383
Technical Committee 101 - Session II /
Comité technique 101 - Session II
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
3.2
Mohr-Coulomb Strength Parameters
Table 3 presents a summary of the Mohr-Coulomb strength
parameters of the Bangkok subsoils (i.e. weathered clay, soft
clay, stiff clay and hard clay) obtained from consolidated
isotropically drained and undrained triaxial compression (CID
and CIU) and extension (CIDE and CIUE) tests reported in the
literature. The notations for the triaxial tests identified in Table
3 are explained. It can be seen that the differences in the applied
stress path have the most significant effect on the Mohr-
Coulomb strength parameters. Initial conditions at the
consolidation state (i.e. isotropic or anisotropic), as well as the
drainage conditions during shear (i.e. drained or undrained),
also have an effect on the strength parameters, but to a lesser
magnitude. Therefore, it needs to be emphasised that the
strength parameters should be carefully selected according to
the applied stress path, resulting from the construction
sequences.
Table 3. Summary of Mohr-Coulomb Strength Parameter of Bangkok
Subsoils
3.3
Stiffness and Strength Parameters of Soft and Stiffness
Bangkok Clays
3.3.1
Soft Bangkok Clay
Two series of isotropically consolidated triaxial compression
tests, CIU and CID, conducted by Balasubramaniam &
Chaudhry (1978) on soft and stiff Bangkok clay, were analysed
in this study. The soil samples were taken from a depth of 6.0 m
below the ground surface. The confining pressures,
′
3
used for
both CIU and CID series were 138, 207, 276, 345 and 414
kN/m
2
for tests S1 to S5, respectively. The angle of the internal
friction (
'
) obtained from the CIU and CID tests were 27
o
and
23.6
o
; whereas, the cohesion (
c'
) was zero for both series. The
drained strength parameters are summarised in Table 4.
The results of the CIU triaxial tests carried out on the soft
clay are plotted in Figure 2. The (
q
,
a
) and (
u
,
a
) relationships
are shown in Figures 2(a) and 2(b), respectively. The deviator
stress and excess pore pressures versus the axial strain
relationships show typical normally to lightly overconslidated
clay behaviour, where the deviator stress and excess pore
pressure reaches their ultimate values at a relatively large strain.
Moreover, all the excess pore pressure plots were located in the
positive range.
Axial strain,
a
(%)
0
5 10 15 20 25
Deviator stress, q (kN/m 2 )
0
50
100
150
200
250
300
CIU S1
CIU S2
CIU S3
CIU S4
CIU S5
Axial strain,
a
(%)
0
5 10 15 20 25
Excess pore pressure, u (kN/m 2 )
0
50
100
150
200
250
300
CIU S1
CIU S2
CIU S3
CIU S4
CIU S5
(a) Deviator stress vs axial strain
(b) Pore pressure vs axial strain
Figure 2. Results of CIU triaxial tests on soft Bangkok clay.
The results obtained from the CID triaxial tests for the soft
clay are shown in Figure 3, with the relationships of (
q
,
a
) and
(
v
,
a
) plotted in Figures 3(a) and 3(b), respectively. It can be
seen that, during the deviator stress applied, the volume of the
soil specimen gradually reduces. The volumetric and axial strain
curves of all the tests seem to coincide up to 10% axial strain,
after that they tend to divert slightly.
Axial strain,
a
(%)
0 10 20 30 40 50
Deviator stress, q (kN/m 2 )
0
100
200
300
400
500
600
CID S1
CID S2
CID S3
CID S4
CID S5
Axial strain,
a
(%)
0 10 20 30 40 50
Volumetric strain,
v
(kN/m 2 )
0
5
10
15
20
CID S1
CID S2
CID S3
CID S4
CID S5
(a) Deviator stress vs axial strain
(b) Volumetric strain vs axial strain
Figure 3. Results of CID triaxial tests on soft Bangkok clay
These values are also summarised in Table 4 together with
the reference initial modulus (
ref
i
E
,
), the reference moduli
at 50% of strength (
,50
ref
i
E
50
ref
E
,
E
), and the failure ratio (
R
f
)
resulting from CID tests as well as the shear strength parameters
(
c'
,
'
) for Soft Bangkok Clay.
,50
ref
u
3.3.2
Stiff Bangkok Clay
The two series of isotropically consolidated triaxial
compression tests, CIU and CID, conducted by Hassan (1976)
on stiff Bangkok clay, are re-interpreted in this study. The
undisturbed soils samples were collected from a depth of 17.4 to
18 m below the ground surface. The pre-shear consolidation
pressures ranged from 17 to 620 kN/m
2
and 34 to 552 kN/m
2
,
for the CIU and CID series, respectively. The angles of the
internal friction (
'
) from the CIU and CID series were 28.1 and
26.3 degrees; whereas, the values of cohesion (
c'
) were 11.4 and
32.8 kN/m
2
, respectively. The drained strength parameters are
summarised in Table 4.
Figure 4 shows the results of CIU tests on the stiff Bangkok
clay. It can be seen from Figure 4(a) that (
q
,
a
) relationships,
up to a pre-shear confining pressure of 138 kN/m
2
(tests CIU F1
to F3), exhibit no strain softening. At a level of confining
pressure from 207 to 414 kN/m
2
(tests CIU F4 to F7), these clay
Reference
Depth (m)
Test type
'
(o)
c'
(kN/m
2
)
Weathered Clay
Balasubramaniam
& Uddin (1977)
2.5 to 3.0
CIUE
U
28.9
0
CIU
22.2
0
CID
23.5
0
Balasubramaniam
et al.
(1978)
2.5 to 3.0
CIUE
U
29
0
Soft Clay
CIU
26
0
Balasubramaniam
& Chaudhry
(1978)
5.5 to 6.0
CID
21.7
0
CIU
24
38
CID
23.5
0
CID
P
23.7
0
CIUE
L
26
0
CIUE
U
21.1
58.7
CIDE
L
26.2
0
Balasubramaniam
et. al.
(1978)
5.5 to 6.0
CIDE
U
23.5
31.8
Stiff Clay
CID
26
30
CIUE
L
18
54
CIUE
U
25
54
Balasubramaniam
et al.
(1978)
16.0 to
16.6
CIDE
U
16.6
11
CIU
28.1
11.4
Hassan (1976)
17.0 to
18.0
CID
26.3
32.8
