Compacted aggregate-clay mixtures are currently successfully used as
the cores of embankment dams. These materials, called composite clays
by Jafari and Shafiee (2004), are usually broadly graded and are composed
of clay as the main body with sand, gravel, cobble or even boulders floating
in the clay matrix. Karkheh and Gotvand dams in Iran are some recent examples
of dams with cores composed of aggregate-clay mixtures.
It is also a current practice to employ a mixture of high plastic clay
with aggregates as impervious blankets for waste disposal projects (Chapuis,
1990; Pandian et al., 1995; Abichou et al., 2002). It is
generally assumed that the coarser fraction of such soils imparts a relatively
higher shear strength and high compacted density while the permeability
of the soil is governed by the proportion and index properties of the
finer fraction (Shafiee, 2008).
Yin (1999) experimentally investigated the behavior of Hong Kong marine
deposits with different clay contents. Test results revealed that the
friction angle of deposits decreases with an increase in plasticity index.
Young`s modulus also increases with an increase in effective confining
pressure but decreases with an increase in clay content. Vallejo and Mawby
(2000) carried out a series of direct shear tests and porosity measurements
on sand-clay mixtures. It was found that the percentage of sand in the
mixtures had a marked influence on their shear strength. It was determined
that when the concentration by weight of the sand in the mixtures was
more than 75%, the shear strength of the mixtures was governed mainly
by the frictional resistance between the sand grains. When the concentration
of sand varied between 75 and 40%, the shear strength of the mixture was
provided in part by the shear strength of the clay and in part by the
frictional resistance between the sand grains. When the sand concentration
was less than 40% by weight, the shear strength of the mixture was entirely
dictated by strength of clay.
Muir Wood and Kumar (2000) conducted drained and undrained triaxial
compression tests on isotropically normal consolidated and overconsolidated
mixtures of kaolin clay and coarse uniform sand. It was found that the
deviator stress, clay volumetric strain and pore pressure were unaffected
by the presence of the sand until the clay content fell below 40%. Jafari
and Shafiee (2004) carried out a series of strain-controlled monotonic
and cyclic triaxial tests on gravel-fat clay and sand-fat clay mixtures
to investigate the effects of aggregate on the mechanical behavior of
the mixtures. Compression monotonic test results revealed that the angle
of shearing resistance increased with aggregate content. Also, when aggregate
content was raised, pore pressure rose in both monotonic and cyclic loading.
It was also found that the presence of aggregates within a cohesive matrix
led to formation of a heterogeneous field of density in the clayey part
of the mixture. Prakasha and Chandrasekaran (2005) conducted an experimental
study on reconstituted Indian marine soils having different proportions
of sand and clay. Test results revealed that sand grains in clay leads
to reduction in void ratio and increase in friction and pore pressure
response resulting in decrease in undrained shear strength.
A review of the published literature reveals that experimental studies
on aggregate-clay mixtures have mainly focused on shear strength parameters,
particularly in compression loading and shear strength either increases
with aggregate content or remains constant until a limiting aggregate
content, then increases as the aggregate content increases. To explore
all features of mechanical behavior, there is a need to investigate pre-failure
along with failure behavior of the mixtures subjected to various loading
paths. The present study describes the pre-failure and failure characteristics
of compacted sand-clay mixtures under monotonic compression and extension
MATERIALS AND METHODS
Materials tested: Pure clay with two mixtures of sand-clay were
used in this study. The physical properties of the materials were measured
just prior to the beginning of the shear tests. The clay had a specific
gravity of 2.70, liquid limit of 42% and plasticity index of 18%. The
grain-size distribution curve for the clay is shown in Fig.
1. X-ray diffraction analysis revealed that the clay was mainly composed
of kaolinite with some illite, montmorilonite and quartz.
