Geotechnical and Geological Engineering 21: 1–28, 2003.
# 2003 Kluwer Academic Publishers. Printed in the Netherlands.
1
Triaxial behaviour of a cemented
gravely sand, Tehran alluvium
E. ASGHARI,1 D. G. TOLL2 and S. M. HAERI3
1
Department of Engineering Geology, University of Tarbiat Modarres, Tehran, Iran
([email protected])
2
School of Engineering, University of Durham, Durham, DH1 3LE, UK
([email protected])
3
Civil Engineering Department, Sharif University of Technology, Tehran, Iran
([email protected])
(Received 9 July 2002; accepted without revision 26 August 2002)
Abstract. Cemented coarse-grained alluvium is present in a vast area of Tehran city, Iran
including its suburbs. This deposit consists of gravely sand to sandy gravel with some cobbles
and is dominantly cemented by carbonaceous materials. In order to understand the mechanical behaviour of this soil, a series of triaxial compression tests were performed on uncemented, artificially cemented and destructured samples. Hydrated lime was used as the
cementation agent for sample preparation to model the Tehran deposit. The tests were performed on cemented samples after an appropriate time for curing. The tests on cemented samples show that a shear zone appears as the shear stress approaches the peak shear strength.
During shearing these samples undergo dilation at confining stress lower than 1000 kPa. However, the uncemented and destructured samples show contraction during shearing. Peak shear
strength is followed by strain softening for all cemented samples. The shear strength increases
with increasing cement content but the influence of the cementation decreases as the confining
stress increases. With increasing cementation the stress-strain behaviour of samples tend
towards the behaviour expected of high-density soils. Test results indicate that the failure
envelope for cemented samples is curved and not linear.
Key words. cemented soil, gravely sand, lime, shear strength, Tehran alluvium.
1. Introduction
Earth slopes and high vertical cuts are frequently observed to be stable for long periods in coarse-grained Tehran alluvium, Iran. Figure 1 shows a very steep cut in
Tehran alluvium, which has been stable for a long period of time. Stability is often
attributed to cementation effects producing increased shear strength in this deposit.
Tehran alluvium exists in the form of hills and valleys, therefore for construction of
highways, roads, high-rise buildings and other projects like underground rails and
roads, construction of tunnels or deep excavations are required. Foundation performance on this deposit is also dependent on the bonded structure of the loaded soil,
which contributes to settlement reduction and increase in bearing capacity. Due to
the presence of some large cobbles in the deposit and variable cementation of the
2
E. ASGHARI, D. G. TOLL AND S. M. HAERI
Figure 1. An example of stable deep trench in the cemented coarse-grained alluvium of Tehran.
soil, it is extremely difficult to obtain undisturbed representative samples. Therefore
in most projects, the designers do not consider the cementation effects on the shear
strength and deformation parameters and usually use test results on disturbed and
reconstituted samples.
The cemented coarse-grained alluvium of Tehran often consists of gravely sand to
sandy gravel with some cobbles, silt and clay. The degree of cementation of this alluvium usually varies not only on a large scale, but also on a small scale.
The effects of low to moderate degree of cementation on the mechanical behaviour
of naturally and artificially cemented coarse-grained soils have been investigated by
several researchers [e.g., Saxena and Lastrico (1978), Clough et al. (1979, 1981),
Dupas and Pecker (1979), Acar and El-Tahir (1986), Leroueil and Vaughan
(1990), Airey (1993), Coop and Atkinson (1993), Gens and Nova (1993), Cuccovillo
and Coop (1997), Malandraki and Toll (2000, 2001), and Schnaid et al. (2001)]. In
almost all the experimental programs reported in the literature, artificially cemented
samples have been used either to establish a fundamental understanding of the behaviour of natural soils or to develop soil stabilization methods using a cementation
agent.
The present work evaluates the mechanical behaviour of an artificially cemented
gravely sand using triaxial compression tests. In this work the cemented coarsegrained alluvium of Tehran is modeled. The weak to medium strength cement of
the Tehran alluvium is carbonaceous material deposited from ground water. Therefore, the soil used in this research was obtained from Tehran alluvium and hydrated
lime was used for cementing the samples. The behaviour of cemented samples is
TRIAXIAL BEHAVIOUR OF A CEMENTED GRAVELY SAND, TEHRAN ALLUVIUM
3
compared with that of the uncemented and destructured soils to examine the influence of cementation. The degree of cementation and the effective stress level are two
major variables examined.
2. Research Background on Cemented Coarse-Grained Soils
Cementation plays an important role in the stress-strain and strength behaviour of
frictional materials (Lade and Overton, 1989). From a mechanical point of view,
cemented soils, weak rocks, and bonded materials constitute an intermediate class
of geo-materials that fall between classical soil mechanics and rock mechanics. Often
considered as nontextbook materials, there is a lack of physical and mathematical
models able to integrate all these materials in a consistent and unified framework
(Schnaid et al., 2001).
In the last decades, however, researchers have made noteworthy experimental contributions in this field [e.g., Saxena and Lastrico (1978), Clough et al. (1979, 1981),
Dupas and Pecker (1979), Acar and El-Tahir (1986), Leroueil and Vaughan (1990),
Airey (1993), Coop and Atkinson (1993), Gens and Nova (1993), Cuccovillo and
Coop (1997), Malandraki and Toll (2000, 2001), and Schniad et al. (2001)].
