384
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
samples behaved as heavily overconsolidated clay showing a
clear peak deviator stress at a low axial strain, followed by a
strain softening. Beyond the confining pressure of 552 kN/m
2
(tests CIU F8 and F9), these samples behaved as lightly
overconsolidated clay.
Table 4. Stiffness and strength parameters from CID and CIU tests for
Bangkok Clays
Axial strain,
a
(%)
0
5 10 15 20 25
Deviator stress, q (kN/m 2 )
0
200
400
600
800
CIUF1
CIUF2
CIUF3
CIUF4
CIUF5
CIUF6
CIUF7
CIUF8
CIUF9
Axial strain,
a
(%)
0 2 4 6 8 10 12 14
Excess pore pressure, u (kN/m 2 )
-100
-50
0
50
100
150
200
250
(a) Deviator stress vs axial strain
(b) Pore pressure vs axial strain
Parameters
CID
CIU
Soft Clay
Confining pressure (kN/m
2
)
138 – 414
138 – 414
ref
i
E
,
(kN/m
2
)
ref
iu
E
,
1343
7690
Initial
m
1.0
1.2
ref
E
50
,
(kN/m
2
)
ref
u
E
50,
690
4831
50%
m
1.1
1.0
R
f
0.72
0.94
'
23.6
27.0
c'
(kN/m
2
)
0
0
Stiff Clay
Confining pressure (kN/m
2
)
34 – 552
17 – 620
ref
i
E
,
(kN/m
2
)
ref
iu
E
,
29676
30109
Initial
m
0.52
0.46
ref
E
50
,
(kN/m
2
)
ref
u
E
50,
14398
11104
50%
m
0.48
0.53
R
f
0.89
0.88
'
26.3
28.1
c
' (kN/m
2
)
32.8
11.4
Figure 4. Results of CIU triaxial tests on stiff Bangkok clay
The relationships between the excess pore pressure and the
axial strain are shown in Figure 4(b). For all clay samples (CIU
F1 to F9), the excess pore pressure increases as the deviator
stress increases, until the peak values are reached at 1 to 4%
axial strain, depending on the confining pressure. The peak
excess pore pressure seems to be reached at a higher axial strain
as the confining pressure increases. As the sample was further
sheared, the excess pore pressure gradually reduced to the
minimum value, at approximately 12% axial strain. Only the
first three samples (tests CIU F1 to F3) reached negative excess
pore pressures.
4. CONCLUSION
In this study, the experimental data on soft and stiff Bangkok
clays available in the literature was reanalysed in order to obtain
the Duncan-Chang stiffness and strength parameters required
for the Hardening Soil Model. Undrained and drained behaviour
of Soft and Stiff Bangkok Clays was modelled using these
parameters.
Axial strain,
a
(%)
0 2 4 6 8 10 12 14 16
Deviator stress, q (kN/m 2 )
0
200
400
600
800
1000
CIDF1
CIDF2
CIDF3
CIDF4
Axial strain,
a
(%)
0 2 4 6 8 10 12 14 16
Volumetric strain,
v
(%)
-3
-2
-1
0
1
2
3
CIDF1
CIDF2
CIDF3
CIDF4
(a) Deviator stress vs axial strain
(b) Volumetric strain vs axial strain
5. REFERENCES
Balasubramaniam A.S. and Hwang Z. M. 1980. Yielding of weathered
Bangkok clay,
Soils and Foundations
, 20(2), 1-15.
Balasubramaniam, A.S. and Chaudhry, A. R. 1978. Deformation and
strength characteristics of Soft Bangkok Clay.
Journal of
Geotechnical Engineering Division, ASCE
, 104, 1153 - 1167.
Balasubramaniam, A.S. and Uddin, W. 1977. Deformation
characteristics of weathered Bangkok Clay in triaxial extension.
Géotechnique
, 27(1), 75 - 92.
Balasubramaniam, A.S., Handali, S., and Wood, D. M. 1992. Pore
pressure-stress ratio relationship for soft Bangkok clay,
Soils and
Foundations
, 32(1), 117-131.
Figure 5. Results of CID triaxial tests on stiff Bangkok clay
The results of CID triaxial tests carried out on the stiff
Bangkok clay are shown in Figure 5. The deviator stress versus
the axial strain relationships of the stiff clay are shown in Figure
5(a). The pre-shear confining pressures of 34, 103, 414 and 552
kN/m
2
were applied. None of the stiff clay samples
demonstrated a well defined peak. However, samples CID F1 to
F3 (with confining pressure of 34, 103 and 414 kN/m
2
) illustrate
some degree of strain softening after the peak deviator stresses
are reached at axial strain levels of 3 to 5%. The plots of the
volumetric versus the axial strain are given in Figure 5(b). The
specimens with a confining pressure of 34 and 103 kN/m
2
(tests
CID F1 and 2) start to dilate at about 1.2 and 3.5% axial strain.
The specimen at 414 kN/m
2
confining pressure consolidates up
to an axial strain level of 8%. After that, the volumetric strain
seems to be constant with an increase in axial strain. The last
specimen with a confining pressure of 552 kN/m
2
consolidates
up to 7% of the axial strain, and then it tends to dilate.
Balasubramaniam, A.S., Hwang, Z. M., Waheed U., Chaudhry, A. R.
and Li, Y. G. 1978. Critical state parameters and peak stress
envelopes for Bangkok Clays,
Quarterly Journal of Engineering
Geology
, 1, 219-232.
Duncan, J.M. and Chang, C.M. 1970. Nonlinear analysis of stress and
strain in soils.
Journal of Soil Mechanics and Foundations
Division, ASCE
, 96(SM5), 1629-1653.
Hassan, Z. 1976. Stress-strain behaviour and shear strength
characteristics of stiff Bangkok Clays. Master Thesis, Asian
Institute of Technology, Thailand.
Schanz, T., Vermeer, P.A., and Bonnier, P.G. 1999. The hardening soil
model: formulation and verification. Beyond 2000 in
Computational Geotechnics. Rotterdam.
Schweiger, H.F. 2009. Influence of constitutive model and EC7 design
approach in FEM analysis of deep excavations. In:
Proceeding of
ISSMGE Int. Seminar on Deep Excavations and Retaining
Structures
, Budapest, 99 - 114.
Surarak, C. 2010. Geotechnical aspects of the Bangkok MRT blue line
project. Ph.D. Thesis, Griffith University, Australia.
The values of
50
ref
E
and
together with the deformation
moduli and the failure ratios resulting from the CIU and CID
series are also summarised in Table 4. It can be observed from
Table 4 that the failure ratio (
R
f
) falls in a narrow range with an
average value of 0.88. The power
m
for both the initial and the
50% moduli are approximately 0.5.
,50
ref
u
E
