En gin eer ing Geology, no.19.

pp 166-1751,

T a b l e of C on

t en t s


T a b l e

of C on t en t s

T





Whiteside, P.G.D. 1983). Pa tt er n of Qu at er na ry sedi ment s reve aled du ri ng

piling w or ks a t Sha Tin, Hong Kong.

Proceedings of th e Meeting on th e

Geoloqy of Su rf ic ia l Deposits in Honq Konq. Hong Kong. p p 153-159.

Published

as

Geological Society of Hong Kong, Bulletin no. 1, ed it ed by

W.W.S. Yim, 1984).

Whiteside, P.G.D. Bra ceg ird le.

D R

1984). Geological ch ar ac te ri st ic s of some

Hong Kong r ock s an d the ir importance in un der gro un d excavation works.

Proc eedi ngs of t h e C onference on Geological Aspe cts of Site

In ves tig ati on, Hong Kong, pp 175-187. Pub lis hed a s Geological Society

of Hong Konq, Bulletin no. 2, edited by I. McFeat-Smith, 1985).

Willis, A.J. Sh ir law. J.N. 1983). Deep alluvial depo sits ben eat h Victoria

Pa rk , Causeway Bay. Proce edin gs of t h e Meetinq on the Geology of

Sur fic ial Deposits in Hong Kong, Hong Kong, pp 143-152. Pub lis hed a s

Geoloqical So ciety of Honq Kong, Bu lle tin no. 1, edi ted by W.W.S. Yim,

1984).

Yim, W.W.S. 1983). sedim ento logi cal s t u d y of t h e sea flo or se di me nt s

exposed du rin g excavation of t h e East

Dam

Site. High Island. Sai Kung.

Proc eedi ngs of t h e Meetinq on

the

Geology

of Surficial Deposits in Hong

Kong, Hong Kong, p p 131-142. Pub lis hed as Geological Society of Hong

Kong, Bullet in no. 1. ed i ted by W.W.S. Yim. 1984).

Yim, W.W.S. Li, Q Y 1983) . Sea level ch ang es an d seaf loor surf icial

dep osi ts off Chek Lap Kok. Ab str act s of t h e Meeting on th e Geology of

Surficial Deposits i n ~ ~ o n qong, Hong Kong, pp. 48-59. Pub lis hed a s

Geological S oc ie ty of Honq Konq, Bulle tin no. 1 , ed i ted by W.W.S. Yim,

1984).

T a b l e of C on

t en t s


T a b l e

of C on t en t s

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[BL NK P GE]

T a b l e of C on t

en t s

T a b l e of C

on t en t s

T a b l e o

f C on t en t s

T a b





T BLES





T




[BL NK

P GE]


T a b l e of C

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T a b l e o

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L I S T

O

T BLES

Table

No.

Page

No.

Guidance on Rock and Soil Description in

Dif fe ren t ~oca t ions

2

Classification of Rock Material Strength

3

Colour Description Scheme

4

Classification of

Grades

Rock Material Decomposition

5

Classification of Solid Rocks and

Deposits in HOng Kong

Superficial

6

Spacing of Planar Str uc tu re s

7

Discontinuity Spacing

8

Une venne ss Small- Scale Roughness) of

Discontinuities

9

Aperture Size

1 Classification of Rock Mass Weathering Zones

11

Procedure for Rapid Identification

Description of Soils

and

12

Soil St re ng t h in Terms of Compactness an d

Consistency

13

Particle Form

14

Particle Angularity

15

Use of Sec onda ry Constitue nts fo r t he Naming

of Composite Soils

16

Methods for Naming Soils Containing Very

Coarse Material

17

Plasticity Terms Based on Liquid Limit

18

Identification

Types

Features for Different Soil

19

Names and Descriptive Letters for Grading

and Plasticity Characteristics


T a b l e of C

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T a b l e o

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Table

No.

2

21

22

23

24

P a g e

No.

Bri t i sh Soi l Class i f icat ion

E n g i n e e r i n g P u r p o s e s

S y s t e m f o r

Symbol s for Rocks and So i l s

Symbol s for Borehole Records

S y m b o l s f o r G e ne ra l P la n ar S t r u c t u r e s

S y m b o l s f o r O t h e r G e o l o g i c a l S t r u c t u r e s a n d

B o u n d a r i e s


T a b l e of C

on t en t s






T a b l e 1 - G u i d a n c e on R o c k and S o i l Descr ipt ion i n D i f f e r e n t Locations

Location F ie ld Ground Inves t iga t ion D ril l ing R ig La bora to ry

RockandS oi l N a t u r a l o r m a n - m a d e e x p os u re

Rock Core Rock Pieces o f core a nd i r regu lar handsamples

Typical

( e . g. t r i a l p i t o r c u t t i n g )

Soi l Samples f rom sp l i t t r i p le - tube core-1So i l Samples f rom sp l i t t r i p le - tube core-bar re ls .

Sample

bar re l s . d r i ven sa m pl er s a nd c ore - e xtr ud ed t hi ck lt hi n- w al !e d s am ple rs , S.PT

b a rr el cu tt in g to es , SPT li ne rs lin ers , han d-cut b lo ck s. t r~ m m ed e st

specimen!

l escr ip t i ve

Mater ia l

Mass Materia l

Mass

Mater ia l

Mass

Scale

Strength S truc ture S trength (s t r u c tu re ) S tre n g th

( N o r m a l l y n o t

Colour D iscontinuities Colour D iscont inu it ies

Colour

appl icable1

(na ture , o r ien ta t i on ,

(na ture . spac ing ,

TexturelFabric

Texture/Fabric

Tex ture IFabr i c

spacing, persistence. roughne ss,

Weathered Sta te W eathered Sta te W eathering S ta te

roughness, a perture, aperture. in fi l l ing .

l te m s f o r A l t e ra t i o n S t a t e i n f i l l ing , seepage) A lte ration S ta te fr ac tu re in dic es ) A lte ra tio n S ta te

Rock

Rock Name

MassWeathering

Rock Name M assW eathering Rock Name

Descr ip t i on

Add i t i ona l l n fo rmat ion

Add i ti ona l l n fo rmat ion

Add itiona l Inform ation Ad d itio na l l n fo rm a tio n

k g . e s u l ts of i n de x t e s ts

(e.g.representabi l i ty

(e .g . resu l ts o f

(e .g . minera logy , resu l ts

o f m a s s s a m ~ l e )

index t e s t s ) of index te s t s )

Add i t i ona l Geo log ica l l n fo rmat ion

(e .g . geo log ica l fo rmat ion . age)

S t rength

S tru c tu re S tren gth (S t ru c tu re )

S t rength (s t ruc ture)

Colour

Discont inui t ies

Colour

(Discont inui t ies)

Colour

(D iscont inu i t i es )

( i te m s a s in ro ck ( i te m s a s in rock (na tu re , o r ien ta t ion .

art ic le Shape

/

P a r t ic le S h a p e / P ar t ic le S h a p e lC o m po s it io n

desc r ip t i on l i s t above

descr lp t l on l i s t spac ing , roughness ,

Compos i t i on

Composi t ion

S o i l N a m e

above)

a p e r t u r e , i n f i l l i n g )

Weathered S ta te

o i l Name So i l Name

( b a s e d o n r a p i d o r d e t a i le d

I t em s f o r

Weathered S ta te (wea thered S ta te)

basedon rap id

A d d i ti o n al I n f o rm a t i o n ( b a s e d o n r a p i d a s s e ss m e n t o f g r a d i n g 1

Soi l

assessment o f

(e.g.p resence of

a s s e s s m e n t o f

p las t i c i t y )

Descript ion

gradingIplast ic i ty)

v o id s , s e e pa g e n o t

g rad ing /p las t i c i t y )

A d d i t io n a l l n f o r m a ti o n

r e l a t e d t o

Add i t i ona l In fo rmat ion

Addi t ional lnformation

(e.g. degreeo f sample

k g . resu ltsof indextests ) (e.g. degree of

d i s tu rbance. minera l0 y .

sam ple dis tu rbance, re su l ts of index te s ts ?

Add i t i ona l Geo log ica l In fo rmat ion

resul tsof indextests)

( e. g. g e o l og i c al o r i g m , t y p e o f d e p o s i t )

Notes

( 1 ) L e s s c o m pr e he n si ve d e s c ri p ti o n s m a y a l s o b e p o s s i b le w i t h l o w e r q u a l it y s a m p le s ( e . g. f l u s hi n g s, j a r o r b u l k s a m p l e s ) .

( 2 ) M a in d e s c ri p ti v e i t e m s m a r k e d i n s q u a r e b r a c k e t s m a y o f t e n n o t b e a p p li c a bl e , d e p e n d in g o n s a m p l e s i z e .

3 ) Fo r s o i l s d e r i v e d f r o m i n s it u r o c k w e a t h e r in g :

i f the or ig ina l rock tex ture /s t ruc ture i s re tained ( i .e . sapro l i tes ) , they should be descr ibed

i n r o ck t e r m s , s u p pl e m en t ed b y a d d it i on a l s o i l t e r m s f o r c o m p a c t n e s s l co n s i s t e nc y a n d g r a d in g ; i f t h e r o c k t e x t u r e l s t r u c t u r e i s c o m pl e te l y

l o s t ( i. e , r e s id u a l s o i l s ) , t h e y s h o ul d b e d e s cr ib e d i n s o i l t e r m s .

( L ) Descr ip t ion o f the or ien ta t ion o f d i scont inu i ti es i n d r i l l core m ay a l so beposs ib le , dep end ingon the ground inves t iga t ion techn iques us ed;

see Geoguide

(GCO,1987a).

)

This tab le prov ides genera l gu idance on ly .For fu r therde ta i l s on par t i cu la r desc r ip t ive i tems, re fer to the re levant te x t sec t ion .


T a b l e of C

on t en t s





Table

-

lassification of Rock Material Str eng th

Approximate Point Load

Strength Index Values (ls(50))

for Granitic & Volcanic Rocks

(MPa1

Genera l ly no t app l icab le

L - 8

8

and ex t reme ly weak c lass es a re app l icab le to so i l s der ived f rom ins i tu weather ing o f rock s .

Field Identification Tests

I

Eas i ly c rumbled by hand; inden ted deep ly by thumbna i l .

Crumbled w i th d i f f i cu l t y by hand ; sc ra tche d eas i ly

by thumbna i l ; pee led eas i ly by pocke t kn i fe .

May be b roken by hand in to p ieces ; sc ra tched by thumbna il ;

pee led by pocke t kn i fe ; deep inden ta t ions up to 5mm made

w i t h p o i n t of g e o lo g i c a l p i c k ; h a n d - h e ld s p e c im e n e a s il y

b roken by s ing le l igh t b low o f geo log ica l hammer .

M a y b e b r o k e n w i th d i f f i c u l ty i n t wo h a n d s ; s c r a t c h e d w i t h

d i f f i cu l t y by thumbna i l ; d i f f i cu l t to pee l bu t eas i ly sc ra tched

b y p o c k e t k n i f e ; s h a l l ow i n d e n t a t i o n s e a s i l y m a d e w i t h

po in t o f geo log ica l p ick ; han d-he ld spec imen usua l ly

broken by s ingle l ight b low of geological hammer.

Sc r a t c h e d b y p o c k e t k n i f e ; s h a l lo w i n d e n t a t i o n s m a d e b y

f i rm b l o w w i th p o i n t o f g e o lo g i c a l p i c k ; h a n d - h e l d

spec imen usua l ly b roken by s ing le f i rm b low o f geo log ica l

hammer.

F i rm b lows wi th po in t o f geo log ica l p ick cause on ly

s u p e r f i c i a l s u r f a c e da m a g e ; h a n d - h e ld s p e c im e n r e q u i r e s

more than one f i rm b low to b reak w i th geo log ica l hamm er .

I

Many b lows o f geo log ica l hammer requ i red to b reak spec imen.

Specimen can only be chipped by blo ws of g eological ham mer.

Descriptive

Term

I

Ext remely

weak

Very weak

I

Weak

Moderately

weak

Moderately

s t r o n g

St rong

Very strong

Ext remely

s t r o n g

I

Note

Uniaxial

Compressive

Strength

( MP a

The very weak


T a b l e of C

on t en t s





T a b l e of C on

t en t s

T a b l e of C

on t en t s

T a b l e

of C on t en t s





Desc r i p t i ve

Grade

Term ;ymbol

Highly

Decomwrrd

Modemlcly

Decomposed

Slightly

Decompascd

Fresh

General Notes

Note5 on Index Tests

Table lassification of Rock Material Decomposition Grades

A d di t io n al T y pi c al C h or a ct e ri s tl c s f o r S p ec i fl c R o c k T y p es

Ge n e r a l Ch a r a c t e r is t i c s f o r G r a ni t ic 8

Vo l con i c Rocks 8 O t he r R o c k s o f

Equivalent S t r e n g t h in t h e F r e s h S t a t e

Gran i te

Gr anod ior ite C oa rs e A sh Cr ys la ll Li th ~c T uf

Fine A s h V it ri c T uf f

Original rock texture complctcly destroyed Reddsh brawn Brown or rrddtsh brown Yellowish brown

Canbe crumbled byhand and f inger pressure mto

Feldsparscompletely destroyed

Quartzonlyremamng prlmory

constituent groins

Quartz is only remolnmg prrmory mmerd

mlnerol; vsvolly dull. etched or plttec

and reduced in s i re compared wi th

fresh condltlon

3rngmol rock texture preserved

Yellow#Shbrown to reddish brown

h l l ow#bh brown to r e dd ish b rown B rown to r cddl sh bmwn

Con be crumbled byhand and f inger pressure m a Fe1d~p.r~powderyto so ft

Plogiochsefeldlpm5powdwy to Soft

Slake5 Slowly n water

conslltucnt 9'0l"S

nand penetrometer Sheor strengthmdc. very eoslly grooved by p#n

Maf8c mmerols soft,dull. dark grew

of pick

<250kPa

O r t h ~~ I a s eeldspars gutty, leS5 eo511

10 b rown, dlstlnguclh

osblym de nt cd by p oi nt g ~o l o g ~~ o l

d#fficultto

Slakes whcn rmmerred I water

Zero reboundfrom N Schmidt hmmer

grooved

Completely discolauredcompared wllh fresh rock

Zero reboundfrom NSchmidthommcl

en be b rok en by h on d ~n t o m dl cr pmrccs Y e ll o w~~hr own 10 ye ll ow8sh o ra ng el

Yctlor~shbrown to y tl lav n~ h rongel V el lo w~ ~hrown Yellowish g rey

%kes dull ~ u n d hen struckbygeolog8sol hommrr brown

brown Moflc mlneroll soft, dull. dark green Surface can bessrotchedby kntfc

NO eosbly indented by point o fgcologico l p ~ k

Feldspars powdery Plogioclose feldspars powdery 10 g i t t l

Doe5 not slake when

immersed

in w l e r Hand p cnelrometcr shear strengt h lndcx N Schmidl reboundvolue 15-30

3ompletely discoloured compared with fresh rock 250 kPm

mSdrvr N Schmidt rebound d u e < 2 5

Connot uruolly be broken by hond; cosily broken by Yellwish brown Yellowish brown Vellowlsh grey White

or hgh l g rey

geobgicol hommcr

Feldspars grltty

plaglochsefeldspars party decomposed

Moflc mmeralS

generally

not~ h lny,

Surfme cannot bescratched by knl

Makes

a

dull or sl igh tr ing lngsoundwhcn Struck by Bmtlte not shiny

to gritty small pieces

soft. black

1

51omed dark brown

geologlcol hammer N Schmtdl rebound value

2 5 - L 5

NSchmidt rcbwnd value

2 5 - 5 0

Completely slolned throughout

v at b ro ke n e a si ly b y g e ol og ic al h ammer F el ds par s h ar d t o S l qh tl y g r lt t y P lo gi oc lo se fel dS par S ~18 gh ll y r8I ty

Light grey or greenish grey Grey, lhght grey or greentsh grey

Uokrs a ringing sound whcnstruckby geolog~col

Orthoclase feldspars often

pink

BiMWond hornblende ~lighl iy

s ta im

Mofic mlnerols shiny, hord, black.

Cloudy appearance

hammer B#ot#telrghlly stornrd and dull oround

and du l l may be s igh t ly s to iocd and du l l

9 e ~ h c k CD~DU~S enerally retained but stained near

edges

N Schmidt

rebound value L5-70

around edges

jomt Lurfoces

N

Sshmldt reboundvalue

> 4 5

Overall

rock

colaur g reylwhrte OverOll r o c k colourgrey

overall rockeatour rangesfromlqht Over~llockcolour block

Feldspors hmrd and shmy

Feldspars hard and shiny

greentshgrey IJSMI to grey IJSM,

~ l o s s yoppe o rm c c

Biotite Shiny. not stalncd

Biotiteand hornblende5hmy,not 5ta m

JYTl

Qvortr colourlcss or grey. g1055y

Quartz solourless

or

g re y. g l ossy FeId5po rs hard ond shi ny

N

Sehmldt reboundvolue >60

Moflsmmerals shiny. hard black

Qumtzcolo~rle55

r

grey, glassy

I 1Not011 these generol chorocler #st#csre oppl~coble

131Based

on

Moyc 119551. Henchet

6

I61 Bared

on

unpubhshed work by

191Based on unpubhrhed

work

bl

torock5whose strength nthefreshstate 15 Martm 11982)ondunpublishedwork the GCO

the GCO

modemte lys trongor less(seeTab le 21 ~l tc rn ot ~a

by the GCO 171 JYT=Y,m

Tln

Tram

Farmotton

~ Iossv f~c~ t bons for such

IL I

S S P S S ~ C ~ ~ ~

f mlnero l~ pp l~~ab le JSMrShlng Mun Formotion

oybemore appropriate

moter lo ll (seeSr ctm

2

3L I

to

med~umnd mrse grolnedgronte

15ee HKGSmops ondmemo8rsI

121 Use of geologrsal hammer oppl8coblemomly to

may be dofflcult

or

mporstb le to

181M0f8cm~ne rals eferredto are

materds mnfcned m

o

fjeld ezooaure

asserr n f w - g r amed grm l tes biotlfe and hornblende

It01 S h k ~est ' romplcr a lreodyc lose to ra turo twn rn~rr tu re con lent are less l ikely tosloke.

1111 Feldsparaltcrot8onirr t: Hard=cannot be

cut

by kn i fe

or

grooved by pin: Gritty:con be

cut

by knife or grooved by pin with prcz3urc; Powdery

easdy grooved by pro. can be crushed to silt

frogmentr in fingers; Soll=eorily grooved by pin, con be moulded very easily to cloy m Imgcrr.

1121

N

Schmldt hommerter t: rebound

values

ore for nomrnrr held pcrpendiculm to rock focc:takeinitial'seating' blowsto

ensure

good contact and record average value from a rnrncmum of flvc consecuflv<

irnpocts. qnaring unusually lowreadfngs

It31

HOM

penefrometer test. press mstrumcnt hcod slowly and smoothly ,nto sample,

take

anoverage of tenvalues and divide bytwo to gweshear rtrength mdex:test maybe improctlcol onvery smoll samples

I 1 0 T es t r e s ~ l t s8n general may be affected by sample moisture content and degree of mrsrofrosturmg


T a b l e of C

on t en t s





78

Table -

Spacing of l a n a r S t r u c t u r e s

Descriptive

Term

Spacing

Very thick

Thick

Medium

Thin

Very thin

Thickly

- Laminated sedimentary

Na rro w metamorphic and igneous

Thinly - laminated sedimentary

Very narr ow metam orphic and igneous

T a b l e of C on

t en t s

T a b l e of C

on t en t s

T a b l e

of C on t en t s





79

Table i s c o n t i n u i t y S p a c i n g

Descriptive

Term

Spacing

Extremely widely spaced

Very widely spaced

Widely spaced

Medium spaced

Closely spaced

Very closely spaced

Extremely closely spaced


T a b l e of C

on t en t s

T a b l e o

f C on t en t s

T a b


T




0

Table -

U n e v e n n e s s

Small-Scale

R o u g h n es s ) o f D i s c o n t i n u i t i e s

lass First Term

Second

Term

Illustration

R o u g h

Stepped

2

S m o o th

S te p p e d

S l ickens ided

Stepped

R o u g h U n d u la t i n g

5

Smooth

U n d u l a t i n g

6

Sl ickens ided

U n d u l a t i n g

Rough P la n a r

8

S m o o t h

P l a n a r

9

S l i cke n s id e d

P la n a r

N o te s

1 )

L e n g th of t h e i l lu s t ra te d

p r o f i l e s i s in t h e r a n g e 1 t o

10

m e t r e s .

2 ) V e r t ic a l a n d h o r i z o n t a l

s c a l e s a r e e q u a l .


