Content uploaded by Dominic E. L. Ong
Author content
All content in this area was uploaded by Dominic E. L. Ong on May 30, 2017
Content may be subject to copyright.
Behaviour of MH silts with varying plasticity
indices
Steven T. Y. Wong BEng(Hons)
Research Scholar, Research Centre for Sustainable Technologies, Faculty of
Engineering, Science and Computing, Swinburne University of Technology
Sarawak Campus, Kuching, Malaysia
Dominic E. L. Ong BE(Hons), PhD, PEng, MIEM CPEng, MIEAust
Associate Professor and Director, Research Centre for Sustainable
Technologies, Faculty of Engineering, Science and Computing, Swinburne
University of Technology Sarawak Campus, Kuching, Malaysia
(corresponding author: elong@swinburne.edu.my)
(Orcid:0000-0001-8604-8176)
Retnamony G. Robinson BEng(Hons), MSc, PhD
Professor, Civil Engineering Department, Indian Institute of Technology
Madras, Chennai, India
It is important for geotechnical engineers to understand the intrinsic and mechanical behaviour of silt as it is presently
recognised that there exist gaps in understanding its fundamental behaviour. The behaviour of kaolin samples with
varying clay and silt contents was investigated in the present study. This study characterised the samples by their
corresponding ranges of plasticity index (P
I
)–namely, P
I
£13% and P
I
> 13%. This outcome is achieved by interpreting the
results of Atterberg limit tests, particle size analysis, isotropically consolidated undrained triaxial tests and oedometer tests
carried out on the kaolin samples with P
I
between 7 and 16%. The results obtained in this study show good reliability
when compared to 65 sets of past significant experimental studies derived from 33 established past research papers.
Notation
A
c
activity
evoid ratio
C
L
% clay content
CL low plasticity clay
CH high plasticity clay
L
L
liquid limit
Mslope of the critical-state line (CSL)
MH high-plasticity silt
ML low-plasticity silt
O
CR
overconsolidation ratios
p0effective normal pressure or mean effective normal
stress
p0
ceffective confining pressure
P
I
plasticity index
P
L
plastic limit
qdeviator stress
q
f
deviator stress at failure
u
f
pore pressure at failure
vspecific volume
Ds1change in total vertical stress
Ds0
1change in the effective vertical stress
Ds3change in the total confining pressure
Ds0
3change in the effective confining stress
Duexcess pore water pressure
e
f
axial strain at failure
kgradient of swelling line
lgradient of compression line
l
CSL
gradient of compression lines of CSL
l
NCL
gradient of compression lines of the normally
consolidated line
l
NCL
/l
NCL
ratios of the gradients of compression lines
s
1
total vertical stress
s0
1effective vertical stress
s0
1=s0
3maximum principal stress ratio
s
3
total confining pressure
s0
3effective confining stress
f0effective angle of shearing resistance friction
Introduction
Based on particle size and plasticity characteristics, soils are
classified as sand, silt and clay. The behaviours of sand and clay
are unique. However, the behaviour of silts lies in between the
behaviours of clays and sands. Some silts behave more like sand,
and some others more like clay depending on their basic
characteristics. It is essential to identify silts behaving as sand-like
or clay-like. Boulanger and Idriss (2006) found that sand-like
(behaving more fundamentally as sand) and clay-like soils
(behaving more fundamentally as clay) have some fundamental
differences in terms of stress–strain behaviour, compressibility
and the slope of the critical-state line (CSL) in the e–ln p0space
against the slope of the normally consolidated line (NCL). The
major observations of Boulanger and Idriss (2004) include the
following.
(a) Sand-like materials exhibit different slopes for the CSL and
NCL, while clay-like materials show a similar slope for the
CSL and NCL in the e–ln p0space.
(b) The effective stress paths of sand-like materials in undrained
monotonic shearing show an initially contractive response
(positive pore pressure increments) followed by a transition to an
incrementally dilative response (decreases in pore pressure).
(c) Sand-like materials have little compressibility such that their
void ratio does not change significantly as the effective
consolidation stress is increased, while clay-like materials are
relatively more compressible.
1
Geotechnical Research
Behaviour of MH silts with varying
plasticity indices
Wong, Ong and Robinson
Geotechnical Research
http://dx.doi.org/10.1680/jgere.17.00002
Paper 17.00002
Received 25/01/2017; accepted 13/04/2017
Keywords: silts/strength & testing of materials/stress path
Published with permission by the ICE under the CC-BY license.
(http://creativecommons.org/licenses/by/4.0/)
Downloaded by [] on [30/05/17]. Published with permission by the ICE under the CC-BY license
In a subsequent study, Boulanger and Idriss (2006) proposed that
materials with P
I
less than 7% would have a sand-like behaviour,
while materials with P
I
greater than or equal to 7% would exhibit
a clay-like behaviour. In addition, it was also described that fine-
grained soils with P
I
values between 3 and 6% may exhibit a
transitional behaviour. As a result, an appropriate procedure for
examining the liquefaction resistance of soils was developed
based on the behaviour of the materials. Boulanger and Idriss
(2006) stated that the Atterberg limits tests are more reliable than
particle size analysis, particularly for clays (i.e. <2 mm), in order
to correlate to the stress–strain characteristics of soils and also to
differentiate between clay-like and sand-like behaviours.
It is also vital to note that Ferreira and Bica (2006) reported that
transitional soils with particle size distributions between those of
clean sands and plastic clays develop their own respective unique
NCL and CSL and are not in accordance with the critical-state
framework described for either sands or clays. In a research study,
Nocilla et al. (2006) recommended identifying a new framework to
describe the behaviour of silts in the transitional form. Such a
framework has yet to be identified or developed due to the behaviour
of silt being perhaps more complex than those of sand and clay.
Much research has been reported on low-plasticity silt (ML), with
a liquid limit (L
L
) lower than 50% –namely, by Wang and Luna
(2012), Boulanger and Idriss (2006), Nocilla et al. (2006) and
Hyde et al. (2006). They dealt with the characterisation and/or
mechanical properties of ML. However, it was observed that very
limited studies have been carried out to understand the behaviour
of high-plasticity silts (MH) with a liquid limit higher than 50%.
In this research, MH silts with L
L
that ranges from 52 to 64%
were tested, and the results are reported.
Nocilla et al. (2006) investigated the behaviour of Italian silt with a
plasticity index (P
I
) of 12% and found that as the clay contents
reduce, the behaviour of silt changes from a clay-like behaviour to
a transitional form demonstrating behaviours between those of
clays and clean sands. For this situation, the silt in the transitional
form demonstrates neither a unique NCL behaviour nor any unique
CSL behaviour. Hyde et al. (2006) reported that the mechanical
properties of low-plasticity silt with P
I
of 6% were sand-like.
Furthermore, Wang and Luna (2012) characterised the low-
plasticity Mississippi River Valley (MRV) silt with P
I
of 6% by
using triaxial compression tests with different overconsolidation
ratios (O
CR
) of 1, 2 and 8 and reported that no unique critical state
could be observed. The CSL in the void ratio (e) against the
logarithm of mean effective normal stress (p0) space was not
parallel to the normal consolidation curve, thus possibly indicating
a sand-like behaviour. Ladd et al. (1977) as cited by Wang and
Luna (2012) stated that deviator stress of some clays can be
normalised by the effective consolidation pressure. The deviator
stresses of the MRV silts with different O
CR
values were
normalised by the effective consolidation pressures, after which the
results indicated that the MRV silts had a unique clay-like
behaviour. The method identified by Boulanger and Idriss (2006)
for distinguishing between sand-like and clay-like behaviours has
worked well on both low-plasticity (ML) silts presented by Nocilla
et al. (2006) and Wang and Luna (2012). Nonetheless, Wang and
Luna (2012) likewise reported that the work of Boulanger and
Idriss (2006) did not capture the effect of O
CR
on silt behaviour.
As such, it is envisaged that the work of Boulanger and Idriss (2006)
can be further refined to accommodate a larger variety of soils that
include high-plasticity silts (MH). In the present study, the authors
carried out a series of experiments on four types of kaolin samples
with different particle size distributions and plasticity values. The
results are analysed along with the data presented in the literature so
as to identify the clay-like and sand-like behaviours.
Laboratory testing programme
Material description
Four types of commercially available kaolin powder with varying
clay contents –namely KM20, KM25, KM35 and KM55 –were
100
90
80
70
60
50
40
30
20
10
0
110 100 1000
Particle size: μm
Silt fraction
Clay
fraction
Sand fraction
KM20
KM25
KM35
KM55
Passing percentage: %
Figure 1. Particle size distribution of kaolinite samples
Table 1. Gradation, L
L
,P
L
and P
I
values of the kaolin samples
Sample Sand
fraction: %
Silt
fraction: %
Clay
fraction: % L
L
:% P
L
:% P
I
:% Soil
classification
Activity = P
I
/clay content%
(Skempton, 1953)
KM20 4·44 84·61 10·95 52 44 7 MH 0·66
KM25 0·88 79·62 19·50 59 48 11 MH 0·55
KM35 0·88 78·00 21·12 62 46 16 MH 0·77
KM55 0·88 74·75 24·38 64 49 15 MH 0·62
2
Geotechnical Research Behaviour of MH silts with varying
plasticity indices
Wong, Ong and Robinson
Downloaded by [] on [30/05/17]. Published with permission by the ICE under the CC-BY license
utilised to set up the reconstituted kaolin samples. The kaolin was
mined at depths between 4·5 and 6·0 m below the ground.
