WALLACE, K. B. (1973). Geotechnique 23, No. 2, 203-218.

Structural behaviour of residual soils of the continually wet Highlands of Papua New Guinea

K. B. WALLACE*

The Paper illustrates how high porosity, a strongly cemented structure and highly hydrated kandite clay minerals control the geotechnical properties of resi- dual sandy to silty clay soils of the Papua New Guinea highlands. These soils are characterized by high field moisture contents, relatively high shear strengths and low sensitivity, and are highly com- pressible when the applied pressure in an oedometer test exceeds a certain critical pressure. Measured values of critical pressure and compression index are compared with those reported for a wide variety of soils, and a linear relationship between volumetric strain and porosity is established for porous soils. The natural inhomogeneity of residual soils is

emphasized. Examination of relationships between geotechnical properties and porosity and moisture content shows that porosity or dry density is a more reasonable basis for description and grouping of these soils for engineering purposes. The behaviour of the soils is interpreted in terms of

an idealized model of the soil structure. The soil is considered to consist of a coarse open skeleton of cemented rock mineral and aggregated clay particles surrounded by a viscous gel. At low applied stresses, when the cemented bonds in the soil skeleton are intact, it is the soil skeleton that deter- mines structural behaviour. At high applied stresses the porous soil skeleton still has a strong influence on structural behaviour.

Cette communication montre comment la porosite BlevBe, la structure fortement cimentCe et les min6raux argileux trbs hydra& du type kandite, rdgissent les propri&s gkotechniques de sols r&i- duels de type sableux & silt argileux dans les hautes terres de Panouasie-Nouvelle GuinBe. Ces sols sent caract&&s par des teneurs en eau en place dlevkes, des rCsistances au cisaillement relativement 6levees et une faible sensibilit6, et sent t&s com- pressibles lorsque la pression appliqu6e depasse une certaine valeur critique lors de lessai oedom&rique. On compare les valeurs mesurees pour la pression critique et lindice de compression B celles qui ont dt6 publiees pour une grande vari& de sols, et on etablit une relation liSaire entre la dkformation volumetrique et la porosit6, pour les sols poreux. On insiste sur lhCt&ogCn&t6 des sols r6siduels.

Lexamen des relations entre les propriCtCs gee- techniques et la porosit6 et la teneur en eau montre que la porositk ou la densite B set forme une base raisonnable de description et de groupement de ces sols pour le genie civil. On interprkte le comportement de ces sols en fonc-

tion dun modkle idkalisd de la structure du sol. On considkre que le sol se compose dun squellette grossier ouvert de minkraux cimentks et daggrtgats de particules dargile entourtes dun gel visqueux. Aux faibles contraintes, les liaisons cimentkes dans le squelette du sol restent intactes et le squelette du sol determine le comportement de la structure. Aux contraintes tlevtes, le squelette poreux du sol a encore une grande influence sur le comportement de la structure.

The residual soils described in this Paper occur at well-drained sites in the Western and Southern Highlands Districts of Papua New Guinea. The Highlands Districts are situated close to the equator (63, 14OE) and consist of undulating valleys (4000-6000 ft above sea level) separated by extremely rugged mountainous terrain. Although these valleys contain about 40% of the countrys population, they are mainly undeveloped with the population de- pending on a simple subsistence agriculture.

The climate is continually wet with an average annual rainfall of about 100 inches, evenly distributed throughout the year. The mean monthly rainfall nearly always exceeds the esti- mated mean evapo-transpiration and rain may be expected on an average of between 70 and

* Research Fellow, Department of Engineering, James Cook University of North Queensland, Townsville, Australia.

204 K. B. WALLACE

90% of all days. At 5000 ft elevation the temperature usually varies from 55F at night to 75F during the day and there is very little monthly fluctuation in temperature. This warm, continually wet climate is the major factor determining the unusual properties of soils in this region.

The zonal nature of these soils has been recognized by earlier pedological studies (Haantjens and Rutherford, 1964), which showed similarities in the morphology and clay mineralogy of soils on such dissimilar parent materials as limestone, siltstone, basalt and andesitic ash. The Authors studies (Wallace, 1971) have confirmed this zonality and have highlighted one dif- ference of engineering consequence. Soil depth on volcanics is generally greater than 20-30 ft whereas in many areas, only 3-6 ft of soil rests abruptly on limestone.

Pleistocene to recent volcanic eruptions have covered large areas of the Highlands with layers of andesitic ash. The combination of volcanic ash and continually wet climate have produced soils which are, in many respects, similar to the New Zealand andesitic ash soils (Birrell, 1952) and to Kanto Loam of Japan (Miki, 1960).

Basic data on these residual soils of the Papua New Guinea Highlands is presented in the next section of this Paper, while the succeeding sections describe their geotechnical properties and go on to discuss the relationship between these properties and the soil structure.