A review of the literature enables one to identify some important characteristics of
the behaviour of cemented soils, especially those of cemented granular material.
Saxena and Lastrico (1978) pointed out that the cemented skeleton initially compresses under a load as would be expected, but with further straining, the cemented
soil skeleton tends to dilate (as would a dense material). Lade and Overton (1989)
stated that because large particles were cemented together from smaller particles
and the large particles were highly interlocked, thus producing greater rates of dilation during shearing at low confining stresses. Saxena and Lastrico (1978) suggested
that at low axial strains (<1%), the cohesion caused by the cement bonding
between particles is the major component of the strength. The cohesive shear
strength vanishes around 1% strain and at the same time the frictional strength
becomes dominant. They reported that a very high confining stress could break
the cementation bonds as well. Clough et al. (1981) suggested that the nature and
amount of cement, confining stress, density, gradation and structure of the soil
are the governing variables for cemented soil behaviour. Cemented sand shows a
brittle failure mode at low confining stresses with a transition to ductile failure at
higher confining stresses.
Leroueil and Vaughan (1990) proposed a conceptual approach to describe the
stress-strain behaviour of soils that show similar characteristics due to the existence
of a common bonded structure. The effect of structure on soil behaviour is similar to
that resulting from over consolidation in clays and could be represented by a yield
surface in stress space. Basically, it comprises initially stiff behaviour followed by
increasingly plastic deformation as the soil approaches the failure point. Huang
and Airey (1993) showed that the yield loci increase in size with increases in cement
content of sand, and the cemented samples become more brittle.
4
E. ASGHARI, D. G. TOLL AND S. M. HAERI
Coop and Atkinson (1993) proposed that the stress-strain behaviour of cemented
soils depends on the position of the initial state of the soil relative to the yield locus
of the bonding, as illustrated in Figure 2. The first class of the behaviour occurs
where the sample has passed its yield point during isotropic compression; subsequent
shearing would produce behaviour similar to that of an initially uncemented soil,
with no yield point. The second class (2 in Figure 2) occurs at intermediate confining
stresses so that the cemented bonds are intact at the start of the test and they yield
during shearing. In this case, the peak state is governed by the frictional behaviour of
the uncemented soil. The stress-strain curve for this type of test might be expected
to show a distinct yield point after an initial elastic section. In the third class (3 in
Figure 2) the sample is sheared at low confining stresses relative to the strength of
the cementation agent. A peak state occurs at small strains well outside the state
boundary surface of the uncemented soil.
Toll and Malandraki (1993) put forward a framework based on a similar approach
(identifying zones of behaviour relating the stress state to the bond yield surface and
failure envelope) and Malandraki and Toll (2000) extended this to look at the behaviour of an artificially weakly bonded soil under a range of different stress path
Figure 2. Idealized behaviour of cemented soils; (a) Stress paths, (b) Stress- strain behaviour (Coop and
Atkinson, 1993).
TRIAXIAL BEHAVIOUR OF A CEMENTED GRAVELY SAND, TEHRAN ALLUVIUM
5
directions. Malandraki and Toll (2001) showed that bond yield is an anisotropic process and that different yield points could be seen if the stress path direction was changed, even if the sample had previously yielded following another stress path direction.
Haeri et al. (2002) carried out some large direct shear tests on uncemented and
artificially cemented gravely sands using lime as a cementing agent. The base soil
sample was obtained from the Tehran alluvium. The results showed that cementation increased the cohesion intercept and that the failure envelope for cemented samples was curved.
3. Engineering Geology of Tehran Alluvium
The soil used in the present study was obtained from the cemented coarse-grained
alluvium of Tehran. Tehran alluvium has been deposited by frequent flow of floods
and rivers originating from mountains to the north of Tehran city. The materials are
deposited in several alluvial fans and the layers consist of different gradings. Coarsegrained particles consist of fragments of tuff, shale and volcanic rocks. The particle
shape is dominantly subangular. Cementation of the Tehran alluvium is a secondary
event, and is predominantly deposited by ground water. Cement materials are
mainly carbonate materials such as calcite. The cementation degree of this alluvium
varies in different series and sites.
Grading curves for the Tehran Alluvium taken from a number of sites in the north
of Tehran are shown in Figure 3. The materials are generally gravely sands and
Figure 3. Grain size distribution curve of some samples obtained from various sites in north Tehran.
6
E. ASGHARI, D. G. TOLL AND S. M. HAERI
sandy gravels and can be classified according to the Unified Classification System as
According to the Unified Classification System as GW-GM, GW-GC, GP-GM,
GM, GC, SW-SM, SW-SC, SM and SC. The natural dry unit weight of undisturbed
samples varied between 16.5 to 19.5 kN/m3, with an average of 18 kN/m3.
4. Experimental Program
As has been discussed, the cemented coarse-grained alluvium of Tehran is heterogeneous both in grading and cementation. Moreover, it is extremely difficult to
acquire undisturbed samples and prepare test specimens for triaxial testing.