Aluminium silicate is the predominant chemical constituent of
kaolin. Reconstituted kaolin samples were found to be ideal in
this study on the grounds that kaolin (a) has appropriate ranges of
particle sizes (silt and clay fractions), (b) is relatively less
expensive than obtaining undisturbed soil samples from the field
and (c) tests can be consistently repeated with confidence.
Soil properties
British standard BS 1377-2:1990 (BSI, 1990a) was adopted to
determine the index properties such as L
L
by using the
Casagrande method as well as plastic limit (P
L
) and particle size
distribution. Both sieve and hydrometer analyses were carried out
to determine the percentages of sand, silt and clay.
The particle size distribution of kaolin samples based on the sieve
and hydrometer analyses is shown in Figure 1. The clay contents
(<2 mm) of KM20, KM25, KM35 and KM55 were found to be
10·95, 19·50, 21·12 and 24·38%, respectively, while the silt
contents (2–60 mm) of KM20, KM25, KM35 and KM55 were
84·61, 79·62, 78·00 and 74·75%, respectively.
The L
L
,P
L
and P
I
of the kaolin samples are summarised in
Table 1. They are found to be directly proportional to the clay
contents of the kaolin samples as shown in Figure 2. The
reconstituted kaolin samples are classified as high-plasticity silt
(MH) in accordance with ASTM D 2487 (ASTM, 2000).
The activity (A
c
)offine-grained soils, the ratio of P
I
to the
percentage clay content, is the amount of water that is attracted to
the surfaces of the soil particles. The amount of water attracted is
largely influenced by the amount of clay that is present in the soil
(Lambe and Whitman, 1969; Skempton, 1953). Skempton (1953)
stated that A
c
provides a convenient value for assessing the
particular minerals found in clay. In other words, before X-ray
diffraction tests are done on the samples, soils with different
minerals, such as kaolin, can be differentiated by their respective
A
c
values since different minerals are characterised by their
unique A
c
values. Table 1 shows that the A
c
of the reconstituted
kaolin samples are 0·66, 0·55, 0·77 and 0·62 for KM20, KM25,
KM35 and KM55, respectively, which are considered reasonable
as they are generally close to the A
c
value given by Skempton
(1953) (A
c
= 0·46) and Ferreira and Bica (2006) (A
c
= 0·66).
Testing procedures
The one-dimensional (1D) consolidation (oedometer) tests were
also conducted in accordance with BS 1377-5:1990 (BSI, 1990b)
and ASTM D 2435 (ASTM, 2011) to examine the compressibility
characteristics. Consolidated isotropic undrained (CIU) triaxial
tests were performed based on BS 1377-8:1990 (BSI, 1990c) and
Head (1998) in order to obtain the shear strength characteristics.
Strips of filter paper were attached on the sides of the specimens for
radial drainage as per specifications by Head (1998) so as to
accelerate the consolidation during the consolidation phase. The
specimens were saturated by applying back pressures until
Skempton’s pore pressure parameter (B) value of at least 0·98 was
achieved. The CIU tests were carried out at different initial effective
LL
PL
PI
80
60
40
20
0
010 20 30
Clay content: %
Water content: %
Figure 2. Linear relationship between clay contents and L
L
,P
L
and
P
I
of kaolin samples
Table 2. Summary of the conducted tests
Test KM20 KM25 KM35 KM55
L
L
,P
L
and P
I
✓✓✓✓
Sieve and hydrometer ✓✓✓✓
Oedometer ✓✓✓✓
CIU ✓✓✓✓
Effective confining pressure, p0
c: kPa
100 KM20-100 KM25-100 KM35-100 KM55-100
150 KM20-150 KM25-150 KM35-150 KM55-150
200 KM20-200 KM25-200 KM35-200 KM55-200
250 KM20-250 KM25-250 KM35-250 KM55-250
300 KM20-300 KM25-300 KM35-300 KM55-300
400 KM20-400 KM25-400 KM35-400 KM55-400
3
Geotechnical Research Behaviour of MH silts with varying
plasticity indices
Wong, Ong and Robinson
Downloaded by [] on [30/05/17]. Published with permission by the ICE under the CC-BY license
confining pressures of 100, 150, 200, 250, 300 and 400 kPa. As the
time taken for consolidation was shorter than 2 h for all specimens, a
minimum of 2 h was considered in order to calculate the required
shearing rates as suggested by Head (1998). As such, a shearing rate
of 0·07 mm/min was adopted. A similar shearing rate was also
reported by Pillai et al. (2011) for tests on kaolin.
During the consolidation phase of the triaxial test, the volume
change of the sample was continuously monitored with time.
The post-consolidation dimensions were determined using
equations 6.3(b) and 6.3(c) of BS 1377-8:1990 (BSI, 1990c).
Once the consolidation was over, the sample was sheared under
undrained conditions. The axial load, axial deformation and the
pore pressures were recorded using a load cell, a displacement
transducer and a pore pressure transducer, respectively. The tests
were continued until axial strain of 20% was reached. The
corrected length was used for computing the strains during the
shearing stage.
CIU tests at six different initial effective confining pressures
of 100, 150, 200, 250, 300 and 400 kPa were carried out for
each type of kaolin sample with varying clay–silt contents
(KM20, KM25, KM35 and KM55). Therefore, a total of 24 CIU
tests were performed. The samples were labelled based on the
type of kaolin and the confining pressure in the triaxial test. For
example, KM20-100 means 100 kPa confining pressure was
applied on the sample KM20. The conducted tests are
summarised in Table 2.
Sample preparation
Reconstituted and undisturbed samples
Burland (1990) demonstrated that the reconstituted clay should be
mixed at higher water contents from 1·00 to 1·50 times its liquid
limit (L
L
) and preferably consolidated one-dimensionally to obtain
the intrinsic properties of the soils. As there is no proper guideline
for reconstituting kaolin silt, the mixing ratio as proposed by
Burland (1990) has been adopted. A comparative mixing ratio
was likewise embraced in the work of Pillai et al. (2011) for
kaolin to prepare normally consolidated reconstituted samples.
In a different work by Hyodo et al. (1994), triaxial compression
and extension tests were conducted on both normally consolidated
undisturbed and reconstituted marine clays with a liquid limit of
124·2%, a plastic limit of 51·4% and a plasticity index of 72·8%.
The monotonic axial load was applied at an axial strain rate of
0·1%/min. The test outcomes demonstrated that undisturbed and
reconstituted samples behaved similarly in terms of deviator
stresses and stress paths. Hyodo et al. (1994) added that the ageing
effects such as chemical bonding or secondary compression for
undisturbed samples were eliminated during the application of
effective confining stress to the normally consolidated state during
the consolidation stage. Therefore, the work of Hyodo et al. (1994)
guaranteed that the behaviour of normally consolidated undisturbed
samples can be decently depicted by reconstituted samples.
Sample preparation
The use of reconstituted kaolin samples prepared by consolidation
of the soil–water mixture in the form of slurry state is a common
approach in the laboratory testing of soils (Pillai et al., 2011;
Wang and Luna, 2012). Firstly, the kaolin powder was mixed with
distilled water at a water content of 1·50 times its liquid limit.
The water–soil mixture was mixed thoroughly in an automatic
soil mixer to form the slurry. For triaxial tests, 38 mm dia.
samples were used. The slurry was directly poured into the
38 mm dia. sampling tube after applying silicone grease on the
inner surfaces of the tubes to reduce side friction. Subsequently,
the sample was gradually loaded to a vertical consolidation
pressure of 100 kPa. The vertical consolidation pressure was
achieved by the application of a dead weight of 11·5 kg on the
sample through a guide rod.
After the consolidation stage, the kaolin samples were extruded
using a universal extruder. The reconstituted kaolin samples were
then trimmed to 38 mm diameter and 76 mm height.
For the 1D consolidation tests, the slurry was carefully poured
into a 60 mm dia. and 20 mm high 1D consolidation ring along
with the collar so that the thickness of the sample was 30 mm.
The slurry was initially consolidated to a consolidation pressure
of 6·25 kPa. Once the consolidation under 6·25 kPa was
complete, the sample was carefully trimmed to a thickness of
20 mm. The sample was then subjected to a 1D consolidation test
with a load increment ratio of 1·0.
