1994_Mahalinga-iyer

Prévia do material em texto

| 
ELSEVIER Engineering Geology 38 (1994) 53-63 
E G NEER G 
GEOLOGY 
Consolidation and shear strength properties of a lateritic soil 
Umarany Mahalinga-Iyer, David J. Williams 
Department of Civil Engineering, The University of Queensland, Queensland 4072, Australia 
Received 24 May 1994; revised version accepted 18 July 1994 
Abstract 
The consolidation and shear strength behaviour of a lateritic soil developed on Tertiary basalt in southeast 
Queensland, Australia has been investigated. Oedometer tests were performed on remoulded specimens of the soil, 
and on a specimen obtained from an undisturbed block sample. Primary consolidation took place rapidly, and the 
total settlement over the applied stress range was small. The consolidation behaviour was similar to that expected of 
a silt, despite the appreciable clay content and relatively minor silt content of the soil. The analysis of secondary 
compression showed that this behaviour can be modelled satisfactorily using existing theory. The results of direct 
shear strength tests performed on remoulded specimens showed that the soil has a high shear strength despite its 
appreciable clay content. This is due to the cementation bonds between the particles at the micro-level. 
1. Introduction 
Laterites and lateritic soil are the products of 
intensive weathering that occurs under tropical 
and sub-tropical climatic conditions. The geotech- 
nical properties of these soils are quite different 
from those of soils developed under cold or 
temperate climates (Gidigasu, 1972, 1976). 
Laterisation involves the leaching out of silica and 
alkali, and the accumulation of hydrated iron and 
aluminium oxides (sesquioxides). Various terms 
and definitions have been applied to lateritic soil, 
by researchers in different countries, according to 
the chemical and mineralogical composition, par- 
ticle size distribution and the state of hardening. 
In this paper, lateritic soil is defined as a soil 
developed by the process of laterisation which is 
rich in sesquioxides, poor in silica and bases, but 
which may contain kaolinite and quartz. It con- 
tains hardened laterite rock or laterite gravel, or 
is capable of hardening on exposure to cycles of 
wetting and drying. In southeast Queensland, 
Australia, lateritic soil is found along the east 
0013-7952/94/$7.00 © 1994 Elsevier Science B.V. All rights reserved 
SSDI 0013-7952(94)00037-9 
coastal area. The geology of this region comprises 
a complex series of rocks of different geological 
age, with lateritic soil formed on Tertiary basalt. 
Samples of lateritic soil have been collected at 
different depths from Victoria Point, located 
about 30 km southeast of Brisbane, the capital of 
Queensland. In this paper, the results of one- 
dimensional consolidation (oedometer) tests and 
direct shear tests carried out on remoulded samples 
collected from different depths within the laterite 
zone, and on a specimen obtained from an undis- 
turbed block sample are presented and discussed. 
2. Experimental program 
2.1. Classification testing 
At Victoria Point, the lateritic soil in situ was 
generally too brittle to allow the collection of 
undisturbed block samples. The soil was collected 
as disturbed bag samples at 0.5 m intervals of 
54 U. Mahalinga-Iyer and D. J. Williams/Engineering Geology 38 (1994) 53 63 
depth from 0.5 to 2.5 m depth. A single block 
sample was collected from 0.5 m depth. X-ray 
diffraction analysis of the soil revealed that it 
comprises kaolinite, quartz, gibbsite and haematite 
and chemical analysis showed that the soil has a 
silica:sesquioxide ratio in the range from 0.59 
to 0.66. 
As the soil is sensitive to oven drying, classifica- 
tion properties were obtained on air-dried samples, 
in accordance with AS 1289 (1977) (This standard 
is equivalent to ASTM standards Vol. 04.08 (1991) 
in all but minor details). Over the depth of the 
profile the soil comprised 23-58% clay sized par- 
ticles, 6-12% silt sized particles, and 1-38% sand 
and gravel sized particles. The in situ moisture 
content was between 20% and 25%. This moisture 
content corresponds to a degree of saturation in 
the range from 72% to 87%. The liquid limit was 
between 56.6% and 63.4% and the plastic limit 
was between 41.6% and 46.3%, indicating a plastic- 
ity index in the range from 15.0% to 17.1%. 
Casagrande's classification was not used as it has 
been found to be unsuitable for application to this 
soil (Mahalinga-Iyer and Williams, 1991 ). An esti- 
mate of the in situ density was determined by the 
toluene displacement method using the soil clods 
(Sibley and Williams, 1989). The in situ bulk 
density was between 1.87 and 2.12 Mg/m 3. Details 
of the soil profile, climate and geology of the area, 
and classification properties of the soil are given 
in Mahalinga-Iyer and Williams ( 1991 ). 