Therefore, to understand the mechanical behaviour and study the effects of cementation on the mechanical characteristics of this alluvial material, samples artificially
cemented with lime were prepared and tested in this research.
A typical base soil with the grading curve shown in Figure 4 was chosen, based on
gradation curves shown in Figure 3 and regarding the limitation on maximum particle size for triaxial testing of 100 mm diameter samples. The physical properties of
the selected soil are given in Table 1. The base soil was a gravely sand with 45%
gravel, 49% sand and 6% fine material. Hydrated lime, Ca(OH)2, was used as the
cementing agent for the artificially cemented samples.
Grain size distribution, dry density and cementation were the main controlling
factors in sample preparation. After preparation of appropriate amount of the base
soil for each sample, it was mixed with the cementing material (hydrated lime) and
the correct amount of distilled water. Sections of high strength P.V.C. perforated
pipes, 200 mm high and 100 mm diameter, were used as moulds for sample
Figure 4. Grain size distribution curve of typical soil used in this study.
TRIAXIAL BEHAVIOUR OF A CEMENTED GRAVELY SAND, TEHRAN ALLUVIUM
Table 1.
7
Average physical properties of the base soil
Property
Value
Specific gravity (Gs)
Effective diameter (D10)
Uniformity coefficient (Cu)
Percent gravel (>4.76 mm)
Percent fine material (<0.074 mm)
Liquid limit of passing 425 mm
Plastic index of passing 425 mm
Unified Classification of Soil
2.58
0.2 mm
27.8
45%
6%
44%
16%
SW-SM
preparation. The prepared soil-lime mixture was then placed in the mould in 10 layers and each layer was compacted statically to achieve the required height. The dry
unit weight of the samples was set to be 18 kN/m3 with 8.5% water content. This is
the average dry insitu unit weight of the deposit and the average optimum water content of the compacted base soil obtained from Proctor compaction tests. The soil was
cured for 6 weeks before testing in order to produce the pozzolanic cementation
compounds formed by reaction between lime and soil silica (fine particles of base
soil), when they are saturated (Boardman et al., 2001). The sample was kept in the
perforated moulds during the curing period. The temperature of the curing water
tank was kept constant at 25 C. In order to avoid the water from the tank leaching
into the sample, the samples were kept inside a plastic bag containing distilled water.
After the curing time had elapsed, the P.V.C. mould was cut and the sample was
ready for testing.
5. Testing Program and Procedure
Cementation effects were studied in three main series of drained and undrained triaxial compression tests on uncemented, cemented and destructured samples. Artificially cemented samples with lime contents of 1.5, 3 and 4.5% of the weight of dry
soil were prepared.
Destructured soil was prepared by breaking down the artificially cemented soils by
hand. The destructured soil may well perform differently to the uncemented soil
since the presence of the cementing agent will affect the overall grading. Therefore,
to study the effects of cementation on various mechanical parameters of cemented
soils it is better to compare the behaviour of the cemented soils with the associated
destructured samples, rather than the uncemented material.
The tests performed are summarized in Table 2. Consolidated drained and
undrained triaxial compression tests were carried out. The samples were set up in
the triaxial apparatus, and were flushed through with the deaired water under a small
pressure gradient (10 kPa). To avoid the flushing water from leaching out the cementation, the flushing was not continuous but the outlet valve was opened for two to
three minutes every one or two hours. This procedure was maintained for two or
8
E. ASGHARI, D. G. TOLL AND S. M. HAERI
Table 2.
Triaxial tests performed
Confining stress (kPa)
Sample type
Drained tests
Undrained tests
Uncemented
1.5% Cemented
3% Cemented
4.5% Cemented
Destructured 3%
25, 55, 110, 300, 500, 1000
110, 300, 500
25, 55, 110, 300, 500, 1000
10, 110, 300, 500
55, 300, 500
110, 300, 500
110
110, 300, 500
110
110, 300, 500
three days in an attempt to remove all air bubbles. After this flushing procedure, the
cell and back pressures were increased in a controlled manner up to 300–400 kPa, so
that the remaining air in the voids would be forced into solution. A check was made
periodically on each sample to determine the pore pressure parameter B (Skempton,
1954). When the B value was greater than 0.90, saturation was assumed to be
complete.
The sample was then consolidated isotropically under the desired confining stress.
The change in volume during consolidation was recorded. Shearing was carried out
at a constant rate of strain of 3% per hour for drained tests and 6% per hour for
undrained tests. A computer controlled triaxial cell with the usual transducers (cell
pressure, back or pore pressure, volume change, displacement and load) was used
to carry out all the triaxial tests, using the TRIAX program developed by Toll
(1999). The axial strains were measured externally using a high-resolution displacement transducer.
6. Analysis of Test Data
The test results have been analysed using s01 , s03 , ea, ev, e, Du, q and p0 , where s01 and
s03 are the axial and radial effective stresses on a cylindrical sample, ea and ev are the
axial and volumetric strains (which are taken as positive for compressional strains),
e is the void ratio, Du is excess pore water pressure, q and p0 are deviatoric and mean
effective stresses. q and p0 are defined as:
q ¼ s01 s03
p0 ¼ ðs01 þ 2s03 Þ=3
A summary of test results is shown in Table 3.