Results and discussions
CIU tests
Figures 3(a), 3(b) and 3(c) show the stress–strain behaviour, stress
paths and pore pressure–strain behaviour of the reconstituted
kaolin samples, respectively. For clarity, separate plots of these
three parameters for the KM20, KM25, KM35 and KM55
samples can be found in Figures 4, 5, 6 and 7, respectively.
When the clay contents decreased (from KM55 to KM20),
the deviator stresses at failure (q
f
) increased as shown in Figure 3(a).
The mean effective normal stress (p0) and deviator stress (q) can be
expressed as Equations 1 and 2a. Equation 2a can then be written in
incremental form as shown in Equation 2b. For the CIU tests, the
change in the total confining pressure (Ds
3
) equals zero. Hence,
Equation 2b can be rewritten as Equations 2c and 2d as the changes
in deviator stress (Dq)aredefined as negative and positive for strain
softening and hardening, respectively.
From Equations 2c and 2d, it is obvious that samples are expected
to exhibit strain softening only if (a) the change in the total
vertical stress (Ds
1
)or(b) the summation of the changes in the
effective vertical stress ðDs0
1Þand excess pore water pressure (Du)
is negative. In other words, the kaolin samples will strain soften,
due to the negative value of Du(dilation) and/or negative value of
Ds0
1(axial unloading).
4
Geotechnical Research Behaviour of MH silts with varying
plasticity indices
Wong, Ong and Robinson
Downloaded by [] on [30/05/17]. Published with permission by the ICE under the CC-BY license
Similarly, the kaolin samples will exhibit strain hardening if
(a) the change in Ds
1
or (b) the summation of Ds0
1and excess
pore water pressure (Du) is positive. The kaolin samples will
strain harden due to the positive values in Du(contraction) and/or
positive value in Ds0
1(compression)
p0¼s0
1þ2s0
3
3
1.
q¼s0
1
−s0
3
2a.
Dq¼Ds0
1
−Ds0
3¼Ds1
−Ds3
2b.
For softening
Dq¼Ds1¼Ds0
1þDu<02c.
For hardening
Dq¼Ds1¼Ds0
1þDu>02d.
The effective stress paths in Figure 3(b) were plotted in the
Cambridge stress space with Equation 1 in the abscissa and
Equation 2a in the ordinate. A similar approach was also adopted
by Wang and Luna (2012) to understand the stress paths of silts.
The effective stress paths of KM20 and KM25 with the lower P
I
500
450
400
350
300
250
200
150
100
50
0
04812 16 20 0
Axial strain, εa: %
Axial strain, εa: %
(a) (b)
Mean effective normal stress, p’: kPa
50 100 150 200 250 300 350 400 450
0
50
100
150
200
250
300
350
400
Deviator stress, q: kPa
Deviator stress, q: kPa
04812 16 20
(c)
250
200
150
100
50
0
Change in sample pressure, ∆u: kPa
KM55-400
KM55-300
KM55-250
KM55-200
KM55-150
KM55-100
KM35-400
KM35-300
KM35-250
KM35-200
KM35-150
KM35-100 KM20-100
KM20-150
KM20-200
KM20-250
KM20-300
KM20-400
KM25-100
KM25-150
KM25-200
KM25-250
KM25-300
KM25-400
Figure 3. (a) Stress–strain, (b) stress paths and (c) pore pressure–strain behaviours of kaolinite samples subjected to CIU tests
5
Geotechnical Research Behaviour of MH silts with varying
plasticity indices
Wong, Ong and Robinson
Downloaded by [] on [30/05/17]. Published with permission by the ICE under the CC-BY license
values of 7% and 11%, respectively, curved away from the origin
of mean effective normal stress (p0), but the effective stress paths
of KM35 and KM55 with higher P
I
values of 16 and 15%,
respectively, curved towards the origin of p0.
Figure 3(c) shows the pore pressure behaviour of the kaolin
samples. Table 3 summarises the changes in effective vertical
stresses and pore pressures. As shown in Figure 3(a), KM20
exhibited strain softening at s0
3values of 100, 150 and 200 kPa
due to the development of negative values of Du(dilation). This is
because of having negative values in both Duand Ds0
1as well as
the summation of Duand Ds0
1being similarly negative, resulting
in the change in deviator stress (Dq) also being a negative value
(see Equation 2c). Theoretically, KM20 is expected to exhibit
strain softening at the aforementioned confining pressures, as
observed in Figure 3(a) or Figure 4 for better clarity. However, on
the contrary, with KM20 tested under confining pressures s0
3of
250, 300 and 400 kPa, the values of Ds0
1become positive
(compression). As such, KM20 is expected to show strain
hardening with positive Dqvalues as per Equation 2d and as
observed in Figure 3(a) or Figure 4 for better clarity.
For KM25, dilation can be observed when the samples are tested
with confining pressures of s0
3of 100, 200, 250, 300 and 400 kPa,
except 150 kPa, with no change in Du.Ds0
1at all effective confining
pressures are negative values (denoting axial unloading behaviour),
except s0
3¼100 kPa, which showed a small positive value (slight
compression). Hence, with Equation 2c, KM25 is expected to show
strain softening behaviour at all the tested s0
3values due to the
summation of Duand Ds0
1or Dqbeing all negative values. By
comparing with Figure 5 for better clarity, this observation holds true.
For KM35, the samples have shown slight dilation at s0
3of 100
and 400 kPa, and no dilation or slight contraction at s0
3of 150,
Axial strain, εa: %
KM20-400
KM20-300
KM20-250
KM20-200
KM20-150
KM20-100
04812 16 20 050 100 150 200 250 300 350 400 450
(b)(a)
Mean effective normal stress, p’: kPa
Deviator stress, q: kPa
0
50
100
150
200
250
300
350
400
Deviator stress, q: kPa
500
450
400
350
300
250
200
150
100
50
0
Change in sample pressure, ∆u: kPa
250
200
150
100
50
0
Axial strain, εa: %
048121620
(c)
Figure 4. (a) Stress–strain, (b) stress paths and (c) pore pressure–strain behaviours of KM20 kaolin samples subjected to CIU tests
6
Geotechnical Research Behaviour of MH silts with varying
plasticity indices
Wong, Ong and Robinson
Downloaded by [] on [30/05/17]. Published with permission by the ICE under the CC-BY license
200, 250 and 300 kPa as shown in Table 3. For KM55, the
samples show slight dilation at s0
3of 100, 200, 250, 300 and
400 kPa; while at 150 kPa it shows no dilation. It can be seen that
the strain softening and hardening of KM35 and KM55 samples
are governed by their more significant and dominant values of
Ds0
1.
For example, KM35 has shown strain hardening at s0
3of 100 and
150 kPa due to the positive values of Ds0
1(compression) greater
than Du; hence, the samples have shown positive values in Dq
based on Equation 2d. For other s0
3values of KM35 and all s0
3
values of KM55, the negative values of Ds0
1(axial unloading) are
greater than Du, so the samples have shown negative values
(strain softening) in Dqof Equation 2c. By observing Figure 3(a),
or for better clarity Figures 6 and 7, the behaviours of strain
softening and strain hardening are very obvious.
Ishihara et al. (1975) defined the phase transformation point as
the point at which the stress path turns its direction in p0–qspace.
They also observed that for cyclic triaxial tests, it is necessary for
a sample to go at least once through this critical value in order to
be taken to a completely liquefied state. In this sense, the phase
transformation point may be considered as a threshold at which
the behaviour of sand as a solid is lost and transformed into that
of a liquefied state.
As shown in Figures 4(b), 5(b), 6(b) and 7(b), all samples
exhibited the initial contraction before the phase transformation
points. As shown in Table 3, all samples exhibit post-peak
dilation except for KM25-150, KM35-150, KM35-200, KM35-
250 and KM55-150. Except for KM20-250, KM20-300, KM20-
400, KM35-100 and KM35-150, all the samples exhibited post-
peak strain softening.
Axial strain, εa: %
Axial strain, εa: %
Deviator stress, q: kPa
500
450
400
350
300
250
200
150
100
50
0
048121620
048121620
(a)
(c)
050 100 150 200 250 300 350 400 450
0
50
100
150
200
250
300
350
400
Deviator stress, q: kPa
(b)
Mean effective normal stress, p’: kPa
Change in sample pressure, ∆u: kPa
250
200
150
100
50
0
KM25-400
KM25-300
KM25-250
KM25-200
KM25-150
KM25-100
Figure 5. (a) Stress–strain, (b) stress paths and (c) pore pressure–strain behaviours of KM25 kaolin samples subjected to CIU tests
7
Geotechnical Research Behaviour of MH silts with varying
plasticity indices
Wong, Ong and Robinson
Downloaded by [] on [30/05/17]. Published with permission by the ICE under the CC-BY license
Critical state
The deviator stresses at failure (q
f
) can be identified when either of
the following conditions are achieved: (a) maximum principal stress
ratio ðs0
1=s0
3Þand (b)‘critical-state’condition –that is, constant
deviator stress and pore pressure. The corresponding values of strain
and pore pressure are axial strain at failure (e
f
) and pore pressure at
failure (u
f
), respectively (Head, 1998). The work of Wang and Luna
(2012) showed that the criterion of maximum principal stress ratio
always gives a consistent estimation of the effective angle of shearing
resistance. Hence, in the present study, the maximum principal stress
ratio method has been used to identify q
f
.