2.2. Consolidation testing 
Consolidation tests were carried out in accor- 
dance with AS 1289 (1977) in standard fixed ring 
type oedometers with a brass ring of 75 mm 
internal diameter and 20 mm height. The side 
friction between the ring and the soil was reduced 
by lubricating the inside of the ring with silicone 
grease. Specimens were taken from each of the 
bag samples and remoulded in the ring, approxi- 
mating the in situ density by tamping, at natural 
moisture content. One undisturbed specimen was 
taken from the block sample collected from 0.5 m 
depth. Porous stones were placed on both ends of 
each specimen to provide two-way vertical drain- 
age, with filter papers placed between the specimen 
and the porous stones to prevent fine particles 
being forced into the pores of the stones. The 
specimens were soaked for 24 h under a seating 
stress of 6 kPa, and 100% saturation was achieved 
during this period. The degree of saturation was 
back-calculated from the moisture content of the 
specimen at the end of the test, the volume of 
water expelled during the test and the specific 
gravity of the solids. The permeability of the 
specimens was of the order of 10 -s m/s which 
corresponds to that of a silt. This relatively high 
permeability ensures that the specimens achieved 
100% saturation within 24 h. After soaking under 
a stress of 6 kPa, each specimen was loaded to 
800 kPa in the sequence 12.5, 25, 50, 100, 200, 400 
and 800 kPa. The duration of each stage was 24 h. 
The resulting settlements were recorded both auto- 
matically and manually. The tests were carried out 
under controlled temperature conditions in the 
range from 19 to 20°C. The settlement behaviour 
due to long term soaking of the Victoria Point soil 
was also investigated by testing a remoulded speci- 
men from 1 m depth in the oedometer with each 
stage of loading maintained for one week. 
2.3. Strength testing 
Direct shear strength tests were carried out on 
remoulded specimens at in situ moisture content 
and under saturated conditions, in a square direct 
shear box of plan dimension 100 mm and depth 
30 ram. All specimens were prepared in the box 
by tamping, to approximate the in situ density at 
natural moisture content. The specimen height of 
30 mm was used to accommodate any gravel pass- 
ing the 9.5 mm size sieve. Tests at natural moisture 
content (that is partially saturated, with a degree 
of saturation in the range from 72% to 87%) were 
carried out at a strain rate of 0.5 mm/min normal 
pressures of 24 kPa, 48 kPa, 100 kPa and at 150 
kPa in order to estimate the in situ shear strength 
parameters. 
Tests were performed on specimens soaked for 
24 h after preparation. The moisture content prior 
to shearing was in the range from 29% to 30% 
indicating that 100% saturation was achieved 
during soaking. The tests were conducted at a 
strain rate of 0.05 mm/min consolidated under 
U. Mahalinga-Iyer and D. J. Williams/Engineering Geology 38 (1994) 53-63 55 
normal pressures of 24 kPa, 48 kPa, 100 kPa and 
200 kPa prior to shearing. The sample from 1 m 
depth was also tested at normal pressuresof 
300 kPa, 400 kPa, 480 kPa and 560 kPa. According 
to Cheung et al. (1988), the effect of strain rate 
on the direct shear strength parameters of lateritic 
soils is not significant over the range of strain rates 
from 0.6 to 0.0072 mm/min. Both series of direct 
shear tests, those at natural moisture content and 
those under saturated conditions, may be consid- 
ered to be drained. 
Suction measurements were carried out on soil 
clods of about 30 mm diameter from each depth, 
using the filter paper method (McQueen and 
Miller, 1968). 
The micro-structure of the soil in the undis- 
turbed state and from the shear plane formed 
during drained testing was obtained using Philips 
SEM505 and JEOL 6400F scanning electron 
microscopes, at the Electron Microscope Centre 
at The University of Queensland, Australia. 
3. Results 
3.1. Consolidation testing 
Fig. 1 shows a typical settlement versus square 
root of time curve for the lateritic soil. This curve 
was obtained for the specimen from 0.5 m depth, 
consolidated to 100kPa. Similar curves were 
obtained for the applied pressure range from 
12.5 kPa to 800 kPa, and for specimens from other 
depths. From Fig. 1, it is clear that the compression 
of the lateritic soil specimen took place in three 
distinct phases, which can be described as initial 
compression, primary consolidation and secondary 
compression. Primary consolidation took place 
rapidly, with the time for 90% primary consolida- 
tion t9o determined by Taylor's square-root time 
method being 1.0 min. 