THE SOILS

The results presented in this Paper are taken from a more general study of field occurrence and geotechnical properties of these soils (Wallace, 1971). Because of the relatively remote and undeveloped nature of the Highlands the field work consisted of sampling from a few representative sites which had been chosen after extensive terrain description and examination of road cuttings in the region. Although the data are inadequate for detailed statistical analy- sis and correlation, it is thought that the careful selection of sampling sites has produced data that are characteristic of the nature of soils on the parent materials within the region.

Soils from four well-drained locations are described in Table 1, which also gives details of field moisture content, liquid limit, plasticity index, percentage sand (as measured by wet sieving undried soil on a 75 pm sieve), clay mineralogy and soil chemistry.

Three of the soils were formed from intermediate to basic volcanics and one from limestone. As all the sites were adjacent to Mt Giluwe, a Pleistocene volcano which is known to have covered the area with andesitic ash showers (Blake and Liiffler, 1971), it is extremely difficult to determine the relative influence of volcanic ash at each site. Examination of quarry exposures, sand mineralogy and local geomorphology indicates that the allophanic sandy clay (Soils No. 2 and No. 3) has been strongly influenced by andesitic ash whereas the halloysitic silty clay (Soil No. 1) is formed from a vesicular basic lava flow with possibly some minor ash contamination from above. The limestone soil (Soil No. 4) has been only slightly contamina- ted by ash (Wallace, 1971).

Clay mineral analysis was done by X-ray diffraction and differential thermal analysis (Amdel, 1971) after free iron removal by a dithionite-citrate system buffered with sodium bicarbonate (Mehra and Jackson, 1960). Table 1 shows that the predominant clay minerals were the highly hydrated kandite minerals, halloysite and allophane with gibbsite occurring in large proportions, close to the surface, especially on limestone. The presence of allophane can remain undetected by available tests on soils which contain both hydrated halloysite and gibbsite, as do Soils No. 1 and No. 4.

The low silica/alumina ratios in Table 1 illustrate the severe leaching of silica during rock weathering. The silty clay, Soil 1, has a silica/alumina ratio of 1.15 compared with a value of

Table

1.

Desc

rip

tion

of s

oils

-

Appro

xim

ate

Pri

nci

pal c

lay

per;

;;zt

ge

min

era

ls

% O

rganic

m

att

er

PH

Hydra

ted h

allo

y-

site

pre

dom

inant

wit

h m

odera

te

quanti

ties o

f gib

bsi

te

14

pH

= 6

.2

Allo

phane a

nd

3

verm

iculit

e

pH

= 5

.5

Allo

phane a

nd

2:

verm

iculit

e

pH

= 5

.4

Gib

bsi

te p

redom

i-

nant w

ith

modera

te am

ount:

of

hydra

ted

hallo

ysit

e

1

pH

= 5

.6

i -

-

I

-

SiO

z A

l,03

Soi~

~~

~dnce

loca

tion

Field

desc

ripti

on

Pla

stic

ity

index

-___

55

Liquid

lim

tit

____

_ 13

5

14

5

16

0

___-

11

0 -

-

Typic

al f

ield

m

ois

ture

co

nte

nt

(10

5G

), %

13

0

85

13

0

l-

No.

1

7

-6

depth

at

Sit

e N

o.

1 o

n

Volc

anic

s

Firm

, mois

t,

yello

w-b

row

n

fiss

ure

d silt

y cl

ay

15

[r

ock

form

ing

min

era

ls)

1.1

5

No.

2

5-

6 d

epth

at

Sit

e N

o.

2 o

n

Volc

anic

s

70

3

5

(rock

form

ing

min

era

ls)

1.6

5

Firm

, m

ois

t,

oliv

e-b

row

n,

fiss

ure

d, s

andy

clay

Firm

, m

ois

t,

oliv

e-b

row

n

:;;t

red,

sandy

Firm

, m

ois

t,

yello

w-b

row

n

slig

htl

y fr

iable

, si

lty

clay

,

No. 3

5

-6

depth

at

Sit

e N

o.

3 o

n

Volc

anic

s

85

4

5

(rock

form

ing

min

era

ls)

1.3

I-

0.6

N

o. 4

3

depth

at

Sit

e N

o.

4 o

n

Lim

est

one

90

3

5

15

(m

ain

ly a

ggre

- gate

d cl

ay

min

era

ls)

206 K. B. WALLACE

2.8 for the parent materials. Ruxton (1968) has concluded that silica/alumina ratio is a good index of the degree of rock weathering in free-draining weathering environments in humid regions. This conclusion is supported by the results of chemical analysis of the present soils (Wallace, 1971). In their chemical classification of lateritic soils, Martin and Doyne (1927) described soils with a silica/alumina ratio between 1.33 and 2.00 as lateritic soils. Soils with a silica/alumina ratio less than 1.33 were called laterite soils, while soils with a silica/alumina ratio in excess of 2.00 were considered to be non-Iateritic.