The outcome of projecting the effective stress paths in Figure 3(b)
onto the semilogarithmic plot of v–ln p0is shown in Figure 8. As
drainage was not allowed during the shearing stage of the sample,
the effective stress paths in v–ln p0were expected only to move
horizontally since undrained shearing occurred at constant specific
volume. It is observed that the CSLs and the NCLs are not parallel
as shown in Figure 8 for the kaolin with lower clay contents
(KM20 and KM25) due to the P
I
values of KM20 and KM25
being lower at 7 and 11%, respectively, compared to other samples,
which have P
I
values greater than 13% (KM35 (P
I
=16%)and
KM55 (P
I
= 15%)). Therefore, it is defined that KM20 and KM25
behave as sand-like materials. A similar observation of non-parallel
CSL and NCL curves was also reported by Wang and Luna (2012)
in their study of triaxial compression test on the MRV silt with P
I
of 6%. Furthermore, it is interesting to observe that as the P
I
values
of the kaolin samples increased starting from KM20 (P
I
= 7%) and
KM25 (P
I
=11%)toKM35(P
I
=16%)andKM55(P
I
= 15%), the
gradients of the respective CSLs seem to be getting more and more
parallel to the gradients of their corresponding NCLs, indicating
that the clay-like behaviour starts to be more prominent
Axial strain, εa: %
Axial strain, εa: %
04812 16 20 050 100 150 200 250 300 350 400 450
(b)(a)
(c)
Mean effective normal stress, p’: kPa
250
200
150
100
50
0
04812 16 20
Deviator stress, q: kPa
Deviator stress, q: kPa
0
50
100
150
200
250
300
350
400
500
450
400
350
300
250
200
150
100
50
0
KM35-400
KM35-300
KM35-250
KM35-200
KM35-150
KM35-100
Change in sample pressure, ∆u: kPa
Figure 6. (a) Stress–strain, (b) stress paths and (c) pore pressure–strain behaviours of KM35 kaolin samples subjected to CIU tests
8
Geotechnical Research Behaviour of MH silts with varying
plasticity indices
Wong, Ong and Robinson
Downloaded by [] on [30/05/17]. Published with permission by the ICE under the CC-BY license
progressively. Therefore, for KM20 and KM25 with the relatively
lesser P
I
values, the non-parallel NCLs and CSLs seem to suggest
that the sample demonstrated a more dominant sand-like behaviour
instead, when compared to those KM samples with greater P
I
values.
As shown in Figure 9 for better clarity, the normalised deviator
stress of KM20 and KM25 cannot be effectively bound by the
effective consolidation pressures ðp0
cÞinto a narrower single band,
particularly at 100 kPa for p0
c, which indicates that they are not
clay-like materials. Comparatively, KM35 and KM55 can be
better normalised by p0
cinto a single band and have shown a
unique behaviour as clay-like materials.
Based on this important observation, since KM35 and KM55 have
higher P
I
values and their NCLs and CSLs are much more parallel
(evident in Figure 8), it is thus postulated that KM35 and KM55
demonstrate clay-like behaviours. In view of this, a P
I
value of equal
to or greater than 13% can be postulated as the boundary when clay-
like behaviour types of soils would start to dominate. Similar results
were also reported by Wang and Luna (2012) and Boulanger and
Idriss (2006) in their works, which showed sand-like materials
exhibiting non-parallel NCL and CSL behaviours.
Table 4 summarises the results of the CIU tests performed on the
reconstituted kaolin soils. Based on the mean effective normal
stress at failure ðp0
fÞand q
f
from Table 4, the CSLs of
reconstituted kaolin soil samples are then determined, as shown in
Figure 10. The slope of the CSL increased when the clay contents
of the reconstituted kaolin soils reduced. The slope of the CSL is
M, and the effective angle of shearing resistance friction (f0) can
be back-calculated using the following equation.
Axial strain, εa: %
Axial Strain, εa: %
(b)(a)
04812 16 20
500
450
400
350
300
250
200
150
100
50
0
400
350
300
250
200
150
100
50
0
Mean effective normal stress, p’: kPa
250
Deviator stress, q: kPa
Deviator stress, q: kPa
200
150
100
50
0
04812
16 20
(c)
KM55-400 KM55-200
KM55-150
KM55-100
KM55-300
KM55-250
Change in sample pressure, ∆u: kPa
0 50 100 150 200 250 300 350 400 450
Figure 7. (a) Stress–strain, (b) stress paths and (c) pore pressure–strain behaviours of KM55 kaolin samples subjected to CIU tests
9
Geotechnical Research Behaviour of MH silts with varying
plasticity indices
Wong, Ong and Robinson
Downloaded by [] on [30/05/17]. Published with permission by the ICE under the CC-BY license
sin f0¼6M
6þM
3.
A similar approach was also used by Wang and Luna (2012) to
find the f0value. The Mand f0values of the kaolin soils are
summarised in Table 4.
The relationship between the clay contents and effective angles of
shearing resistance is indirectly proportional as shown in
Figure 11. The correlation between clay contents (C
L
%) and
effective angle of shearing resistance (f0) for the kaolin is
f0¼346422 −03323 CL%4.
with coefficient of determination (R
2
) equal to 0·931. Similarly,
Nocilla and Coop (2008) conducted CIU triaxial tests on alluvial
sediments from the floodplain of Po River in Italy with C
L
%
ranging from 16 to 25% and P
I
ranging from 13 to 16%. It was
also found that the back-calculated f0showed a similar inverse
correlation with C
L
%. However, the observed scatters in their data
set could be due to the presence and the variation in silt and sand
contents.
One-dimensional consolidation tests
The compressibility of the reconstituted kaolin was determined
using 1D consolidation tests. The semilogarithmic e−log s0
v
(effective vertical stress) relationships obtained from the 1D
consolidation testing are plotted in Figure 12. The initial void
ratio is directly proportional to the L
L
of the soils. If the L
L
is
higher, the initial void ratio will also tend to be higher due to the
higher water content present within the void of the samples. The
results of the 1D consolidation tests are summarised in Table 5.
Furthermore, the gradient of compression line (l) and the gradient
of swelling line (k) show a directly proportional relationship with
the C
L
% of the samples, as shown in Figure 13.
Table 3. Changes in effective vertical stresses and pore pressures
Sample Axial strains: % Dr0
1DuDr0
1þDu
KM20-100 19·0–20·0 −2·95 −3·95 (d) −6·89 (s)
KM20-150 18·0–20·0 −1·82 −1·00 (d) −2·82 (s)
KM20-200 15·5–20·0 −22·45 −5·00 (d) −27·45 (s)
KM20-250 12·5–20·0 41·49 −6·00 (d) 35·49 (h)
KM20-300 16·0–20·0 7·43 −2·00 (d) 5·43 (h)
KM20-400 12·0–20·0 74·82 −12·00 (d) 62·82 (h)
KM25-100 7·0–20·0 0·67 −10·00 (d) −9·33 (s)
KM25-150 18·5–20·0 −1·95 0·00 (—)−1·95 (s)
KM25-200 11·0–20·0 −30·82 −3·00 (d) −33·82 (s)
KM25-250 17·5–20·0 −15·19 −2·00 (d) −17·19 (s)
KM25-300 12·0–20·0 −93·66 −5·00 (d) −98·66 (s)
KM25-400 19·0–20·0 −7·71 −1·00 (d) −8·71 (s)
KM35-100 10·5–20·0 23·86 −4·00 (d) 19·86 (h)
KM35-150 12·5–20·0 13·43 0·00 (—) 13·43 (h)
KM35-200 16·5–20·0 −47·85 0·00 (—)−47·85 (s)
KM35-250 11·5–20·0 −30·75 0·00 (—)−30·75 (s)
KM35-300 13·0–20·0 −83·37 10·00 (—)−73·37 (s)
KM35-400 14·5–20·0 −59·88 −5·00 (d) −64·88 (s)
KM55-100 12·5–20·0 −2·79 −1·00 (d) −3·79 (s)
KM55-150 19·0–20·0 −1·95 0·00 (—)−1·95 (s)
KM55-200 12·0–20·0 −36·16 −1·00 (d) −37·16 (s)
KM55-250 12·0–20·0 −61·47 −1·00 (d) −62·47 (s)
KM55-300 12·0–20·0 −56·27 −3·00 (d) −59·27 (s)
KM55-400 16·5–20·0 −27·77 −1·00 (d) −28·77 (s)
d, dilation; c, contraction; s, softening; h, hardening
2·5
2·4
2·3
2·2
2·1
1·9
1·8
2·0
Specic volume, v
100 1000
Mean effective normal stress, p’: kPa
KM20-NCL KM20-NCL
KM20-CSL KM20-CSL
KM25-CSL KM25-CSL
KM25-NCL KM25-NCL
KM35-CSL KM35-CSL
KM35-NCL KM35-NCL
KM55-CSL KM55-CSL
KM55-NCL KM55-NCL
Figure 8. CSLs and isotropically NCLs of kaolin samples derived
from CIU tests
10
Geotechnical Research Behaviour of MH silts with varying
plasticity indices
Wong, Ong and Robinson
Downloaded by [] on [30/05/17]. Published with permission by the ICE under the CC-BY license
Gradient of compression lines of NCL and CSL from CIU
tests
The gradients of the compression lines of the reconstituted kaolins
were back-calculated from the NCLs and CSLs of the CIU tests
as shown in Figure 8. Both the gradients of the compression lines
of NCLs and CSLs are directly proportional to the clay contents
as shown in Table 6. Furthermore, the gradient of the compression
lines of NCL (l
NCL
) converges to the gradient of the compression
lines of CSL (l
CSL
) when the clay content of kaolin increases.