T a b l e of C

on t en t s

T a b l e o

f C on t en t s

T a b





Table

perture S i z e

Aperture Distance between Disconlinuity Walls

m m

Zero


T a b l e of C

on t en t s

T a b l e o

f C on t en t s

T a b


T




82

Table 1 lassification of Rock Mass Weathering Zones

Zone Zone Zone Characteristics

Description Symbol

Residual soil derived from insitu weathering;

Residua l

So i l

mass structure and material texture

/

fabric

completely destroyed

:

100% soil

Less than 30

%

rock

Soil retains original mass structure and

material texture

/

fabric ( i . e . saprol ite)

0 1 3 0 %

Rock con tent does no t a f f e c t shear behav iour o f

Rock

mass, bu t r e l i c t d is cont in u it ie s in so i l may do so

Rock content may be s ign if ican t f o r i nves ti ga ti on

and cons t r u c t io n

U

-

r

u

a,

30

lo

to 50

%

rock

a,

r

30150

C

O

Rock

Bo th r oc k c o nt en t a nd r e l ic t d is c on tin u it ie s may

r

a f f e c t shear b ehav io ur of mass

Z

A

-

,

L

a

50 % to 90

lo

rock

50190

Rock

I nt er l ocked s t r u c t u re

Greater than 90 '10 rock

90 1100

Rock

Sma ll amount o f t he ma te ria l c o nver te d t o s o i l

a long d iscon t inu i t ies

100 % rock

Unweathered

Rock

May show s l l gh t d~sc o lou ra t i ona long d i scon t~nu l tl es

T a b l e of C on

t en t s

T a b l e of C

on t en t s


T a





- -

+00

Table 1 1 P r o c e d u r e f o r R ap id I d e n t i f i c a t i o n a n d D e s c r i p t i o n o f S o i l s

Par t ic le

P a rt ic le

Structure

and

Weathering

h s i c Soil

Type

Size Visual ldentif

ication

Shape and

C o l o u r

I m m l

Plasticity

O u a n t l t o t l v e S c a l e s

P a r t l c l e

mkof

Ycmdary

with Coarse Soil5

OULDERS

Only seen complete ~n potsor exposurer

S h a p e

Homo

-

Byinspectionof voids

gmCOus

and porticlepocking

COBBLES

Oftendaffisultto rcswrr fromboreholes

orm

Equidlmmsionl

F l a t

dtermt8ng layersof

Elongotr

asilyvisible to naked eye; w r t i c l r w i n g typer or with

Flatand

andr or lensesof

be descr~bed

clongote

thermotermlr .

Well-graded:w ide rangeot grainsizes.

Angulor!ly

GRAVELS

ngular

partic lesl ies betweennarrow l tmi ts ;

Subangular

Svbrrrunded

or gap-graded:

on

intwmedimtrsize

Rounded

Fine

of port is le is markedly under

-

represented. I

SurfoseTcxtvri

Visible to caked ere

rrrv

l i t t l e or no Smooth

omse

fraction mayo l ro be

, . .

Requirespickfor

Thinly-

m.,.r cohcsmwhen dry: grading can be ROU*

I

laminated

<

Hue

Glassy

I C C O ~ ~ O ~ Y

whereonstituent

described.

Hmeycornbed

0.6

Wel l-graded:wide rsnge of groinsi res

Pltted

For composite types dcacribed

o r :

Striated

SANDS

Poorly-graded- not well grodcd. lMoy :Myq ' fines

are

plortic.cohcsive;

Mcd,um

be ~ n i t o l m : IZC

of

most p o r l l s l e ~

INCS

I

etween norrowlimits; orgap-grodcd.

0 2 mtcrmcdmtcslzc ofpart ls l r

1s

markedlyunder-rrprcsenlcd.1

Fine

rraksintopolyhedral

mrre Only soarsc r l l t bare ly

visible

to naked

rogmentr dong

- 0. 02 e ye ; e l h i b m li t t le p ds lt ic it y m d r mr kc d

i'lsures

dilotonsy:slightlygranular or s i l ky to

Gravel

or

SILTS Mcd,um u mt i to t i ve xa l e

the touch. Oismtcgroter

tn

w d e r .

Non

-

plssta

I spasmgof

-0 0 06 l umps dry q ui ck ly ; p osse ss coh es io n b u t or LOW

Closely-

iscontinuities may

can

bepnvdcrcdeasily betwee' fingers

plostisity

Spaced

Flne

used.

c3

Moltled

o ry l u mp ~

an

be broken but not

Streaked

pow&wd betweenfingers; lheyak a

frnger pterr urr

dlsmtcgrotc undermtn but moreslm ly

Homo

eposit con3ists

thansilt; smooth lo lhctouch; erhibttr

'ull explanmion ofthe

use

of Cannot be moulded by OLneour

sscntbllyofone type

Drrcmt#nu#t#erGeneral

plusticity butno dilatonsy;sticks to

ieconchryeonstttuents m Stiff Cge - be Me nl ed

I

select one

CLAYS

fingns ond drwsslowly, shrmks :ompOste Soils IS gwrn~

I nt er - l tn mt in g layers o f

appveciably

on

drying,urua llyshowing

loblcs 15 and 16. st lo t i t ied

o r l i n g t yp e r .

cracks. Intermediateand high plasticit y

1t~rv.1 S m k for

IOYS

prowrthes ahowthe% l o

0 I nt el - u ck ne rr o f l ay er r

modcmteand hlghdegree,r crpedlvely.

High

l am in ated l ay b e u se d.

qualitledby

plasticity

Weathering a term f o r

ORGAN lC

Cmtalnssub~to ntl dmountrof organfs

IFat clay

I

COmwcl Fibre5 already

CLAY,SlLT

Varies vegetable matter Often has nolicroblc

compressed together

or SAND

smellandchmgcr colour on modation.

I

....

.

In warre soils : &scribe

.pp pr~otc.

v er y c o m ~ c l s i b l e n d F ib ro us

Ian1rema,ns

overall dissolourot~onf See

Predominantly plantremains; ul u~ ll y Spongy

open structure.

:cognizable and

soiland dcgree of

E A T S Varies dark brown

or

bhck incolwr.oftenwith

:tam some rtrcnalh

on bemoulded m hand.

decompositi& of grovel

di5llnnlvesmell; low bulk density

Plasl,s

~ ~ . ~ ~ h ~ ~ !

and smearsfingers and larger fragmcntr


T a b l e of C

on t en t s





Table 2 Soi l Strength in Terms of ompactness and onsis tenc y

T a b l e of C on

t en t s

T a b l e of C

on t en t s

T a b l e

of C on t en t s





Table

4

escriptive Term

Subangular

Subrounded

Rounded

Table

3

Particle F o r m

escriptive Term

lllustr tion

Equidimensional

F l a t

0

ELongate

Flat and ELongate

@ @

Particle ngularity

Illustration

T a b l e of C on

t en t s

T a b l e of C

on t en t s

T a b l e

of C on t en t s





Table

5 -

Use of Secondary Const i tuents for the Naming of Compos ite Soi ls

'ercentage

1

Terminology Term for

Secondary

Seque nce Secondary Constituent

Consti tuent

W i th a l it tl e

S e c o n d a r y +

c o n s t i t u e n t s

W it h s om e

a f t e r p r in c ip a l

W i t h m u c h

S l ig h t ly ( s i l t y , c l a ye y o r

s i l t y l c l a y e y

I

- ( s i l t y , c l a y e y o r

s i l t y l c l a y e y * )

S e c o n d a r y

V ery [ s i l t y , c la y e y or

c o n s t i tu e n t s b e fo re

s i l t y / c layey* ]

p r i n c i p a l ( e x c l u d i n g

c o b b le s a n d AND

OR

b o u l d e r s 1

S li gh tl y [ g ra v e ll y o r s o n d y *

( g ra v e ll y o r s a n d y *

Very (g rave l ly o r sandy

*

S e c o n d a r y

S l igh t ly (g rave l l y o r sandy

c o n s t i t u e n t s b e fo re

o r b o th *

p r i n c i p a l ( e x c l u d i n c

c o bb le s a n d

( g r a v e l ly o r s a n d y

*

o u l d e r s

Examples :S l i g h t ly s i l t y l c l a y e y , s a n d y GRAVEL

S l ig h t ly c la y ey , g ra v el ly S AN D

V e ry g r a ve l ly S AN D

Sandy S ILT

S l ig h tl y g ra ve lly , s l i g h t ly s a n d y S l LT lC L A Y


T a b l e of C

on t en t s






-

Estimated oulder or

Principal Soil Type Term

Cobble Content of

Very Coarse Fraction

BOULDERS

>

5 0% i s of boulder

s ize

>

200mml

Very coarse soils

COBBLES > 5 0% is o f cobble

(BOULDERS

&

COBBLES]

i.e.>50% of material is

s ize ( 6 0 -

200mm

very coarse

>

60mm)

BOULDERS m ay be q ua li f ie d a s 'cob bly ' and

COBBLES a s

'

bouldery'

Full Method for Naming Composite Soils Containing Very Coarse Material

Note

:

W h en t h e f u l l m e t h od i s u s e d, t he n am e of t he f in er m a te ri al s ho ul d

b e g iv en i n p a r en th e se s w h e n i t i s t h e m in or c o ns ti tu e nt , a s s ho wn

be low 1 .

Examples

:

Sandy GRAVEL w ith occasiona l boulde rs

C ob bly BOULDERS w ~ t h om e f m e r m a te ria l ( sl ig h tl y g ra ve lly s an d

)

BOULDERS w ith much f lner ma te r ~a l [ s i lt y l c layey , ve r y sandy g r ave l

T a b l e

16 -

M e t h o d s f o r N a m i n g S o i l s C o n t a i n i n g V e r y C o a r s e M a t e r i a l

Rapid Method for Naming Very Coarse Soils

1 Irincipal Soil Type

BOULDERS (o rCOBBLES)

BOULDERS (o rCOBBLES1

BOULDERS (o rCOBBLES)

FlNER MATERIAL

FlNER MATERIAL

FlNER MATERIAL

Term

BOULDERS (or COBBLES] with a

l it tle f in er m a te ri al

BOULDERS (o rCOBBLES] with

some f iner ma te ri al

BOULDERS (or COBBLES] with

m uch fin er m ate ria l

F lNER MATERIAL w i th

boulders (o r cobbles

I

FlNER MATERIAL w i th

boulders (o r cobbles

F lNER MATERIAL w i th

boulders

o r cobbles)

many

some

occasional

Composition

<

5% fin er m ate ria l

5%-2 0% finer material

20%- 50% f iner material

5 0% -2 0% b ou lde rs

( o r cobb les

I

20%

-

5% b ou ld ers

( o r cobb les )

< 5 boulders

(o r cobbles)

T a b l e of C on

t en t s

T a b l e of C

on t en t s

T a b l e

of C on t en t s





Table 17 - Plasticity Terms ased o Liquid

Limit

Range of

Liquid

LimitDescriptive Term for Plasticity

( la

Low plasticity

Intermediate plasticity

High plasticity

Very high plasticity

Extremely high plasticity

Note :

Ctassification in terms of plasticity is based on Liquid limit, in

accordance with BS 5930

I 98

.

T a b l e of C on t

en t s

T a b l e of C

on t en t s

T a b l e o

f C on t en t s

T a b





T a b l e 18 Ident i f icat ion

F e a t u r e s

for

Dif ferent

S o i l T y p e s

eature I Soil

T v ~ e

.

Soils

Der ived

f r om

Weathered

Insitu

Rock

Group

Weather in

ill

b uv'um Co~~uv ium

esidual

Saprolits

So iI

a.Foreign, but presentupslope

RockTypeof

Bouldersand

Cobbles

b. Totally foreignor man-made

1brick,etc.)

a. Jointsandveinsi n boulders

b. Joints andveins persistent

throughmatrix

Relict

Structure

c. Remnantsof disruptedveins

Id.Layering

controlled by original joint

pattern

b. Jumbledarrangement of rock

fragments with some point

contact

Texture1

c. Gradationalchangeinstrengtt

and

grc~dingrom rock

Fabric

fraomentstomatrix

d. Sharpchangei nstrength and

gradingfrom rock fragments

tomatr ix

e. Matrixmineralogyltexture

differentfromrock fragments

Stratigraphic a.Overliesboulder-free zone

Prof i le or layer

a.Topsoil Layer

Basal

Features

b.Particle alignment

Springs

(a.At contactswi thother soils

a.Fan-or lobe-shaped

b *

Geomorphology

b. Anomalous topography

h

Legend

:

*

Featureindicatessoil type, but soiltypedoes not always show a particularfeature

Note : Table adapted fr om Huntley

&

Randa ll 11981

1

T a b l e of C on

t en t s

T a b l e of C

on t en t s

T a b l e

of C on t en t s





9

T a b l e 19 - Names a n d D e s c r i p t i v e L e t t e r s f o r G r a d i n g a n d

P l a s t i c i t y C h a r a c t e r i s t i c s

Soil

Compo-

nents

n

C

0

E

@J

0

0

V

al

C

0

U.

m

..-

U

.

5 g

P a

E

0

U

Legend :

Descriptive

Name

Letter

GRAVEL

SAND

We lI - graded

Poor l y -g raded

U n i f o r m

Gap

graded

FINE SOIL, FINES

may be d i f f e ren t i a ted i n to M or C

SILT

(

M

-

SOIL) *

p l o ts b elo w A - l in e o f p l a s t i c i t y c h a r t

of F igure

8

(o f res t r i c ted p l as t i c range ]

Terms

Main terms

Qual i fy ing

t e r m s

Main te rms

Qua l i f y i ng

t e r m s

Main te rm

Qual i f y ing

t e r m

CLAY

plo ts above A

L o w p l a s t i c i t y

l in e ( f u l l y p l a st ic

1

I n te rmed ia te p l as t i c i t y

H i g h p l a s t i c i t y

Very h igh p la s t i c i t y

Ex t remely h igh p las t i c i t y

Upper p las t i c i t y range *

incorporat ing groups

PEAT

Organic

I , H ,

V

and E

may be su f f i xed to any g roup

See N ote in Table 20

This te rm is a usefu l gu ide when i t

is

not poss ib le or not requi red

to designate th e range of l iqu id L imi t more c losely , e. g. du r ing the

*

rapid descr ipt ion of so i ls

T a b l e of C on t

en t s


T a b l e o

f C on t en t s

T a b


T




Table 20 - rit ish Soi l Class if ication System for Engineering Pur pose s

Soil Groups ( 1 1

Subgroups and Laboratory ldent if ication

GRAVELar S A N D

may

b e

Fines

q u a l i f i e d an additional

G r ou p Subgroup

(% l e ss L iq u i d

s e c o n d a r y c o ~ t i t u e n tf o r c o a r s e

S y m b o l S ym b o l

than

Limit

N a m e

f r a c t i o n w ! re a p p r o p r i a t e

121131 121

0 . 0 6 m m ) 1% )

I

ble151

S l i gh t l y s i l t y o r

We l l - g raded GRAVEL

G

G W GPg

W

layey GRAVEL G P GPu

Po orly -gra de dlun iform /Gap-graded GRAVE

Si l ty GRAVEL

G-M GWMGPF

Well groded1Poorly graded si l tyGRAVEL

G - F

layey GRAVEL

G-C GWC GPC

Well gradedIPoorly graded cla ye y GRAVE1

Verys i l ty GRAVEL

Very si l t yGRAVEL subdividea s for GC

GC IG M L, etc

in termedia te .

h igh .

v e ry h i g h .

e x tr e m e ly h i g h p l o s t i c ~ t y

Very clay ey GRAVEL

GF G M GCL

Veryclaye y GRAVEL

I

c l a y o f l o w ,

S l ~ g h t l y i l t y o r

Wel l-graded SAND

c layev SAND

Poorlv-araded/Uniform GOD-aradedSAND

Si l ty SAND

Well-graded/Poorly-graded s i l t y SAND

Clayey SAND

Well-graded/Poorly-qraded clayey SAND

Very s i l ty SAND Very s i l t y SAND : s u b di v id e a s f o r SC

Very clayey SAND Very clayey SAND

I

c l o y o f l o w .

i n te rmed ia te .

SC H h i g h .

SCv

v e ry h i g h ,

SCE

e x t re m e l y h i g h p l a s t i c i t y

I

Grave l ly SILT Grave l ly S lLT : subdi vide as fo r CG

Grav el ly CLAY14)

Gravel ly CLAY

I

o f l o w.

in termedia te .

h i g h ,

v e ry h l g h .

e x t r em e l y h i g h p l a s t i c i t y I

Sandy S ILT (41

Sandy S lLT : s u b di v id e a s f o r CG

Sandy CLAY

l 4

Sandy CLAY

:

subd i v ide as fo r CG

S lLT IM-so i l 1

M M L, e tc

SlLT

:

subdi vide as fo r C

F

CLAY

151I 6 1 171

C CL CLAY l o f lo w ,

CI

i n te rmed ia te ,

CH

high.

C

v

v e r y h ~ g h .

C E e x t r e m e ly h i g h p l a s t i c i t y

Desc r i p t i ve Le tte r '

ti ? d t o b e a s i g n if i c a nt c o n s t it u e n t

l r g a n i c S o i ls

a ny g ro up o r s u b -g r o up s ym b ol . E xa mp le MHO

:

Organic

: S ILT o f h ig h p l a s t i c i t y .

P e a t

P t P ea t s o il s c o ns is t p r ed o m in a n tl y o f p la n t r em a in s w h ~ c h m ay b e f i b r o u s o r a m o r ph o u s.

L

Notes :

(1

1

Thename o f t he so i l g roup shou ld a lways be g iven when desc r ib i ng so i l s , supplemented . i f r equ ired , by

the g roup symbo l . a l though fo r some app l ico t tons 1e.g .d iag rams1 i t moy be conven ien t t o u se the

g r ou p s y mb o l a l o n e .

(21 The g roup symbol o r sub -g roup symbol should be p laced i n b racke ts ~ f Labo ra tory me thods have no t

beenused fo r i den t i f i ca t ion , e . g . ( G CI

(31 The des igna ti onF INE SOIL o r F INES. F may be used i n p lace o f S ILT . M , o r C LAY. C, w he n i t i s n o t

p o ss ib le o r n o t r e q ui re d t o d i s t i n g u ~ s h be tween them

( 41 G r a ve l ly i f m o re t h a n 5 0 % o f c o ar se m a t e r i a l i s o f g r av e l s i z e . S an dy i f m o re t h a n 5 0 % o f

c oa rs e m a t e r ia l i s o f s a nd s i z e .

151 SlL T i M - s o i l ) . M i s m a te ri al t h at p l o t s b e lo w t h e A - l in e , a n d h a s a r e s t r i c t e d p l a s t i c r o n g e i n r e l a t ~ o n

t o i t s l i q u i d l i m lt . a n d r e l a t iv e l y l ow c o h e s i on . F i ne s o i ls o f t h i s t y p e

i n cl u de c l e a n s i l t - s i z e d

m o t e r i a k a n d r o c k f l o u r , m i ca ce o us a n d d ia to m ac e ou s s o i ls , p u mi ce , a n d v ol ca n ic s o i l s , a n d s o i l s

c o n t a ~ n ln g a l l o y s ~ t e . h e a l t e r n a t ~ v e e r m ' M - s o i l ' a vo id s c o n fu s io n w i t h m a t e ri a ls o f p r ed o m ~ n an t ly

s i l t s i z e, w h ic h f o r m o n l y a p a r t o f t h e g r o u p .

O rg an ic s o ll s a l s o u s u a ll y p lo t b elo w t he A - l i n e o n t h e p l a s t ~ c i t y h a r t , w h en t h e y a r e d e si gn a te d

ORGANIC SILT, MO.

161CLAY.C i s ma te r i a l t ha t p lo t s above the A - l i ne , and i s f u l l y p las t i c i n re la t i on to i t s l i qu id l im i t.

I 71 S lL T a n d CLAY m a y b e q u a l ~ f i e d s s l i g h t l y s a n dy . o r s l i g h t l y g r a v e ll y , o r b o t h , w he re appropriate

[Tab le 151 .

T a b l e of C on

t en t s

T a b l e of C

on t en t s

T a b l e

of C on t en t s





T a b l e 21 - S ym b ols f o r

Rocks

a n d S o i l s

ROCKS

Igneous Rocks Pyroclastic Rocks

o v o o v a

Granite

T

v v V

I

Pyroc1astic brec c ia

Granodiorite , Syenite. Coarse a s h tu f f ,

I::::I Monzonite

Lapi l l i tu f f

v

' . .V . .

Gabbro , Lamprophyre

L l F in e a s h t u f f

;i..+:~'-.* Dac i te , La t i te .

.*

c'.'*'.'X

Andesi te , Trachy te , Trachy ande s i te

y

, . Basal t

Sedimentary Rocks

Conglomerate

Sed imentary b recc ia

...........

..........

..........

.......... Sandstone..........

Sil tstone

Shale

Limestone

F i l l (made g round ]

Bou lde rs and cobbles

Gravel

SO,d

. . . .

. .

. . . . . .

.

s i l t

1

1

Metamorphic Rocks

Metamorph ic rock s -

[M regional

Phyl li te , Mylonite

Schist

............

.....

..........

Gneiss

.....

-::: I

....

Quartz i te

......

Marble

M e t am o r ph ic r o c k s

-

c o n ta c t le .p . H o m f e l s l

SOILS

c l a y

Peat

Note : Composite so il t ypes to be

s ign i fied by combined symbols ,

e.g.