Table 1 shows the range of t9o values, and the 
range of the coefficient of consolidation cv over 
the applied pressure range from 12.5 to 800 kPa, 
and the values of preconsolidation pressure Pc' and 
over consolidation ratio OCR for all depths. Fig. 2 
shows the variation of void ratio with logarithm 
of pressure for the remoulded and undisturbed 
specimens from 0.5 m depth. The initial void ratio 
of the oedometer specimens before soaking has 
been included in Fig. 2, which shows that the 
settlement due to soaking is not significant. The 
undisturbed specimen was lightly fissured, leading 
to a higher void ratio and lower bulk density (by 
about 5%) than the remoulded specimen. However, 
the values of preconsolidation pressure are approx- 
imately the same for both undisturbed and 
remoulded specimens. 
Fig. 3 shows the variation of the compression 
index with percentage clay. For temperate zone 
soils the clay fraction has a great influence on the 
geotechnical properties of the soil. Over the profile, 
the clay fraction varies from 23% to 58%. However, 
Fig. 3 does not show any systematic variation 
between the parameters. Also, there is little change 
in the shape of the settlement versus time curves, 
and primary consolidation was essentially com- 
plete within 2.0 min for all of the specimens, 
despite their different clay contents. This indicates 
0.00 
~ 0.01 
"~ 0.02 
0.03 
, , ,J 
0.04 
0.05 
Square r o o t o f t ime _ 0 . (mLla .5) 
0 1 2 3 4 5 6 7 8 9 10 
P l I i I 1 i I t I 
0 0 o 
0 0 0 
Table 1 
Consolidation data 
Depth Range of tgo Range of cv Pc' OCR 
(m) (min) (m2/year) (kPa) 
0.5* 0.25-1.04 37-200 300 32.70 
0.5 0.13-1.44 30-368 325 35.70 
1.0 0.09-1.25 38-578 195 10.35 
1.5 0.25-1.69 30-216 150 5.08 
2.0 0.04-1.10 41-1255 270 6.49 
2.5 0.09-1.21 38-550 205 4.15 
Fig. 1. Typical settlement versus time curve. *Undisturbed specimen. 
56 U. Mahalinga-lyer and D. J. Williams~Engineering Geology 38 (1994) 53-63 
o 
1.08 
1.04 
1.00 
0.96 
0.92 
0.88 
0.84 
0.80 
• Initial 
~ _ o Undtzturbed 
I I 
10 100 
Prenare (kPa) 
Fig. 2. One-dimensional consolidation curves. 
I 
1000 
0.12 
0.10 
0.08 
0.08 
0.04 
8 
0.02 
0 
O 
0.0 I t I I I i I I 4 
20 25 30 35 40 45 50 55 60 65 
Clay content (~) 
Fig. 3. Variation of compression index with % clay. 
that the clay fraction has little influence on consoli- 
dation properties for this soil. 
Fig. 4 shows the settlement versus logarithm of 
time curves during secondary compression. Gibson 
and Lo (1961) proposed a theory of secondary 
consolidation and, based on this, Lo (1961) pro- 
posed three different types of curves for secondary 
consolidation. For the type 1 curve, the rate of 
secondary compression gradually decreases with 
time; for the type 2 curve, the rate is proportional 
to the logarithm of time, for a considerable range 
of time and then rapidly decreases, and for the 
type 3 curve, the rate increases with time, then 
gradually diminishes. From Fig. 4, it is clear that 
for the lateritic soil tested the variation of the rate 
of secondary compression with time is similar to 
Lo's type 1 curve. According to Gibson and Lo 
(1961) the settlement St at any time t after the 
application of pressure increment Ap is given by: 
St = HAp{a + b[ 1 - exp( - 2t/b)]} ( 1 ) 
where a, b and 2 are soil parameters and H is the 
thickness of the soil specimen. Thurairajah and 
Wijekulasuriya (1979) showed that: 
log(ST--St) = -0.434(2/b)t+log(HApb) (2) 
where is Sr the settlement at a large time T after 
the application of Ap. 