The sand used in the study was retrieved from a riverbed and composed
of subrounded particles with a specific gravity of 2.65. The aggregates
used as sand material passed through a 4.75 mm sieve and was retained
on a 3.35 mm sieve, with minimum and maximum void ratios of 0.667 and
0.803, respectively. The gap graded gradation was considered for the aggregates
to minimize the effect of particle size distribution of sand on the mechanical
behavior of the mixture. Figure 1 shows the grain-size
distribution curve of the parent granular material from which the sand
The clay was mixed with different amounts of sand to obtain different
mixtures. Three mixtures were obtained by mixing 100, 60 and 40% of clay
with sand in volumetric proportions. A minimum of 40% clay content was
considered since this is a limit value for materials used as cores in
Specimen preparation: The specimen preparation technique was chosen
to model as precisely as possible the in situ condition of the core materials
of embankment dams. All specimens, typically 38 mm in diameter and 76
mm in height, were prepared with a dry density of 95% of the maximum dry
density obtained from the standard compaction test method and water content
of 2% wet of optimum. Table 1 presents the initial dry
density and water content of the specimens.
||Particle size distribution of materials used in the
|| Specimen properties
Appropriate amounts of clay and sand for each layer were first thoroughly
mixed. Each layer was then mixed with water at least 24 h before use and
sealed. The material was poured in six layers into a cylindrical mold
and compacted. To achieve a greater uniformity of specimens, a procedure
similar to the undercompaction technique (Ladd, 1978) was used. For each
layer, the compactive effort was increased toward the top by increasing
the number of blows per layer. Each layer was then scored after it was
compacted for better bonding with the next layer.
To reduce the effect of cap friction during the triaxial test, two thin
rubber sheets coated with silicone grease were placed between the lower
and upper porous stones and the specimen. Further, the sheets in contact
with the specimen were divided into four sectors. This was done to let
the specimen deform more easily in the lateral direction. Five drainage
holes of about 5 mm in diameter were also provided in the rubber sheets
to facilitate the saturation and consolidation process. The specimen preparation
technique was verified when repeated testing of similar specimens yielded
Test procedure: The specimens were saturated with a Skempton B
value in excess of 97%. To facilitate the saturation process, CO2
was first percolated through the specimens (this was more effective for
saturation of the low clay content specimens), then de-aired water was
flushed into the specimens. Lastly, a back pressure of 200 kPa was incrementally
applied to accelerate the saturation rate. The specimens were then isotropically
consolidated under effective stresses of 100, 300 and 500 kPa.
Following consolidation, undrained monotonic triaxial tests were carried
out under strain-controlled conditions either in compression or extension.
An advanced automated triaxial testing apparatus was used to conduct the
monotonic tests. The specimens were tested at an axial rate of 0.01 mm
min-1. The loading rate was chosen so that pore pressure equalization
throughout the specimen was ensured. The compression and extension monotonic
tests were continued until an axial strain of at least 15% was achieved.
RESULTS AND DISCUSSION
Stress-strain characteristics: Figure 2 shows
the variation of stress ratio, q/p`0, versus axial strain,
εa, in compression and extension tests at various initial
confining stresses, q/p`0, where q is the deviator stress and
defined as (
being the principal effective stresses). As can be seen, sand content
has a dramatic influence on the stress-strain behavior of the mixtures
in compression and stress ratio increases remarkably with sand content.
For example, the addition of 60% sand into the clay leads to 280, 210
and 170% increases in stress ratio at an axial strain level of 3% (where
the behavior starts to stabilize) for initial confining stresses of 100,
300 and 500 kPa, respectively.
It is also found that, regardless of the sand content and strain level,
the stress ratio decreases when the initial confining stress is raised.
In the extension tests, stress ratio slightly decreases with sand content
for strain levels of less than 3%. In high strain levels, the effect of
sand content on the behavior gradually diminishes and all curves converge
to the same value. It appears that in the extension test, as opposed to
the compression test, the undrained shear strength is unaffected by the
Figure 3 presents the variation of normalized undrained
shear strength, Su/p`0, with sand content in compression
and extension tests at various initial confining stresses, where Su
refers to the undrained shear strength and is defined as half of the deviator
stress at an axial strain level of 15%. Since stress-strain curves do
not exhibit a clear peak, deviator stress at a high axial strain is selected
as an indication of shear strength. Figure 3a clearly
indicates that, in compression loading, the undrained shear strength increases
with sand content. The increase is remarkable, when sand content reaches
as high as 60% and the initial confining stress is low (i.e., 100 kPa).
On the other hand, sand content does not remarkably affect the undrained
shear strength in extension loading and shear strength exhibits a gradual
increase with sand content for all confining stresses (Fig.