This also indicates that the NCLs and CSLs of KM55 and KM35
are relatively more parallel than the NCLs and CSLs of KM20
and KM25. As shown in Figure 14 and Table 6, the ratios of the
gradients of compression lines (l
NCL
/l
CSL
) of KM20, KM25,
KM35 and KM55 are 0·702, 0·806, 0.846 and 0·948,
respectively. A value of l
NCL
/l
CSL
closer to 1·000 indicates that
the NCL and CSL are more parallel. The R
2
of the l
NCL
/l
CSL
and
Axial strain, εa: %
Normalised deviator stress, q/p’
c
2·0
1·5
0·5
KM20-400
KM20-300
KM20-250
KM20-200
KM20-150
KM20-100
1·0
0
04812 16 20
(a)
2·0
1·5
0·5
1·0
0
04812
16 20
Axial strain, εa: %
KM25-400
KM25-300
KM25-250
KM25-200
KM25-150
KM25-100
Normalised deviator stress, q/p’
c
(b)
2·0
1·5
0·5
1·0
0
Normalised deviator stress, q/p’
c
KM35-400
KM35-300
KM35-250
KM35-200
KM35-150
KM35-100
04812 16 20
Axial strain, εa: %
(c)
2·0
1·5
1·0
0
04 8 12 16 20
0·5
Axial strain, εa: %
Normalised deviator stress, q/p’
c
KM55-400 KM55-200
KM55-150
KM55-100
KM55-300
KM55-250
(d)
Figure 9. (a) Normalised deviator stress–strain plots for KM20; (b) normalised deviator stress–strain plots for KM25; (c) normalised deviator
stress–strain plots for KM35; (d) normalised deviator stress–strain plots for KM55
11
Geotechnical Research Behaviour of MH silts with varying
plasticity indices
Wong, Ong and Robinson
Downloaded by [] on [30/05/17]. Published with permission by the ICE under the CC-BY license
clay contents (C
L
%) for the kaolin samples is rather strong at
0·931 and the proposed correlation equation is
lNCL
lCSL
¼0500 þ0017 CL%
5.
Distinguishing between clay-like and sand-like soil
behaviours through plasticity index (P
I
)
A sand-like behaviour is observed from the reconstituted kaolin soils
of KM20 and KM25, characterised by their lower P
I
values of 7 and
11%, respectively, as well as their CSL and NCL being non-parallel
(see Figure 8). In the event that the method of Boulanger and Idriss
(2006), which was developed for distinguishing clay-like (P
I
greater
Table 4. Results of CIU tests, slopes of CSLs and effective angles of shearing resistance
Sample p0
c: kPa p0
f: kPa q
f
: kPa u
f
: kPa d
af
:% Me:°
KM20 100 134·16 189·47 23 18·0 1·2496 30·73
150 169·40 211·20 51 17·5
200 199·01 258·02 83 13·0
250 265·09 309·28 88 20·0
300 290·10 360·31 124 17·0
400 367·76 464·29 187 20·0
KM25 100 106·27 141·82 39 12·5 1·1448 28·83
150 156·22 153·67 45 18·0
200 173·88 206·65 95 10·5
250 219·54 250·63 114 17·0
300 257·86 278·57 134 11·5
400 337·70 401·11 196 19·0
KM35 100 87·92 113·75 47 18·5 1·0996 27·75
150 115·77 134·30 79 20·0
200 195·44 211·33 75 16·5
250 211·59 226·77 114 16·0
300 222·80 239·41 154 10·0
400 317·19 351·56 200 14·5
KM55 100 78·69 89·07 47 12·5 1·0300 26·02
150 98·61 112·82 89 19·0
200 159·46 160·38 94 9·5
250 198·24 207·72 121 14·5
300 223·05 231·16 149 10·5
400 307·13 309·40 196 16·5
500
400
300
200
100
0
0100 200 300 400 500
Mean effectiive normal stress at failure, p’
f: kPa
Deviator stress at failure, qf: kPa
KM20
CSL: KM20
CSL: KM25
CSL: KM35
CSL: KM55
KM25
KM35
KM55
Figure 10. CSLs of tested kaolin samples
36
32
28
24
Kaolinites
Effective angle of shearing resistance, φ’: °
φ’ = –0·3323CL + 34·6422
R2 = 0·931
010 20 30 40
Clay contents, CL: %
Figure 11. Relationship between clay content and effective angle
of internal friction
12
Geotechnical Research Behaviour of MH silts with varying
plasticity indices
Wong, Ong and Robinson
Downloaded by [] on [30/05/17]. Published with permission by the ICE under the CC-BY license
than or equal to 7%) and sand-like (P
I
smaller than 7%) behaviours
of silt based on P
I
, was applied to the high-plasticity silt (MH) of
KM20 and KM25, then KM20 and KM25 would be ‘incorrectly’
classified as having a clay-like behaviour.
To evaluate further the reliability of the original work of
Boulanger and Idriss (2006) on MH, various P
I
and L
L
data
points obtained from well-established papers (see Table 7) are
subsequently used to develop Figure 15 together with the
classification rules for sand-like and clay-like behaviours as
reported by Boulanger and Idriss (2004). As illustrated in Figure
15, apparently, some of the well-documented sand-like soils are
lying slightly above the P
I
= 7% boundary suggested by
Boulanger and Idriss (2006), which evidently demonstrates the
conflict in trying to categorise the well-documented sand-like data
to be within the P
I
= 7% boundary. Therefore, it is now proposed
that the boundary for describing sand-like behaviour soils be
raised from P
I
= 7% (Boulanger and Idriss, 2006) to P
I
= 13% so
that the well-documented data points (see Table 7) for soils with a
sand-like behaviour can be fulfilled, including that of KM20 (P
I
=
7%) and KM25 (P
I
= 11%), whose sand-like behaviour is clearly
demonstrated in Figure 8. The boundary P
I
= 13% is also
supported by Nocilla et al. (2006), who conducted reliable triaxial
compression tests on Italian silts. In their work, the samples with
clay contents of 4% (L
L
= 37% and P
I
= 13%) and 8% (L
L
= 34%
and P
I
= 13%) had evidently demonstrated a sand-like behaviour
as the effective stress paths showed an initially contractive
response (due to increase in pore pressure) followed by a
transition to an incrementally dilative response (due to decrease in
pore pressure).