GEOTECHNICAL PROPERTIES

Field moisture content In contrast to their firm moist appearance, these soils have extremely high field moisture

contents. This is illustrated by Soil 3 (Table 1) which contains 45% of sand particles but has a moisture content of 130%. This is very much higher than the values of 20-70% moisture con- tents that are typical of a wide range of sedimentary clays of low to quite high plasticity. Generally the moisture content of the present soils varies between 80-190%, and there is no consistent pattern of variation of moisture content with depth or between sites. Field dry density of the sub-soil is correspondingly low, varying between 35 and 55 lb/cu. ft.

The soils are not saturated. The measured degree of saturation of undisturbed samples was generally between 85 and 95%.

The field moisture content of most samples is between 80 and 100% of the liquid limit. Many of the samples tested had a moisture content which was about 10% lower than the liquid limit. When the consistency limits are plotted on a Casagrande chart all soils are well below the A-line and would therefore be classified as highly plastic silts.

Shear strength The immediate, undrained shear strength has been determined using a Torvane. Generally

the Torvane shear strength of undisturbed samples tested at field moisture content ranged from 800 to 1200 Ib/sq. ft. The strength of the allophanic sandy clay (Soils 2 and 3) was slightly greater than that of the halloysitic silty clay (Soil 1). Several samples of Soil 2 had Torvane shear strengths of 2500 lb/sq. ft but other samples from the same location gave values of 1100 Ib/sq. ft. The undrained shear strength is much higher than the values of 300 lb/sq. ft which are common for equivalent, normally consolidated, sedimentary soils (Skempton, 1957 and Osterman, 1959). The reIatively high shear strength is also indicated by the existence of stable, nearly vertical road cuttings up to 30 feet high.

There was general agreement between the Torvane shear strength measurements and the cohesive component of shear strength as measured in drained shear box tests on soaked samples of similar soil. From this it was concluded that moisture suction does not contribute to the comparatively high undrained shear strength. In a later section of this Paper the high undrained shear strength is attributed to cemented bonds between the soil particles.

Stress-deformation curves for drained shear box tests which were carried out at two different applied pressures on soaked, undisturbed samples of Soil 2, are shown in Fig. 1. It can be seen that the behaviour of the soil was similar to that of an overconsolidated clay, in that at low applied pressure the soil dilated during shearing and there was a pronounced peak shear strength. At higher applied pressure the sample was compressed during shearing and there was no sharp peak shear strength, the maximum shear strength occurring at a large deformation.

STRUCTURAL BEHAVIOUR OF RESIDUAL SOILS OF THE CONTINUALLY WET HIGHLANDS OF PAPUA NEW GUINEA 207

wN= 15 Ib/rq. in.

--o---l UN=5 Ib/rq. in.

Horizontal movemenr in inches

Fig. 1. Direct shear box test results for loads of 5 and 15 lb/sq. in. on Soil No. 2

Table 2 illustrates how the undrained shear strength is affected by soaking and remoulding. The strength of undisturbed samples was not appreciably altered by soaking, but soaking greatly reduced the strength of remoulded samples.

Both Table 2 and Fig. 1 indicate that, for soils of such high porosity, these soils are relatively insensitive, having a sensitivity of about 2. This is very much less than the sensitivity of the cemented Canadian sedimentary clays (Sangrey, 1972b) but not much lower than that of many residual soils. Lohnes et al. (1971) report low sensitivity for a range of Puerto Rican lateritic soils.

Residual values of the angle of internal friction obtained from drained shear box tests on soaked, undisturbed samples of sandy clay were:

Soil 2 +d = 38

Soil 3 fjd = 29

Comparison of the descriptions of the two soils given in Table 1 shows that the higher strength of Soil 2 may be attributed to the lesser degree of weathering as indicated by the lower field moisture content and higher silica/alumina ratio of Soil 2.

Table 2. Torvane shear strength before and after remoulding and soaking

Soil Test condition

Undisturbed I Remoulded at field moisture content

1 Unsoaked, lb/sq. ft / Soaked, lb/sq. ft / Unsoaked, lb/sq. ft / Soaked, Ib/sq. ft

No. 1 640 600 300 100 No. 3 980 1040 420 300

208

Compressibility

K. B. WALLACE

Typical compressibility characteristics for these soils are shown in Figure 2 which presents the e-logp curves for oedometer tests on undisturbed and remoulded specimens of Soil 2. At low applied pressure the compressibility is low but when the applied pressure exceeds a certain critical pressure (pc) the compressibility of the soil is very high. The remoulded compressibility is also very high but is lower than that of the undisturbed soil at pressures exceeding the critical pressure. On removal of the load the soil does not swell very much. The resultant rebound curve, for the least disturbed samples, was roughly parallel to the com- pression curve for low applied pressures.