3b). It is also worth noting that, regardless of the initial confining
stress, the undrained shear strength in extension tests is less than for
the compression test at an identical sand content. In addition, normalized
undrained shear strength decreases with confining stress in both compression
and extension loading. Keeping in mind that the value of Su/p`0
(in compression) for normally consolidated clays is only related to the
plasticity index (e.g., Chandler, 1988), it can be inferred that the tested
materials are overconsolidated (because of the high compaction energy
transferred to them during specimen preparation) with a preconsolidation
pressure of between 100 to 300 kPa.
||Stress ratio-strain curves of sand-clay mixtures tested
under different initial confining stresses
||Undrained shear strength of sand-clay mixtures in (a)
compression and (b) extension
As seen in Fig. 3a, when p`0 is more than
100 kPa, the tested materials behave as a normally consolidated soil and
Su/p`0 remains nearly constant, independent of initial
confining stress. The rate of decrease in Su/p`0
with confining stress increases with sand content in compression; however,
the rate is nearly identical for all mixtures in extension. Figure
4 also indicates the extent of the undrained shear strength anisotropy
of compacted sand-clay mixtures. The anisotropy in undrained shear strength
is defined as the ratio of shear strength in extension to compression
loading, Sue/Suc (Duncan and Seed, 1966). It should
be noted that aggregate-clay mixtures, whether used as the core of embankment
dams or as the materials in soil liner systems, are inherently anisotropic
owing to the method of compaction (the technique used herein for the specimen
preparation). As can be seen, the strength anisotropy is manifested when
sand content is raised. The undrained shear strength anisotropy ranges
from 0.66 to 0.72 for pure clay, while it varies from 0.34 to 0.52 for
the mixture containing 60% sand. Test results also reveal that the influence
of the sand content on the strength anisotropy is more pronounced at an
initial confining stress of 100 kPa.
||Undrained shear strength anisotropy of sand-clay mixtures
To explore the effect of plasticity of the clay on undrained shear strength,
a comparison was made with the shear strength of aggregate-fat clay mixtures
studied by Jafari and Shafiee (2004), where the clay had a plasticity
index of 38%. As shown in Fig. 5, when the plasticity
of the clay is reduced, the undrained shear strength increases. This is
caused by the tendency of the mixture to show more contractive behavior
(leading to an increase in pore pressure build-up), when the plasticity
of the clay is increased. For example, normalized pore pressure in 60%
aggregate -40% fat clay mixtures ranges from 0.65 to 0.68 at an axial
strain of 15% and an initial confining stress of 500 kPa (Jafari and Shafiee,
2004), while it is 0.52 for the sand-clay mixture at an identical axial
strain and initial confining stress (Fig. 6c). It is
also interesting to note that the effect of clay plasticity on the undrained
shear strength is more evident when the aggregate content is raised (Fig.
||Effect of the plasticity of clay on the undrained shear
strength of aggregate-clay mixtures
Pore pressure-strain characteristics: For a fundamental understanding
of the undrained behavior of sand-clay mixtures, it is prudent to observe
the pore pressure generation pattern in the mixtures. Figure
6 and 7 compare normalized pore pressure, uN
(pore pressure normalized to initial confining stress), in terms of axial
strain for all mixtures under compression and extension monotonic loading
paths, respectively, at different confining stresses. As shown, regardless
of the loading path and initial confining stress, pore pressure generally
increases with sand content, so that uN is highest for the
mixture containing 60% sand and lowest for pure clay. The difference in
uN is more pronounced at higher confining stresses (Fig.
6c, 7c). The reason behind this behavior can simply
be explained: since the compressibility of the clayey matrix is greater
than for individual grains, all of the specimen deformations take place
in the clay. Hence, during strain-controlled loading, the clayey matrix
of the specimens containing more aggregate experiences more deformation
for the same strain level, directly leading to more pore pressure generation.
Similar observations were made by Jafari and Shafiee (2004) from the strain-controlled
compression monotonic triaxial tests on sand-fat clay and gravel-fat clay
mixtures. If one assumes that all of the specimen deformations occur homogeneously
in the clayey matrix of the mixtures and in-contact grains are not sufficient
to form a granular skeleton that can deform, then all of the deformations
take place in the clayey matrix. In this case, the strain in the clayey
matrix, called effective axial strain, εeff, can be defined
as (Jafari and Shafiee, 2004):
||Variation of pore pressure in sand-clay mixtures under
a triaxial compression loading path
where, c is the percent of volumetric clay. Hence, if pore pressure is
compared with εeff, it is anticipated that the direct
effect of sand volume will be eliminated and the uN versus
εeff curves for different specimens will coincide.
