Environmental Engineering Reference
In-Depth Information
120
D. G. Fredlund et al. (1996)
100
80
Goh et al. (2010)
60
Oberg and Sallfors (1997)
40
20
Vanapalli et al. (1996b)
0
0
50
100
150
200
250
300
Matric suction (
u
a
-
u
w
), kPa
Figure 12.29
Comparison of estimations of shear strength from various proposed equations and
measured data for compacted kaolin with volume change measurements (after Thu et al., 2007a).
12.2.5.4 Data on Madrid Grey Clay
(Escario and Juca, 1989)
Escario and Juca (1989) used a modified direct shear box
to measure the shear strength of three types of clay over
an extended suction range up to 15,000 kPa. All specimens
were statically compacted at the same water content and den-
sity conditions. The specimens were then allowed to come
to equilibrium under predetermined total normal stresses as
well as applied air and water pressures. The specimens were
then sheared to failure under constant suction in the mod-
ified direct shear apparatus. One set of data is shown that
compares the measured results with the estimation proce-
dures for Madrid grey clay when the net normal stress was
300 kPa. The SWCC of the Madrid grey clay was obtained
from drying tests and is shown in Fig. 12.30 along with the
best-fit Fredlund and Xing (1994) parameters. Figure 12.31
shows a plot of the volumetric water content versus soil
suction for Madrid grey clay.
The shear strength parameters for saturated Madrid grey
clay are as follows:
φ
=
25
.
3
◦
,
c
=
30 kPa (Escario and
Juca, 1989). The additional parameters used for comparing
various strength equations are listed in the Table 12.6.
The predictions of shear strength under net normal stresses
of 300 kPa are shown in Fig. 12.32. The measured shear
strength appears to reach a peak around suction values of
10,000 kPa and then decreases slightly. The D. G. Fredlund
et al. (1996) provides a reasonably close estimation of the
unsaturated soil shear strength. The prediction using the Vana-
palli et al., (1996) equation proves to be quite sensitive to
the estimation of residual suction. The results in Fig. 12.32
correspond to an estimated residual suction of 30,000 kPa.
There are other experimental data on unsaturated soil
shear strengths in the literature; however,
the results
120
100
80
SWCC fitting parameters
60
a
f
n
f
m
f
y
r
(kPa)
30,000
600
07
1.0
40
20
Test data (after Escario and Juca, 1989)
Fredlund and Xing (1994)
0
10
6
0.1
1
10
100
1000
10,000
100,000
Soil suction, kPa
Figure 12.30
SWCC for Madrid grey clay in terms of degree of saturation versus soil suction
(after Escario and Juca, 1989).
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