. . . . .

. I .I .I. I Si l ty sand

T a b l e of C on

t en t s

T a b l e of C

on t en t s

T a b l e

of C on t en t s





Tab le Sym bols f o r Bor eho le Recor ds

Fault

Slip surface

a

Shell band

Examples

:

Granite faulted against

gn iss

Fault in sandstone

..I

Slip surface in sandstone

Slip surface in shale


T a b l e of C

on t en t s

T a b l e o

f C on t en t s

T a b





Table 3 - Symbols f o r General Planar Str uct ure s

Horizontal stratum

Inclined stratum with dip in degrees (long axis is strike direction 1

Vertical stratum (Long axis is strike direction )

Foliat ion or cleavage, horizontal

Foliat ion or cleavage. inclined, wi th dip in degrees

Long axis is

strike direction)

Foliation or cleavage, vertical [long axis is strike direction)

Joint, horizontal

Joint, inclined, with dip in degrees Long axis is st ri ke direct ion )

Joint, vertical (long ax is is strike direction)

Flow fabric, horizontal

Flow fabric, inclined, w it h dip in degrees (Long axis i s

strike direction )

Flow fabric, vertical (long axis is strike direction)


T a b l e of C

on t en t s






Table 4 Symbols for Other Geological Structures and Boundaries

Geological boundary, superficial deposit

Geological boundary, solid rock

broken lines

denote uncertainty

Mineral vein

Axial trace of anticline

Axial trace of

syncline

Fault, crossmark on downthrow side, with

dip in degrees and throw in metres

broken lines

denote uncertainty

Fault, with horizontal component of

elative movement

Photogeological lineament


T a b l e of C

on t en t s






B L N K

P GE]

T a b l e of C on

t en t s


T a b l e

of C on t en t s





FIGUR S

T a b l e of C on

t en t s

T a b l e of C

on t en t s

T a b l e

of C on t en t s





[BL NK P GE]


T a b l e of C

on t en t s

T a b l e o

f C on t en t s

T a b





Figure

No.

1

2

3

4

5

6

7

8

9

LIST O

FIGUR S

Page

No.

Example of a Discontinuity Data Sheet

Shape of

Mass

Rock Blocks in a Discontinuous Rock

Scales of Discontinuity Roughness

Schematic Illustration of F ra ct ur e Logging

Terms

Mass Weathering Profiles and Zonal Weathering

Classification of a Mass Exposure

Qua nti ta tiv e Classification of Parti cle Form

Types of Bedding

Plasticity Chart for Classification of Fine

Soils a nd t h e Fin er P a rt of Composite Soils

Grading Chart for Soils with Grading Curves

of Selected Soil Types

T a b l e of C on

t en t s

T a b l e of C

on t en t s

T a b l e

of C on t en t s





[BL NK P GE]

T a b l e of C on

t en t s

T a b l e of C

on t en t s

T a b l e

of C on t en t s





GENERAL INFORMATION

Day Month Year

lecord Discontin uity Data

No.

Sheet No.

Of

? 1 ite I

N Y W E R E O a t e ~ O p e r a t O r ~

F l

Location

:

Chainage

Dip

n or, N direction Dip Persistenc e;

1 , 7 , 8

5 , 6

1 3 ; s

0 1 2 , 6

2 , 7

I 1 1 ; s

2 , 3 , 5

8 , 6

I

i 9

1 , 7 , 6 6 , 6 i 7

I

2 , 4 , 6 8 , 4

I

1 4 i 3

1 , 4 , 9 5 , 5

1 2 i 4

O 1 6 , L L I L

, 2 , 5 ; 0

l 1 4 4 g 1 0 1 1 i 9

2 ,

3 , 4 8 , 2 I

1 3 i 3

2 , 3 , 2

8 l

i 4

1 , L I 4

6 , 2 I 1 1 i 3

7 , 1 ; 0 ; 0 1 5 , 5 , 0 1 I I I ~ I I I I I I I I I I

age estimate 5 litreslmin

2 , 6 , 3 8 , O 1 , 8 ; 0

I I

ype Dip direction, Dip Persistence Aperture Nature of Infill ing Consistency of Infilling Unewnes s Waviness W a t ~

0.Fault zone Expressed in Maximum I Wide 1>2 00m ml 0. Clean Soil strength Rock strength Small- scale roughness1 [large- scale I Dry

1.Fault degrees

dimenwon of 2.Mod. wide (60-200mml 1 Surface staining Very SOH

6

Extremely weak

1.

Rough stepped roughness1 2. Damplw et

2. Joint trace length 3. Mod. narr ow(20 -60mm ) 2. Deromposcd disintegrated rock

2.

Soft

7.

Very weak 2. Smoot h stepped Express 3. Seepage

3. Cleavage

m exposed 4.Narrow (6- 20m ml

3 Non-cohesive soil

3 Firm

8. Weak 3. Slick ensid ed stepped wavelength present

4.Schislosily face expressed 5. Very narrow (2 - 6mml L ohesive roil 4. Sti ff 9. Moderately weak Rough undulatmg amplilude (estimate

5. Shear plane in metres 6. El l. narrow

b

-2mml 5 Quart z 5. Very stiff 10. Modcrotcly strong 5 Smooth undulating in metres quantity

6. Fissure

7.

Tight (zero1 6. Calcite or hard 11. Strong

6.

Slickensided undulating

separately

7. Tension crack 7. Manganese

12. Very strong

7. Rough planar in lilres/se

8. Folialion 8. Kaolin 13. Extremely strong

8.

Smooth planar

r

Iitreshn.

O Bedding 9 Other -specify 9. Slickensided planar

Figure

1 -

Example

of

a iscontinuity ata Sheet


T a b l e of

C on t en t s





Tabular

B l o c k y

Polyhedral

Columnar

N o te N u m b e rs i n d i a g r a m s r e f e r t o d i s c o n t i n u i t y s e t s .

F i g u r e 2

-

S h a p e o f R o c k B l o c k s i n a D i s c o n t i n u o u s R o c k M a s s

Small-scale uneveness

Notes 1 Waviness ca n be des cr ibe d by es t im at ion measurement o f

wave leng th and amp l i t ude .

( 2 )

Uneveness can be desc r i bed us ing the te rms g i ven i n Tab le 8 .

F i g u r e 3

-

S ca le s o f D i s c o n t i n u i t y R o u g h n e s s

T a b l e of C on

t en t s

T a b l e of C

on t en t s

T a b l e

of C on t en t s





Figure 4 Schematic Illustration of Fracture Logging



.

I

.

Figure 5 Mass Weathering Profiles and Zonal Weathering Classification of a Mass Exposure


T a b l e of

C on t en t s





Flat and

Elongat

Elongate

i6

1

Flatness Rotio

Legend Partic le diameters

x Longest diameter

Intermediate diameter

Shortest diameter

Fig ure Quant ita tive Classification of Particle Form

Regular Bedding

Graded Bedding

Lenticular Bedding

Figure

Flat Equidimensional

Cross Bedding

Slump Bedding

Wavy Bedding

Types

o f

edding


T a b l e of C

on t en t s

T a b l e

of C on t en t s

T a





line

4

Liquid Limit

( I

Notes:

(1 The letter is add ed to the symbol

of

any material containin g a significant proportion of organic ma tter e.g.

MHO.

2

Plasticity measurements are made on materia l passing

L 2 5

p m BS sieve.

Figure 8 Plasticity Chart for Classification of Fine Soils and the Finer Part of Composite Soils







Figure 9 Grading Chart for Soils with Grading Curves of Select



[BL NK P GE]


T a b l e of C

on t en t s

T a b l e

of C on t en t s

T a





PL TES

T a b l e of C on t e

n t s

T a b l e of C on

t en t s


T a b l e of

C on t en t s




[BL NK P GE]


T a b l e of C

on t en t s

T a b l e o

f C on t en t s

T a b





Plate

NO

1

2

3

4

5

6

7

8

9

1

11

LIST

O PL TES

Rock Textures

Microfractures

Decomposition Grades of Rock Material

Rock Material Examples

Rock Planar Structures

Corestones

Complex Rock Mass Exposures

Rock Mass Examples

Contrasts between Unweathered Marine and

Weathered Alluvial Sediments

Colluvium/Insitu Decomposed Rock Boundaries

Soil Examples

Page

NO

3

4

5

7

9

2

22

23

24


T a b l e of C

on t en t s

T a b l e o

f C on t en t s

T a b


T




[BL NK P GE]

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t en t s


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Equigranular

B : lnequigranular C: Megacrystic

I

D

:

Aphanitic

:

Cryptocrystalline

Natura l

smk

Note

:

Porphyrit ic tex ture represents a specia l case o f rnegacrystic tex ture

(see Glossary

l

I t is ra re ly poss ib le to d is tingu ish between the two

by observation alone; additional geological information on the

composition

o f the large g ra ins lc rys tals relat ive to the matrix is

usuaiiy required.

I

late ock xtur s

T a b l e of C on

t en t s


T a b l e

of C on t en t s

T





A : Microfractures Caused by General

Mechanical Weathering in Highly

Decomposed Gronite a t Kwai Chung

ri tor ie s New Terr i tor ies

C

:

Curved Microfroctures Caused by

D Microfractures Caused b y Tectonic

Mechanical Weathering Ex fo liat ion i n

Ac tivity in Slightly Decomposed

a Granite Corestone ot Lung Kwu

Granite at Siu Lam. New Territories

Chou Island. New Terri tories

Note For fu rth er information. see Section 2. 3. 3

P l a t e - M i c r o f r a c t u r e s


T a b l e of C

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Descriptive

Term

Coarse-grained Medium-grained Fine-grained

Granite Granite Granite

Grade Symbol

Completely

Decomposed

Highly

Decomposed

Moderately

Decomposed

Slightly

Decomposed

F r e s h

atural scale

Plate - Decomposition Grades o f Rock Material Sheet of

2

T a b l e of C on

t en t s


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Descriptive Term

Coarse Ash Fine Ash

Granodiori te

Tuff

Tuff

rade

Symbol

Residual

Soil

7

Completely

Decomposed

Highly

Decomposed

Moderately

Decomposed

Slightly

Decomposed

Fresh

atural

scale

Plate 3

-

Decomposition Grades o f Rock Material (Sheet 2 of 2

T a b l e of C on

t en t s


T a b l e

of C on t en t s

T





A :

Igneous Rock from Lai King New Territor ies

B : Pyroclastic Rock from Chai Won. Hong Kong Island

Note For f u ll descriptions see Section 2 . 3 . 7 .

plate

4 Rock

Material Examples S h ee t 1

o

2

T a b l e of C on

t en t s


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of C on t en t s

T





D : Sedimentary Rock from a Liu Shui New Territories

Note For fu ll descriptions see Section

2 3 7

Plate

Rock

Material Examples Sheet 2

of

2

T a b l e of C on

t en t s


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of C on t en t s

T





at Sai Kung Hoi. New Territories

Massive

D Flow -ban ded

Lapilli tuff with impersistent joints

Rhyolite ot Clear Water

Bay

at Ting Kou. New Territories) Peninsula. New Territories

Plate

5

Rock Planar Struc tures Sh eet

1 of 2)

T a b l e of C on

t en t s


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G :

Cleaved

(

Phyllite a t Tuen Mun,New

Territories,showingslightly

undulating cleavage)

F

Fol ia ted

Schist from Tuen Mun. New

Territor iesas seen in dr i l lcore)

H :

Banded

(Schistoseandesite at Tuen Mun.

New Territories, showing alternating

Layersof different

grainsizeI

mineralogy

I

Plate 5 - Rock Planar Structures Sheet 2 of 2)

I

I

T a b l e of C on

t en t s


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A : Corestones in a Natural Coast01 Exposure of Quartz Syenite at

Tai Miu Wan CLear Water Bay Peninsula New Territories

Plate Corestones

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Sranite Cutslope at Jat s Incline. East Kowloon. Showing a

Partially Weathered PW

3 15

Rock Mass Zone with Large

Corestones Overlying a Partially Weathered PW

13

Rock

Mass Zone

B

wrs(ope In Coarse nsn

~ u r rat M uav~s, ong KOng Islano.

Showing Voriotion in Degree

of

Rock Mass Weathering and

Spacing Orientation of Major Discontinuities Vegetation and

chunam surfacing obscure the relatively more weathered zones

Plate - omplex Rock ass Exposures

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t en t s


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of C on t en t s

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A : Pyroc

ck MassExposed

in

an Excavationat Choi Wan,

Hong

IgneousRock Moss Sampled

by

Dril ling at TsimSha Tsui .

Kowloon

Note

For fu ll descriptions,s Section 2 . 4 . 6

late 8 RockMass

xamples

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A :

Uniform Grey or Greenish Grey Marine Muds Deposited in a

Reducing Environment

i

e. have not been exposed to su b-a eri al

weather ing

Plate

9

Contrasts between Unweathered Marine and

Weathered Alluvial Sediments

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A :

Boundary in Split Mazier Samples fro m Borehole a t Peak Rood,

ne ar Magazine Gap, Hong Kong Island Lig ht brow n.

structureless colluvium containing occusional cobbles of highly

decomposed ash tuf f over ly ing darker, var iab ly co loured,

complete ly decomposed ash t u f f w i th prominent b lack -sta ine d

and kao l in

? I

in f l led re l ic t jo in ts

B :

Boundary in a Tr ia l Pi t a t Ngou

Chi

Wan, East Kowloon

(Va r iab ly co loured

I

mot t led 1 volcanic co l luvium conta in ing large

decomposed boulders of ash tuff overlying uniform, l ight pinkish

grey, com pletely decomposed medium-grained gra nite 1

P la t e 1

-

C o l l u v i u m / I n s i t u D e c o m p o s e d R o c k B o u n d a r i e s

T a b l e of C on

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A :

Hand Sample of Residual Soil from Chai Wan, Hong Kong Islond

: Hand Sample of Completely Decomposed Gra nite from Ho Man

7

Kow loon

Not e For fu ll descriptions, see Section

3 . 9 .

Plate

Soil

Examples Sheet o f 4 )

T a b l e of C on

t en t s


T a b l e

of C on t en t s

T





C : Hand Sample of Marine

ud

from

Junk

Bay, New Territories

Note

For full descriptions. see Section 3 . 9

late oil xamples Sheet

o

4


T a b l e of C

on t en t s

T a b l e o

f C on t en t s

T a b





.

Chau

ang

Kong Island

' ~ e l dExposure of Alluvlurn in Stream Bonk neor Tong Yon Son

Ts ue n . Ne w Te r r~ to r~e s

Note :

For full descriptions. s Section 3 . 9

I

Plate 1 1 Soil Examples S h ee t 3 of 4 )

I

T a b l e of C on

t en t s


T a b l e

of C on t en t s

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G : Exposure of Layered Fi l l in o Trial Pit a t

Peak Rood, near Ma ga zin e Gap, Hong

Kon

Island

Note :

For full descriptions, see Section 3

Plate Soil

Examples

Sheet 4 of 4)

T a b l e of C on

t en t s


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BL NK P GE]

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PPENDIX

N TURE ND OCCURRENCE OF

HONG KONG ROCKS ND SU PE R FI C I L DE PO SIT S


T a b l e of C

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[BL NK

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1 3 3

CONTENTS

P a g e

NO.

TITL

P GE

CONTENTS

A 1 I N T R O D U C T I O N

A 2 I G N E O US ROCKS

A . 2 . 1 N a t u r e

A . 2 . 2 O c c u r r e n c e

A . 2 . 3 C o m p o s i t i o n

A.2.4 C h e m i c a l T y p e s

A . 2 . 5 T e x t u r e s

A . 2 . 6 A l t e r a t i o n

A.2.7 N a m e d V a r i e t i e s

A 3

P Y R O C L A S T I C R O C K S

A . 3 . 1 N a t u r e

A . 3 . 2 C o m p o s i t i o n

A . 3 . 3 T y p e s

A 4 SED IMENTARY ROCKS

A . 4 . 1 N a t u r e

A.4.2 D e t r i t a l S e d i m e n t a r y R o c k s

A . 4 . 2 . 1 T y p e s

A .4 .2.2 R u d a c e o u s R o c k s

A .4 .2 .3 A r e n a c e o u s R o c k s

A .4 .2.4 A r g i l l a c e o u s R o c k s

A.4.3 Ch e m i c a l

nd

B i oc h e m i ca l S e d i m e n t a r y R o c k s

A . 4 . 3 . 1 T y p e s

A .4 .3.2 L i m e s t o n e a n d D o l o m i t e

A . 4 . 3 . 3 C h e r t

A . 4 . 3 . 4 E v a p o r i t i e s

A 5 M E T A M O RPHIC ROCKS

A . 5 . 1 N a t u r e

A.5.2 C o n t a c t M e t a m o r p h i s m

A .5 .3 D y n a m i c M e t a m o r p h i s m

A.5.4 R e g i o n a l M e t a m o r p h i s m

A 6 S U P E R F I C I A L D E P O S I T S

A . 6 . 1 T y p e s

A .6 .2 M a s s W a s t i n g D e p o s i t s

A .6 .3 F l u v i a l D e p o s i t s

A.6.4 M a r i n e D e p o s i t s

A .6 .5 O r g a n i c D e p o s i t s

A 7 S T R U C T U R A L G E O L O G Y

A.7.1 G e n e r a l A s p e c t s

A.7.2 F a u l t s a n d O t h e r F r a c t u r e s

A . 7 . 3 F o l d s

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P a g e

NO

A 8 WEATHERING

A 8 1 G e n e ra l A s p e c t s

A 8 2 M e c h a n i c a l W e a t h e r i n g

A 8 3 C h e mi ca l W e a t h e r i n g

A 8 4 W e a t h er i n g F e a t u r e s

A 9 R E F E RE N C E S

A 10 BI BLI O GRAPH Y

L I S T O F T A B L E S

L I S T O F F I G U R E S

L I S T O F P L A T E S

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A l

INTRODUCTION

~ o c k s n d soils may be des cr ibed for engineer ing purposes by th e

methods given in Chapters 2 and 3 respe cti vely . The pur pos e of th is Appendix

is to explain th e nat ure and occu rren ce of Hong Kong rocks and superf icial

deposits from the geological viewpoint.

Geological classifi cations of nat ura l ea rt h materials a r e bas ed on

characte r ist ics suc h as l i thology, chemistry, mode of formation and occurrence,

an d age. On t h e geological maps of t h e Hong Kong Geological Su rv ey, a

distinction is made between t he solid l i thif ied rocks, which include th ei r insit u

weathered mantles, and t he tr ans por te d superf icial deposits . To most

geologists, soil is t h e nat ura l material which o cc urs above t h e lower limit of

biological activi ty, and i t is not inc luded on t h e geological maps; an e ngi nee r

te nds t o refe r t o th is mater ia l a s topsoil . The d if ferences between t he

engineering and geological us es of t h e term s rock . soil an d superficial

deposi ts ar e d iscussed in Section

1 2 1

Solid roc ks a r e classif ied into t he following fou r broad typ es, based on

th e i r mode of formation

Igneous , which ar e crys ta l l ine or g lassy rocks t ha t ar e

formed by t h e solidification of molten material known as

magma . They ar e eith er intr usive , solidifying beneath

th e ea r th s s u r face , o r ex t ru s ive , e r up t in g a t t h e s u r f ace

before cooling.

Sedimentar y, which a re formed ei th er from fragm ente d

rock o r mater ia l par t ic les tha t have been t ransp or te d by

gra vit y , water , wind o r ice, or f rom chemical precipi tates

from solutions or secretion s by organisms. Sediments a re

often well s tratif ied or have structures which indicate

th ei r mode of de position.

Pyroclasti c, which a r e formed of frag ment s and part icles

of magma and pre-existing rocks th at a re ejecte d

explosively from a volcano and which sett le a t th e

sur f ace by sed imentation thro ugh a ir o r water . These

roc ks s ha re some fe at ur es of both sedi mentary ro cks (i.e.

they are f ragmental and may be s t ra t i f ied) and igneous

rocks (i .e . they a re e rup ted a t th e su r face) .

Metamorphic, which ar e deriv ed from pre-existing roc ks

by mineralogical, chemical an d st ru ct ur al chang es.

Metamorphism is cause d b y t h e effect s of ch angi ng

temperature . p ressure , shear ing s t ress and chemical

envi ronm ent acting on solid rocks.

Superf icial deposits commonly mantle and obscure the underlying,

o lder

solid rocks. Most superf icial deposits a r e sediments which, because they a r e

geologically ver y young , ha ve not ye t been lithified to form solid sedimentary

rocks. Both solid roc ks an d superficial deposi ts can be modified by

weathering.

The following sec tion s giv e a brief acc oun t of eac h of t h e fo ur bro ad

rock t yp es an d of t h e superf icial deposits , plus some gene ral information on

st ru ct ur al geology and weathering. More detailed information of specific


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relevance to Hong Kong can be obtained from Bennett (1984a. 1984b. 1984~)

an d from numerous oth er ref ere nce s given by Brand (1988). The most detailed

acc ou nts of th e dis tri bu tio n an d n at ur e of geological materials in Hong Kong

a r e given in t he s eri es of maps and memoirs pro duce d b y t h e Hong Kong

Geological Su rv ey . The geological classification of ro ck s an d sup erfic ial

deposi ts used in th e maps an d memoirs is given in Table

A l .