Figure 8 compares pore pressure versus effective axial
strain in compression tests. The figure indicates that, when pore pressure
is plotted against effective axial strain, the curves tend to converge
(compare with the curves in Fig. 6), particularly in
low strain levels, but do not coincide. This means that the assumption
of a uniform field of deformation in the clayey matrix is no longer valid
and coarse particles floating in a fine matrix can induce a non-uniform
field of density and consequently, deformation in the fine matrix. This
was previously addressed by Fragaszy et al. (1992) for sandy gravels
containing oversized particles and by Jafari and Shafiee (2004) for cohesive
materials containing aggregates.
It is interesting to note that, in extension tests (Fig.
7), negative pore pressure develops at the beginning of the test.
The phenomenon may be attributed to the elastic response of the compacted
materials to the reduction of the deviator and mean stress. The range
of axial strains, where negative pore pressure develops, decreases with
sand content. In addition, regardless of the loading path and initial
confining stress, all specimens more or less exhibit a peak in pore pressure
that is an indication of dilative behavior (Fig. 6,
7). The dilative behavior is more evident in extension
than compression tests and when sand content is raised, particularly at
a low confining stress (100 kPa). The axial strain to the peak decreases
with sand content, however, the peak tends to migrate to higher strain
levels with an increase in initial confining stress.
||Variation of pore pressure in sand-clay mixtures under
a triaxial extension loading path
||Pore pressure in terms of effective strain in sand-clay
mixtures under a triaxial compression loading path
The test results presented in Fig. 2 and 6
reveal that both deviator stress and pore pressure increase with sand
content. Hence, the pore pressure parameter, A, can be regarded as a suitable
parameter for an appropriate description of the undrained behavior of
compacted sand-clay mixtures. In triaxial tests, A is expressed as follows
(Skempton and Bishop, 1954):
||Variation of pore pressure parameter, A, in sand-clay
mixtures under different initial confining stresses
||Pore pressure parameter at failure, Af, in
terms of sand content under (a) compression and (b) extension
where, u is pore pressure. As shown in Fig. 9, regardless
of initial confining stress, A decreases generally with sand content.
Thus, A is lowest for the mixture containing 60% sand and highest for
Figure 10 also displays the pore pressure parameter
at failure, Af, versus sand content at different initial confining
stresses. Af is calculated at an axial strain level of 15%.
The Fig. 10 indicates that, for both types of loading
path and regardless of initial confining stress, the value of Af
decreases with sand content. In compression, the value of Af
varies from 0.14 to 0.49 for pure clay and from 0.03 to 0.44 for the mixture
containing 60% sand. In addition, the value of Af depends on
the initial confining stress; in compression tests it increases with confining
stress (Fig. 10a), while in extension tests it decreases
with confining stress (Fig. 10b).
||Stress paths of sand-clay mixtures in compression tests:
(a) pure clay, (b) 40% sand -60% clay and (c) 60% sand -40% clay
Effective stress path: The interesting features of the behavior
of compacted sand-clay mixtures, shown by the stress-strain and pore pressure-strain
curves, can be better represented by effective stress paths. The effective
stress paths for all mixtures in compression and extension tests are presented
in Fig. 11 and 12, respectively.
The failure line is the locus of the deviator stress, q and mean effective
at an axial strain of 15%. The phase transformation line, PTL (Ishihara
et al., 1975), is where the behavior changes from contractive to
dilative. These are superimposed on the figures. Since, in most cases
(Fig. 2, 6, 7),
a plateau in stress-strain and pore pressure-strain is reached, the failure
line can be regarded as the critical state line, CSL (Roscoe et al.,
1958). The CSL and PTL shown in these figures have been established by
fitting the best line (passing from origin) to the q and p` data at critical
state and phase transformation, respectively. As seen in Fig.
11 and 12, all specimens experience dilative behavior
either on compression or extension loading paths prior to critical state.
In addition, regardless of the loading path, the slopes of the CSL and
PTL increase with sand content. In addition, the plots show that PTL approaches
CSL when the sand content is raised.
Table 2 also presents the values of the mobilized angle
of shearing resistance at critical state (φ`) for sand-clay mixtures.