Finally, a threshold of P
I
greater than 13% is now suggested for
soils behaving in a clay-like manner. This would then include the
2·1
2·0
1·9
1·8
1·7
1·6
1·5
1·4
1·3
110 100 1000
Effective vertical stress, σ’
v: kPa
Void ratio, e
KM20
KM25
KM35
KM55
Figure 12. Stress–void relationship during 1D consolidation tests
Table 5. Results of 1D consolidation tests
Sample kj
KM20 0·148 0·035
KM25 0·201 0·042
KM35 0·160 0·036
KM55 0·186 0·037
0·10
0·08
0·06
0·04
0·02
0
0 5 10 15
λ
κ
Clay content, CL: %
20 25
Compressibility
Figure 13. Relationship between clay content and compressibility
Table 6. Gradients of compression lines of NCLs and CSLs
Sample k
NCL
k
CSL
k
NCL
/k
CSL
KM20 0·09354 0·13324 0·702
KM25 0·08163 0·10117 0·807
KM35 0·10887 0·12863 0·846
KM55 0·10484 0·11055 0·948
1·0
0·9
0·8
0·7
0·6
0·5
0102030
Clay content, CL: %
Compression indices’ ratio, λNCL/λCSL
λNCL/λCSL = –0·0172CL% + 0·500
R2 = 0·931
λNCL/λCSL
Figure 14. Relationship between compression indices’ratio and
clay content
13
Geotechnical Research Behaviour of MH silts with varying
plasticity indices
Wong, Ong and Robinson
Downloaded by [] on [30/05/17]. Published with permission by the ICE under the CC-BY license
Table 7. Documented Atterberg limits for soils with sand-like and clay-like behaviours (continued on next page)
Soil name Soil classification, ASTM
D 2487 (ASTM, 2000)
L
L
:
%
P
I
:
%Tests Categories and
evidence References
Cohesive or clay-like behaviour
1 B6 marine clay –James Bay CL 37 13 Triaxial and direct
simple shear
Cohesive, Figure 9 Ladd (1991); Boulanger
and Idriss (2006)
2 Natural London Clay, 5·2 m BGL CH 69 44 HCA stress path C3, Figure 4 Nishimura et al. (2007)
3 Natural London Clay, 10·5 m
BGL
CH 70 44 HCA stress path C3, Figure 4 Nishimura et al. (2007)
4 Speswhite kaolin MH 62 30 Triaxial C3, p. 8, column 2,
line 4 –p. 9,
column 1, line 1
Georgiannou et al. (1990)
5 Aeolian silt CL 37 18 Triaxial C3, Figures 8–13 Cui and Delage (1996)
6 Sleech silt (3 m depth) CH 58 36 Triaxial and 1D
consolidation
C3, Figure 3(a);
C1, Figure 2
Lehane (2003)
7 Sleech silt (6 m depth) CH 70 48 Triaxial and 1D
consolidation
C3, Figure 3(a);
C1, Figure 2
Lehane (2003)
8 Sub-Apennine Blue Clays, By CL 49 26·4 Triaxial and 1D
consolidation
C3, Figure 13 Cotecchia et al. (2007)
9 Sub-Apennine Blue Clays, Bg CL 51·1 27·7 Triaxial and 1D
consolidation
C3, Figure 13 Cotecchia et al. (2007)
10 Sub-Apennine Blue Clays, P9 CH 69·3 38·4 Triaxial and 1D
consolidation
C1, Figure 5 Cotecchia et al. (2007)
11 Sub-Apennine Blue Clays, P19 CL 51·8 28·8 Triaxial and 1D
consolidation
C1, Figure 5 Cotecchia et al. (2007)
12 Sub-Apennine Blue Clays, P25 CH 65 35 Triaxial and 1D
consolidation
C3, Figure 11 Cotecchia et al. (2007)
13 Sub-Apennine Blue Clays, P33 CH 53·4 27·5 Triaxial and 1D
consolidation
C1, Figure 5 Cotecchia et al. (2007)
14 Sherbrooke laminated clay CL 45 18 Triaxial C3, Figure 7(a) Long (2006)
15 Compacted clayey silt fill CL 45·6 20·1 Triaxial C3, Figures 5(c)
and 5(d)
Almeida et al. (2012)
16 Bengawan Solo fill D1 MH 54 18 Triaxial C3, Figure 8 Mountassir et al. (2011)
17 Bengawan Solo fill D2 MH 53 16 Triaxial C3, Figure 8 Mountassir et al. (2011)
18 Mixtures of kaolin, sodium
bentonite and London Clay
CL 28 18 Triaxial C3, Figure 6 Cunningham et al. (2003)
19 Mexico Clay –oven-dried MH 93 23 1D consolidation C1, Figure 16 Mesri et al. (1975)
20 Residual London Clay CH 80 51 Triaxial Cohesive, Figure 11 Skempton (1985)
21 KM35 MH 62 16 Triaxial C2, Figure 5 This paper
22 KM55 MH 64 15 Triaxial C2, Figure 5 This paper
23 Grey organic clay CL 38 19 Triaxial C3, Figure 11 Long and O’Riordan (2001)
24 Bolkin silt CL 29·4 15·6 Triaxial C3, Figure 1 Wang et al. (2002)
25 Hong Kong marine deposits C4 CL 60 32 Triaxial and 1D
consolidation
C3, Figure 12 Yin (1999)
26 Kaolin soil 68-32 ML 47 17 Triaxial and 1D
consolidation
C3, Figure 6 Anantanasakul et al. (2012)
27 MSM10-3 CL 35 15 1D consolidation C1, Figure 8 Biscontin et al. (2007)
28 MSM10-6 CL 38 19 1D consolidation C1, Figure 8 Biscontin et al. (2007)
29 MSM10-14 ML 49 20 1D consolidation C1, Figure 8 Biscontin et al. (2007)
30 MSM10-43 CL 42 22 1D consolidation C1, Figure 8 Biscontin et al. (2007)
31 MSM10-48 CL 36 13 1D consolidation C1, Figure 8 Biscontin et al. (2007)
32 MSM10-52 CL 38 19 1D consolidation C1, Figure 8 Biscontin et al. (2007)
33 MSgM1-2 CL 34 14 1D consolidation C1, Figure 8 Biscontin et al. (2007)
34 MSgM1-3 CL 32 13 1D consolidation C1, Figure 8 Biscontin et al. (2007)
35 MSgM1-10 MH 56 24 1D consolidation C1, Figure 8 Biscontin et al. (2007)
36 MSgM1-22 MH 62 28 1D consolidation C1, Figure 8 Biscontin et al. (2007)
37 MSgM1-22b MH 62 28 1D consolidation C1, Figure 8 Biscontin et al. (2007)
38 MSgM1-24 CL 41 19 1D consolidation C1, Figure 8 Biscontin et al. (2007)
39 MSgM2-11mb ML 46 15 1D consolidation C1, Figure 8 Biscontin et al. (2007)
40 Completely decomposed tuff ML 43 14 Triaxial –
consolidation
C2, Figure 3(a) Chiu and Ng (2012)
41 Natural soil CL 38 16 Triaxial C1, Figure 3 Cetin and Soylemez (2004)
42 Nancy North-west silt MH 56 25 Triaxial C3, Figure 2 Ltifiet al. (2014)
14
Geotechnical Research Behaviour of MH silts with varying
plasticity indices
Wong, Ong and Robinson
Downloaded by [] on [30/05/17]. Published with permission by the ICE under the CC-BY license
Soil name Soil classification, ASTM
D 2487 (ASTM, 2000)
L
L
:
%
P
I
:
%Tests Categories and
evidence References
43 Gorgon muddy silt ML 45 15 Monotonic
simple shear
C3, Figure 2 Mao and Fahey (2003)
44 Mud cake in silt CH 58·5 28·8 Triaxial C3, Figure 5 Zhang et al. (2009)
45 In situ soil in silt CH 52·6 25·3 Triaxial C3, Figure 5 Zhang et al. (2009)
46 Mud cake in clay CL 43·9 20 Triaxial C3, Figure 6 Zhang et al. (2009)
47 Italian silt (clay content 25%) CL 46 22 Triaxial C3, Figure 7 Nocilla et al. (2006)
48 Italian silt (clay content 45%) MH 60 33 Triaxial C3, Figure 7 Nocilla et al. (2006)
Cohesionless or sand-like behaviour
1 KM20 MH 51 7 Triaxial S2, Figure 5 This paper
2 KM25 MH 59 11 Triaxial C2, Figure 5 This paper
3 Residual soil from
Botucatu Sandstone
CL-ML 20 6 Triaxial and 1D
consolidation
S1, Figure 5 Ferreira and Bica (2006)
4 Brown laminated clay ML 35 12 Triaxial S3, Figure 11 Long and O’Riordan
(2001)
5 MRV silt CL 28 6 Triaxial S3, Figure 6 Wang et al. (2011)
6 Silt at Moss Landing B7-03 ML 36 11 Cyclic triaxial S4, Figure 11 Boulanger et al. (1998)
7 Silt at Moss Landing B7-03 ML 31 6 Cyclic triaxial S4, Figure 11 Boulanger et al. (1998)
8 Delhi silt S60M40 ML 27·5 4·5 Triaxial S3, Figure 15 Usmani et al. (2011)
9 Delhi silt S20M80 CL 30 8 Triaxial S3, Figure 15 Usmani et al. (2011)
10 Kaolin soil 45-55 CL-ML 28 7 Triaxial S3, Figure 6 Anantanasakul et al.
(2012)