6.1.
FAILURE MODE
All uncemented samples showed barrelling failure modes during shear and underwent
contraction, except the sample tested at the lowest confining stress i.e. 25 kPa. The
destructured samples also showed similar modes to the uncemented samples except
that tested at the lowest confining stress i.e. 55 kPa. These two exceptions showed the
presence of a shear zone in the sample and demonstrated a clear peak in shear stress.
10
110
C4.5 – D
C4.5 – D
–
–
–
–
–
–
–
–
–
25
55
110
300
500
1000
110
300
500
3
3
3
3
3
3
3
3
3
D
D
D
D
D
D
U
U
U
C
C
C
C
C
C
C
C
C
110
300
500
110
C1.5
C1.5
C1.5
C1.5
D
D
D
U
Un1 – D2
Un – D
Un – D
Un – D
Un – D
Un – D
Un – U
Un – U
Un – U
–
–
–
–
25
55
110
300
500
1000
110
300
500
Test
1115.5
1514.0
684.5
765.7
978.1
1747.9
2463.6
3551.9
1871.9
2208.3
3030.8
916.43
1679.9
2334.0
1625.7
155.8
209.8
439.6
816.2
1518.8
2930.9
332.1
651.1
555.3
At peak
89.7
450.1
160.5
249.3
484.9
1105.5
2011.5
2741.7
1684.0
1800.1
2535.6
389.9
1033.5
1736.7
1430.4
135.0
196.7
431.2
816.2
1518.8
2930.9
328.4
638.1
549.1
At ultimate
state
Deviatoric stress (kPa)
Summary of test results
Confining
stress
(kPa)
Table 3.
382.7
616.5
253.5
312.7
439.7
882.9
1323.5
2184.3
1031.4
1159.2
1678.2
417.68
862.1
1277.7
922.1
79.9
124.5
256.9
572.1
1005.0
1976.6
195.1
384.6
337.4
At failure
40.4
260.0
78.6
138.4
270.8
669.0
1172.7
1913.9
1010.8
1100.7
1595.6
240.7
644.8
1078.6
885.3
70.4
120.1
254.2
572.1
1005.4
1976.6
194.9
387.9
342.3
At ultimate
state
Mean effective stress (kPa)
0.468
0.443
0.431
0.436
0.427
0.424
0.422
0.423
0.442
0.438
0.418
0.443
0.438
0.425
0.451
0.439
0.448
0.445
0.434
0.406
0.357
0.449
0.422
0.407
After
consolidation
0.467
0.447
0.429
0.437
0.432
0.419
0.413
0.411
0.459
0.425
0.411
0.444
0.442
0.434
0.405
0.346
0.276
At
failure
Void ratio
0.620
0.521
0.558
0.543
0.511
0.445
0.424
0.398
0.500
0.463
0.423
0.466
0.443
0.433
0.405
0.346
0.276
At ultimate
state
(Continued)
Shear zone
Shear zone
Shear zone
Shear zone
Shear zone
Shear zone
Barrelling
Barrelling
Shear zone
Shear zone
Shear zone
Shear zone
Shear zone
Barrelling
Shear zone
Barrelling
Barrelling
Barrelling
Barrelling
Barrelling
Barrelling
Barrelling
Barrelling
Barrelling
Prevailing
failure
mode
TRIAXIAL BEHAVIOUR OF A CEMENTED GRAVELY SAND, TEHRAN ALLUVIUM
9
55
300
500
110
300
500
D
D
D
D
D
D
2
1
288.7
1171.6
1800.0
759.8
931.5
1329.2
2216.7
2970.7
2236.0
At peak
210.3
1077.1
1745.0
702.3
900.8
1233.0
1402.6
1996.6
1797
At ultimate
state
Deviatoric stress (kPa)
152.9
690.7
1099.8
443.9
567.8
780.3
1039.7
1491.3
1102.1
At failure
126.8
658.7
1080.4
426.5
555.2
747.6
768.0
1165.9
1067.6
At ultimate
state
Mean effective stress (kPa)
0.473
0.430
0.415
0.439
0.427
0.407
0.437
0.426
0.438
After
consolidation
0.473
0.415
0.376
0.434
0.433
At
failure
Void ratio
Un (Uncemented sample), C (Cemented sample), D (Destructured sample), 1.5, 3 and 4.5 (Cementing material percentage)
D (Drained test) and U (Undrained test)
D
D
D
U
U
U
C4.5 – D
C4.5 – D
C4.5 – U
–
–
–
–
–
–
300
500
110
Test
3
3
3
3
3
3
Confining
stress
(kPa)
Table 3. (Continued)
0.495
0.418
0.374
0.465
0.452
At ultimate
state
Shear zone
Barrelling
Barrelling
Barrelling
Barrelling
Barrelling
Shear zone
Barrelling
Shear zone
Prevailing
failure
mode
10
E. ASGHARI, D. G. TOLL AND S. M. HAERI
TRIAXIAL BEHAVIOUR OF A CEMENTED GRAVELY SAND, TEHRAN ALLUVIUM
11
Cemented samples, however, showed a brittle failure mode accompanied with a
shear zone at low confining stresses with a transition to a barrelling failure mode
at higher confining stresses. In cemented samples with confining stresses lower than
110 kPa, shearing was accompanied with a shear zone without significant barrelling.