The relative compressibility and rebound of soils may be described by the following indices :

C, the compression index of the soil in its natural state (that is the slope of the e-logp curve at pressures exceeding the critical pressure, pJ C, the compression index of the soil after remoulding

and C,, the swelling index (the slope of the rebound curve).

Values of these indices are given on Fig. 2 and the values obtained from oedometer tests on each of the four soils are summarized in Table 3. Table 3 also gives some indication of the relative disturbance of undisturbed samples. This is a subjective assessment based on the shape of the e-logp curves. Soils that show a sharp change in slope of the curve over a narrow range of pressure around the critical pressure are described as least disturbed. This assessment was found to agree well with the relative difficulty of preparing oedometer speci- mens from 4-inch tube samples.

It should be noted that in all cases the critical pressure was much greater than the present overburden pressure. In this respect, the compressibility characteristics are similar to those of overconsolidated soils but there is no geomorphological evidence to suggest that any of the soils have ever experienced overburden pressures of similar magnitude to the critical pressure. The continually wet climate and the presence of highly hydrated allophane and halloysite sug- gest that desiccation has not caused overconsolidation. This critical pressure may therefore be attributed to cemented bonds between the soil particles as is discussed in a later section of this Paper.

The magnitude of the critical pressure measured on samples of the various soils ranged from about 1 to 34 ton/sq. ft and this may be compared with values reported for other residual soils. Gradwell and Birrell (1954) report values of 1.1 to 2.7 ton/sq. ft for a wide range of volcanic clays. Vargas (1953) shows that the critical pressures for residual clays on gneiss, basalt and sandstone in Southern Brazil are widely scattered between O-6 and 4.5 ton/sq. ft. Values for residual soils in the South-eastern United States (Sowers, 1963) vary between 1 and 5.5 ton/sq. ft. Finally, the Japanese soil, Kanto Loam (Koizumi and Ito, 1963), which is mineralogically

Table 3. Compressibility characteristics

Soil Critical pressure,

, ton/sq. ft

Compression index Relative disturbance

No. 1 ~ 1.6, 2.3 i 1.31, 1.93 0.10, 0.07 most disturbed No. 2

/ 3.5, 3.6 079, 093 i 0.06, 0.07 0.39-1.29 least disturbed

No. 3 1.7-1.8 1.01-2.3 1 0.06 0.59 least disturbed No. 4 0.9-2.3 0.74, 1.19 0.04, 0.05 j - most disturbed

I

STRUCTURAL BEHAVIOUR OF RESIDUAL SOILS OF THE CONTINUALLY WET HIGHLANDS OF PAPUA NEW GUINEA 209

I.0 I/ O! 02 05 I.0 2.0 50

Applied pressure in ton.sq. fr

Fig. 2. Typical e-logp curves for oedometer test on Soil No. 2

and geotechnically similar to the present soils, has critical pressures within the range 2-5.5 ton/sq. ft. From these diverse results it can be seen that although there is general agreement on the order of magnitude of critical pressures for a wide variety of residual soils, there is a large variation in values for any particular soil type. Large point to point variations in residual

soil properties are discussed in a later section of this Paper.

Compressibility correlations On Fig. 3, the measured values of compression index are plotted against initial void ratio for

both undisturbed and remoulded samples. It can be seen that there is a good linear relation-

ship between compression index and initial void ratio. When the applied pressure exceeds the

critical pressure the compressibility of the undisturbed samples with initial void ratios greater than 2 can be described by the relationship

C = ae,-b . . . . . . . . . (1)

where a=0*6 and b=0.7. Similar correlations have been obtained by others as a result of more extensive tests on a

variety of soils. Some of these other correlations between compression index and initial void ratio are compared with the present results on Fig. 4. These correlations indicate that for

soils with an initial void ratio in excess of I.0 (or a saturation moisture content greater than 40%) there is a reasonably linear relationship between compression index and initial void ratio.

For comparison of the compressibility of various soils at loads in excess of the critical pressure, the settlement, S, of a layer of thickness, H, due to an increase in pressure from p. to p. + Ap may be described by the approximate relationship

&$-olog,op~ . . . . . . . . (2)

210 K. B. WALLACE

0 I 2 3 4 5 6

lnltial void ratio, co

Fig. 3. Relationship between initial void ratio and compression index of undisturbed and remoulded soils

0 1 2 3 5 Initial Void Ratlo, e,

(I) The soils discussed in this Paper

(2) Sowers (1963): residual soils of south-eastern USA

(3) Penta et al. (1961): Italian volcanic ash soils

(4) Arango (Lambe and Whitman. 1969): soils of western USA and Columbia

(5) Kay and Krizek (1971): sedimentary roils:of USA

Fig. 4. Compression index correlations with initial void ratio

(This approximation appears to be reasonable when considered with respect to the relatively low compressibility below the critical pressure and the uncertainty in estimating the critical pressure.)