All the rock

ty pe s found in Hong Kong ar e i l lustrated in Plate Al.

Additional information on geological processes, and on the geological and

engineering classif icat ions of rocks and soi ls, can be found in the sources

listed in Sections A.9 and

A . l O .

In pa rti cul ar, t h e importance of geological

pro cess es in engi neer ing has been well reviewed by Blyth de Freit as (1984)

an d Legg ett Karrow (1983).

A s with th e remain der of th is Geoguide, t h e meanings of all th e

specialised geological term s u sed in t h e following se ctions ar e given in th e

Glossary. Most of t he e ntr ies in th e Glossary a r e based on th e defini tions

give n by Bates Jack son (1980).

A.2 IGNEOUS ROCKS

A.2.1 Nature

Ig neo us roc ks a r e formed by th e solidification of magma. They may be

ex t rus ive o r in t rus ive , and these two types a re d i s t ingui shed by t he l a rge -

scale form of t he rock mass a nd i t s relat ionship to adj acen t rocks.

A s

t h i s

form may not always be readily appar ent , t h e rock names used ar e not

depe nden t on mode of occu rren ce (e.g. i t is possible to have a basal t intrusion

o r a basal t extrusion). The normal methods of classifying igneous rocks ar e

based on t he relat ive ab unda nce of selected minerals and t h e chemical

composition. This is often supplemented by stu dies of th e text ure, as seen in

the f ield and under a microscope.

A.2.2 Occurrence

In tr us iv e igne ous ro ck s, which ar e ve ry common in Hong Kong, a r e

usually markedly crystall ine. The grain size can va ry from ver y f ine

<

0.06

mm) to coarse

>

6 mm), and may be pegmatitic

>

20

mm .

The intrusions of

gr an it e foun d in Hong Kong usually display a wide ran ge of g rai n siz es,

indicating a complex cooling histor y. In th e simplest case s, t h e cooling of an

int rusion res ul t s in a f iner-gra ined margin n ear th e contac ts with other rocks.

Minor intr usio ns, which a r e usually v er y f ine-grained. may occur as d ykes ,

which ar e near vert ical , or as si l ls , which ar e roughly f lat- lying. These small

in t rus ions cu t the o lde r rocks in which they a r e found .

Extrusive igneou s roc ks , more generally de scri bed a s lavas , have flowed

from a volcanic v e n t o r fissur e. Lavas may occu r a s a single flow o r a

successio n of flows, and may be inte rbe dde d with sediments.

A.2.3 Composition

The composition of igne ous ro cks , which is th e basis fo r the ir

classif icat ion, can be d escribed in terms of t he minerals p res ent o r th e

chemical composition. Most igne ous ro ck s ar e de ri ve d from magma ri ch in

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silica, so nearly all t h e minerals a r e sil icates.

The commonest are quartz and

feldsp ar (felsic) , which a re l ig ht in colour, but dist inct ive da rk minerals

(mafic) , suc h a s biot ite , can al ter t he ap pearan ce of a rock when p re se nt in

small amounts. Thus, a simple division of t h e ign eou s ro ck s bas ed on mineral

con ten t ca n b e made in ter ms of colour leucocratic (l ight), mesocratic

(medium) and melanocratic (dark).

The most widely-used classification of t h e crystalli ne igneous ro cks

(Strec keisen , 1974) is based on th e rela tive p ropor tions of qu ar tz ( Q) , alkali

fe ldspar

A )

an d plagioclase f el ds pa r (P ), from which s uc h common names as

grani te , granodiori te and gabbr o ar e defined. The proport ions of these

minerals ar e obtained b y modal analysis, i .e. b y measuring t h e actual

percentage mineral composition, and t h e res ul ts ar e plotted on a QAP

triangular diagram (Figure

A l ) .

The ver y f ine o r glassy igneous rock s, whose

individual crystals cannot be dist inguished, are classif ied on the basis of

chemical composition (Cox e t al, 1979). The rock t yp es de fined, s uc h as

rhyolite, dacite a nd basalt , have the ir equiva lents in th e QAP classification

(Strec keisen , 1980). These equiva lents ar e given in Figure Al, but , because

differ ent methods of classification a r e used. t h e corres ponde nce with th e

chemical classification is not exact.

A.2.4 Chemical Types

Igneous roc ks can be grouped tog ether in related famil ies based o n the ir

chemical composition.

If t h e composition is n ot known in detail, th e following

simple sys tem of classification can be us ed fo r Hong Kong ro ck s

(a ) Basic ro ck s, which a r e melanocratic, with usually more

th an abo ut 30% dar k minerals, an d

4 4

t o 54% silica (e.g.

gabbro) .

(b ) Intermediate roc ks, which ar e usually mesocratic, with

le ss th an 50% da rk minerals. a nd 54 to 62% silica (e.g.

andesi te and syeni te ) .

(c) Acid r ock s, which a r e often leucocratic. with les s than

20% da rk minerals a nd more th an 62% silica (e.g. gr an it e

and granodior i te ) .

A.2.5 Textures

The texture of an igneous rock is concerned with the size, shape and

disposi tion of t he const i t uent minerals. Int rus ive rocks ar e predominantly

crysta lline, with g rain boundaries int erlock ed, while ext rus ive

rocks, which

have cooled rapidly a t th e su rface, ar e part ly o r dominantly glassy.

The textural feature of most importance in igneous rock classification is

th e dominant grain size of th e groundmass. The ver y f ine-grained rocks,

(aphanitic), with

a

gr ain size of le ss t ha n 0.06

mm.

have crysta ls th a t cannot

be dist inguished with th e naked eye. For la rg er grain sizes, th er e is a

division into fine-, medium-, an d coars e-grai ned rock (Table

A l ) .

The very

coarse -grain ed (pegmatitic) roc ks have g rai ns la rg er tha n 20 mm.

Within th e groundmass , the re a r e of ten s igni ficant ly la rg er c ryst a ls ,

termed 'megacrysts' . There is a wide var iet y of megacr ystic tex tur es, each


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indicating a d iffer ent mode of occ urr enc e of t h e megac ryst , for example

po rph yri tic and xenocrystic. Megacrysts may be aligned parallel to th e flow

direction in a rock; t his is commonly see n in th e sy eni tes and monzonites, and

also in lavas and narrow dyk es. Where these textural differences res ul t in a

visible layering o r band ing, an igneous rock is said to have a ' f low-banded'

s t r uc t u r e .

A.2.6 Alteration

t

a late st age in t h e crystal l izat ion of an igneous rock, t he r elease of

accumulated hot l iquids and gase s may al ter th e rock extensively. typical

example is kaolinization of gran ite, in which th e feld sp ar is alte red to

kaolinite. Alteration is usually controlled by existing discontinuities in t h e

rock mass, an d t h er e may be a grada tion from, fo r example, completeiy altere d

gran i te adjacent to a d iscont inui ty outwards in to f re sh grani te.

A.2.7 Named Va riet ie s

The var iet ies of ign eou s ro ck s found in Hong Kong a re listed below.

These notes are intended to highlight the differences between variet ies.

(a) Granite, which is th e most widespread igneous rock type,

is a leucocratic, sil ica-rich (acid ), cryst alline rock

composed of q uar tz, feld spa r an d dar k biotite mica. The

dominant fe ld sp ar is usually alkali. Granite forms major

and minor in t rusions, including v ery narrow dykes.

Aplite dykes are generally granitic in composition, and

ar e charac ter i sed by an equigranular f ine-gra ined texture .

Pegmatite is also usually gran itic in composition, b u t is

charac ter i s t ical ly ve ry coarse-grained.

( b )

Granodiorite is a mesocratic, sil ica-rich (acid), crys tallin e

rock composed of quart z , feld spar and a bun dan t biot i te

(which res ul t s in a da rk er colour than gran ite) . The

dominant fel dsp ar is plagioclase. Granodiorite forms

major int rus ion s, typically see n a t Tai Po, and sometimes

forms dykes.

(c) Qua rtz syen ite is a leucocratic to mesocrat ic crystal l ine

rock with intermediate silica content. in which there

is

l e ss than 20% quart z. The rock i s mostly felds par, with

alkal i felds par dominant . The rock occu rs a s intru sion s,

for example at D'Aguilar Peak, and a s larg e dyk es, f or

example a t Wong Chuk Hang. Qu ar tz monzonite is

related to quartz syenite , but plagioclase and alkal i

fe ldspar a re pres ent in roughly equal amounts.

Examples

ca n b e fo un d a t Tai Wai, Sha Tin.

( d) Rhyolite is th e ve ry f ine-grained equivalen t of granite .

The megacrysts of quar tz (q uar tz phyr ic) or fe ldspar

(fe lsparphyr ic) g ive the di f ferent rhyol i tes the i r

chara cter . Rhyolite is found as narrow dyke s, for

example a t Kwai S hing a nd t h e Lower Shing Mun

Reser voir, a nd a s lava flows in t h e Clear Water Bay

Peninsula and the Sai Kung Country Park.

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Dacite is the mesocratic, very fine-grained equivalent of

granodiorite. There a re usually megac rysts of q uar tz and

feld spa r, and biotite is often clearly seen. Dacite is

either associated with the rhyolite as narrow dykes. as

on Tsing Yi, or forms the margin to the granodiorite.

Rocks which cannot be di stingu ished a s eith er rhyoli te o r

dacite a re called rhyod acite , and can be seen on Mt.

Stenhouse. Lamma Island.

Qu art z t rac hy te is th e mesocrat ic, v ery fine-grained

equiv alent of qu artz syenite. The rock is characte rised

by alkali feldspar megacrysts. I t occurs as dykes,

for

example a t Aberdeen, and on th e margin s of qu art z

syenit e intru sio ns, as a t Cape D Aguilar. Qu ar tz latite,

th e ver y fine-grained equiv alent of qua rtz monzonite, is

a related rock.

Trachyandesite is intermediate, usually melanocratic and

is ve ry fine-grained. Megacrysts of alkali feldsp ar ar e

common. I t is found a s lava flows in the Clear Water

Bay area.

Andesite is .intermediate, usually melanocratic or

mesocratic, and is ve ry fine-gr ained. Megacrysts of

fe ld sp ar and mafic minerals ar e common. Andesite is

found as lava flows within t h e tu ff s, a s a t Ma Wo

(Tai

Po) and Tuen Mun, and a s dy ke s, as a t Tsing Lung Tau.

Gabbro and i t s very fine-grained equivalent , basal t , are

bas ic, melanocratic rocks composed of an in te rg rowt h of

plagioclase feld spa r and mafic minerals. These rock s are

commonly found as narrow dykes; for example, gabbro at

Diamond Hill and ba sa lt a t Siu Lam.

Lamprophyre is a basic, melanocratic rock characterised

by the abundance of mafic minerals, with feldspar only

pr es en t in th e groundmass. It is occasionally found as

narrow dykes. for example at Rennies Mill.

A.3 PYROCLASTIC ROCKS

A.3.1 Nature

Pyrocl astic roc ks a re formed by th e lithification of material which has

been ejecte d explosively from a volcanic ve nt . Materials from non-explosive

volcanic eruptions are lavas, which are classified as igneous rocks (see Section

A.2.2). Pyrocl asti c rock material is composed of glass an d pumice, broken

cry st al s and rock fragmen ts. The rock fragm ent s may be solidified magma

from th e ven t, or material which formed t he si des of o r choked th e ven t. The

majority of t he material in a pyrocl astic rock is of ign eous orig in, bu t since

t h e rock is composed of fra gment al material and i s sedimented. i t is classified

in a manner similar to that used for sedimentary rocks.

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A.3.2 Composition

The normal method of classifying pyroclastic rocks is on the basis of

composition and size ra ng e of th e individual components or pyroclasts (F igures

A2 and A3).

Pyroc last ic ro ck s may conta in sedim enta ry material. Rocks with

roughly equal amounts of pyroclastic and primary sedimentary material are

' tuffites' , and are usually given the sedimentary rock name with ' tuffaceous' as

a prefix (e.g. tuffaceous sands tone) .

The la rg es t pyr ocla sts a r e blocks and bombs, and , when lithified, become

'pyroclastic breccia' and 'agglomerate' respectively (Figure A3). Lapilli,

equiva lent in sediment grai n size to gravel, a r e lithified t o a 'lapilli tuff' , less

commonly called 'lapillistone'. Ash, which is eq ui va le nt in gr ai n size t o sa nd

and mud, is l ithified to 'coarse ash tuff ' and ' fine a sh tuff ' respectively.

I t i s

ve ry common t o find poorly-sorted roc ks containing a mixture of diff eren t-

sized pyroclasts, a nd the se ar e covered by th e names ' tuff-breccia' , ' lapilli-ash

tuf f' a nd 'ash-lapilli tuff'. When t h e composition i s known in gre at er detail. i t

is possible t o refine th is nomenclature to give such rocks a s 'lapill i-coarse ash

tuff ' and 'coarse ash-fine ash tuff ' .

Pyroclastic rock names are qualified by a term which reflects the

composition of t h e dominant var iet y of pyrocl ast. This is ei th er 'vitric '

(gla ss), 'crystal' o r ' li thic ', bu t in rocks older than Tertiary (c. 60 million

years) i t i s very unl ikely that g lass wi l l survive, s i t rapidly becomes s table

an d microcrystalline. There fore , in t h e pyrocla stic ro cks of Hong Kong, which

ar e Jurass ic in age (much older than t he Tert iary) , the term 'v i tr ic' i s used t o

describe fragments that are recognized from their shape and texture to have

been gl ass when th e rock was fi rs t deposi ted. An example of su ch a rock is

eutaxite, a variety

of

vi t r ic tuff , which

is

found, for example, on t Kellett

an d Razor Hill. The terms 'crys tal' and 'lithic' r ef e r t o pyroc last s composed

of crystals (or crystal fragments) and rock fragments respectively.

A.3.3 Types

Pyroclastic fragments a r e created by th e explosive expansion of gase s in

a magma, by fragmentation of adjacent magmatic rocks from previous volcanic

erupt ions , or by th e break-up of the basement rocks under t he volcanic vent

o r fissure. On ejection from t h e ven t or fissur e, th e fragments become eit her

'fallout deposits' o r 'pyrocl asti c flow deposits', a s follows

(a)

Fallout deposits have many s tr uc tu re s th at resemble those

of sedimentary rocks. They a r e generally well-sorted

when deposited in water, with well-defined, rapidly

alte rnati ng beds. Such deposit s a r e found in Hong Kong.

but they are rare, only being seen at Lai Chi Chong,

Sham Chung and Clear Water Bay.

(b ) Pyroclastic flow deposits a r e formed of hot, gaseous,

de ns e masses of material th a t move rapidl y away from a

volcanic ven t. The material is usually a highly

conce ntrat ed mixture of gas es an d solids. The res ult ant

deposi t may be str ati fie d, b u t in Hong Kong i t more

usually forms massive, poorly-sorted units of g re at

thick ness. Most of the thick se quenc es of poorly-sorted

tuffs in Hong Kong originated as pyroclastic flow

depos its. However, t h e process of welding, in which


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there is viscous deformation of vitric fragments, can

re su l t in marked planar fabrics , which is a charac terist ic

of eutaxi te. When wat er has de-stabil ised an existi ng

unli thif ied pyroclast ic deposi t . th e resul t ing water-

t ra nsp ort ed, poorly-sorted material is known as a

'laharic ' deposit . Both pyroclastic flows and , to a les ser

extent , lahars a re found in Hong Kong; th e la t ter , for

example, a t Hong Lok Yuen.

A.4 SEDIMENTARY ROCKS

A.4 .1

Nature

Sediment is produced by t he weathering a nd erosion of pre-exist ing

roc ks, o r by chemical o r biochemical precipitation. Sedimentary roc ks

produced by t he l ithification of t he t ran spo rte d produ cts of weathering ar e

termed 'detrital ' sedimentary rocks. Those produ ced by chemical precipitation

o r biochemical action a r e 'chemical' an d 'biochemical' sedime ntary r ock s; fo r

example, sa lt depos its and limestone. Generally, when a sediment ha s been

deposited b ut not l i thif ied, t is called a 'superficial deposit ' (see Section A.6).

Sediments, and th e roc ks produced from them, ar e classified on th e basis

of th e si ze of t h e con sti tue nt particles, mineralogical composition an d origin.

The system ado pted by t h e Hong Kong Geological Su rv ey is based on t h e 2-6

gra in s ize divisions which a r e commonly use d fo r th e engin eering descri ption

of soils (Table A l .

A.4.2 Detrit al Sed imen tary Rocks

A.4.2.1 Types

Detri ta l sedimentary rocks a re divided on th e basis of grain size into

'rudaceous' (gravel ly) , 'arenaceous' (s an dy ) and 'argi llaceous' (c layey and si l ty) .

A.4.2.2 Rudaceous Rocks

Lithified deposits of gr avel, which may include cobbles and boulde rs, a r e

called 'conglomerate' when th e particles ar e ro un de d, an d 'sedimentary breccia '

when they a r e angul ar . The coarse part ic les may all be one ty pe of rock, o r

th ey may b e derived from more tha n one sou rce rock. The matrix, which is

subordinate , is e i th er san d o r si l t and may be cemented. Conglomerates can be

found a t Harbour Island an d a t Brides Pool, and sedimentary brecci as on Yim

Tin Tsai in Tolo Harbo ur. Sedim entary breccia i s so-called to dis tin gui sh it

from faul t breccia , pyroclast ic breccia and other genet ic types.

A.4.2.3 Arena ceous Rocks

Lithified depos its of material in which sa nd is t h e dominant g rai n siz e

a r e sands tones. The re is commonly fine material (sil t o r clay) between t h e

san d grains. and when t he amount of thi s f iner matrix is less than

1 5 ,

t h e

san dst one is called an 'arenite ' . When th e matrix exceeds 15 . the rock is a

'wacke'. The cement which bind s th e san d particles toget her to form a rock

is e i ther si l ica , i ron oxides, c lay or carbonates.


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142

Arenaceous roc ks in Hong Kong can be sp li t in to the following fou r

main types

(a)

Quartz ose sandst one is nearly all quart z, cemented by

silica. I t is generally well-sorted, well-rounded and clay-

free, probably being composed of material that has

travell ed a long way from the source rock . Examples

can be seen on Bluff Head.

Feldspathic sandstone contains many feldspar grains,

indicating relatively rapid erosion and deposition close to

th e source . Examples can be seen at The Chinese

University and Tai Po Kau.

Lithic sandstone is very variable, containing recognizable

fragments of othe r rock s. Although usually associated

with rive rs , i t can be deposited in any environment.

Examples can be seen a t Sham Chung and on Yim Tin

Tsai.

Calcareous sandstone is a sandstone cemented by

calcareous material in which the clasts are not

themselves calcareous. An example is th e beach rock

containing tuff clasts and calcareous cement found at

Tau Chau. Repulse Bay.

A.4.2.4 Argillaceous Rocks

Both siltstone and claystone can be recognized in Hong Kong, but,

because of t he difficulty in distinguishing grai n sizes of lithified material, i t is

usual to restrict the term 'siltstone' to rocks composed of the coarser silt

grai ns, and to use 'mudstone' o r 'shale' for all finer mixes. Mudstones ar e

non-fissile, while shale is fissile; shale should not be confused with slate,

which has a metamorphic fissility (cleavage). Apart from fissil ity, th e

important characteris tics of th e argillaceous rocks a re colour, sedimentary

st ru ct ur es and non-clay material (e.g. sand g rai ns, organic matter, fossils).

Good examples of si lt stones ar e found on Ping Chau.

Mudstones can be seen

a t Fei Ngo Shan. and gra phi te-bea ring mudstones can be found on Mo To

Chau.

A.4.3 Chemical and Biochemical Sedimentary Rocks

A.4.3.1 Types

The dominant type s of chemical and biochemical rocks are limestones

(calcium carbonate ) and dolomites (calcium magnesium carbonate). There a re

also siliceous rocks and evaporites in this group.

A.4.3.2 Limestone and Dolomite

Although essentially chemical or biochemical in origin, these rocks may

contain fragmented material, e.g. broken calcareous fossil s. Non-carbonate

material, such as sand g rai ns and c he rt, may also be p resent in small amounts.

Limestone occurs in Hong Kong beneath the alluvium in the Yuen Long area.

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Most of t h e limestone has been metamorphosed t o marble. Dolomite is found

offs hore from Ma Shi Chau an d possibly in th e Ma On Shan mine.

A.4.3.3 Chert

Chert is a n o rga nic o r inorganic precipi tate of silica; th e silica is mostly

cryptocrystalline quartz, but may be amorphous in part (opal).

Impurities in

chert give it different colours, and flint is synonymous with one of the darker

varieties. Chert is eithe r bedded o r nodular; nodular ch er t is common in

limestone, and bedded c he rt can be found on Ping Chau. Chert lenses

associated with pyroclastic roc ks a r e found on the we stern sho re s of Ju nk Bay.