At higher confining stresses, however, the samples undergo considerable barrelling
but still showed a shear zone. As would be expected, the brittle behaviour increased
with increasing cementation and decreased with an increase in confining stress. The
thickness of the shear zone in cemented samples varied between 2 and 5 cm. It can be
observed that the thickness of shear zone is dependant on the amount of cementation
and confining stress level of testing. Figure 5 shows a picture of the typical failure
modes of uncemented and cemented samples.
6.2.
STRESS-STRAIN BEHAVIOUR
Figures 6(a), 7(a), 8(a), 9(a) and 10(a) show deviatoric stress (q) plotted against the
axial strain for uncemented, cemented and destructured samples respectively.
Figure 5. Prevailing failure modes of samples; (a) barrelling in uncemented samples, (b) shear zone in
cemented samples.
12
E. ASGHARI, D. G. TOLL AND S. M. HAERI
Figure 6. Behaviour of uncemented samples during shearing at various confining stresses; (a) deviatoric
stress-axial strain, (b) volumetric strain-axial strain for drained tests, (c) stress ratio-axial strain,
[D ¼ Drained and U ¼ Undrained test].
TRIAXIAL BEHAVIOUR OF A CEMENTED GRAVELY SAND, TEHRAN ALLUVIUM
13
Figure 7. Behaviour of 1.5% cemented samples during shearing at various confining stresses; (a) deviatoric stress-axial strain, (b) volumetric strain-axial strain for drained tests, (c) stress ratio-axial strain,
[D ¼ Drained and U ¼ Undrained test].
14
E. ASGHARI, D. G. TOLL AND S. M. HAERI
Figure 8. Behaviour of 3% cemented samples during shearing at various confining stresses; (a) deviatoric
stress-axial strain, (b) volumetric strain-axial strain for drained tests, (c) stress ratio-axial strain,
[D ¼ Drained and U ¼ Undrained test].
TRIAXIAL BEHAVIOUR OF A CEMENTED GRAVELY SAND, TEHRAN ALLUVIUM
15
Figure 9. Behaviour of 4.5% cemented samples during shearing at various confining stresses; (a) deviatoric stress-axial strain, (b) volumetric strain-axial strain for drained tests, (c) stress ratio-axial strain,
[D ¼ Drained and U ¼ Undrained test].
16
E. ASGHARI, D. G. TOLL AND S. M. HAERI
Figure 10. Behaviour of destructured 3% samples during shearing at various confining stresses; (a) deviatoric stress-axial strain, (b) volumetric strain-axial strain, for drained tests (c) stress ratio-axial strain,
[D ¼ Drained and U ¼ Undrained test].
TRIAXIAL BEHAVIOUR OF A CEMENTED GRAVELY SAND, TEHRAN ALLUVIUM
17
Drained tests are shown prefixed by ‘D’ and undrained tests are prefixed by ‘U’. The
number following the prefix is the initial effective confining stress ( p0i ). The test results
show that the strain associated with peak strength increases with an increase of
confining stress. There is no clear peak in deviatoric stress for uncemented samples
(Figure 6) but the presence of a peak for all cemented samples is evident (Figures 7–10).
For a given confining stress, the strain associated with peak strength decreases with
an increase in cementation. In cemented samples the peak strength is followed by
strain softening. An important characteristic shown by the stress-strain curves is that
the post peak response is highly dependant on the degree of cementation and the
confining stress. For undrained tests the stress-strain curves closely approach states
at which deviatoric stresses remain constant for continuing strains, but in drained
tests, deviatoric stresses continue to increase for uncemented samples or decrease
slowly for cemented samples even at axial strains of the order of 20% or more.
Figures 6(c), 7(c), 8(c), 9(c) and 10(c) show stress ratios (q/p0 ) plotted against axial
strains for uncemented, cemented and destructured samples. Undrained tests show
higher peak stress ratios relative to drained tests. The stress ratios of the uncemented
samples show a gradual increase up to a maximum ultimate value. The destructured
samples show stress ratios reaching a constant ultimate value. At higher confining
stresses (above 300 kPa) they follow a similar pattern to the uncemented material
but at lower stresses they reach slightly higher initial values before dropping down
to ultimate conditions. However, the cemented samples generally show clear peaks
in stress ratio before dropping to a lower ultimate value, similar to that of the third
class described by Coop and Atkinson (1993) and shown in Figure 2. However, at
higher stresses (above 500 kPa) the behaviour is more like the first or second class.
Figure 11 shows the stress ratios for all tests carried out at the same initial effective
confining stress ( p0i ¼ 110 kPa). Stress ratio curves associated with all undrained tests
shown in Figure 11 fall close to a single stress ratio (about 1.68 for p0i ¼ 110 kPa) at
large axial strain levels. This is generally true for drained tests as well, although the
curve associated with 3% cementation does show a higher value. In fact the results of
drained tests do show greater scatter in the q/p0 ratio at large strains, and the value of
Figure 11. Influence of cementation on shearing behaviour; (a) drained tests, (b) undrained tests.
