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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
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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
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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
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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
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P a g e
NO.
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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12
to good ground investigation practice
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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.
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
T a b l e of C on t en t s
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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
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
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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 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
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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 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
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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 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
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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 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
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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 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
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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
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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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 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
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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
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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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
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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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 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
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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
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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) .
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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
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f C on t en t s
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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
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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
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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
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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
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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.
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[BL NK P GE]
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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 ) ,
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(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
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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 ) .
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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
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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
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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
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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
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a b l e of C on t en t s
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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
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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
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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
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T a
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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
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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 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
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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 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
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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 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
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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 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
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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 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
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( 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 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
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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
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
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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 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
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( 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 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
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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 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
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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 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
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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 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
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[BL NKP 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 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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T a
b l e of C on t en t s
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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
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T (1982). S l o ~ e ailures in colluvium overlying weak residual soils in
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T a b l e of C on
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[BL NK P GE]
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T BLES
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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
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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
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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
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
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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
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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
T a b l e of C on t en t s
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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
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
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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
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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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 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
T
a b l e of C on t en t s
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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 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
T a b l e of C on t en t s
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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 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
T
a b l e of C on t en t s
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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 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
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- -
+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
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
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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
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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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
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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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
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
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-
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
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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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
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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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
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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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 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
T
a b l e of C on t en t s
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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 of C on t en t s
T a b l e of C on t en t s
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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
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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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 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
T a b l e of C on t en t s
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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
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
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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
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
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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
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
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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
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
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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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
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
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
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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
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
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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
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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Figure 4 Schematic Illustration of Fracture Logging
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.
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
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
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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 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
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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
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
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Figure 9 Grading Chart for Soils with Grading Curves of Select
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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
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
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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
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
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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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 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
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
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
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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 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
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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 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
T a b l e of C on t en t s
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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
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
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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 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
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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
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
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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
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
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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
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
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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
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
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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
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
T a b l e of C on t en t s
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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
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
T a b l e 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
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
T a b l e of C on t en t s
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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
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
T a b l e of C on t en t s
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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
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
T a b l e of C on t en t s
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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 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
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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 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
T a b l e of C on t en t s
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.
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
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
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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
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
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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
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
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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 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
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
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
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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
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
T a b l e of C on t en t 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
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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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
former ar e th e res ul t of unusually high waves (e.g. due to typhoons ) and th e
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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
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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.
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
T a b l e of C on t en t s
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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 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
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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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 .
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
T a b l e of C on t en t s
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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
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
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
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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Table A1 Classificatio n of Solid Rocks and Superficia l Deposit
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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
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
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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
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 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
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
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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
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
T a b l e of C on t en t s
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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
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
T a b l e of C on t en t s
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6
LIST OF PL TES
Page
Plate
NO
NO
A
Hong Kong
ock T y p e 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 o
f 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
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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
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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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
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
T a b l e of C on t en t s
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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
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
T a b l e of C on t en t s
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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
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
T a b l e of C on t en t s
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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
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
T a b l e of C on t en t s
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GLOSS RY
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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
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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
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
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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 .
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 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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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.
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
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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
usually double convex in cross-section.
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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 .
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