The value of φ` is determined by the following equations (Roscoe
and Burland, 1968):
where, M is the slope of the CSL. As seen in Table 2,
regardless of the loading path, φ` increases with sand content and
the presence of 60% sand leads to an 11° increase in φ`. In addition,
φ` in compression is slightly more than for extension for all mixtures.
A comparison of the φ` of the sand-lean clay mixtures of this study
with that of the aggregate-fat clay mixtures studied by Jafari and Shafiee
(2008) shows that the plasticity of the clay does not affect φ` remarkably
||Stress paths of sand-clay mixtures in extension tests:
(a) pure clay, (b) 40% sand -60% clay and (c) 60% sand -40% clay
Deformational characteristics: Strain-dependent soil stiffness
is an important pre-failure property that controls soil deformations.
It is well known that the static deformation modulus is mainly governed
by strain amplitude and initial confining stress (Yasuhara et al.,
1982). Herein, the secant modulus at 50% shear strength, E50,
is used to compare the deformational properties of the mixtures. E50
is usually used in elastic-perfect plastic models of soil materials. It
is an average stiffness corresponding to a stress state intermediate between
the beginning and end of the test (Muir Wood, 2004). Variations of E50
with sand content at different initial confining stresses on both compression
and extension loading paths are presented in Fig. 14.
From these plots it is clear that E50 increases with sand content
and initial confining stress in both compression and extension. As shown,
the value of E50 in extension is more than that for compression.
However, the difference does not depend on sand content.
||Effect of the plasticity of clay on the angle of shearing
||Effect of sand content on secant modulus at 50% shear
|| Angle of shearing resistance at critical state
An experimental study was performed on compacted mixtures of sand-clay
at different initial confining stresses under triaxial compression and
extension loading paths to thoroughly investigate the various aspects
of the undrained behavior of the mixtures. The following conclusions may
be drawn based on this experimental study:
||The undrained shear strength in compression, Suc,
increases with sand content, particularly when the sand content reaches
as high as 60% and the initial confining stress is low. On the other
hand, undrained shear strength in extension, Su, gradually
increases with sand content. This would increase the extent of the
undrained shear strength anisotropy (Sue/Suc)
with sand content. In addition, Su in extension is less
than in compression at an identical sand content.
||Plasticity of the clay and initial confining stress affect the undrained
shear strength. It can be shown that Su decreases when
the plasticity of the clay increases. The effect of clay plasticity
on Su is more evident when the aggregate content is raised.
The shear strength normalized to the initial confining stress, Su/p`0,
depends on confining stress and decreases with confining stress in
both compression and extension loading. The rate of decrease in Su/p`0
with confining stress increases with sand content in compression,
however, the rate is nearly identical for all mixtures in extension.
It is also found that the tested materials are overconsolidated, as
demonstrated by the fact that Su/p`0 depends
on the initial confining stress.
||An increase in sand content causes more deformation of the clayey
matrix at the same strain level, leading to more pore pressure generation
in compression and extension. Even if pore pressure is compared with
the same strain level in clay, it still increases with sand content.
This shows the non-uniformity of the deformation field in the clayey
matrix of the mixture.
||Negative pore pressure develops in extension at the beginning of
the test. The range of axial strain, where negative pore pressure
develops, decreases with sand content. In addition, all the specimens
more or less exhibit dilative behavior. The dilative behavior is more
evident in extension than in compression and when the sand content
is raised, particularly at low confining stresses. The axial strain
to the peak in pore pressure response decreases with sand content,
however, the peak tends to migrate to higher strain levels with an
increase in initial confining stress.
||The pore pressure parameter at failure, Af (pore pressure
divided by deviator stress at failure) decreases with sand content
in compression and extension. The value of Af depends upon
the initial confining stress; in compression it increases with confining
stress, while in extension it decreases.
||The slopes of the CSL and PTL in the p`-q plane (where p` and q
are mean and deviator stress, respectively) increase with sand content.
The PTL approaches the CSL when sand content is raised. In addition,
the mobilized angle of shearing resistance at critical state increases
with sand content, however, its value in compression is slightly more
than in extension.
||The secant modulus at 50% shear strength, E50, increases
with sand content and initial confining stress in both compression
and extension. The value of E50 in extension is more than
that for compression.