11 Kaolin soil 24-76 CL-ML 20 4 Triaxial S3, Figure 6 Anantanasakul et al.
(2012)
12 Manglerud quick clay ML 27 8 Direct shear S3, Figure 9 Bjerrum and Landva
(1966)
13 Adapazari silt CL 30·5 5·5 Triaxial S3, Figure 15 Arel and Onalp (2012)
14 Limestone powder CL-ML 24 6 Triaxial S3, Figures 5
and 6
Hyde et al. (2006)
15 Norwegian glaciomarine silt CL 33 12 Triaxial S3, Figure 9 Long et al. (2010)
16 Italian silt (clay content 4%) CL 34 12 Triaxial S3, Figure 7 Nocilla et al. (2006)
17 Italian silt (clay content 8%) CL 37 13 Triaxial S3, Figure 7 Nocilla et al. (2006)
18 Fraser River silt CL 30·4 4·1 Cyclic direct
simple shear
S4, Figure 5 Wijewickreme and Sanin
(2010)
19 Blended silt mixture 1 ML 26 0 Triaxial S3, Figure 9 Boulanger and Idriss
(2006)
20 Blended silt mixture 2 ML 30 4 Triaxial S3, Figure 9 Boulanger and Idriss
(2006)
21 Blended silt mixture 3 ML 36·5 10·5 Triaxial S2, Figure 8
(replotted
including all
scatter points)
Boulanger and Idriss
(2006)
Extracted from Boulanger and Idriss (2004)
BGL, below ground level; HCA, hollow cylinder apparatus
S1: sand-like; sands have a small enough compressibility that their void ratio does not change significantly as the effective consolidation stress is increased
S2: sand-like; the slope of the CSL in void ratio (e) against the logarithm of mean effective stress (p0) space is different from the slope of virgin consolidation line
S3: sand-like; the effective stress paths for sand in undrained monotonic shearing often show an initially contractive response (positive pore pressure increments
since volume change is zero) followed by a transition to an incrementally dilative response (decreases in pore pressure)
S4: sand-like; during the undrained cyclic stress–strain loops, the sands develop a very flat middle portion (where the shear stiffness is essentially zero) that is
observed for sands after the excess pore pressure reaches a limiting value, which corresponds to the sample temporarily having zero effective stress (r
u
= 100%)
C1: clay-like; clays have a large enough compressibility that their void ratio is highly dependent on the effective consolidation stress and consolidation stress history
C2: clay-like; the slope of the CSL in void ratio (e) against the logarithm of the mean effective stress (p0) space is the same as the slope of virgin consolidation line
C3: clay-like; the effective stress paths for clay in undrained monotonic shearing not following S3 behaviour
C4: clay-like; clays show a very plastic stress–strain response (nearly constant shear stress after yield) for O
CR
of 1–8, while sands show a range of strain softening to
strain hardening behaviour that depended on the sand’s relative density and confining stress
C5: clay-like; during the undrained cyclic stress–strain loops, the clays do not develop a very flat middle portion (where the shear stiffness is essentially zero) that is
observed for sands after the excess pore pressure reaches a limiting value, which corresponds to the sample temporarily having zero effective stress (r
u
= 100%)
Table 7. Continued
15
Geotechnical Research Behaviour of MH silts with varying
plasticity indices
Wong, Ong and Robinson
Downloaded by [] on [30/05/17]. Published with permission by the ICE under the CC-BY license
well-documented soils sourced from the literature summarised in
Table 7, as well as KM35 (P
I
= 16%) and KM55 (P
I
= 15%) as
included in Figure 15. Indeed, Figure 8 evidently shows that the
apparent clay-like behaviours of KM35 and KM55 have parallel
CSLs and NCLs.
As all the well-documented data points, including those from this
study, can be simultaneously included within the newly postulated
framework as shown in Figure 15, this study has been proven to
be reliable, consistent and beneficial to engineers seeking a less
expensive classification method for identifying silt behaviour.
Conclusions
Reconstituted kaolin samples of high-plasticity silts (MH) with
varying clay contents were used to investigate the transitional
behaviour of silts. Atterberg limit, dry sieving, hydrometer, CIU
and oedometer tests were conducted on these reconstituted
samples to characterise the behaviours of the samples in terms of
clay-like or sand-like behaviours.
In the v–ln p0space derived from the CIU tests, the gradients of the
respective CSLs seem to be approaching the same gradient of the
NCLs when the P
I
of the reconstituted kaolins increased
progressively from KM20 (P
I
= 7%), KM25 (P
I
= 11%), KM35
(P
I
=16%)toKM55(P
I
= 15%). KM35 and KM55 demonstrate a
clay-like behaviour with P
I
values greater than 13%, while KM20
and KM25 exhibit a sand-like behaviour with P
I
values smaller
than 13%. Therefore, with the support of well-documented soils
sourced from the 33 papers summarised in Table 7 and the research
work carried out in this paper, thresholds for (a) sand-like and (b)
clay-like silts are thus proposed as (a)P
I
£13% and (b)P
I
> 13%,
respectively. Based on the newly postulated framework, all the
well-documented data points from Table 7 as well as the MH data
points from this research can be simultaneously included.
Based on the mentioned important findings of this study, if the
values of Atterberg limits are the only available data, silts can
then be characterised in terms of sand-like or clay-like behaviours
by using the newly proposed framework before an appropriate
KM35
KM55
KM25
KM20
0
0102030405060
70 80 90 100 110
Liquid limit, LL: %
10
20
1
43
2
5
7
910 8
11
Sand-like and clay-like boundary (this study)
Boulanger and Idriss (2006)
‘A’ line
Sand-like behaviour (literature; see Table 5)
Clay-like behaviour (literature; see Table 5)
KM – Sand-like behaviour
KM – Clay-like behaviour
80
70
60
50
40
30
Plasticity Index, Pl: %
Number LL: % Pl: % References
1
2
3
64
5
6
7
8
9
10
11 37
34
33
35
36
59
36
27
30
28
51 7
7
8
8
10
11
11
12
12
12
13 Nocilla et al. (2006)
Nocilla et al. (2006)
Long et al. (2010)
Long and O’Riordan (2011)
Boulanger et al. (1998)
This paper (KM25)
Boulanger and Idriss (2006)
Bjerrum and Landva (1966)
Usmani et al. (2011)
Anantanasakul et al. (2012)
This paper (KM20)
Figure 15. Boundaries for clay-like and sand-like behaviours (this study and others)
16
Geotechnical Research Behaviour of MH silts with varying
plasticity indices
Wong, Ong and Robinson
Downloaded by [] on [30/05/17]. Published with permission by the ICE under the CC-BY license
advanced soil model is selected to represent its behaviour. With
this knowledge, for example, the modified Cam Clay model may
not be wrongly applied to silts with a sand-like behaviour. The
results obtained in this study show good reliability when
compared to 65 sets of past significant experimental studies
derived from 33 established past research papers.
REFERENCES
Almeida MSS, Filho MGR, Futai MM and Filho MR (2012) Laboratory and
in situ studies on a compacted clayey silt fill. Proceedings of the
Institution of Civil Engineers –Geotechnical Engineering 165(2):
107–119, http://dx.doi.org/10.1680/geng.10.00050.
Anantanasakul P, Yamamuro JA and Kaliakin VN (2012) Stress–strain and
strength characteristics of silt–clay transition soils. Journal of Geotechnical
and Geoenvironmental Engineering 138(10): 1257–1265, http://dx.doi.
org/10.1061/(ASCE)GT.1943-5606.0000692.
Arel E and Onalp A (2012) Geotechnical properties of Adapazari silt.
Bulletin of Engineering Geology and the Environment 71(4): 709–720,
http://dx.doi.org/10.1007/s10064-012-0443-6.
ASTM (2000) D 2487: Standard practice for classification of soils for
engineering purposes (unified soil classification system). ASTM
International, West Conshohocken, PA, USA.
ASTM (2011) D 2435: Standard test methods for one-dimensional
consolidation properties of soils using incremental loading. ASTM
International, West Conshohocken, PA, USA.
Biscontin G, Cola S, Pestanam JM and Simonini P (2007) Unified
compression model for Venice lagoon natural silts. Journal of
Geotechnical and Geoenvironmental Engineering 133(8): 932–942,
http://dx.doi.org/10.1061/(ASCE)1090-0241(2007)133:8(932).
Bjerrum L and Landva A (1966) Direct simple-shear tests on a Norwegian
quick clay. Géotechnique 16(1):1–20, http://dx.doi.org/10.1680/geot.
1966.16.1.1.
Boulanger R and Idriss IM (2004) Evaluating the Potential for Liquefaction
of Cyclic Failure of Silts and Clays. Center for Geotechnical Modeling,
University of California, Davis, CA, USA.
Boulanger R and Idriss IM (2006) Liquefaction susceptibility criteria for
silts and clays. Journal of Geotechnical and Geoenvironmental
Engineering 132(11): 1413–1426, http://dx.doi.org/10.1061/(ASCE)
1090-0241(2006)132:11(1413).
Boulanger RW, Meyers MW, Mejia L and Idriss IM (1998) Behaviour of a
fine-grained soil during the Loma Prieta earthquake. Canadian
Geotechnical Journal 35(1): 146–158, http://dx.doi.org/10.1139/t97-
078.
BSI (1990a) BS 1377-2:1990: Methods of test for soils for civil
engineering purposes. Classification tests. BSI, London. UK.
BSI (1990b) BS 1377-5:1990: Methods of test for soils for civil
engineering purposes. Compressibility, permeability and durability
tests. BSI, London. UK.
BSI (1990c) BS 1377-8:1990: Methods of test for soils for civil engineering
purposes. Shear strength tests (effective stress). BSI, London. UK.
Burland JB (1990) On the compressibility and shear strength of natural
clays. Géotechnique 40(3): 329–378, http://dx.doi.org/10.1680/geot.
1990.40.3.329.