The graph of l o g ( S t - S 0 versus t should plot 
as a straight line with gradient -0.434 2/b and 
intercept log(H Apb) from which the soil param- 
U. Mahalinga-Iyer and D. J. Williams~Engineering Geology38 (1994) 53-63 57 
0.0 
0.1, 
.,-, 0.2, 
0.3 
0.4 
0.5 
10 100 1000 
' , i M i 
IO00O IOOOOO 
i i 
50 kPa 
100 kPa 
2 0 0 kPa 
4 0 0 kPa 
BOO kPa 
Fig. 4. Settlement versus time curves highlighting secondary compression. 
eters a, b, 2 can be obtained. Table 2 shows the 
values of soil parameters a, b and 2 obtained using 
this approach for the lateritic soil. In the estimation 
of these parameters, the value for T was taken as 
15,000 min. The values of b and 2 decrease with 
increasing consolidation pressure. A similar trend 
was observed for lateritic soil by Thurairajah and 
Wijekularsuria (1979). 
3.2. Strength testing 
Table 3 shows the values of cohesion intercept 
and angle of shearing resistance obtained from the 
unsaturated (apparent parameters ca, ~ba) and the 
saturated (effective stress parameters c', ~') direct 
shear tests carried out on remoulded lateritic soil 
specimens. Suction values {pF =log (kPa)+ 1 } at 
the natural moisture content are also given in 
Table 3. The variation in the values of soil para- 
meters with depth shows that the soil is not homo- 
geneous within the laterite zone. 
Table 3 shows that both the cohesion intercept 
and the angle of shearing resistance for specimens 
from all depths are reduced by saturation. This is 
Table 2 
Parameters for Gibson and Lo theory 
Pressure a b 2 
(kPa) (kPa-~.10 -5) (kPa-l-10 -s) (kPa-Xmin-l-10 -9) 
25 12.10 4.97 11.20 
50 5.48 4.34 8.61 
100 4.12 1.09 3.56 
200 5.77 0.872 2.78 
400 8.19 0.287 0.78 
800 5.95 0.109 0.31 
Table 3 
Shear strength parameters 
Natural moisture content Saturated 
Depth c~ ~a Suction c' q~' 
(m) (kPa) (°) (pF) (kPa) (o) 
0.5 87 42 3.74 40 31 
1.0 76 40 3.60 37 32 
1.5 125 37 3.50 48 30 
2.0 131 43 3.50 45 35 
2.5 152 37 3.46 43 32 
due mainly to the loss of soil suction. A similar 
loss in shear strength for lateritic soil was observed 
by Foss (1973). 
4. Discussion 
4.1. Consolidation testing 
Although the specimens contain a substantial 
amount of clay sized particles, the settlement 
versus time curves, as shown in Fig. 1 were similar 
to that expected for silt. Micro-aggregation and 
micro-voids are responsible for this behaviour. 
During laterisation, sesquioxide is precipitated on 
the surfaces of the clay particles as an amorphous 
gel. The kaolinite particles have a plate-shaped 
micro-structure. The sesquioxide coating changes 
the plate-shaped micro-structure into micro- 
aggregation, resembling tiny balls, as shown in 
Fig. 5. Several micro-voids can also be seen in this 
figure. Drainage of water through pure kaolinite 
is similar to drainage through a stack of plates, 
58 U. Mahalinga-lyerand D. J. Williams/Engineering Geology 38 (1994) 53 63 
Fig. 5. Microstructure of the lateritic soil. 
whereas drainage through lateritic soil is similar 
to flow through beads and voids. Consequently 
primary consolidation of lateritic soil takes place 
much more rapidly than that of kaolinitic clay. 
The totalsettlement of the lateritic soil speci- 
mens over the applied pressure range was between 
1 mm and 1.3 mm. The compression index lies in 
the range from 0.03 to 0.16, whereas that for 
kaolinite lies in the range from 0.19 to 0.28 
(Mitchell, 1976). This indicates that the lateritic 
soil is relatively incompressible. Olson and Mesri 
(1970) showed that the compressibility of pure 
kaolinite subjected to low stress levels is mainly 
due to particle bending, sliding and compression. 
As the kaolinite particles are plate shaped, they 
can slide over each other and can bend easily 
under the application of stress. Terzaghi (1948) 
also reported that compressibility increases greatly 
with increasing percentages of scale-shaped par- 
ticles such as mica. In lateritic soil, the sesquioxide 
coating of the kaolinite plates changes their shape 
to spheroidal, and forms bonds (bridges) between 
the particles. The spheroidal shaped particles can 
not bend as easily as plate shaped particles. Also, 
the bonds between the particles restrict the move- 
ment or sliding of particles. Consequently the 
compressibility of lateritic soil comprising kaolinite 
is much less than that of pure kaolinite. 