18
E. ASGHARI, D. G. TOLL AND S. M. HAERI
q/p0 at large strains was different for various initial confining stresses. Nevertheless, it
can be seen from Figure 11 that the peak stress ratio, (q/p0 )max, increases with
increasing cementation.
6.3.
VOLUME CHANGES
Volume changes or volumetric strains (ev) during shearing in drained tests are shown
in Figures 6(b), 7(b), 8(b), 9(b) and 10(b). At the lowest confining stresses, both
cemented and uncemented samples undergo dilation during shear and in the highest
confining stress (1000 kPa) both cemented and uncemented samples showed contraction during shear. Under the remaining confining stresses uncemented samples
showed contraction and cemented samples showed dilation during shear. Figure 12
shows a comparison between the volume change behaviour for uncemented material
and for samples with 3% cementation. As can be observed from Figure 12, more
cementation causes a greater amount of dilation.
In cemented samples the maximum rate of dilation, (dev/dea)max, take place after
the maximum stress ratio, (q/p0 )max. However, these two points coincide in uncemented and destructured samples. Leroueil and Vaughan (1990) showed that when dilation is due to dense packing, these two points coincide but when the peak strength is
controlled by cementation rather than density, the maximum rate of dilation takes
place after the bonding has yielded. This is clearly shown in Figure 13.
Figure 12. Influence of cementation on volume changes during shear (drained testes with p0i ¼ 500 kPa).
TRIAXIAL BEHAVIOUR OF A CEMENTED GRAVELY SAND, TEHRAN ALLUVIUM
19
Figure 13. In cemented samples maximum rate of dilation occur after peak strength; (a) dilation rate–
axial strain, (b) stress-strain curve with maximum rate of dilation regions.
6.4.
PORE WATER PRESSURE CHANGES
In undrained tests on cemented and destructured samples the excess pore water pressure remained positive (consistent with a tendency for the sample to contract) but in
cemented samples, after a first stage of positive excess pore water pressure, negative
excess pore water pressure was produced (consistent with a tendency for the sample
20
E. ASGHARI, D. G. TOLL AND S. M. HAERI
to dilate). Figure 14 shows a comparison between uncemented and 3% cemented
samples showing this clear change in behaviour. Figure 15 shows that amount of
negative excess pore water pressure increases with an increase in cementing of the
samples.
For cemented samples the maximum rate of excess pore water pressure changes,
(du/dea)max, occur after the point of (q/p0 )max, while for uncemented and destructured
samples these points coincide. This is shown in Figure 16 for 3% cemented and
destructured 3% samples tested under 110 kPa initial confining stress. The region
representing the maximum rate of excess pore water pressure changes in undrained
tests corresponds to the region of maximum rate of dilation (dev/dea)max for the
drained tests. Where bonding between particles influences the strength, the maximum rate of excess pore water pressure changes take place after the point of (q/p0 )max
max in undrained tests, as was shown by Malandraki (1994).
6.5.
STRESS PATHS
Effective stress paths for uncemented and cemented samples are shown in Figure 17.
These stress paths show many of the features commonly associated with uncemented
and cemented samples. The stress path for drained test is a straight line with a slope
3/1 in q-p0 space. The highest point on that line is the failure point for uncemented
and cemented samples. However, for cemented samples the test continues with strain
softening behaviour where the stress condition decreases along the same stress path
to reach a lower ultimate stress.
Figure 14. Excess pore water pressure changes for uncemented and 3% cemented samples during shear.
TRIAXIAL BEHAVIOUR OF A CEMENTED GRAVELY SAND, TEHRAN ALLUVIUM
21
Figure 15. Influence of cement material content on excess pore water pressure changes during shear (with
p0i ¼ 110 kPa).
For the uncemented material, the stress paths from undrained tests define a similar
failure envelope to that defined by the drained tests. However, in the cemented material, the undrained tests cross the failure surface defined by the qmax points from
drained tests, showing that the material can attain higher stress ratios when sheared
in an undrained condition. However, the qmax points for the undrained tests agree
quite well with the drained tests in terms of stress ratio. The difference between limiting
stress ratios from undrained and drained tests is likely to be due to the fact that volumetric straining (as happens in the drained tests) will contribute to breakdown of the
cemented bonds (Malandraki and Toll, 2001). When volume strains are prevented (in
undrained tests) a higher stress ratio can be sustained before the bonds break down.
Effective stress paths of undrained tests on uncemented and 3% cemented samples
are shown in Figure 18. Figure 18a shows the stress paths for uncemented samples.
As shown in this Figure, the stress path for p0i ¼110 kPa is similar to that expected of
soils that are ‘dense’ compared to the critical state whereas the stress path for
p0i ¼500 kPa is similar to that expected of ‘loose’ sands showing positive excess pore
water pressure. The test with p0i ¼300 kPa shows intermediate behaviour, with very
little pore water pressure change. In comparison, the behaviour of all 3% cemented
samples tested under the same confining stresses are shown in the Figure 18b and
show behaviour similar to that which would be expected of ‘dense’ soils. This is
in agreement with the observations of Saxena and Lastrico (1978) and Lade and
Overton (1989) that cemented soils often exhibit dilatent behaviour.
