Environmental Engineering Reference
In-Depth Information
Comparisons can be made between measured shear
strength data and the estimation of unsaturated soil shear
strengths (Sheng et al., 2009). The estimation equations
used in the comparison are those proposed by (i) Oberg
and Sallfors (1997), (ii) D.G. Fredlund et al., (1996),
(iii) Vanapalli et al., (1996b), and (iv) Goh et al., (2010).
Comparisons between four shear strength data sets are
presented: (i) reconsituted silty clay (Cunningham, et al.,
2003), (ii) compacted kaolin assuming no volume change
(Thu et al., 2007a), (iii) compacted kaolin measuring
volume change (Thu et al., 2007a), and (iv) Madrid grey
clay (Escario and Juca, 1989).
200
160
120
80
40
Satija (1978)
Gan et al. (1988)
0
12.2.5.1 Data on Reconstituted Silty Clay
(Cunningham et al., 2003)
Cunningham et al., (2003) presented triaxial compression
tests performed using various confining pressures on the
reconstituted silty clay comprised of a mixture of 20% pure
Speswhite kaolin, 10% London clay, and 70% silica silt. The
slurry soil was isotropically preconsolidated to 130 kPa. The
SWCC in terms of degree of saturation versus soil suction
is presented in Fig. 12.20. The air-entry value of the soil
determined using an empirical construction procedure was
approximately 450 kPa. However, the degree of saturation
starts to reduce at about 250 kPa. The
a
f
fitting parameter
is 800 kPa, and it designates the inflection point associ-
ated with fitting the Fredlund and Xing (1994) equation.
Figures 12.21 and 12.22 show the SWCC for the reconsti-
tuted silty clay plotted in terms of gravimetric water content
and volumetric water content, respectively.
The original shear strength data published by Cunningham
et al. (2003) for the reconstituted silty clay were presented
in terms of the maximum deviator stress applied to the soil.
Figure 12.23 shows the shear strength data presented in
terms of the apparent cohesion intercept versus the applied
matric suction for net confining pressures ranging from 50
to 400 kPa. The matric suction versus shear strength is quite
nonlinear, tending to level off before residual suction condi-
tions are reached. The data can be compared with the shear
strengths estimated when using various proposed estimation
equations.
The soil parameters from the SWCC required for each
of the estimation procedures are tabulated in Table 12.3. It
is noted that while all procedures rely upon the SWCC, the
estimation equations depend upon different variables associ-
ated with the SWCC. The parameters for the saturated soil
are
φ
=
0
200
400
600
Soil suction, kPa
(a)
200
s
-
u
a
=
200 kPa
160
s
-
u
a
=
100 kPa
120
s
-
u
a
=
25 kPa
80
40
Vanapalli et al. (1996)
0
0
200
400
600
Soil suction, kPa
(b)
Figure 12.19
Comparison of experimental data with Vilar (2006)
estimation of shear strength envelopes: (a) shear strength data from
Satija (1978) and Gan et al. (1988); (b) shear strength data from
Vanapalli (1994).
Comparisons are made between predicted shear strengths
and measured shear strengths when using some of the
proposed estimation equations. The soil data sets used for
comparative purposes are independent of the data used in
the development of the empirical shear strength equations.
Table 12.2 Estimation Equation Results Compared
with Measured Test Results
32
◦
,
c
=
0 kPa, as given in Cunningham et al.
Equation
Suction Soil Parameters
(2003).
The results from triaxial compression tests on the recon-
stituted silty clay tested with a net confining pressure of
100 kPa are used for comparison with the estimated shear
strength results. Figure 12.24 shows the estimations of shear
strength from the procedures listed in Table 12.3 along with
the shear strengths measured when the confining pressure
was 100 kPa. The measured shear strength increases with
matric suction in a nonlinear manner as anticipated.
S
tan
φ
Oberg and Sallfors (1997)
θ
θ
s
κ
tan
φ
D.G. Fredlund et al. (1996)
θ
tan
φ
−
θ
r
Vanapalli et al. (1996b)
θ
s
−
θ
r
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