Combining equations (1) and (2) gives

S ae,-b PO+AP -=l+elog,opc * * H 0 Substituting initial porosity, no, for e,/l +e,

. . . . . . (3)

; = [(a+b)n,-b] logIop+

STRUCTURAL BEHAVIOUR OF RESIDUAL SOILS OF THE CONTINUALLY WET HIGHLANDS OF PAPUA NEW GUINEA 211

PO+4 = A(no-B) log,,--- . . . . . . . . . PC

(4)

where A and B are constants such that A = (a + b) and B = b/(u + b). For the present soils, A= l-3 and B=0.55. It should be emphasized that the preceding approximations are not suitable for close pre-

diction of soil properties. The scatter of results obtained by the various investigators named on Fig. 4 suggests that individual values may vary by f 30% or more from these correlations. However it is thought that equations (l), (3) and (4) are a reasonable basis for analysing the structural characteristics of the soil. For example, a comparison of equations (3) and (4) shows that the compression of the soil is more linearly related to porosity than to void ratio. This linearity is discussed further in a later section of this Paper which considers the con- sequences of high moisture contents.

A simple interpretation of equation (4) suggests that B represents an apparent minimum porosity for the particular group of soils to which the equation is applied and the parameter A is the flexibility of the soil skeleton. This flexibility (which is the inverse of the stiffness of the soil skeleton) is defined in terms of porosity by equation (4).

Creep Some measurements of the rate of creep of oedometer specimens (the oedometer rings being

lubricated with silicone grease and Ap/p = 1) have yielded the following conclusions :

(a) at loads greater than the critical pressure the creep rate of undisturbed samples (C, -0*006) was roughly ten times the creep rate at loads less than the critical pressure (C, N 0*0002-0~0005),

(b) at loads greater than the critical pressure the creep rate was reasonably independent of applied pressure increment whereas, below the critical pressure, the creep rate increased roughly linearly with applied pressure increment,

(c) disturbance during sampling greatly increased measured creep rates at applied pres- sures less than the critical pressure.

The general effect of drying on these creep characteristics is discussed later in the Paper, with respect to interpreting the general structural behaviour of the soil.

EFFECT OF DRYING

Without exception these soils become non-plastic when air-dried or oven-dried and do not regain their plasticity when wetted again. Frost (1967) has emphasized the need for appro- priate preparation prior to testing these soils. He has also observed that as a result of air- drying, the soaked, remoulded CBR of one soil from the region was increased from 2 to 60%.

To check the effect of drying on permeability, samples were dried and then remoulded and compacted to their original dry density. The permeability measured after saturation of the compacted sample was then compared with that of the same soil remoulded from the natural state without drying. The results of these tests are shown on Table 4, where it can be seen that as a result of dehydration and aggregation of the clay particles on drying, the permeability is increased more than one hundredfold.

This aggregation of soil particles on drying is illustrated on Table 5 which shows that results of particle size analysis of natural and dried samples of Soils 1 and 3. In these tests the dis- persing agent was sodium hexametaphosphate.

212 K. B. WALLACE

Table 4. Effect of drying on permeability

Soil Soil remoulded from natural state and Soil dried, remoulded and saturated saturated

No. 1 No. 3

Dry density, lb/cu. ft

:;:

Permeability, cm/s

5,7x 10-T 2.4 x lo-

Dry density, Ib/cu. ft

::

Permeability, cm/s

;:;; ;;I:

Table 5. Effect of drying on particle size

Soil Tested from natural state Tested after drying (105C)

Sand, % 1 Silt, % 1 Clay, % Saud, % 1 Silt, % 1 Clay, %

No. 1 1

No. 3 /

3: :: 47 :z 45

18

j 20

~ 3

It is often suggested that this dramatic improvement of remoulded soil properties by drying could be used to considerable advantage in construction. However, the continually wet climate which is responsible for formation of the unusual clay minerals usually persists throughout construction and limits the practicality of soil improvement by drying to the most compact sites such as airports or dam sites.

DISCUSSION

Variability of residual soil properties As is typical of many residual soils, the properties of soils sampled from a given depth in a

given test pit varied considerably. The results of six measurements of the moisture content of parts of a two-inch lump of Soil 3 ranged from 108% (n=0*76) up to 146% (n=O*80). Several samples of Soil 2 had undrained shear strengths of about twice the typical average value for this soil. These point to point variations in soil properties were often as large as varia- tions with depth at a given site or variations between sites.