22
E. ASGHARI, D. G. TOLL AND S. M. HAERI
Figure 16. Maximum rate of excess pore water pressure changes in undrained tests, (a) Rate of excess
pore water pressure changes during shearing with p0i ¼ 110 kPa on 3% cemented and destructured 3%,
(b) Stress-strain curve and limits of maximum rate of excess pore water pressure changes for the same
samples.
TRIAXIAL BEHAVIOUR OF A CEMENTED GRAVELY SAND, TEHRAN ALLUVIUM
23
Figure 17. Effective stress path; (a) Uncemented samples, (b) 1.5% cemented samples, (c) 3% cemented
samples, (d) 4.5% cemented samples.
Figure 19 shows the influence of cementation on undrained path of soil tested at
the same initial effective confining stress (110 kPa). The stress paths are all similar in
shape, but it can be seen that with increased cementing, the stress path climbs to
much higher stress ratios before reaching a peak value of deviator stress and dropping back. A difference in behaviour between the uncemented and the destructured
can be seen. Even though the ultimate stress ratios are very similar, the destructured
sample shows a higher strength due to lower excess pore water pressure being
24
E. ASGHARI, D. G. TOLL AND S. M. HAERI
Figure 18. Comparisons between effective stress paths, (a) undrained tests on uncemented samples,
(b) undrained tests on 3% cemented samples.
Figure 19. Influence of cementation on effective stress paths of undrained tests ( p0i ¼ 110 kPa).
TRIAXIAL BEHAVIOUR OF A CEMENTED GRAVELY SAND, TEHRAN ALLUVIUM
25
developed. This is consistent with the difference in volume change between uncemented and destructured samples tested in drained conditions (Figure 12). This may
be due to the difference in grading and plasticity produced by the addition of lime.
Boyenton (1966) showed that the lime’s reaction with soil is twofold. First, it
agglomerates the fine clay particles into coarse, friable particles (silt and sand sizes)
through base exchange with the calcium cation displacing sodium or hydrogen ions.
Next, it provides a cementing (hardening) action in which the lime reacts chemically
with available silica and some alumina in the soil. Some parts of the first effect
remain in the destructured soil but the second effect disappear by breaking down.
6.6.
FAILURE ENVELOPE
The failure envelopes in q-p0 space for uncemented, cemented and destructured 3%
samples defined from drained and undrained samples are shown in Figure 20. These
envelopes have been defined by plotting points representing the maximum deviatoric
stress, qmax, rather than the maximum stress ratio, (q/p0 )max. As has shown, the
undrained tests results showed that the maximum stress ratios occur before the
Figure 20. Failure envelopes of samples in q: p0 space.
26
E. ASGHARI, D. G. TOLL AND S. M. HAERI
maximum deviatoric stress but that the samples continue to sustain increasing shear
stress until a maximum deviator stress was reached. For comparison between
drained and undrained tests we have used the stress condition associated with qmax
to define the failure envelopes. However, it should be recognised that the soil could
sustain higher stress ratios if tested in undrained conditions.
The failure envelope for the uncemented and destructured samples is almost linear,
whereas those for cemented samples are curved. With increasing cementation the
failure envelope moves to higher stress levels. The results of the tests for the cemented samples show an apparent cohesion, which increases with increasing cementation. However, there are few tests at very low confining stresses, so it is not clear
if the failure envelopes would curve down to nearer the origin at low stresses.
The curvature of the failure envelope for the cemented samples is best illustrated
by the 3% cemented samples as this has the widest range of stress level (confining
stress up to 1000 kPa). The slope of the failure envelope decreases as p0 increases,
indicating a loss of the cementation strength at failure. This is due to cementing
bonds being broken down by consolidation to high stresses, as has been seen for
other cemented materials (e.g. Malandraki and Toll, 2000). This means that the
influence of cementation is greater at low confining stresses and this effect reduces
with an increase in confining stress. Figure 21 shows this clearly by plotting qmax
Figure 21. Influence of confining stress on the increase of strength due to cementation.
TRIAXIAL BEHAVIOUR OF A CEMENTED GRAVELY SAND, TEHRAN ALLUVIUM
27
for the cemented material as a ratio of qmax for the uncemented material at the same
confining stress. The peak strength of cemented samples reduces with increasing confining stress and it is expected that at a confining stress of about 1200 kPa, the failure
envelope of 3% cemented and uncemented samples would coincide.
7. Conclusion
A sandy gravel that is representative of the Tehran alluvium has been artificially
cemented with lime in order to study the mechanical behaviour of the cemented
material. Drained and undrained triaxial compression tests were conducted on uncemented, artificially cemented and destructured samples. The test results show that
cemented samples develop a shear zone during shear and undergo dilation at lower
confining stresses (lower than 1000 kPa), but show a transition to barrelling failure at
higher confining stresses i.e. transition from brittle to ductile failure modes. Meanwhile, uncemented samples demonstrated contraction and barrelling during shear.