A.4.3.4 Evaporites

Evaporites include gypsum, anhydrite and halite. They a re often asso-

ciated with mudstones and siltstones, forming in shallow basins which are

periodically flooded and drie d out ; th is association can be seen on Ping Chau,

although only evaporite mineral pseudomorphs can be seen.

A.5 METAMORPHIC ROCKS

A 5 1

Nature

Metamorphism describes the process of production of new minerals.

s t ru ct ur es and text ures in pre-existing rocks, excluding th e processes of

weathering. There ar e th re e ty pes of metamorphism, based on the variables of

pressure due to depth of burial , temperature, s train result ing from str ss

applied during deformation, and fluid pressure

(a) thermal or contact metamorphism. charact erised by high

temperature, low pressure and low strain,

(b )

dynamic metamorphism, char act eri sed by high st ra in and

high fluid pressure, and

(c ) regional metamorphism, cha rac ter ise d by high tempera-

tu r e and h igh pressure .

These th re e ty pe s ove rlap considerably, bu t thermal and dynamic metamorphism

ar e restrict ed to localised areas , respectively, along the e dges of lar ge intr u-

sions and on narrow thrusts and faults .

A.5.2 Contact Metamorphism

Both heat a nd hot fluids from a larg e intr usio n of igne ous rock a ffe ct a

narr ow belt of cou nt ry rock surr oun di ng th e intrusi on. Thermal metamorphism

takes place within this contact aureole, affecting different country rocks in

different ways. Mudstones and impure carbonates show t h e gre ate st

mineralogical changes, e.g. the mudstones at The Chinese University which

have been affected by th e major gran ite intrusion in th e Sha Tin area. The

least affected are those possessing mineral assemblages which are stable at

tempera tures as high a s those of t he intruding granite, e.g. sandstone s and

vitric tuffs on Victoria Peak.



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In mudstones , the se change s s t a r t as spot t ing of th e rock, of ten caused

by new mineral gr ow th, while close to th e intrus ion complete recrystallis ation

gives a hornfels, a har d glas sy rock with no fabric. Thermally metamorphosed

limestones become marbles, a s a t Yuen Long, and s k ar n s (calcium-bearing

silicate minerals), a s rep ort ed in th e Ma On Shan mine. San dsto nes become

quartzi te , e .g. a t San dy Bay, bu t impuri t ies in t h e sediment can give small

quan tit ie s of new minerals s uc h as sill imanite, and alus ite an d muscovite, e.g.

behind Belcher s St re et , Kennedy Town.

A.5.3 Dynamic Metamorphism

The high sh ear s t re ss in faul t zones resul t s in c rus hing of t he wall

rock s. allowing mobile fluids to develop high fluid pre ss ur es . Temp eratu res can

be raised local ly, b u t the re is no regional heat ing. The processes an d rock

typ es assoc iated with faul t ing can be s pl i t in to th re e typ es

Bri tt le faul ts , which give cataclasi tes su ch a s faul t

breccia and faul t gouge. These rock s ar e non-fol iated,

an d can be s een in places su ch a s Lai Chi Kok a nd

northwest Tai Lam Country Park.

Ductile fau lts, which give mylonites,

i .e. finely crystalline

rocks contain ing surv ivor megacrysts . These rocks a re

genera l ly fo lia ted , and can be seen thr oug hout th e

gr an it e of th e Castle Peak area . When gre en (chlorite-

r ich ) and shiny, they ar e ca lled phyll ites . e .g. in the

Lok Ma Chau Formation se di me nt s of Mouse Is lan d. Tuen

Mun.

Ductile flow, which gives metamorphic rocks character-

ised b y a penetrat ive fol ia tion su ch as schis t . Examples

of sch is t can be se en within t h e metatuffs of t h e

no rt he rn New Territories.

Although th ru s ts and faul ts ar e limited in width, often a l arge nu mber of

them can be found in bel ts seve ral kilometres wide. Shear st re ss es and f luid

pre ss ur es in t hese be l t s can lead to th e format ion of such minerals as ser ic i te

(f ine muscovite) , pyri te a nd calcite . All the se fea tur es can be found in the

no rth ern New Terri torie s.

A.5.4 Regional Metamorphism

Regional metamorphism is achiev ed by ductile flow un de r high

tempera tur e and pre ss ur e in broad be l t s of fo lded or sheare d rocks .

broad

bel t of metamorphic rock s, which includes sch ists , metatuffs an d phyll i tes,

occurs in th e sout hern pa r t of Guangdong Province and extends in to th e

no rt he rn New Terr itor ies.

A.6 SUPERFICIAL DEPOSITS

A.6.1 Types

Superf ic ia l deposi t s a re thos e sediments th a t have not been l i th i fied to

form rock s.

The classification of superficial deposits and sedimentary rocks is

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essentially th e same. The most important ty pe s of superficial deposit s in Hong

Kong a re mass wasting deposits o r colluvium (s ee below), fluvial deposits, an d

marine deposits. Some small quan titie s of or ga ni c depo sits ar e also foun d.

A.6.2 Mass Wasting Depos its

In Hong Kong, mass wastin g dep osi ts (commonly called colluvium ) a r e

predominantly debris flow deposits and comprise heterogeneous mixtures of

sediment and rock . They ar e formed by t he rap id downslope movement of

sa tu ra te d masses of material, p redominantly by flow (i.e. th e moving mass does

not conta in discre te sh ear or s l ide surfaces a nd has t he genera l appearance of

a body t h a t has behaved a s a fluid). Other ty pe s of slope movement (e.g. rock

slides. de br is s lide s) and slow soil cr ee p also cont rib ute to th e formation of

mass wast ing deposi ts. These deposi ts usually col lect in val leys and a t th e

base s of slopes , fo r example in the Mid-levels area . The deposits fre que ntl y

gra de into r iv er deposi ts (al luvium) o r marine deposi ts

a t th e foot of a slope.

Some oth er , less common. ty pe s of mass wasting deposit a r e bould er

fields and s cre es. Boulder fields a r e accumulations of bou lder s on a slope,

which re su lt from lar ge pieces of rock being weathered a nd ero ded from

outcr ops high er upslope, o r by t he eluviation of fines from a weathered mantle

o r from sh eet s of bo ulder-rich deb ris flow deposi ts.

Boulder fields commonly

gr ad e downslope int o boulder s trea ms along valleys and depr essi ons. Examples

can be seen at Cape D Aguilar an d Lin Fa Shan . Talus o r scre e i s coa rs e

material which has weathered and fallen from a rock face and accumulated on

o r a t t h e bas e of a slope; good examples ca n be s ee n below Lion Rock.

A.6.3 Fluvial Deposits

These dep os its ar e collectively known as alluvium . In Hong Kong two

ag es of fluvial depos ition a r e reco gniz ed; Holocene alluvium, fou nd n ext to

exist ing riv ers and stream course s, and older alluvium, found in higher ter rac es

an d offs hore be nea th th e Holocene marin e depo sits . Both a r e composed of

similar materials, dominantly silt , bu t with significan t amounts of san d, gr ave l

an d clay. The olde r alluvium is evi den ce of a more exten sive floodplain.

Examples can be seen ar ou nd Yuen Long an d Shek Kong. wide spre ad

development of t hi s olde r alluvium occ ur s offshore, beneath marine deposits.

which indic ates a much lower sea-level a t t h e time of it s deposition. Small

patches of alluvium can also be foun d on high gr oun d, where a nat ura l

constriction in an uplan d valley has r esu lte d in th e valley being infilled by

locally reworked colluvial debris.

A.6.4 Marine Deposi ts

Marine superficial deposits in Hong Kong have accumulated on older

alluvial deposits an d th e pre-Holocene ero ded ro ck s urfac e. The commonest

material is a l ight o r dark g rey , or greenis h gr ey, mud. Deposits of san d are

also found on the f loors of contemporary deep-water channels and in other

areas of s t ron g current s . Older sand deposi t s can a lso be found bur ied

beneath mud. Close to th e pres en t coastline, th e most distinctive marine

deposi ts ar e san d beaches, which ar e accumulat ions of f luvial ly-derived san d

washed onto th e shor e by waves and cur ren ts. Storm beaches and raised

beaches ar e two ot he r beach ty pes, b ut thes e ar e much less common. The

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la tt er a r e remnants of olde r beaches associated with periods of h ighe r sea-

level . Estuarine fans an d deltas of sand a nd si l t a r e oth er types of dist inctive

marine deposit. Examples of beach deposits, estu ari ne fans an d a delta can be

found at Lung Kwu Tan.

Tai Po and Nim Wan respectively.

A.6.5 Organic Deposits

The main ty pe of o rg an ic deposit in Hong Kong is peat. which is a da rk

accumulation of organic material that has not fully decayed because of its very

high moisture content . Peat is derive d from organi c deb ris which has

accumulated in poorly-dr ained level sites. There is usually some sedimen t

within the peat, and the depositional environment is often similar to that of

ri ve r deposits. Peat can be found int erb edde d with sediments south-w est of

Yuen Long.

A.7 STRUCTURAL GEOLOGY

A.7.1 Genera l As pect s

Geological structures in rocks and superficial deposits can be divided

into fault s and ot he r fra ct ur es , and folds. Associated with folds ar e minor

structures such as foliat ions, l ineations and mineral fabrics.

A

.

7.2 Faults and Other Fractures

Rock fractures (discontinuities) are the commonest of geological

structures, and can be defined as surfaces in a rock mass across which the

cohesion of t h e rock material is lost. The two most importa nt ty pe s of

fr ac tu re are faults and joints. Where th er e has been visible movement along

th e surface, th e frac tur e is a ' fault ', otherwise i t is a ' joint' . This distinction

is somewhat arbitrary, since nearly all fractures involve some movement.

however slight.

t or near the surface , faul ts can be c lass i f ied in to three types ,

depending on the orientations of the principal str ss s

(a) normal fa ul ts , with t h e maximum compre ssive

str ss

vertical,

(b ) low-angle re ve rs e faul ts o r th ru st s, with t he maximum

compressive stress horizontal, and the minimum vertical.

and

(c) strike -slip o r wrench faults, with t he maximum and

minimum compressive stresses both horizontal.

Faults are often found arranged in s ts (i.e. in groups with similar

orientation). Major fau lts may have associated minor fau lts . Fault planes va r y

from single sh ea r plan es, which may be polished and smoothed, to fau lt zones

in which the associated rocks are broken to fault breccia or fault gouge, or

converted to a mylonite (see Section A.5.3).

Another fe at ur e associated with faulting is 'slickensiding'. Slickensides

are polished and finely striated surfaces that result from frict ion along a fault

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plane. Although slickensides ar e used as movement direction indicators, th ey

are not reliable. t best they only indicate the direction of movement during

th ei r formation, which may not b e th e main movement phase.

Joints commonly develop in regularly-spaced sets, which may be

geometrically r ela ted t o tecton ic

stress

and the form of the rock body.

However. i t is virtual ly impossible to esta bli sh t h e rela tiv e ag es of joints of

dif fer ent orientatio ns, which makes systematic analy sis difficult. The following

th re e main t yp es of jointing can be recognized

(a) Tectonic joints, which ar e reg ula r set s produced by

regio nal compression o r extension. Their orien tation can

give an indication of th e st r es s field. They are relate d

in origin to faul ts or folds, and there is often a

symmetrical arran gemen t of th es e th re e featu res. In

Hong Kong, such joints ar e well displayed in th e

granites, particularly at Castle Peak.

( b ) Cooling joints, which re su lt from th e contraction of an

igneous, pyroclastic o r oth er heated rock body. These

joints may form polygonal columns which have their axes

perpendicular t o th e sur fac e of t he hot rock mass, b ut

the y may also be paral lel t o th e su rfa ce of t he body.

well-known example is th e marked columnar jointing in

the trachyandesites and welded tuffs in the High Island

ar ea of t he Sai Kung peninsula.

( c ) Unloading o r sheet ing joints, which res ult from expansion

of t h e rock mass as the confining pre ssu re is reduced.

usually by erosion. These joints ar e usually parallel o r

near-paral lel to the erosional surface, and are well

displayed on Po Toi Island,

t

Cape D'Aguilar an d a t Siu

Lam.

The surfaces of joints can vary widely in texture and may have been

altere d, weathered, o r coated with minerals. Individual joints a r e usually

reasonably s traight , but may be curved or show sharp changes in direct ion.

Joints close to th e su rfa ce may b e opened b y weathering and infilled by

superficial deposits o r th e prod uct s of insitu weathering.

A 7 3 Folds

fold is a cu rv e or bend in th e rock s tru ctu re, an d i ts recognition

req uir es th e presence of a planar f eat ure such as rock s trat if icat ion. foliat ion

o r cleavage. Although a relatively homogeneous rock mass, such as a gra nit e

intrusion, may be folded, if there are no planar markers within the rock mass

the fold cannot be seen. Fold st ru ct ur es may be complex when t he rock s have

been affected by more than one period of folding.

Folds a re classified by at t i tu de into thr ee main typ es

(a)

syncli nes, which a re folds th at close downwards, with t he

beds younging towards the centre.

(b ) ant iclines, which ar e folds th at close upwards, with t he

beds younging away from the centre, and


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(c) neutral folds, which are folds that close sideways.

The geometry of folds can be descri bed f ur th er by t h e angle of dip of

the axial plane from vertical to horizontal (using the terms 'upright' ,

' inclined' . 'overfolded' an d 'recumbent'), and by t he angle between t he

opposing fold limbs from

O

to over

12 • ‹

(us ing th e terms 'gentle'. 'open'.

'close', ' ti ght ' and 'isoclinal'). These terms ar e defined in t h e Glossary.

Major folds may b e many kilometres ac ro ss , as is t h e Tolo Channel

Anticline, o r hu ndr ed s of met res across, a s a r e tho se found on Victoria Peak.

Minor folds. visible in small exposures, often mirror the form of the major

folds an d ar e th en called 'para sitic' folds. Good examples of the se folds can

be se en on Ma Shi Chau.

A.8 WEATHERING

A.8.1 Genera l Aspects

Weathering is the process responsible for the breakdown and alteration

of materials near the earth 's surface. In igneous, pycroclastic and

metamorphic rocks. i t is the response of rocks to lower temperatures and

stresses than those that prevai led

at

th e time th ey were formed. In most

sedimentary rocks, whose co nst itu ent minerals have previously been weathered

to some extent, i t is chiefly the response of the cementing agent in the rock

to atmospheric conditions (i.e. th e prese nce of oxygen and weak acids). In

superficial deposits, the weathering of individual minerals may still be

continuing at th e prese nt. The weathering process can be divided into th e two

main categories

(a)

mechanical weathering (or disi ntegr ation) , which is

caused by stresses, from both within the rock and as

appl ied external ly , that d isrupt the rock fabric , and

( b ) chemical weathe ring (o r decomposition), which involves

chemical reactions t ha t transfo rm minerals t o more stabl e

forms in the new environment.

The susceptibility of diffe rent rock ty pe s t o disintegration an d

decomposition may differ markedly. Where two o r more rock t yp es a r e pr es en t

together, e.g. where there is an igneous intrusion into another rock, relat ively

more weathered rock may occur beneath o r adja cen t to less weathered rock.

and such a sequence may be repeated.

Weathering profil es may be of c onsi der abl e ag e on a geological time

scale. Consequently, the y do not necessarily reflect th e res pons e of t h e roc ks

to th e pres en t climate. Also, th ey may have been partly removed by

subsequent erosion.

Rock exposed in a recent excavation may be affected by

subsequent mechanical or chemical weathering effects, or both. under

prevailing climatic conditions.

A.8.2 Mechanical Weather ing

Mechanical weathering is b ro ug ht a bout chiefly by cha nge s of

stress and

temper ature a t o r near th e exposed rock surf ace. The important physical

proces ses involved ar e expansion of wate r on freezing in rock pores o r cr ac ks ,


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reduction in confining stress by erosion of overlying material, and differential

expansion of t he rock o r rock minerals when st ro ngl y heated by insolation.

The expansion of certain minerals in joints is also caused by chemical

reactions suc h as hydrati on and oxidation, so t h a t in some res pec ts mechanical

and chemical weathering are not easily separated and produce similar effects.

A

common form of mechanical weathering is exfoliation, which is the scaling

o r peeling-off of flakes an d cur ve d shell s of rock blocks. a s can be se en a t To

Kwa Wan. The biological components of mechanical weathering incl ude

breakdown of rocks by plant roots and animals.

A 8 3 Chemical Weathering

Chemical weathering is br ou ght abo ut mainly b y t he action of subs tan ces

dissolved in rainwater and circulating groundwater.

The intensify of chemical

weathering is controlled by the rates of decomposition of individual minerals

and the removal of decomposed minerals from the rock. Silicate minerals, the

most important rock-forming group, a r e broken down by hydroge n ion

introd uction, oxidation of fe rr ou s to fe rr ic ions, an d hydrati on. Clay minerals

ar e th e chief resid ual pro duc ts of f eld spa r decomposition. while clay, chlorite

and limonite are produced from the decomposition of mafic minerals such as

biotite. These pr odu ct s a r e commonly removed by eluviation an d erosion.

which allows the proces s of chemical wea the rin g to progr es s. The biological

components of chemical weathering include changes in soil p and the

formation of complex organic-mineral substances.

In limestone or marble, solution is the dominant aspect of chemical

weathering. Distinctive landforms a re produc ed, notably kar st topogra phy.

This is characterised by sinkholes, caves and und ergro und drainage, and has

been found buri ed beneat h superf icial deposits a t Yuen Long.

A 8 4 Weathering Features

The following examples of weathering features are found in Hong Kong

Weathered mantle, which is the entire depth of the

weathering profile, excluding any transported material t

the top.

Weathering fro nt, which is an essentially p lanar s ur fa ce

a t th e downward limit of activ e weathering within th e

rock mass. A sha rp well-defined weathering f ro nt i s a

relatively rare feature, but good examples can be seen on

Tai Tam Reservoir Road.

Colour banding, and t he more str uct ure d spheroidal

weathering, which ar e caused by alternating enrichment

an d depletion of iro n oxides. Colour banding can be

seen in sediments on the west side of Three Fathoms

Cove.

Joint harde ning. c aused by t h e migration an d deposition

of ferr omagne sian minerals, which makes t he joints s ta nd

ou t on erosional surf ace s. Examples can be seen at Ma

Shi Chau and on the east side of Deep Bay.


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Weathering pits, cause d by th e prefe rent ial weatheri ng of

diff eren t lithologies, e.g. mudstone lapilli in tuf f, o r of

differen t crys tal s in homogeneous r ocks such a s granite.

Examples ca n b e see n in th e tuf fs nor th of Tsuen Wan

and in granite on Hammer Hill.

Mineral boxwork, which is similar to joint hard enin g, b u t

in th is case the ha rd substance is an unal tered i ron

mineral deposit. This can be seen in the granodiorite of

Cape D Aguilar.

Tors and cores tone s, which ar e piles of jointed roc ks

(t or s) , for example a s a t To Kwa Wan. o r s ingle blocks

(corestone s). However, th e t e r m corestone should be

applied only t o blocks within th e weather ed mantle which

a r e not in contac t with solid rock.

Solution grooves and basins, which are normally

associated with soluble ro cks su ch

as

limestone, b u t may

also develop on siliceous rock s. Examples ca n be see n in

granite on Hammer Hill and in tuff in the Tai Po Kau

Nature Reserve.

Kar st topog raph y, desc ribed un de r chemical weatheri ng in

Section A.8.3.

A.9 REFERENCES

Bates, R L Jack so n, J.A. (E di to rs ) (1980). Glossary of Geology. American

Geological In st it ut e, Falls C hur ch, Virginia, 749 p.

Benne tt, J.D. (1984a). Review of an d Weathering in Hongu ~ e r f i c i a lD ~ D O S ~ ~ S

Kong G C O Publication No. 4/84, Geotechnical Control Office, Hong

Kong, 51 p.

Bennett. J.D. (1984b). Review of Hong Kong S t r a t i g r a ~ h y . G C O Publication

No. 5/84, Geotechnical Con tro l Office, Hong Kong, 86 p.

Bennett, J.D. (1 98 4~ ). Review of Tectonic History, S tr uc tu re a nd Metamor-

phism of Honq Kong. G C O Publication No. 6/84. Geotechnical Control

Office, Hong Kong, 63 p.

Blyth, F.G.H. d e Fr ei ta s, M H (1984). A Geology fo r Ennineers. (Sev enth

edi tio n). Edwa rd Arnold, London. 325 p.

Brand, E W (1988). Bi bl io gr a~ hy n th e Geolony and Geotechnical Enni neer ing

of Hons Konq t o December 1987. G C O Publication No. 1/88,

Geotechnica l Control Office. Hong Kong. 150 p .

Cox, K.C., Bell. J.D. Pa nk hu rs t, R.J. (1979). The I n t e r ~ r e t a t i o nof Igneous

Rocks

George Allen Unwin. London. 450 p.

Fisher. R V Schmincke, H U (1984). Pyro clas tic Rocks. Sp ri ng er Verlag.

Berlin, 472 p.