A similar lack of homogeneity has been observed for decomposed granite (Lumb, 1962), and for the New Zealand volcanic ash clays (Gradwell and Birrell, 1954). This inhomogeneity is due to some or all of the following factors which are basic to the occurrence of residual soils, particularly when the soil is formed under well-drained conditions:

(a) variations in grain size and mineralogy of the parent material (b) irregular chemical weathering and leaching of weathering products due to non- uniform seepage of ground water through the variable and fissured, weathered parent material

(c) the dependence of the undisturbed properties on the nature of cemented bonds, which are only a very small part of the total volume of the soil particles and therefore may be more variable than the mineralogy or the soil texture

(d) the occurrence of fissures which are often close to vertical: these fissures could be the remnants of cracks or joints in the parent material or they could have been caused by slumping or lateral unloading of the soil as the landscape is dissected by streams; such fissures will often determine the stability of road cuttings

STRUCTURAL BEHAVIOUR OF RESIDUAL SOILS OF THE CONTINUALLY WET HIGHLANDS OF PAPUA NEW GUINEA 213

(e) disturbance of the cemented soil structure during sampling and during trimming of laboratory specimens which will add to the variability of test results

This variability of residual soil properties suggests that site investigations for extensive engineering works should concentrate on more intensive evaluation at fewer sampling sites. The location of these sites would be chosen according to changes in the basic soil- forming factors: parent material, drainage, slope and, where applicable, climate. Because of the irrelevance of conventional classification systems with respect to residual soils (Wallace, 1970) and the important influence of macroscopic structure, it is particularly important that site investigation reports should recognize residual soils as such and contain objective des- criptions of their undisturbed strength (firm, soft, stiff etc.) and their macroscopic structure (friable, fissured, fragmented etc.).

High moisture contents The most remarkable characteristic of these soils is their unusually high field moisture con-

tents, which of course represent proportionally high void ratios and higher than normal porosities. To many soils engineers who intuitively associate high field moisture content with low shear strength, high sensitivity and high compressibility, these soils appear to be unusually strong. Much of the stiffness and strength of these soils may be attributed to the cemented structure which is described in the next section; however, at loads high enough to break down this cemented structure, the strength and stiffness can still be surprisingly high.

This raises a simple question of practical importance. Is it reasonable to expect propor- tionally high compressibility and low drained shear strength at high field moisture contents? High field moisture contents (that is moisture contents in excess of 40%) may be due to the presence of a highly active clay fraction or, as is the case for the present soils which contain clay minerals of known low activity, high moisture content may be due to an unusual fabric of high porosity. The following remarks are relevant to the latter type of soil.

Analysis of compressibility correlations in a preceding section has shown that for the present soils, and for a wide variety of other porous soils, the relative compressibility is linearly depen- dent on porosity rather than on void ratio and moisture content. The drained shear strength of these porous soils is a complicated function of the cross sectional area of the soil skeleton and the degree of interlocking of soil particles. The cross sectional area will be linearly related to porosity while the previous relationship between compressibility and porosity suggests that the interlocking component of shear strength could also be more linearly related to porosity. Alternatively, it could be argued that the basic mechanism of large volume changes during compression is one of local shear, and therefore compressibility and drained shear strength could be expected to have a similar primary dependence on soil structure.

The highly non-linear relationship between saturation moisture content and porosity for porous soils is reproduced in Fig. 5. This figure indicates that, compared with variations of porosity with moisture content in the range of moisture contents between 20 and 70x, an increase of moisture content from 100 to 150% represents a relatively small increase in poro- sity. Therefore, as compressibility and probably also shear strength are linearly related to porosity, it can be concluded that it is not reasonable to expect that soils with high moisture contents will have proportionally low shear strength and compressibility. This conclusion indicates that porosity or dry density is a more reasonable basis for description and grouping of these extremely porous soils for engineering purposes.

214

I.0

0.8

0.6 x c 8 8 a.

0.4

0.2

0 20 50 70 100 150 Moisture Content I%1

K. B. WALLACE

Fig. 6 (below). Idealized residual soil structure

LCernented Bonds

Fig. 5 (left). Variation of porosity with moisture content of a saturated soil (G,=27)

One consequence of the high moisture content in construction on the present soils is that they require comparatively small amounts of additional water to bring them to the liquid limit. A typical highly plastic sedimentary clay (LL = 60, PL = 30) at a natural moisture content of 40% would require the addition of a volume of water equal to about 25% of its bulk volume to bring the soil to the liquid limit. A sandy clay, similar to the present soils, at a natural moisture content of 12OA (LL= 135), would reach the same consistency after the addition of less than 10% (by total volume) of water. This illustrates how the surface of trafficked earth- fill will be reduced to a quagmire by relatively light rainfall.