There is an evident peak in shear stress for all cemented samples that it is followed
by strain softening. For a given confining stress, the strain associated with the peak
strength decreases with an increase in cementation. The maximum rate of dilation in
drained tests and maximum rate of excess pore water pressure in undrained tests,
takes place after the maximum stress ratio, (q/p0 )max. However these two points coincide in uncemented and destructured samples. The strength increases with increasing
cementing material but the influence of the cementation decreases as the confining
stress increases.
The failure envelope for the uncemented and destructured samples is almost linear,
but the envelope is curved for cemented samples. The results of tests on the cemented
samples suggest an apparent cohesion and this cohesion increases with increasing
cementation.
Acknowledgements
The triaxial tests described in this paper were performed in the Engineering Geology
Laboratory, School of Engineering, University of Durham, UK. The financial
support of Ministry of Science, Research and Technology of Iran to the first
author is acknowledged. The contributions of Dr A. Uromeihy, Dr S. Yasrebi,
Mr. B. McEleavy and Mr S. Namazi to this research is acknowledged as well.
References
Acar, Y. B. and El-Tahir, A. E. (1986) Low strain dynamic properties of artificially cemented
sand, Journal of Geotech. Engrg. Div., ASCE, 112(11), 1001–1015.
Airey, D. W. (1993) Triaxial testing of naturally cemented carbonate soil, Journal of Geotech.
Engerg. Div., ASCE, 119(11), 1379–1398.
Boardman, D. I., Glendinning, S. and Rogers, C. D. F. (2001) Development of stabilisation
and solidification in lime-clay mixes, Geotechnique, London, 50(6), 533–643.
28
E. ASGHARI, D. G. TOLL AND S. M. HAERI
Boynton, R. S. (1966) Chemistry and technology of lime and limestone, John Wiley and Sons.
Clough, G. W., Kuck, W. M. and Kasali, G. (1979) Silicate-stabilized sands, Journal of Geotech. Engrg. Div., ASCE, 105(1), 65–82.
Clough, G. W., Sitar, N., Bachus, R. C. and Rad, N. S. (1981) Cemented sands under static
loading, Journal of Geotech. Engrg. Div., ASCE, 107(6), 799–817.
Coop, M. R. and Atkinson, J. H. (1993) The mechanics of cemented carbonate sands, Geotechnique, London, 43(1), 53–67.
Cuccovilo, T. and Coop, M. R. (1997) Yielding and pre-failure deformation of structured
sands, Geotechnique, London, 47(3), 481–508.
Dupas, J. and Pecker, A. (1979) Static and dynamic properties of sand cement, Journal of Geotech. Engerg. Div., ASCE, 105(3), 419–436.
Gens, A. and Nova, R. (1993) Conceptual bases for a constitutive model for bonded soils and
weak rocks, Proc., Int. Symp. on Geotech. Engerg. of Hard Soils- Soft Rocks, Vol. 1,
Balkema, Rotterdam, The Netherlands, 485–494.
Haeri, S. M., Yasrebi, S. and Asghari, E. (2002) Effects of cementation on the shear strength
parameters of Tehran alluvium using large direct shear test, Accepted for 9th IAEG Congress, Durban, South Africa.
Huang, J. T. and Airey, D. W. (1993) Effects of cement and density on an artificially cemented
sand, Proc. Geotechnical Engineering of Hard Soils- Soft Rocks, Balkema, Roterdam, 1,
553–560.
Lade, P. V. and Overton, D. D. (1989) Cementation effects in frictional materials, Journal of
Geotech. Engerg. Div., ASCE, 115(10), 1373–1387.
Leroueil, S. and Vaughan, P. R. (1990) The general and congruent effects of structure in natural soils and weak rocks, Geotechnique, London, 40(3), 467–488.
Malandraki, V. (1994) The engineering behaviour of a weakly bonded artificial soil, PhD thesis, University of Durham, Durham, U.K.
Malandraki, V. and Toll, D. G. (2000) Drained probing triaxial tests on a weakly bonded artificial soil, Geotechnique, London, 50(2), 141–151.
Malandraki, V. and Toll, D. G. (2001) Triaxial tests on weakly bonded soil with changes in
stress path, Journal of Geotechnical and Geoenvironmental Engineering, ASCE, 127(3),
282–291.
Saxena, S. K. and Lastrico, R. M. (1978) Static properties of lightly cemented sand, Journal of
Geotech. Engerg. Div., ASCE, 104(12), 1449–1465.
Schnaid, F., Prietto, P. D. M. and Consoli, M. H. T. (2001) Characterization of cemented sand
in triaxial compression, Journal of Geotechnical and Geoenvironmental Engineering, ASCE,
127(10), 857–867.
Skempton, A. W. (1954) The pore pressure coefficients A and B, Geotechnique, London, 4,
143–147.
Toll, D. G. (1999) A data acquisition and control system for geotechnical testing, in computing
developments in civil and structural engineering, (eds. B. Kumar and B.H.V. Topping),
Edinburgh: Civil–Comp Press, 237–242.
Toll, D. G. and Malandraki, V. (1993) Triaxial testing of a weakly bonded soil, Proc., Int.
Symp. on Geotech. Engerg. of Hard Soils-Soft Rocks, Balkema, Rotterdam, The
Netherlands, 817–823.