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Legge tt R.F. Karrow P.F. 1983). Handbook of Geology in Civil

En gi neer in g. McGraw-Hill New York

50

chapters .

St reck ei se n A. 1974 ) . Classification and nomenclature of plutonic rocks

IUGS Subcommission on th e Syste mat ics of Ign eou s Rocks. Geologische

Rundschau vol. 63, pp 773-786.

Streckeisen A.

1980) .

Classification a nd nome nclat ure of volcanic roc ks.

lam prop hyr es car bon ati tes an d melilitic roc ks IUGS Subcommission on

t h e Syste mati cs of Igneou s Rocks. Geologische Runds chau vol.

69.

pp 194-207.

A.10 BIBLIOGRAPHY

Blatt H. Middleton G.V. Murray R C

1980 ) .

Origin of Sedimentary Rocks.

Prent ice-Hall Englewood Cliffs New Je r sey

782

p.

Fry

N.

1984) . The Field Description of Metamorphic Rocks. Open Un iv er si ty

Press Milton Keynes.

U K 110

p.

Gillen C. 1982) . Metamorphic Geoloqy An In tr od uc ti on to Metamorphic an d

Tectonic Pr oces se s. George Allen Unwin London

144

p.

Holmes

A. 1965 ) . Pri nci ple s of Phy sic al Geoloqv. Nelson London 1288 p.

Jennings. J.N. 1985) . Ka rs t Geomorphology. Basil Blackwell Oxford. 293 p.

Mackenzie W.S. Donaldson. C.H. Guildford

C.

1982 ) . Atlas of Igneous

Rocks an d t he ir Textu res. Longman Harlow UK 148 p.

Middlemost E.A.K. 1985 ) . Magmas a n d Magmatic Rocks An In tr od uc ti on t o

Ign eou s Petrology. Longman London.

266

p.

Nockolds S.R.. Knox. R.W.OIB. Chinner

G.A.

1978 ) . Petrology for Students

Cambridge University Pre ss

Cambridge

435

p.

Oilier. C.D. 1975). Weatherin g. Longman Group London 304 p

Park. R G

1983) .

Founda tions of St ru ct ur al Geology. Blackie. Glasgow

135

p.

Pettijohn F.J. 1975) . Sedimentary Rocks. Ha rp er Row New York 628 p.

Price. N . J . 1966) . Fault and Joint Development in Brittle and Semi-Brittle

Rock

Pergamon Press. Oxford.

176

p.

Ramsay J.G. 1967 ) . Folding and Fr ac tur in g of Rocks. McGraw-Hill New

York

568

p.

Thorpe . R.S. Brown

G.C.

1985) .

The Field Description of Igneous Rocks.

The Open Un iv er si ty P re ss Milton Keynes. UK

154

p.

Tucker M.E.

1982 ) .

The Field Description of Sedimentary Rocks. he Open

Uni ver sit y Pr es s Milton Keynes

UK .

112 p.


T a b l e of C

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f C on t en t s

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Twi da l e ,

C R

( 1 9 8 2 ) . G r a n it e L a n d f o r m s . E l s e v i e r S c i e n t i f i c P u b l i s h i n g C o,

Ams terda m, 3 7 2 p

Wise,

D . U . .

D u n n . D . E . . E n g e l d e r , J . T . , G e i s e r , P . A . , H a t c h e r , R . D . , K i s h , S . A . ,

O dom , A .L . S c h o m e l. S . ( 1 98 4 ) . F a u l t - r e l a t e d r o c k s s u g g e s t i o n s f o r

t e r m i n o l o g y . G e o l oq y . v o l . 1 2,

pp

3 9 1 -3 9 4 .

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153

L ST

O

T BLES

Table

Page

No No

A

Classification of Solid Rocks and Superficial

155

Deposits in Hong Kong


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Table A1 Classificatio n of Solid Rocks and Superficia l Deposit



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57

LIST OF FIGUR S

Figure

No

A

A

2

A3

3

Classification of Hong Kong Igneous

Pyroclastic Rock Composition

yroclastic Rock Names

Rocks

Page

No

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Legend

A

P

I

Notes :

Silica minerals

Alkali feldspar

Plagioclase feldspar

1 Plutonic in capitals; volcanic in lower case

1 2 Classification after Streckeisen 197L. 1980 .

F i g u r e A 1 Classi f icat ion of Hong Kong Igneous Rocks


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Pumice glass

VlTRlC TUFF

CRYSTAL TUFF LITHIC TUFF

Crystals

Rock

crysta l

trogmcnts

fragments

Figure A 2 Pyroclastic Rock omposition

Blocks ond bombs

Over 60mm

A

YROCLASTIC

TUFF- BRECCIA

TUFF- AGGLOMERATE

ASH- LAPlLLl

Lopil l i

2

6 0 m m

75 5 0 25 [Coarse ash 0.06-2mn

Fine a sh under 0.06mm

Note : Figure adapted f r o m Fisher and Schmincke

l 1 9 8 L .

Figure

A

- Pyroclastic Rock Names


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6

LIST OF PL TES

Page

Plate

NO

NO

A

Hong Kong

ock T y p e s

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[BL NK P GE]



T a b l e o

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T a b


T




A Fine- grained Granite 8 Medium grained Granite

E:

Pegmatite

[intruding gmnod iorite 1

G:

Rhyolite

H :

Rhyodacite

C: Coorse grained Granite

F: Granodiorite

Natural scale

Plate

A 1

Hong

Kong

ock Types

Sheet

of 4

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K:

Quartz Syenite

N: Trachyandesite Andesite

P :

Gabbro

Q : Basalt

R Lamprophyre

Natural

scale

late A 1

ong

ong

Rock T y p e s

Sheet

2

of

4


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T : Co Ash

Tuf f

U Lapil li Tu ff

V : Pyroclast ic Breccia W

:

Mudstone

X Sandstone

( ~ 1 1 2 )

Y :

Conglomerate

Z

Sedimentary Breccia

AA :

Cher t

Natural

scale

I

lateA1 - ong Kong Rock

Types

Sheet of

4 )

I I

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AB: Limestone

AC:

Dolomite

AD: Evapor i te

AE:

M y l o n i te

AF: Phyll i te

AG: Schist

I

A Quar tz i te

AJ :

F a u l t a r e c

late A ong ong Rock Types Sheet 4

of

4

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GLOSS RY


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A cid . C he mic al t e r m f o r a n i g n e o u s r o c k c o n t a i n i n g m o re t h a n 6 2 s i li c a a n d

u s u a l l y l e s s t h a n 20 d a r k m i n e r a l s . ( C o n t r a s t w i t h ' i n te r m e d i a t e ' a n d

' b a s i c ' ) .

A g g l o m e ra t e . P y r o c l a s t i c r o c k c o m p o s ed p r e d o m i n a n t l y of r o u n d e d b o m b s of

m a t e r i a l g r e a t e r t h a n 6 0 mm a v e r a g e d im e n si on . ( C o n t r a s t w i t h ' p y r o -

c l a s t i c b r e c c i a ' ) .

A lk ali f e l d s p a r . G r o u p o f f e l d s p a r s c o m p o se d of m i x t u r e s , o r m ix ed c r y s t a l s , of

po tas s ium fe ld sp a r (KAlSi,O ,) a n d sod ium fe ld sp ar (NaA1Si30,). (Se e

' f e l d s p a r ' ) .

A llu viu m . D e t ri ta l m a t e r i a l of a n y g r a i n s i z e t r a n s p o r t e d a n d d e p o s i t e d d u r i n g

c o m p a r a t i v e ly r e c e n t g e o lo g ic a l t im e b y a r i v e r o r s t re a m .

A m o rp ho us . T erm f o r a m i n er al o r o t h e r s u b s t a n c e t h a t l ac k s c r y s t a l l i n e

s t r u c t u r e a n d h a s n o c h a r a c t e r i s t i c e x t e r n a l fo rm . S t r u c t u r a l t e r m f o r

a n o r g a n i c s o il w i t h n o r e c o g n i z a b l e p l a n t r e m a i ns .

A n d a l u si te . B r ow n , ye ll ow , g r e e n , r e d o r g r e y s i li c a te m in e r al w h ic h o c c u r s i n

t h i c k , n e a r ly s q u a r e p r is m s i n s c h i s t s . g n e i s s e s a n d h o r n f e l se s .

A n d e s i t e . D a r k - c o l o u re d , v e r y f i n e - g r a i n e d , i n t e r m e d i a t e i g n e o u s r o c k . O f te n

o c c u r s i n t h e fo r m of l a v a f lo w s . C om monly c o n t a i n s m e g a c r y s t s of

p l a g i o c l a s e f e l d s p a r a n d m a f i c m i n e r a l s .

A n g u la r . S h a p e t e r m f o r a r oc k p a r t ic l e wi th s h a r p e d g e s a n d c o r n e r s .

A n h y d r i t e . W h ite o r l i g h t - c o l o u r e d m i n e r a l c o n s i s t i n g of c a lc i um s u l p h a t e

( C a S 0 4 ) . E s s e n t ia l l y a s l i g h t l y h a r d e r a n d l e s s so l u b l e f o rm of g y p s u m .

A n ti cl in e . F old i n t h e f or m of a n a r c h w h o s e c o r e c o n t a i n s t h e s t r a t i -

g r a p h i c a l l y o l d e r r o c k s .

A p h a n it ic . T e x t u r a l t e r m f o r a r o c k in w hi ch t h e i n d i v i d u a l c o n s t i t u e n t s a r e

n o t v i s ib l e t o t h e n a k e d e y e .

A p li te . L i g h t - c o lo u r e d , e q u i g r a n u l a r , f i n e - g r a i n e d i g n e o u s ro c k of g r a n i t i c

c o m p o s it io n . V e r y u n i f o rm a n d s m o o t h - t e x t u r e d a p p e a r a n c e . C om m on ly

o c c u r s i n t h e fo rm of n a r r o w d y k e s .

A r e n a c e o u s . T e rm f o r a s e d i m e n t a r y r o c k c o m p o s e d w h ol ly o r p r e d o m i n a n tl y

o f s a n d - s i z e d g r a i n s .

A r e n i t e . A r e n a c e o u s s e d i m e n t a r y r o c k c o n t a i n i n g l e s s t h a n

15

s i l t a n d c l a y

m a t e r i a l. ( C o n t r a s t w i t h ' w a ck e ') .

A r g i l la c e o u s . T e rm f o r a s e d i m e n t a r y r o c k c om p o s e d w h o ll y o r p r e d o m i n a n t l y

of s i l t - a n d / o r c l a y - s i z e p a r t i c l e s .

A s h. P y r o c l a s t i c r o c k m a t e ri a l of s a n d - , s i l t - a n d c l a y - s i z e ( i.e . 2 m m ),

s u b d i v i d e d i n t o c o a r se a s h f o r s a n d - s i z e a n d f in e a s h f o r s i l t - a n d c la y -

s i z e . D e s c r i p t i v e t e r m f o r t u f f c o m p o s e d w h ol ly o r p r e d o m i n a n tl y of

t h e s e g r a i n s i z e s .

A u re ol e. Z one s u r r o u n d i n g a n i g n e o u s i n t r u s i o n i n w h ic h t h e c o u n t r y r o c k

s h o w s t h e e f f e c t s of t h e r m a l o r c o n t a c t m e t am o rp h is m .

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Axial plane. Plane t h a t con nec ts th e points of maximum cu rv at ur e of th e

bedding planes o r othe r s t ruc tu ra l rock surfaces in a fold.

Banded. Str uct ura l term for a rock with alternati ng layers of material of

differ ing colour o r text ur e, possibly of differ ing mineral composition

also.

Basal. Pertai ning to. situ ate d at, o r forming t he base of a geological

str uct ure . 'Basal layer ' ref ers to th e lowest layer in a layered rock or

soil.

Basalt. Dark colou red, v e r y fine-grai ned igneo us rock composed mainly of

plagioclase fel ds pa r an d mafic minerals. Often occu rs in th e form of

lava flows.

The very fine-grained equivalent of gabbro.

Basic. Chemical ter m fo r an ign eou s rock con tai nin g t o 54% silica an d

usually more th an 30% da rk minerals. (Co ntr ast with 'acid' and 'in ter -

mediate').

Bedded. Str uct ura l term for a sedimentary rock or superficial deposit formed,

ar ra ng ed o r deposited in lay ers o r beds 20 mm thick.

Biotite. Black, da rk brown or dark g re en mineral of th e mica gro up. Forms

distinctive sh iny thin prisms o r flakes. Very common in crystalli ne

igneous and metamorphic rocks.

Block. Rock fr agme nt der iv ed from th e si de s of a volcanic ven t. Commonly

an gu la r or su ba ng ul ar . Restricted t o pyr ocl ast s 60 mm diameter. Also

a more gene ral term fo r indiv idual pieces of rock bound ed by

discontinuities in rock mass.

Blocky. Shape term f or a rock mass with th re e approximately orthogonal and

equally-spaced joint s ets , such th at individual rock blocks tend t o be

roughly equidimensional.

Bomb. Partl y molten material from a volcanic v en t which solidifies in fligh t or

sho rtl y af te r landing. Restricted to pyroc lasts 60 mm diameter.

Boulders. Rock frag me nt s gr ea ter tha n 200 mm in size.

Breccia. Coarse-grained rock composed of an gu la r broke n rock fra gme nts held

toget her by a mineral cement o r in a f ine-grained matrix. (Contrast

with 'conglomerate'). May be of sediment ary o r pyroc lasti c origi n, o r

may be formed by crushing of any type of rock in a fault zone.

Cataclastic. Term for th e s tr u ct ur e of a rock which has been brok en up

severe ly b y st ro ng dynamic metamorphism o r faulting.

Common features

ar e bent, broken o r grou nd- up minerals. 'Cataclasite ' is th e name for

any rock showing cataclastic structure.

Calcareous. Term applied t o a rock conta inin g an appreciabl e amount of

calcium carbonate, e.g. calcareous sandstone.

Calcite.

White, light grey. yellow or blue, common carbonate mineral

t h e

ca rbon ate of calcium (CaCO,). Glassy appe ar ance . Eff ervesc es in

hydrochloric acid. The principal con sti tue nt of chalk and most

limestones.

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Carbonate. Term applied to a mineral compound cha rac ter ise d by a n ionic

s t r u c t u r e of C0,-2. Calcite an d dolomite a r e examples of ca rb on at e

minerals. Also applied to a rock consis ting chiefly of carb on ate

minerals. Limestone and dolomite ar e examples of car bon ate roc ks . (Se e

also calcareous ).

Cemented. Term fo r a sedimentary rock whose grai ns a r e bound tog ethe r in a

co her en t mass by mineral cements. Most cements a r e chemically pre -

cipitated. The most common cements ar e iro n oxides. silica (q ua rt z.

opal, chalcedony), carbonates (calcite, dolomite) and clay minerals.

Chalcedony. Silica mineral which is t h e cryp tocr yst allin e va rie ty of qu art z.

Has a wide ra ng e of colours. Several vari eties u sed as semi-precious

s tones (e.g. jasp er, carnel ian. agate. onyx).

Chert. Hard, dens e, dul l to sl ight ly shiny , cryptocrys tal l ine sedimentary rock

consistin g of or gan ic o r inor gani c prec ipita tes of sil ica. Occurs

commonly as

small ir re gu la r lumps in limestones an d dolomites, b u t may

also form extensive bedd ed deposi ts.

Chlorite. Group of platy micaceous minerals, usually gr ee n in colo ur an d

containing much fe rr ou s iron. Often associated with an d resembling

biot i te; crystals cleave into small thin f lakes.

gra de metamorphic rocks, o r found as

ferromagnesian minerals in any rock type.

Widely di st ri bu te d in low-

al terat ion products of

Chroma. Brilliance o r in te ns it y of a colour.

Clastic. Term fo r a rock composed of br ok en fragm ents th a t are deri ved from

pre-exist ing rocks or minerals and th a t have been t rans por ted f rom thei r

places of origin.

Clay. Soil pa rt ic le s smal ler tha n 0.002 mm in size.

Claystone. Sedi ment ary rock composed predominantly of clay -siz e particle s.

Text ure and composition similar to shale, bu t lacks fin e lamination o r

fissili ty. (See also mudstone ).

Cleavage.

Prop erty o r tende ncy of a rock to spl i t easily along al igned, usual ly

closely-spaced fr ac tu re s produced by metamorphism o r deformation.

Cleavage planes a re seconda ry fe atu res an d may differ in spacing and

orientat ion from primary rock st ru ct ur es such as bedding. Also used to

describe the breaking of a mineral along i ts crystal planes.

Close fold.

Fold w ith a n int er-li mb ang le b etwe en 30•‹ and 70•‹.

Cobbles. Rock fr ag me nt s 60 t o 200 in size.

Cohesion. Pro per ty of a soil which exis ts by vir tu e of na tur al attra ctio n

between some fine soil particles , a nd which enabl es th e soil to form a

cohe rent mass, and t o remain as suc h without th e applicat ion of external

forces.

Cohesive. Term fo r a soil which pos se ss es cohesion. (C on tr as t with

granula r ) .


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Colluvium. Deposits formed b y th e downslope movement of ea r t h materia ls

essentia lly un de r th e action of gravi ty. Typical colluvial deposits in

Hong Kong are structureless, mixed accumulations of soil and rock

fragments or iginally deposited on an d a t th e base of na tura l s lopes.

Columnar. Shape term f or a rock mass with typically five to seve n joint se ts

of similar dip th a t combine to form parallel columns of rock .

(See also

columnar joint ing ).

Columnar jointing . Parallel prismati c rock columns, polygonal (oft en six-si ded)

in cros s-se cti on, caus ed by contract ion du ri ng cooling. Common in basic

lavas but a lso found in o the r igneous and pyroclas tic rocks .

Conglomerate. Coarse-gra ined sedi men tary rock composed of ro un de d t o

subangu la r f r agmen ts la rger than 2

mm

ave rage dimension se t in a sand

or f iner-grai ned matr ix which is of ten cemented. (Con tras t with

sedimentary b reccia ) .

Consistency. Degree of re sis ta nce of a fine -gra ined soil t o flow o r t o

deformation in general.

Cooling joint. Joint formed by th e cooling of an igne ous, pyroclast ic o r ot he r

heated rock body.

Country rock. Rock int rud ed by and surro und ing a n igneous intrusion .

Cross bedding. St ruc tu re formed by a ser i es of bedding planes inclined a t an

angle to t he main planes of stra tificat ion in a sedimen tary deposit.

Planes ar e usually curved a nd tru nc at ed in cross-section by overlapping

se ts .

Cryptocrystall ine. Textural term fo r a rock consisting of crys tal s th at ar e too

small to be recognized and distinguished separate ly un de r an ordin ary

microscope.

Crystal. Homogeneous solid chemical element or compound having a re gu la r

atomic st ru ct ur e expres sed by symmetr ically-arranged external plane

faces. Term for a pyroc las tic rock composed predomina ntly of pyrocla sts

in t h e form of crys ta ls or crys t a l f ragments .

Crystall inity . Degree to which cry st al s

are

developed in a roc k, especially in

igneous rocks.

Dacite. Medium-coloured. ve ry fine -gra ine d, acid igneous rock. The ve ry

fine -gra ined equi vale nt of granodiori te. Often contains m ega crys ts of

quar tz and fe ldspar .

Dappled. e r m for non-uniform colour distrib utio n of a rock or soi l where the

secondary colour consti tu ent forms irregular ly-shape d blotches or marks

of widely diffe ring size.

Decomposition grad e. Class assigne d to rock material on th e basi s of it s degr ee

of decomposition (chemical weat he rin g), in te rm s of physi cal

character is t ics such as s t re ngt h , disco lourat ion . s lakeabil i ty , p resence o r

abse nce of orig inal ro ck t ext ure , a nd decomposition state of individual

minerals.


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Detrital. Term for a rock o r sediment formed of fragmental material which i s

der ive d from older roc ks a nd moved from its place of origin by

weathering a nd erosion.

Dolerite. Dark-coloured, medium-grained, basic igne ous rock with th e same

composi tion as basal t a nd gab bro, bu t with a tex tur e of intergrow n

plagioclase an d pyroxene.

Dolomite. Generally white, sometimes slig htly yellow. br ow n, pink o r gr ey

car bo nat e mineral th e car bo nat e of calcium an d magnesium

(CaMg(CO,),). Forms cu rv ed , saddle-like cry st als . Also, t h e term fo r a

car bon ate sedimentary rock of which more than 50% cons ists of th e

mineral dolomite. (C on tr as t with 'calcite' an d 'limestone').

Dyke. Shee t - or table- like body of in t rus ive igneous rock which cut s across

th e bedd ing o r o the r s t ruc t u ra l p l anes of t he coun t ry rock .

Elongate. Sha pe term for a rock particle in which t h e lo nge st diameter is much

grea ter than the in termedia te or shor tes t d iameter . Expressed

quanti ta t iv ely as ' f la tness rat io ' > 0.66 an d 'elongation rat io' 0.66.

Elongation ratio. Ratio of t h e intermediate to longest diameters of a particle.