Cetin H and Soylemez M (2004) Soil-particle and pore orientations during
drained and undrained shear of a cohesive sandy silt–clay soil.
Canadian Geotechnical Journal 41(1): 1127–1138, http://dx.doi.org/
10.1139/t04-055.
Chiu CF and Ng CWW (2012) Coupled water retention and shrinkage
properties of a compacted silt under isotropic and deviatoric stress
paths. Canadian Geotechnical Journal 49(1): 928–938, http://dx.doi.
org/10.1139/t2012-055.
Cotecchia F, Cafaro F and Aresta B (2007) Structure and mechanical
response of sub-Apennine Blue Clays in relation to their geological
and recent loading history. Géotechnique 57(2): 167–180, http://dx.
doi.org/10.1680/geot.2007.57.2.167.
Cui YJ and Delage P (1996) Yielding and plastic behaviour of an
unsaturated compacted silt. Géotechnique 46(2): 291–311, http://dx.
doi.org/10.1680/geot.1996.46.2.291.
Cunningham MR, Ridley AM, Dineen K and Burland JB (2003) The
mechanical behaviour of a reconstituted unsaturated silty clay.
Géotechnique 53(2): 183–194, http://dx.doi.org/10.1680/geot.2003.53.
2.183.
Ferreira PM and Bica AVD (2006) Problems in identifying the effects of
structure and critical state in soil with transitional behaviour. Géotechnique
56(7): 445–454, http://dx.doi.org/10.1680/geot.2006.56.7.445.
Georgiannou VN, Burland JB and Hight DW (1990) The undrained
behaviour of clayey sands in triaxial compression and extension.
Géotechnique 40(3): 431–449, http://dx.doi.org/10.1680/geot.1990.40.
3.431.
Head KH (1998) Effective Stress Tests. Manual of Soil Laboratory Testing.
Wiley, Singapore, vol. 3.
Hyde AF, Higuchi T and Yasuhara K (2006) Liquefaction, cyclic mobility,
and failure of silt. Journal of Geotechnical and Geoenvironmental
Engineering 132(6): 716–735, http://dx.doi.org/10.1061/(ASCE)1090-
0241(2006)132:6(716).
Hyodo M, Yamamoto Y and Sugiyama M (1994) Undrained cyclic
behaviour of normally consolidated clay subjected to initial static
shear stress. Soils and Foundations 34(4):1–11, http://doi.org/10.
3208/sandf1972.34.4_1.
Ishihara K, Tatsuoka, Fumo and Yasuda S (1975) Undrained deformation
and liquefaction of sand under cyclic stress. Soils and Foundations
15(1):29–44, http://doi.org/10.3208/sandf1972.15.29.
Ladd CC (1991) Stability evaluation during staged construction. Journal
of Geotechnical and Geoenvironmental Engineering 117(4): 540–615,
http://dx.doi.org/10.1061/(ASCE)0733-9410(1991)117:4(540).
Ladd CC, Foott R, Ishihara K, Schlosser F and Poulos HG (1977) Stress-
deformation and strength characteristics. In Procedings of 9th
International Conference on Soil Mechanics and Foundation
Engineering. Japanese Society of Soil Mechanics and Foundation
Engineering, Tokyo, Japan, vol. 2, pp. 421–494.
Lambe TW and Whitman RV (1969) Soil Mechanics. Wiley, New York,
NY, USA.
Lehane BM (2003) Vertically loaded shallow foundation on soft clayey
silt. Geotechnical Engineering 156(1):17–26, http://dx.doi.org/10.
1680/geng.2003.156.1.17.
Long M (2006) Sample disturbance effects on medium plasticity clay/silt.
Geotechnical Engineering 159(2):99–111, http://dx.doi.org/10.1680/
geng.2006.159.2.99.
Long MM and O’Riordan J (2001) Field behaviour of very soft clays at
the Athlone embankments. Géotechnique 51(4): 293–309, http://dx.
doi.org/10.1680/geot.2001.51.4.293.
Long M, Gudjonsson G, Donohue S and Hagberg K (2010) Engineering
characterization of Norwegian glaciomarine silt. Engineering Geology
110(3–4):51–65, https://doi.org/10.1016/j.enggeo.2009.11.002.
LtifiM, Abichou T and Tisot JP (2014) Effects of soil aging on mechanical
and hydraulic properties of silty soil. Geotechnical and Geological
Engineering 32(4): 1101–1108, http://dx.doi.org/10.1007/s10706-014-
9784-1.
Mao X and Fahey M (2003) Behaviour of calcareous soils in undrained
cyclic simple shear. Géotechnique 53(8): 715–727, http://dx.doi.org/
10.1680/geot.2003.53.8.715.
Mesri G, Rokshsar A and Bohor BF (1975) Composition and
compressibility of typical samples of Mexico City clay. Géotechnique
25(3): 527–554, http://dx.doi.org/10.1680/geot.1975.25.3.527.
Mountassir GE, Sanchez M, Romero E and Soemitro RAA (2011) Behaviour
of compacted silt use to construct flood embankment. Proceedings of the
Institution of Civil Engineers –Geotechnical Engineering 164(3):
195–210, http://dx.doi.org/10.1680/geng.10.00055.
17
Geotechnical Research Behaviour of MH silts with varying
plasticity indices
Wong, Ong and Robinson
Downloaded by [] on [30/05/17]. Published with permission by the ICE under the CC-BY license
Nishimura S, Minh NA and Jardine RJ (2007) Shear strength anisotropy of
natural London clay. Géotechnique 57(1):49–62, http://dx.doi.org/10.
1680/geot.2007.57.1.49.
Nocilla A and Coop MR (2008) The behaviour of sub-soils from the Po
Rivers embankments: an example of transitional behaviour in natural
soils. Italian Geotechnical Journal 42(1):49–58.
Nocilla A, Coop MR and Colleselli F (2006) The mechanics of an Italian
silt: an example of ‘transitional’behaviour. Géotechnique 56(4):
261–271, http://dx.doi.org/10.1680/geot.2006.56.4.261.
Pillai R, Robinson RG and Boominathan A (2011) Effect of microfabric on
undrained static and cyclic behaviour or kaolin clay. Journal of
Geotechnical and Geoenvironmental Engineering 137(4): 421–429,
http://dx.doi.org/10.1061/(ASCE)GT.1943-5606.0000442.
Skempton AW (1953) The colloidal ‘activity’of clays. Proceedings of the
3rd International Conference on Soil Mechanics, Zurich, Switzerland,
vol. 1, pp. 57–61.
Skempton AW (1985) Residual strength of clays in landslides, folded
strata and the laboratory. Géotechnique 35(1):3–18, http://dx.doi.org/
10.1680/geot.1985.35.1.3.
Usmani A, Ramana GV and Sharma KG (2011) Experimental evaluation of
shear-strength behaviour of Delhi silt under static loading conditions.
Journal of Materials in Civil Engineering 23(5): 533–541, http://dx.
doi.org/10.1061/(ASCE)MT.1943-5533.0000203.
Wang S and Luna R (2012) Monotonic behaviour of Mississippi River
Valley silt in triaxial compression. Journal of Geotechnical and
Geoenvironmental Engineering 138(4): 516–525, http://dx.doi.org/10.
1061/(ASCE)GT.1943-5606.0000603.
Wang Q, Pufahl DE and Fredlund DG (2002) A study of critical state on
an unsaturated silty soil. Canadian Geotechnical Journal 39(1):
213–218, http://dx.doi.org/10.1139/t01-086.
Wang S, Luna R and Stephenson RW (2011) A slurry consolidation
approach to reconstitute low-plasticity silt specimens for laboratory
triaxial testing. Geotechnical Testing Journal 34(4):1–9, https://doi.
org/10.1520/GTJ103529.
Wijewickreme D and Sanin M (2010) Postcyclic reconsolidation strains in
low-plastic Fraser River silt due to dissipation of excess pore-water
pressures. Journal of Geotechnical and Geoenvironmental
Engineering 136(10): 1347–1357, http://dx.doi.org/10.1061/(ASCE)
GT.1943-5606.0000349.
Yin J (1999) Properties and behaviour of Hong Kong marine deposits
with different clay contents. Canadian Geotechnical Journal 36(1):
1085–1095, http://dx.doi.org/10.1139/t99-068.
Zhang ZM, Yu J, Zhang GX and Zhou XM (2009) Test study on the
characteristics of mudcakes and in situ soils around bored piles.
Canadian Geotechnical Journal 46(1): 241–255, http://dx.doi.org/10.
1139/T08-119.
How can you contribute?
To discuss this paper, please submit up to 500 words to
the editor at journals@ice.org.uk. Your contribution will be
forwarded to the author(s) for a reply and, if considered
appropriate by the editorial board, it will be published as a
discussion in a future issue of the journal.
18
Geotechnical Research Behaviour of MH silts with varying
plasticity indices
Wong, Ong and Robinson
Downloaded by [] on [30/05/17]. Published with permission by the ICE under the CC-BY license