Cemented soil structure The fabric or macrostructure of the volcanic soils (as viewed at X20 magnification) consists

of a coarse open skeleton of rock-forming minerals (hornblende, plagioclase and quartz) surrounded by a viscous gel of highly hydrated clay minerals and sesquioxides. The fabric of the limestone soil can be considered to be similar except that the skeleton is comprised of aggregated clay mineral particles rather than the original rock-forming minerals. Other residual soils will have a skeleton that consists partly of rock-forming mineral? and partly of aggregated weathering products. This idealized structure of fine-grained residual soils is sketched in Fig. 6. The proposed structure is different from that of many sedimentary sandy and silty clays, which may be considered to consist of coarse particles dispersed throughout a clay matrix (Trollope and Chan, 1959).

The results of compression and shear tests described earlier show that the particles of the primary soil skeleton are cemented together at their contacts to form a continuous three- dimensional structural framework. The nature of this bond has not been definitely estab- lished but the absence of carbonates, the low organic content, the presence of bonds in the com- pletely residual limestone soil and successful dispersal of the clay fraction after free iron removal suggest that the bonds are associated with iron and aluminium hydroxides precipitated during rock weathering. Measured free iron content of the soil gave no indication of strength or stiffness but such a correlation would not be expected, as the amount of iron oxides required

STRUCTURAL BEHAVIOUR OF RESIDUAL SOILS OF THE CONTINUALLY WET HIGHLANDS OF PAPUA NEW GUINEA 215

to form the bonds could be much smaller than the measured total free iron contents of between one and ten per cent. Higher free iron contents would also be associated with more advanced weathering of the soil skeleton.

Sesquioxide cementing agents have also been indirectly detected in some African clays (Terzaghi, 1958 and Newill, 1961) and also in cemented Canadian clays (Quigley, 1968 and Sangrey, 1972b). It is interesting to note that the rock-forming minerals present in the cemen- ted Canadian clays are the same as those found in andesitic ash soils. Therefore it is expected that at low stresses, when the soil skeleton strength controls drained stress-strain characteris- tics, there would be similarities between the behaviour of the two types of soil.

Relative roles of cemented bonds and viscous gel When the present soils are dried the clay minerals aggregate so that the viscous gel is per-

manently destroyed. It was thought that if oedometer specimens could be prepared from soil which had been dried and then resaturated without disturbance, these specimens would possess cemented bonds without the viscous gel and a comparison with the behaviour of nor- mal undisturbed samples would reveal something of the relative roles of the bonds and gel. Consolidation tests on six undisturbed dried, resaturated oedometer specimens of Soil 2 pro- duced the following results. (In the following discussion low pressures and high pressures refer to applied pressures which are less than or greater than the critical pressure, respectively.)

(a) Removal of the gel only slightly lowered the critical pressure from 3.5 to 2.8 ton/sq. ft. This reduction could be explained by the greater disturbance of the dried-resaturated specimens.

(b) Compressibility at high pressures was not affected by removal of the gel. (c) Consolidation of the dried-resaturated soil was almost instantaneous. This is due to the large increase in permeability noted earlier in this Paper.

(d) Removal of the gel reduced the creep rate at high pressures to about 20% of that of the natural soil. Creep rates for the dried-resaturated soil at low pressures were similar to those at high pressures.

General structural behaviour Before discussing the observed soil behaviour in terms of the soil structure the earlier con-

clusions concerning the structural behaviour of the undisturbed natural soil are summarized as follows.

(e) At low pressures the soil compressibility is low and is similar to the rebound. (f) The volumetric strain at high pressures in the oedometer test was directly proportional to _4(n,, -B), where n,, is the initial porosity, B is an apparent minimum porosity and A is the flexibility of the soil skeleton.

(g) The creep rate at low pressures is approximately linearly proportional to the applied pressure increment.

(h) The creep rate at high pressure is roughly ten times greater than at low pressures and is reasonably independent of applied pressure increment.

(i) The undrained shear strength is comparatively high and is not affected by soaking. (j) The soil is relatively insensitive. (k) Drained shear strength is comparatively high.

From the preceding statements it can be interpreted that the bonds are independent of the viscous gel and that at low pressure the bonds are intact and the structural behaviour is con- trolled by the stiff, elastic soil skeleton. Sangrey and Townsend (1969) have shown that the

216 K. B. WALLACE

volumetric strain of intact Canadian clays is directly proportional to applied pressure. The lower compressibility of the present soils together with normal experimental variability pre- cludes any more definite conclusions than that the volumetric strain of the present soils, when intact, increases with applied pressure and is to a large extent recoverable.