Eluviation. Downward movement of solu ble o r su sp en de d material in a soil o r

superficial deposi t by gro undw ater percolat ion.

Equidimensional. Shape ter m for a rock part ic le in which t h e th re e diameters

a r e of approximately equal leng th. Expressed quanti tat ively a s ' f la tness

rat io ' > 0.66 an d 'elongation ratio ' > 0.66.

Equigranular .

Textura l te rm for a rock charac ter i s ed by cr ysta ls o r gra in s of

th e same size o r approximately t h e same size.

Eutaxit ic . St ruc tur al term fo r some pyroclast ic rock s char acte rise d by a

s t re aked or banded appearance , due to pumice c las ts or o ther mater ia l

be ing s t re tche d ou t whi ls t s ti l l in a hot v iscous s ta t e , and subseq uent ly

preserved by welding.

Evaporite. Sedimentar y rock cons isti ng of minerals res ul tin g from th e

evaporation of saline wate r.

Exfoliation. Process by which thi n, curv ilin ear scales or shell s of rock are

successi vely spal led or st r i pp ed away from t h e ba re su rf ace of a rock

mass o r boulder un de r th e act ion of mechanical and /o r chemical

weathering and release of confining pr es su re by erosion. Often res ul ts

in a roun de d rock mass. Commonly seen i n gr ani te corest ones .

Ext rusive . Term fo r an igneous rock th a t has been erup ted onto th e ear t h 's

su rf ac e (e.9. ro cks formed from lava flows). Also applies to all

pyroc las t ic rocks . (Contras t with ' in t rusive' ) .

Fan. Gently-sloping mass of detr i ta l material deposi ted a t locat ions of a br up t

decr ease in slope gradient . Forms a part-cone sh ap e in cro ss - section

and i s fan-shap ed in p lan . f alluvial o r colluvial origin.

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Feldspar. Group of abu nd an t alumino-silicate rock-form ing minerals of gene ra l

composition MAl(A1,Si),Os whe re

is commonly potassium, sodium or

calcium. Crystals ar e usually white o r nearly white (b ut fre quen tly

coloured by impurities) , tra nsl uce nt, and p osse ss good cleavage in two

directions inte rs ec tin g a t o r nea r 90 . They occu r commonly in many

rock typ es and decompose readily t o clay.

Feldsparphyric. Textural term for a rock containing la rge megac ryst s of

feldspar. e.g. feld spar phyr ic rhyoli te.

Feldspathic.

General term for any rock o r othe r mineral aggre gate containing

feldspar .

Felsic. General term for light-coloured minerals (e.g. qua rtz , feld spar s.

muscovite), o r an igneous rock composed chiefly of th es e minerals.

(Contrast with 'mafic').

Ferromagnesian. Term fo r any rock-forming minerals containing iron o r

magnesium.

Fibrous. Structural term for organic soils like peat which contain recognizable

fi br es , i.e. pl ant remains composed gene ral ly of elongated stems and

roots. (Con tra st with 'amorphous').

Fissility. Pr ope rty possess ed by some rock s, such a s shale, of splitting easily

int o thin lay er s along closely-spaced, approximately planar , parallel

surfaces . It s presence dist inguishes shale from mudstone.

Fissure.

Open cra ck o r fra ct ur e in a roc k o r soil mass.

Also used to descr ibe

a volcanic ve nt in t he form of a crac k.

Flat.

Shape term for a rock part icle in which t he sho rte st diameter

is

much

smaller tha n the intermediate o r longest diameter. Expressed quanti-

tati vely a s 'fla tnes s ratio' 0.66 an d 'elongation ratio' > 0.66.

Flat and elongate. Shap e term for a rock particle in which t he longest.

intermediate and sho rt est diameters a r e al l of significantly differ ent size.

Expressed quanti tat ively a s ' f latness rat io ' 0.66 and 'elongation ratio'

0.66. (C on tr as t with 'equidimensional') .

Flatness ratio. Ratio of th e sho rt es t to intermediate diameters of a particle.

Flint. Dark gre y o r black vari ety of ch er t.

Flow-banded. Stru ctur al term for

a

rock formed by alternating layers of

diff ere nt colour, composition an d/ or te xtu re a s a re su lt of t he flow of

molten rock . Most common in ign eou s ro ck s, bu t sometimes visib le in

pyroclastic flow deposits.

Foliated.

S t ruc tu ra l

term

for th e layered, planar arra ngement of t he consti-

tu en t gra ins of a rock formed by flattenin g of minerals du e to

metamorphism.

Friable. Term for a soil t ha t crumbles ve ry easily in th e hand.

Gabbro. Dark-coloured, fine- to coarse-gr ained, basic intr usiv e igneous rock

composed principally of plagioclase f el ds pa r and mafic minerals.

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Gentle fold. Fold with an inter- limb an gl e betw een 120•‹ a nd 180•‹.

Glassy. Shape term for a rock part ic le with a sur fac e text ure t ha t looks and

feels l ike g lass o r quar tz . Surface i s typical ly shiny, s t ra i gh t o r

smoothly curved and lacks d is t inc t c rysta l shapes .

Gneiss. Coar se-g raine d foliated rock formed by regional metamorphism, in

which band s of gr anu lar minerals a l ternat e with ba nds of f la t ten ed.

elongated minerals showing p ref er red orientat ion paral le l t o t h e banding.

Gouge. Fine-grain ed (sil t- an d clay-size) material formed of rock gr oun d down

by se ve re ear th movement. Commonly fou nd in fault zones an d known

as faul t gouge . Also known a s rock flour . Does not beh ave as a

si l t /c lay if t h e original rocks ar e not argi llaceous.

Graded bedding.

St ru ct ur e evid ent in a bedded sedimentary deposi t in which

each layer shows a gradual and p rogr essi ve change in part ic le size.

usually from relatively coarse a t th e base of t he bed to relat ively f ine

a t th e top (e .g . f ine san d grading t o c lay , cobbles grading to coarse

sa n d ) .

Grading.

Par t ic le s ize d is tr ibut ion , defined as th e pe rcentages of th e var ious

grain sizes present in a soi l as determined by sieving and sedimentat ion

(BSI. 1975).

Granite. Light coloured, fine- t o coar se-g rain ed, acid igneous rock composed

principally of alkali feldspar, quartz and biotite, with some plagioclase

feld spar . Commonly forms both major in tr us iv e bodies an d minor in tr u-

s ions such a s dykes .

Granodiori te . Medium-coloured, f ine- t o coarse-gra ined, acid igneous rock

composed principally of plagioclase fel ds par , qua rt z an d abu nd an t biotite.

with some alkal i fe ldspar and hornblende.

Typically contains more mafic

minerals than grani te .

Granular.

Engineering term for a cohesionless soil , i .e. one which cannot form

a coh ere nt mass. (Co ntra st with cohesive ). Geological term fo r t h e

te xt ur e of a rock t h at cons ists of mineral grai ns of approximately equa l

size.

Graphite. Grey to black, opaque, sh iny , six-sided mineral. natu rall y-

occu rrin g crystal l ine form of carbon. Common as cry st als o r thin f lakes

in veins and in many metamorphic rocks.

Gravel. Soil par tic les

2 t o

60

mm in s iz e.

Groundmass.

Relat ively f ine- grained glassy o r crysta l l ine material between th e

megacryst s in a megacryst ic igneous rock. Also known as th e rock

matrix .

Gypsum. White o r colourless sof t mineral composed of h yd ro us calcium

su lp ha te (CaS0,.2HZO). The commonest su lp ha te mineral. Often forms

extensive beds of evapori te int erst rat i f ied with limestone, shale and clay.

Halite. Ev ap or it e mine ral composed of sodium ch lo ride (NaC1). Also known a s

rock s alt or common s alt .

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Halloysite.

Clay mineral made up of v e r y small hollow tu be s, a s shown by t h e

electron microscope.

Holocene. Most re ce nt p a r t of geological time, from t h e en d of th e

Pleistocene

(approximately

8

000 to 10 000 y ear s ago) t o th e pres ent .

Honeycombed. Shape term fo r a rock part ic le with a ve ry un even su rfa ce

text ure cha rac ter i sed by vis ib le pores or cavi t ies .

Commonly caused by

preferent ia l weathering and erosion of different minerals.

Hornblende. Black, dar k gr ee n or gree nish black, ferromagne sian sil icate

mineral . Crystals may be gran ular , f ibr ous or columnar. Primary

constituent of many acid and intermediate igneous rocks, and a common

metamorphic mineral in gneiss and schist .

Hornfels. Glassy, general ly ve ry f ine-grained, rock produced by contact

metamorphism. Shows no cleav age, sc his tos ity or alignment of minerals.

Hue. Basic colour or a mixture of ba sic colours.

Hydrat ion. Chemical react ion tha t re su l ts in t h e t r an sf er of water from th e

fluid phase into th e st ru ct ur e of a mineral .

Hydrothermal activity.

Circulation of hot fluids a nd g as es, usually associ ated

with movement of magma. Fluids ofte n cont ain var iou s minerals in

solut ion which precipi ta te in rock joints and f issures.

Igneous. General term for any rock formed by the solidification of magma.

Inclined fold. Fold whose axial plane is inclined from t h e vert ical. One fold

l imb is commonly st eep er th an t h e other , b u t th e st eep er limb is not

overfolded.

Inequigr anular . Textural term for a rock characte rise d by a mixture of

cr ys tal s o r grain s of signif icant ly different sizes.

Insi tu. Originally two Latin words (in si tu ) meaning 'in place' or ' in i ts

original position'. Compressed he re to a sing le word fo r conv enie nt

English usage. Dist inguishes rock s and soi ls found in th eir original

posi tion of formation, as opposed to t ran sp or ted materials.

Inte rbed ded. St ru ctu ral term for beds in a sedimentary deposi t with mean

spac ing > 20 mm lyin g betw een, o r alte rna tin g with, ot he r bed s of

di f fe rent charac ter (e .g . sand wi th in te rbedded c lay) .

Interlaminated. St ruc tur al term similar to ' interbed ded' , except applied t o ve ry

thin beds with mean spacing

20

mm

Intermediate . Chemical term for an igne ous rock containing

54

to 62 silica

an d usually less tha n 50 dar k minerals. (Contras t with 'acid ' and

'basic ') .

In te rs t ra t i f ied .

General st ru ct ur al term for sedimentary deposi ts consist ing of

al ternat ing layers of different char acter . Covers both ' interbedded' a nd

'interlaminated'.

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Intru sive. Term for an igneous rock t ha t has been forced into pre-exist ing

rocks and solidified from magma un de rg ro un d. (Co ntr ast with

ext rusive ) .

Isoclinal fold. Fold whose limbs ar e parallel (i.e. t h e inter- limb a ng le is O D .

Isotropic. Term for rock and soil with th e same physical prop ert ies in all

directions.

Ju ra ss ic. Period of geological time betw een approx imately

190

and

135

million

years ago.

Kaolin.

Group of clay minerals derived mainly by alteration of alkali feldspars

and micas. Appearance is th at of a sof t , white o r nearly white

nonplastic clay. Commonly oc cu rs a s a th in coating o r filling in joints

in igneous roc ks , bu t extreme altera tion may co nv er t whole rock mass to

kaolin.

Kaolinized. Alteration term fo r a rock contai ning minerals, especially feld spa rs

and micas, replaced by, o r a l tere d to, kaolin a s a res ul t of hyd rother mal

activity.

Karst topography. Topography charac terise d by sinkholes, caves, solut ion

fea tur es an d la rg e und erg rou nd dra ina ge syst ems . Common in limestones,

ra re in o ther rocks .

Lahar. Mudflow in volcanic material. Caused by water sa tu ra ti on (e.g. by

intense rainfal l ) of unl ithif ied lava o r pyroclast ic deposi ts on th e f lanks

of a volcano.

Laminated. Str uct ura l term fo r a sedimentary rock or superficial deposi t

formed, a r r ang ed or deposited in ve ry th in l ayers

2 mm

thick.

Lamprophyre. Dark-coloured, ve ry f ine- to coa rse -gr ain ed, basic rock char ac-

ter ise d by hig h per cen ta ge s of rnafic minera ls which often form

megacry sts in a finer matrix of similar minera ls plus alt ere d f eld spa rs.

Lapilli. Pyrocl astic rock material of gr av el size (i.e. 2 t o 60

m m .

Descriptive

term for tuff composed wholly or predominantly of this grain size.

Laterite. Residual soil , usually red dis h in co lour, rich in se co nd ar y oxides of

iron and/or aluminium. A produ ct of in tensiv e ins i tu rock weather ing

th ro ug h leaching of more solub le element s. Common in tro pica l area s

with strong seasonal rainfall .

Lava.

General term fo r molten ex tru siv e magma e ru pt in g non-explosively from

a volcanic ven t or f is sure. Also, th e ter m fo r th e rock sol idified from

this magma.

Leaching.

Separat ion and removal of the soluble const i tuents in a rock by the

natura l action of percolating gro un dw at er .

Lenticular bedding.

Beds in a sedimentary rock o r superficial deposi t formed

by discontinuous lens- shaped bodies of on e material su rr ou nd ed by

anothe r ty pe of material , e .g. sa nd len ses in a c lay deposi t . Lenses are

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Leucocrat ic . Light-coloured as applied to igneous rocks. Most f ine- to

coar se-g rain ed acid rock s ar e leucrocratic . (Contras t with mesocratic

and melanocratic ).

Limb. One flank o r si de of a fold.

A

simple fold has two limbs

Limestone. Se dim ent ary rock composed wholly o r pred omin antly of calcium

car bon ate , mainly in th e form of t h e mineral calcite.

Limonite. Usually d ark brown or yellowish brown (may be yellow, re d or

nearly black), amorphous hyd rate d i ron oxide material ( fe rr i c oxide). A

ve ry common weath erin g (oxidation) pr od uc t of all iro n-b ear ing minerals.

Lineation. General term for any rock str uc tu re arra nge d in l ines. Also, th e

term for th e appearance of st ret ch ed- ou t , f la t tened minerals in

metamorphic rocks.

Liquid limit L L ) .

Moisture conte nt a t which a soil passes from t h e plastic to

th e liquid s tat e , as determined by t he l iquid l imit te s t B S I . 1975).

Lithic. Relating t o o r made of existing rock frag men ts. Term fo r a tuff

composed predominantly of fr agm ent s of previousl y-formed rock s.

Lithified. Term fo r a rock which has been co nve rte d into a coh ere nt solid

mass from a newly-deposited loose sediment by s uc h proc esse s as

cementation, compaction, and crys talli zati on. Lithification may occ ur

con cu rre nt with, soon aft er or long aft er deposit ion.

Lobate.

Term for a long, roun ded, tongue-l ike shape.

Often applicable to t h e

sh ap e of colluvial deposi ts.

Macrostr ucture. St ruc tur al featu res of a soi l mass which can be ident if ied by

th e naked eye . (Contras t with microst ru c ture ) .

Mafic.

General term f or d ark-colour ed, ferroma gnesian minerals, o r an igneous

rock composed chiefly of t he se minerals.

(Co ntra st with felsic ) .

Magma. Molten rock material formed within th e e ar th . Solidifies a t o r nea r

th e ea r th s c r us t t o p roduce ex t rus ive and in t rus ive igneous rocks .

Ext rus ive magma becomes lava .

Marble.

General ly l igh t coloured (often stai ned by impuri t ies) , f ine- t o coarse -

grai ned cry stal line metamorphic rock c onsi stin g mainly of recr ysta llize d

calcite an d / o r dolomite. Metamorphosed limestone.

Massive. St ru ct ur al term fo r an igne ous or metamorphic rock with homo-

geneous tex tur e over l arge areas , i .e . with no layering, fol ia t ion o r other

planar st ru ct ur es . May also be applied to sedimentary rocks with no

evidence of stratification (i .e. no beddin g or lamination).

Matrix.

Finer-grained material enclosing, or f i l l ing the spaces between, the

lar ge r g rain s or pa rt ic les in a mixed sedimentary rock o r superficial

deposi t . Synonymous with groundma ss in an igneous rock.

Megacryst . Any cry st al o r grain in an igneous o r metamorphic rock th at is

s ignif icant ly la r ger tha n t he s urro undi ng groundmass o r mat rix. A

general , non-genet ic term.

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M e g a c r y s t i c . G e n e ra l t e x t u r a l t e r m f o r a r o c k c o n t a i n in g m e g a c r y s t s . ( S e e

a l s o p o r p h y r i t i c a n d x e n o c r y s t ic ) .

M e l a n oc r at ic . D a r k - c o l o u r e d , a s a p p l i e d t o i g n e o u s r o c k s .

All b a s i c r o c k s a r e

m e l a n o c r a ti c . ( C o n t r a s t w i t h l e u c o c ra t i c a n d m e s o c r a t i c ) .

M e s o c r a t i c . M e d i u m - c o l o u r ed ( i . e. c o m p o s e d of r o u g h l y e q u a l a m o u n t s of l i g h t

a n d d a r k c o n s t i t u e n t s ) , a s a p p l i e d t o i g n e o u s r o c k s . M o st i n t e r m e d i a t e

r o c k s a r e m e s o c r a t i c . ( C o n t r a s t w i t h l e u c o c ra t i c a n d m e l a n o c r a ti c ) .

M e t a -. P r e f ix u s e d w i th a n i g n e o u s , p y r o c l a s t i c o r s e d i m e n t a r y ro c k n a m e t o

i n d i c a t e t h a t t h e r o c k h a s b e e n p a r ti a ll y m e t a m o r p h o s e d , e . g . m e t a t u f f .

M e t a m o rp h i c. G e n e r a l t e r m f o r a n y r o c k f o r m e d b y m i n e ra l o gi c al , ch e m i ca l ,

a n d s t r u c t u r a l a d j u s t m e n t of p r e - e x i s t i n g r o c k s d u e t o c h a n g e d p h y s i c a l

a n d c he m ic a l c o n d i t io n s ( e x c l u d in g n e a r - s u r f a c e w e a t h e r i n g a n d

c e m e n t a t io n e f f e c t s ) .

M i c ro c ry s t a l l i n e . T ex tu ra l t e r m f o r a c ry s t a l l i n e r o c k w i t h c r y s t a l s t h a t a r e

t o o sm al l t o b e s e e n b y t h e n a k e d e y e , b u t w h i ch c a n b e d i s t in g u i s h e d

s e p a r a t e l y u n d e r a n o r d i n a r y m i c r os c op e .

M i c r o f r a c t u r e s . G e n e r a l t e r m f o r al l s m a l l- s ca l e d i s c o n t i n u i t i e s i n t h e r o c k

f a b r i c . I n c l u d e s c r a c k s , f i s s u r e s a n d p la n e s of s e p a r a t i o n t h r o u g h o r

b e t w e e n i n d i v i d u a l g r a i n s .

M i c r o s t r u c t u r e . S t r u c t u r a l f e a t u r e of a s oi l m a s s w hi ch c a n n o t b e i d e n t i f i e d

c o m pl e t e l y b y t h e n a k e d e y e ; t h e u s e of a m i cr o s co p e i s r e q u i r e d f o r f u ll

a s s e s s m e n t . ( C o n t r a s t w i t h m a c r o s t r u c t u r e ) .

M i n er al . N a tu r al ly o c c u r r i n g i n o r g a n i c e l e m e n t o r co m p o u n d w it h a n o r d e r l y

i n t e r n a l s t r u c t u r e , a n d c h a r a c t e r i s t i c c h em i ca l c o m po s it io n a n d p h y s i c a l

p r o p e r t i e s .

M i n e r a l b o xw o rk . W e a t h er i ng f e a t u r e r e s u l t i n g f r om h a r d m in e ra l d e p o s i t s

f o r m e d i n r o c k jo i n t s s t a n d i n g o u t p r o m in e n tl y o n a w e a t h e r e d s u r f a c e .

M i n e r a l is e d . A l t e r a t i o n t e r m f o r a r o c k c o n t a i n i n g n e w m i n e r a l s fo r m e d e i t h e r

b y c o n v e r s i o n of e x i s t i n g m i n e r a l s , o r b y f i ll in g of d i s c o n t i n u i t i e s w i t h

n e w s u b s t a n c e s .

M o t tl ed . T e r m f o r n o n - u n i f o r m c o l o u r d i s t r i b u t i o n of a r o c k o r s o il w h e r e t h e

s e c o n d a r y c o l o u r c o n s t i t u e n t f o rm s b l o t c h e s o r m a r k s o f a p p ro x i m a te l y

e q u a l s i z e .

M u d s t on e . S e d im e n t a r y r o c k c o m po s ed p r e do m i n a n t ly of s i l t - a n d / o r c l ay -

s i z e p a r t i c l e s . m o r e g e n e r a l t e r m t h a n s i l t s t o n e o r c l a y s to n e .

M u s c o v it e . C o l o u r l e ss , y el lo w o r l i g h t b r o w n m i n er a l of t h e m ic a g r o u p .

F o rm s d i s t i n c t i v e s h i n y t h i n p r i s m s o r f l a k e s . V e r y co mm on i n g n e i s s e s

a n d s c h i s t s , a n d so me a c i d i g n e o u s r o c k s .