The total settlement obtained during the ex- 
tended duration test was approximately the same 
as that obtained in the standard duration test. 
This indicates that the bonds between the particles 
are not affected by soaking for a longer time under 
laboratory conditions. 
The values of preconsolidation pressure Pc' given 
in Table 1 are higher than the present effective 
overburden pressure. There is no geomorphologi- 
cal evidence to suggest that the area has been 
eroded in the past (Beckman et al., 1987). Many 
authors (Lipman and Waynic, 1916; Grant, 1974; 
Allam and Sridharan, 1981 ) indicate that climatic 
changes and repeated wetting and drying cause 
bonds between particles. These bonds are due to 
chemical compounds of iron, calcium, aluminium 
and other elements. Lambe (1960) stated that 
particle cementation is aided by drying as this 
tends to precipitate soluble cementing materials 
from the pore water. The effects of drying are 
commonly attributed to capillary precompression, 
but may actually be due to cementing. Kenny et al. 
U. Mahalinga-lyer and D. J. Williams/Engineering Geology 38 (1994) 53-63 59 
(1967) showed that chemical removal of iron 
compounds from a clay caused a decrease in 
the apparent preconsolidation pressure. 
Therefore, the bonding within the lateritic soil 
accounts for the high values of preconsolidation 
pressure observed. 
The values of preconsolidation pressure for 
undisturbed and remoulded specimens are approxi- 
mately the same (Fig. 2). Vargas (1953) performed 
oedometer tests on undisturbed specimens and 
remoulded specimens of a lateritic soil from Brazil. 
He showed that the soil possessed a much higher 
value for preconsolidation pressure in the undis- 
turbed state than in the remoulded state. However, 
for the Victoria Point soil, the preconsolidation 
pressure values are approximately the same for 
both the undisturbed and remoulded states indicat- 
ing that the soil is not sensitive to the method of 
reconstitution. Lumb (1962) also reported that the 
lateritic soil found in Hong Kong was insensitive 
to remoulding and no significant difference in 
engineering properties was found between undis- 
turbed and remoulded specimens. However, the 
soil was susceptible to sampling disturbance. 
Townsend et al. (1971) reported that remoulding 
of the lateritic soil produced smaller micro- 
aggregates and influenced the index properties, but 
he observed little change in the strength character- 
istics of the soil. 
4.2. Strength testing 
From the saturated drained shear tests it is 
observed that the soil has an effective cohesion 
intercept (Table 3). Cohesion can be due to edge 
to face flocculation, to potassium ion bonding due 
to drying or to cementation by naturally occurring 
materials such as carbonates and iron compounds 
(Lambe, 1960). The potassium ion bonding and 
cementation permanently link adjacent clay crys- 
tals. In the lateritic soil tested, the sesquioxide 
coating and the bonds between particles give rise 
to the effective cohesion intercept. 
Despite the appreciable amount of clay present 
in the soil tested, it has a high angle of shearing 
resistance resulting in high shear strength. The 
high angle of shearing resistance is attributable to 
the grading of the soil which gives high density 
and good interlocking at the macro-level. At the 
micro-level the particles are aggregated and the 
inter-particle bonds give increased resistance to 
sliding. 
The shearing resistance of soils is controlled by 
the effective contact stress C gi,ben by (Sridharan 
and Venkatappa-Rao, 1979): 
C=~'+~r" (3) 
where a' is the conventional effective stress and a" 
is the intrinsic effective stress. An increase in 
results in an increase in the shear strength of 
the soil. 
The intrinsic effective stress is caused by 
London-Van der Waal's attraction and cementa- 
tion bonds between the particles at the micro-level. 
However the contribution from the London-Van 
der Waal's attractive force to the strength may be 
negligible for degrees of saturation greater than 
40% (Sridharan, 1968, quoted in Krishnamoorthy 
et al., 1987). Allam and Sridharan (1981) showed 
that repeated wetting and drying of a lateritic soil 
under laboratory conditions generated chemical 
bonds between the clay particles. When consoli- 
dated undrained triaxial tests were performed on 
specimens after reaching a degree of saturation of 
99.5%, an increase in shear strength was observed 
with an increasing number of cycles of wetting 
and drying. Allam and Sridharan attributed the 
increase in shear strength to an increase in the 
intrinsic effective stress caused by the cementation 
bonds generated. This again confirms that the 
lateritic soils possess high shear strengths despite 
their appreciable clay content, because of their 
cementation bonds. 