The strength of the cemented bonds in residual soils is quite variable, as is shown by the variation in critical pressures and undrained shear strengths. This does not allow close definition of yield criteria such as those given by Sangrey (1972a) for finer grained, more homogeneous, cemented sedimentary soils. General field observations of residual soils indi- cate that even if rough criteria could be established in the laboratory, these criteria would be modified by discontinuities at the design site.

The dependence of the rate of creep of the intact soil on the applied pressure, together with the similarity of the creep of dried-resaturated soil above and below the critical pressure, suggests that at low pressures the creep mechanism is one of readjustment of stresses in the primary skeleton and fracture of the weaker bonds. Similar progressive microfracturing has been studied in detail in investigations into the creep of brittle rock at low temperature and pressure (Scholz, 1970).

The effect of the viscous gel in producing a much greater and apparently pressure-indepen- dent creep at high pressures could be attributed to a micropore mechanism (Barden, 1968) and indicates that when the bonds are fractured the gel supports a portion of the load at contacts between the coarser particles. The low sensitivity and high drained shear strength indicate that, even in a highly remoulded state, there is considerable interlocking of the coarse particles and that the proportion of the total load carried by the gel is small. This indicates that the viscous gel can drain freely through the porous skeleton and prevent any significant pore pres- sures on shear planes. The high value of apparent minimum porosity indicated by oedometer tests suggests that the porosity of the soil will be high, even in a highly compressed or sheared state. However, the Author has observed several specimens in which the soil skeleton has tended to collapse to much lower porosities under repeated shear stresses.

From this discussion, it is concluded that the structural behaviour of the soil is consistent with the proposed model of the soil structure and that, at low applied stresses and, to a lesser extent, at high applied stresses it is the porous soil skeleton which is most important in deter- mining structural behaviour.

CONCLUSION

The conclusions arrived at in the preceding discussion are based on a limited number of tests on residual soils formed in a continually wet environment. Compression and shear tests have concentrated on the sandier volcanic soils for which it was easier to prepare good undisturbed samples but similar results have been obtained on fewer samples of volcanic silty clay and the more friable silty clay on limestone. All the soils tested were well-drained lateritic soils. It is appropriate therefore to conclude by commenting on the breadth of application of the con- clusions reached in this Paper.

From the literature it appears that many residual soils have the cemented structure idealized herein. High shear strength and a link between shear strength and degree of weathering are frequentlyreported for residual soils. It is expected that point to point variability of soil pro- perties will be high for most residual soils. The primary dependence of compressibility on the porosity and critical pressure is well established for a variety of porous sedimentary and

STRUCTURAL BEHAVIOUR OF RESIDUAL SOILS OF THE CONTINUALLY WET HIGHLANDS OF PAPUA NEW GUINEA 217

residual soils. Low sensitivity whenremoulding at field moisture content is associated with the coarse cemented soil structure but it is not known to what extent residual soils can be generally assumed to have a coarse, porous skeleton, although it is considered that such a structure is associated with formation under well-drained conditions.

One aspect of the behaviour of the present soils which is thought to be peculiar to a much narrower range of residual soils is the stability of the cemented bonds on soaking or drying. This is thought to be associated with soil formation under continually wet conditions and will not apply to soils formed under seasonal climates. Sowers (1963) postulates a mechanism of fracture of bonds through unequal expansion of low plasticity residual soils when wetted, while the Author has observed that residual soils in the seasonally wet Eastern Highlands of New Guinea are much more difficult to sample in an undisturbed state and disintegrate readily on soaking.

Accumulated soils engineering experience with sedimentary soils has shown that correlations of the properties of the soils with simple parameters are extremely useful for determining rela- vant description and classification systems and for preliminary estimates of soil properties, but are subject to typical variations of & 30% around the predicted values. Residual soil properties will be more variable but any simple working generalizations on structural behaviour will be valuable aids in soils engineering. Such generalizations can only come from wider discussion of the formation, nature and structural behaviour of residual soils from a variety of sources. The Author believes that when the general characteristics of residual soils are better known much of their behaviour will be interpretable in terms of our much wider knowledge of the mechanics of sedimentary soils.

ACKNOWLEDGEMENTS

The work that has been described in this Paper was completed while the Author was a member of the staff of the Papua New Guinea Institute of Technology. The Author is grateful to the Institute for its generous support of the project. The geotechnical tests were carried out in the Institutes Soil Mechanics Laboratories with the assistance of Mr M. J. Roland, Mr J. P. Rizzi and Mr J. Gaya.

The valuable support of the Southern Highlands District Commissioner, Mr Des Clancy, and his staff, of the Catholic Mission, Mendi, and of civil engineering students Mr John Kavagu, Mr Gavera Morea and Mr Thomas Tohiana during the field work is also acknowledged.

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