M y lo ni te . V e r y f i n e - g r a i n e d c r y s t a l l i n e m e t am o r p h ic r o c k w i th s t r e a k e d o r

b a n d e d t e x t u r e p r o d u c e d b y s h e a r i n g a n d f r a c t u r i n g of o r i g in a l g r a i n s

d u r i n g i n t e n s e d y n a m ic m et a m o rp h i sm .


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Neutral fold. Fold with it s axial plane more o r les s horizontal. Neither an

anticline nor a syncline.

Nodule. A small, ir re gu la r, roun ded lump of a mineral or ro ck, usually con-

tr as ti ng in composition with th e material in which it is embedded e.g.

nodular chert in l imestone.

Normal fault . Dipping fau lt in which th e overlying face or wall app ea rs to

have moved downward relat ive to th e underlyin g face.

The angle of the

faul t is usual ly 5 O to 90•‹.

Opal. Amorphous silica mineral. Softer, less den se , less tr an sp ar en t and lacks

crystal l ine st ru ct ur e compared with qua rtz . Occurs in nearly al l colours.

Transparent coloured variet ies used as gemstones:

Open fold. Fold with an inter-l imb an gl e between 70•‹ and 120•‹

Overfolded. Term fo r a fold, o r th e limb of a fold, t h at has t i l ted beyo nd t h e

perpendicular .

Oxidation. Chemical weat heri ng proces s involving t h e reaction between r ock s

an d atmospher ic oxygen, t h e oxygen usually being dissolved in water.

The main p rodu cts a re oxides and hydroxides. Iron is the mineral most

obviously affected; i ron oxidat ion prod ucts a r e characterist ically brown.

re d and yellow in colour. (Con tras t with 'reduc tion ').

Par asi tic fold. Small fold on th e limb of a lar ge r fold.

Pegmati te . Light coloured, ve ry coars e-grained igneous rock , general ly of

gran itic composition. Commonly occ urs as irreg ula r dyk es or veins.

especia lly a round th e edge s of la rge in t rusio ns .

Phyllite. Fine-graine d metamorphic rock with well-developed slightly

undulat ing cleavage. Commonly gr een , gre y o r red dis h, brown in colour.

Chlorite and serici te cry st als often form a dist inct ive s hiny , smooth

su rfa ce on cleavage faces.

Pi t ted. Shape term for a rock part ic le with an uneven sur fac e tex tur e

cha rac ter is ed by numerous small depr ess ions . Commonly caus ed by

pref eren tial weathering and erosion of di ffer ent minerals.

Plagioclase fel ds par . Group of sodium-calcium fel ds pa rs of ge ner al composition

(Na,Ca)Al(Si,Al)Si,O.. (S ee 'f el ds pa r' ).

Plasticity. Pr op ert y which enabl es a soil or ot he r material to be deformed

continuously and permanent ly wi thout ru pt ur e .

Plastic limit (PL). Moisture con ten t a t which a soil becomes too dr y to b e in

a plastic condition, as determined by the plastic l imit test (BSI, 1975).

Plei stoc ene. Geological time period between appr oxim ately million an d 8 000

t o 10 000 years ago. i.e. immediately prior to the Holocene.

Plutonic. Pertaining to, o r th e general term fo r, any rock formed a t

considerable dep th below t h e earth 's su rf ace by crystal l izat ion of magma

and/or by chemical alteration.

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Polyhedral. Shape term for a rock mass with no consi sten t joint set s, suc h

th at individual rock blocks usually va ry widely in sh ape and size.

Porphyrit ic. Textural term for an igneous rock containing large cry st als

(p he no cr ys ts ) t h a t ar e compatible in composition and mode of formation

with th e groundma ss o r matrix in which th ey occur. (Cont rast with

xenocrystic ) .

Pseudomorph.

Mineral which oc cur s in th e cr ys ta l form of a not her mineral as

a resu lt of alte rati on, o r solution and replacement, within t he same

crys ta l shape.

Pumice. Light-coloured gl ass y rock formed from acid lava. Contains ab un da nt

voids or ca viti es, which means i t is often sufficiently buoyant to float

on water.

Pyrit e. Light brown or dark yellow iron sulph ide mineral

FeS,).

Often forms

cub e-s hap ed, str iat ed cr ys ta ls with a br ig ht metallic surfac e. Common in

veins and fault-zone rock s. Often mistaken for gold.

Pyroclast. Indivi dual rock fra gment o r parti cle ejec ted explosively from a

volcanic vent.

Classified by size into fin e as h, coarse ash , lapilli. blocks

and bombs.

Pyroc last ic. General ter m for any rock composed of material ejec ted explo-

sively from a volcanic vent.

Pyroxene. Groups of mafic silicate mine rals. Commonly app ea r as dark gr ee n

or black prismatic crystals displaying cleavage in two directions parallel

to the c rys ta l faces and in tersect ing a t approximately 90• ‹

Qua rtz . Colourless (often coloured by impur itie s), glass y, hard mineral

composed of cry st al li ne silica (SiO,). Commonly ap pe ar s ei th er as six-

s ided t ra nsp are nt crys t a ls o r a s a dense crys tal l ine mass lack ing

disti ncti ve s ha pe . Very common in all t yp es of ro ck s and mineral veins .

Qua rtz it e. non-foliated metamorphic roc k consi stin g mainly of qua rtz .

Formed by recrystallizat ion of sa nd st one du e to contact or regional

metamorphism.

Qua rtz lati te .

Medium-coloured, ve ry fin e-g rai ned , intermediate igneous rock.

The ve ry fine- grain ed equiv alen t of q ua rt z monzonite.

Qu ar tz monzonite. Medium-coloured, f ine- to coarse-grained, intermediate

igneous rock containing roughly equal amounts of plagioclase and alkali

feldspar .

Quar tzphyr ic .

Textural term for a rock containing l arg e megacrysts of q uartz .

e .g . quar tzphyr ic rhyol i te .

Qu ar tz syeni te. Medium-coloured, fine- to coarse-g raine d, intermediate igneous

rock. Feld spar component is predominantly alkali feldspar.

Qu ar tz tra chy te . Medium-coloured, ve ry f ine-grained, intermediate igneous

rock. The ve ry f ine-grained equi vale nt of qu artz syenite.


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Qu at er na ry . Geological time period from approximately two million ye ar s ago

up to th e prese nt . Split into two pa rt s the Pleistocene and th e

Holocene.

Recrystallization.

Formation of new cry sta lli ne mineral gr ai ns in a rock du e t o

metamorphism o r proce sses involving percolating ground water . New

cry sta ls may have the same o r a dif fer ent composition from th e original

crysta ls .

Recumbent fold. Ove rtu rne d fold whose axial plane is hori zonta l or near ly

horizontal.

Reduction. Chemical process whereby oxy gen is removed in rock s and th e

leached p ar ts of soils. Related t o t h e continuous pr ese nce of water.

which makes oxygen scarce, e.g. by re duc ing fe rri c iron (Fe,O,) to

fer rou s iron (FeO). Characterist ic colours of reduced soils ar e green s

and gr eys . Often associated with st ro ng bacterial activ ity in th e soil.

(Co ntr ast with oxidation ).

Regular bedd ing. Alternating lay ers of materials of di ffer ent gra in size in a

bedded sedimentary deposi t . Grain size within each lay er is essential ly

uniform.

Residual soil. Soil der ive d from ins itu roc k weath ering in which all tr ac e of

th e or iginal rock texture , fabr ic and s t r uc tu re has been dest royed.

(Con tras t with saproli te ; re pr es en ts a more advanced sta ge of

weather ing than saprol i te ) .

Reverse fault . Dipping fault i n

which t he over lying face or wall app ear s to

have moved upward relat ive to th e under lying face. Fault plane usually

dips a t a low angle.

Rhyodacite. Medium-coloured, ve ry fine- grain ed, acid igneous rock . In te r-

mediate in composition between rhy oli te and dacite. Contains les s alkali

felds par than rhyoli te and less plagioclase feldspa r tha n daci te . Often

contains megac ryst s of q uar tz and feldspar.

Rhyolite. Medium-coloured, ve ry fine- grain ed, acid igneous rock. The ve ry

fine- grain ed equivalen t of grani te. Often contains meg acr ys ts of qu ar tz

and feldspar.

Rind. Discoloured, relatively thi n, often loose an d flaky ou te r layer on th e

su rf ac e of a boulder o r rock block c ause d by weathering .

Rough. Shape term for a rock part icle with a surf ace tex tur e th at feels

uneven, corrugated or lumpy,

i.e. th at lacks smoothness.

Rounded. Shape ter m for a rock particle with markedly roun ded ed ge s and

c o rne r s .

Rudaceous. Term fo r an y sedi menta ry rock composed wholly or predominantly

of grave l and larger-sized grai ns.

Sand. Soil par tic les 0 0 6 to 2 mm in size.

Sands tone. Sedimentary rock composed predominantly of sand -si ze particles.

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S a p r o li te . S oil d e r i v e d fr om i n s i t u r o c k w e a t h e r i n g w h ic h r e t a i n s e v i d e n c e of

t h e o ri g in a l ro c k t e x t u r e , f a b r i c a n d s t r u c t u r e .

( C o n t r a s t w i th ' r e s i d u a l

so i l ' ) .

S c h i s t. M ediu m- t o c o a r s e - g r a i n e d , f ol ia t ed , c r y s t a l l in e m e t a m o rp h ic r o c k .

S p l i t s r e a d i l y i n t o f l a k e s o r s l a b s d u e t o p a r a ll e l a r r a n g e m e n t of m o st of

t h e c o n s t i t u e n t m i n e r a ls . C o a r s e r a n d m o re u n d u l a t i n g fo li at io n

c o m p a r e d w i th ' p h y l li t e' ; f i n e r a n d o f t e n n o t b a n d e d c o m p a r e d w i th

' g n e i s s ' .

S c h i s to c i ty . F o li at io n i n s c h i s t o r o t h e r c o a r s e - g r a i n e d c r y s t a l l i n e m e t am o r p h i c

r o c k s d u e t o t h e p a r a ll e l, p l a n a r a r r a n g e m e n t of p l at y a n d p r i s m a t ic

m i n e r a l g r a i n s ( e . g . m i c a) .

S e d i m e n t a r y . G e n e ra l t e r m f o r a n y ro c k f o rm e d b y t h e d e p o s i ti o n of s e d i m e n t ,

i .e . so l id , f r a g m e n t e d m a t e ri a l t r a n s p o r t e d b y g r a v i t y , w i nd , w a t e r o r i c e ,

o r m a t e r i a l a c c u m u l a t e d b y c h e m ic a l p r e c i p i t a ti o n o r s e c r e t i o n b y

o r g a n i s m s .

S e r i c i t e . W h i t e, f i n e - g r a i n e d m i n e r a l of t h e m i ca g r o u p . S i m i la r c o m p o s i ti o n

t o m u s c o v i te . Common i n f a u l t g o u g e a n d o t h e r r o c k s a s s o c i a t e d w i th

d y n a m i c m e t a m o r p h i s m .

S h a l e .

M u d s t on e w i th a f in e ly - la m i n at e d d e p os i t io n a l s t r u c t u r e t h a t g i v e s t h e

r o c k f i s s i li t y , o r t h e t e n d e n c y t o b r e a k i n t o t h i n l a y e r s p a ra l l e l t o t h e

l a m i n a t i o n p l a n e s .

S h e a r p l a n e . S u r f a c e a l o n g w h i c h d i f f e r e n t i a l m o v e m e n t h a s t a k e n p l ac e

p a r a l l e l t o t h e s u r f a c e .

S h e a r z o n e. B e lt of ro c k of s i g n i f i c a n t t h i c k n e s s t h a t h a s b e e n c r u s h e d a n d

c o n t o r t e d b y s h e a r m o ve m en t.

S h e e t i n g j o in t . J o i n t f o rm e d b y p r e s s u r e r e l e a s e d u e t o r e m o v a l of o v e r ly i n g

r o c k b y w e a t h e r i n g a n d e r o s i o n . A lso ca ll e d a n ' u n l o a d i n g j o in t '.

S il ic a. S il ic on d io x i d e (S iO ,). O c c u r s n a t u r a l l y a s c r y s t a l s ( e . g . q u a r t z ) , i n

c r y p t o c r y s t a l l i n e fo rm ( e . g . c h a l c e d o n y ) a n d i n a m o r p h o u s f o rm ( e . g .

o p a l ) . C o m bin ed in s i l i c a te s a s a n e s s e n t i a l c o n s t i t u e n t of m a n y

m i n e r a l s .

S i li c a te . Co m p o u n d m a t e r i a l c o n s i s t i n g of o n e s i li c o n a n d f o u r o x y g e n a t o m s

a r r a n g e d in t r i a n g u l a r p y r a m i ds , e i t h e r i s o l at ed o r jo in ed t h r o u g h o n e o r

m o re of t h e o x y g e n a to m s t o fo rm c h a i n s , s h e e t s o r

s t r u c t u r e s w i t h m e ta ll ic e l e m e n t s s u c h a s a lu m in iu m .

a r e t h e m o s t com mon r o c k - f o r m i n g c o m p o u n d s

a p p r o x i m a t e l y

9 5 %

of t h e e a r t h ' s c r u s t .

t h r e e - d i m e n s i o n a l

S i l i c a t e m i n e r a l s

a n d m ak e u p

S i li ce o u s. T e rm f o r a r o c k c o n t a i n i n g a b u n d a n t s i li c a .

Sil l .

T a b le - li ke b o d y of i n t r u s i v e i g n e o u s r o c k t h a t c o n f o rm s t o t h e b e d d i n g

o r o t h e r p l a n a r s t r u c t u r e s of t h e c o u n t r y r o c k i n w h ic h i t i s i n t r u d e d .

S il li m an it e. B r o w n , g r e y , l i g h t g r e e n o r w h i t e s il i c a t e m i n e r a l . F o r m s l o n g

n e e d l e - l i k e c r y s t a l s . O fte n f o u n d i n h i g h t e m p e r a t u r e , c o n t a c t -

m e t a m o r p h o s e d s e d i m e n t a r y r o c k s .

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Silt. Soil pa rt ic le s 0 0 0 2 t o 0.06 mm in s ize.

Siltstone. Sedim entar y rock composed predominantly of silt- size parti cles.

(See also mudstone ).

Skar n. Thermally metamorp hosed imp ure limestone chara cteri sed by p res enc e

of sil icate minerals con tai nin g calcium.

Slaking.

Breaking-up or disintegration of a rock o r soil when sat ur at ed with

o r immersed in water.

Slate. Fine- grain ed metamorphic rock with a v e r y well-developed parallel

cleavage. Spli ts into ve ry thin plates or f lakes. Most slates ar e

metamorphosed shales.

Sl ickenside. Smooth str i ated surf ace caused by friction duri ng relat ive

movement of rock along t h e sur fa ce (e. g. along a fault plane).

Striations ar e normally low linear grooves an d ridg es parallel to t h e

direction of movement. Surfa ce often app ea rs shiny or polished.

Slump bedding. Beds in a sedimentary deposi t which have been dist urb ed or

deformed by slumping of t h e newly-deposited sediment un de r wate r,

usually on a sloping surface.

Smooth. Shape ter m fo r a rock particle with

a

sur face t ex ture tha t fee l s

even, with no lumps or corrug ation s, i .e . lacks roughnes s. Results from,

for example, being water-worn or t he clean fr ac tu re of ver y f ine-grained

rock.

Solution. Chemical wea the rin g proces s in which minerals a r e diss olved by

percolating or static groundwater, e.g. removal of calcium carbonate in

limestone o r chalk b y carb onic acid (weakly acid rai nwate r).

Sorted.

Term for a loose sedi ment o r sed ime nta ry rock composed of pa rti cle s

of essentially uniform size. Well-sorted re fe rs to ve ry uniform sor tin g.

(Co ntr ast with poor ly-s orted ). Note sor ted in geological use is t h e

opposite of gra ded in engin eerin g use.

Spott ed. Term for non-unifo rm colour distr ibuti on of a rock or soil wher e th e

secondary colour const i tuent forms small rounded spots.

Strat if ied. General stru ct ur al term for a sedimentary rock or superficial

deposi t formed, arr ang ed or deposi ted in laye rs or beds of any thic kness .

(See also bedde d an d laminated ).

St re ak ed . Term for non-uniform colour dis tri but ion of a rock or soil wh ere

the secondary colour const i tuent forms elongated, discontinuous, some-

times branching, l ines.

Striated. Shape term for a rock part icle with a surf ace textu re chara c-

ter ise d by a ser ies of fine, parallel grooved lines. Caused, for example.

by slickensiding in a fault zone.

Str ike . Direction in which a horizontal l ine can be drawn on a st ru ct ur al rock

surface.

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Stri ke-s lip fault . Fault on which th e movement is parallel to th e st ri ke of t he

fault .

Str iped.

Term for non-uniform colour dis tribu tion of a rock o r soil wh ere t he

secondary colour const i tuent forms elongated, continuous, nonbranching

lines.

St ru ct ur al domain. Portion of a rock mass chara cteri sed by a relatively

uniform arr ang eme nt of discontinuities.

Subangular . Shape term for a rock part ic le with sl ight ly sh ar p (sl ight ly

angu la r ) edges and corne rs .

Subr ound ed. Shape term for a rock part ic le with sl ight ly rounded ed ges an d

corners .

Syncline. Fold in th e sh ap e of a basin whose core contains th e str at i-

graphical ly younger rocks.

Tabular. Shape term fo r a rock mass with a singl e, dominant, flat-lying join t

se t , such th at t he mass co nsists of a serie s of table-l ike she ets of rock.

Tectonic activ ity. Movements of th e out er pa rt of th e ear th s c ru st . Some

associated geological featu res a r e earth qua kes , major faul ts an d folds,

tectonic joints and certa in rock typ es suc h a s mylonite .

Tectonic joint. Joi nt formed by tecto nic activ ity. The orien tatio n of tec ton ic

joints is usually controlled by t h e directio ns of th e principal regional

s t r e s se s .

Te rt ia ry . Geological time perio d between approxi mately 60 an d 2 million years

ago.

Throw. Amount of vert ica l displacem ent on a fau lt.

Th ru st . Low-angle re ve rs e fau lt with a dip of les s th an 45O.

Tigh t fold. Fold with an inter-limb ang le between

O

and 30•‹.

Trachyandesi te . Usual ly dark-coloured, v ery f ine-grained, intermediate igneous

roc k. Commonly conta ins meg acr yst s of alkali fel dsp ar.

Tuff. General rock name for all lithified pyrocla stic ro ck s composed of rock

fragments of grave l or f iner size

<

60 mm). Subd ivide d acc ord ing to

dominant grain sizes into lapil li , coars e-as h and f ine-ash t yp es .

Tuffaceous. Term for a sedimentary rock containing up to 50 tuff material.

Tuffite. Mixed sedimen tary/ pyroc last ic rock containing roughly equal amounts

of sedimentary material and tuff material.

Unloading joint. (See shee ting joint ).

Up rig ht fold. Fold whose axial plane is vertic al o r near- verti cal.

Value. Relative li gh tn es s of a colour. Grey has a neu tra l value, white t h e

high est value and black th e lowest.

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V ein . M i n e ra l f il li n g a f a u l t , j o i n t o r o t h e r f r a c t u r e in a r o c k ; t h e v e i n i s f o rm e d

l a t e r t h a n t h e h o s t r o c k . Com monly h a s a t ab le - o r s h e e t - l i k e f or m .

O f te n a s s o c i a t e d w it h a l t e r a t i o n of t h e h o s t r o c k . M o s t v e i n s a r e of

i g n e o u s o r i g i n .

V e nt . O p e n in g a t t h e e a r t h ' s s u r f a c e t h r o u g h w h i ch v o l c an i c m a t e r ia l s a r e

e x t r u d e d .

V e si cl e. C a v i ty of v a r i a b l e s h a p e i n a l a v a , f o r m e d b y t h e e n t r a p m e n t of a g a s

b u b b l e d u r i n g t h e s ol id if ic a ti o n of t h e l a v a.

V i t ri c . T e rm fo r a p y r o c l a s t i c r o c k c o mp o s e d p re d o mi n a n t l y of v o l c a n i c g l a s s

f r a g m e n t s .

V olc an ic . G e n e ra l t e r m f o r a n y e x t r u s i v e i g n e o u s o r p y r o c l a s t i c r o c k .

W a ck e. A r e n a c e o u s s e d i m e n t a r y r o c k c o n t a i n i n g m o r e t h a n 15 s i l t a n d c l a y .

' d i r t y ' s a n d s t o n e . ( C o n t r a s t w i th ' ar e n i t e ') .

W avy b e d d i n g . B e d s i n a s e d i m e n t a r y d e p o s it w i th m a r k e d l y u n d u l a t i n g

b e d d in g s u r f a c e s , i.e , t h e be d s u r f a c e s a r e n o t s t r a i g h t a s i n r e g u l a r o r

g r a d e d b e d d i n g .

T a b l e of C on

t en t s

T a b l e of C

on t en t s

GeoGuide 3 Rock and Soil Descriptions Hong Kong

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