Fig. 6 shows the shear stress versus horizontal 
displacement curves for the lateritic soil specimen 
taken from 1.0 m depth, for the range of normal 
stresses applied for the saturated tests. At low 
normal stress levels, the shear stress increases 
rapidly to a peak, which occurs at a low displace- 
ment level. With further straining, the shear stress 
reduces rapidly before stabilising at a lower level. 
At higher normal stress levels beyond the precon- 
solidation pressure, the shear stress increases 
rapidly at first but the peak is reached only at 
relatively large displacement. Fig. 7 shows the 
effective Mohr-Coulomb failure envelope for the 
60 U. Mahalinga-lyer and D. J. Williams/Engineering Geology 38 (1994) 53-63 
400 
| 554 kPa 
350 l ~ 
~" / ~ 480 kP~ 
~3°° l 
~ 26°t ~ 3~7 ~ 
V ~ 24 k.Pet 
o ~ 
0 1 2 3 4 5 6 ? 8 9 10 11 12 
Horizonta l d i s p l a c e m e n t (mm) 
Fig. 6. Stress versus displacement curves. 
400 
~-, 300 
"~ 200 
-N 
=~ 100 
l ) 
I I I 1 I I 
100 200 300 400 500 B00 
Normal stress (kPn) 
Fig. 7. Effective Mohr~Coulomb envelope. 
lateritic soil to be curved at low normal stress 
levels and linear at high normal stress levels. This 
type of stress-strain behaviour for lateritic soils 
was observed by Raju and John (1975). They 
suggested that at low confining pressure, the shear 
stress has no effect on the cementation bonds, but 
at high confining pressure cementation bonds are 
destroyed during the application of the confining 
pressure and the material behaves like uncemented 
soil. Figs. 8a and 8b show the microstructure of 
the soil from the shear planes of the specimenstested at normal stresses of 200 kPa and 480 kPa, 
respectively. These figures show that intact bonds 
between the grains remain after shearing. However, 
hydrometer analysis showed that there was an 
increase in the clay fraction (passing 2/~m) of 
about 4% for the sample taken from the shear 
plane. This indicates a small degree of breakdown 
of micro-bonding. Townsend et al. (1971) showed 
that chemical removal of sesquioxide from lateritic 
soil changed the micro-structure form from sphe- 
roidal to plate-like. Therefore, it may be concluded 
that the latedtic soil would not behave like a 
completely uncemented soil unless the sesquioxide 
coating and cementation bonds are chemically 
removed. 
u. Mahalinga-lyer and D. J. Williams~Engineering Geology 38 (1994) 53-63 61 
C a ) 
(b) 
Fig. 8(a). Scanning electron micrograph from shear plane (Normal stress 200 kPa), 8(b) Scanning electron micrograph from shear 
plane (Normal stress 480 kPa). 
62 U. Mahalinga-lyer and D. J. Williams/Engineering Geology 38 (1994) 53 -63 
5. Conclusions 
The lateritic soil from southeast Queensland, 
comprising kaolinite, quartz, gibbsite and haema- 
tite, is relatively incompressible, with a compres- 
sion index less than that of kaolinite. Primary 
consolidation was complete within 2.0 min for 
specimens sampled at different depths down the 
profile. The consolidation behaviour was similar 
to that expected of a silt, despite the appreci- 
able clay content and relatively minor silt content 
of the soil. The micro-structure, bonds due to 
sesquioxide coating, and the micro-voids are 
responsible for this behaviour. The high apparent 
preconsolidation pressure is also due to the micro- 
structural bonding. The engineering properties of 
the soil are not sensitive to remoulding. The 
amount of clay present does not influence the 
compression index. This is in contrary to the 
behaviour of temperate zone soils. The secondary 
consolidation behaviour can satisfactorily be mod- 
elled by the Gibson and Lo (1961) theory, the rate 
of settlement decreasing with time similar to the 
type 1 curve proposed in the theory. The soil has 
a high angle of shearing resistance. This is due to 
both the grading and the micro-structural bonding 
between the particles. Only a few bonds break 
down during the application of confining normal 
stresses higher than the preconsolidation pressure 
and shearing. The variation in the soil strength 
and consolidation parameters with depth shows 
that the soil is not homogeneous within the later- 
ite zone. 
Acknowledgements 
The authors wish to express their gratitude to 
Dr P.H. Morris, Department of Civil Engineering, 
The University of Queensland, for his reading of 
the draft paper and his helpful comments. The 
electron micrograph was obtained with assistance 
from staff of the Electron Microscope Centre at 
The University of Queensland. 
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