Environmental Engineering Reference
In-Depth Information
of the portion of the SWCC for soil suctions less than the
air-entry value is defined back to 1 kPa of suction, assuming
that the air-entry value is greater than 1 kPa. Therefore,
w
s
and
S
1
can be considered as two known values.
The bending point on an SWCC at residual conditions
occurs at a suction of approximately
(
2
.
7
a)
1
/b
. The user
can either choose to set the residual soil suction value
ψ
r
as
(
2
.
7
a)
1
/b
or select the residual suction value as an indepen-
dent curve-fitting parameter. If
ψ
r
is replaced by
(
2
.
7
a)
1
/b
,
the curve has only three additional curve-fitting parameters:
w
r
,a
, and
G
s
)
. Plots of Eq. 5.73 with various soil parame-
ters are shown in Fig. 5.57.
used in the testing program were prepared as slurry. The
classification properties of some of the soils tested at the
University of Saskatchewan, Saskatoon, are presented in
Table 5.4. The grain-size distribution curves for these soils
are presented in Fig. 5.58.
The seven soils were best fit using themeaningful parameter
SWCC equation (Eq. 5.72). The parameter
w
s
was estimated
for each soil and a curve-fitting procedure was used to find the
following soil properties: (i) slope of the SWCC at soil suc-
tions less than the air-entry value,
S
1
; (ii) slope of the main
portion of the SWCC (i.e., desaturation zone),
S
2
; (iii) air-
entry value of the soil,
ψ
aev
; and (iv) residual soil suction
ψ
r
.
Best-fitting curves for Regina clay (Fredlund, 1964), Jos-
signy silt (Fleureau et al., 1995), and kaolin (Fleureau et al.,
2004) are shown in Fig. 5.59. Best-fitting curves for the four
soils tested at the University of Saskatchewan, Saskatoon, are
shown in Fig. 5.60. Curve-fitting parameters associated with
the seven soils are summarized in Table 5.5.
The best-fit results for the seven soils using the simpli-
fied SWCC equation (i.e., Eq. 5.73) were also computed.
The slopes of the SWCCs at soil suctions less than the
air-entry values for the seven soils were estimated prior to
5.6.3 Pham and Fredlund (2011) Equation Fit
of Soil Data
The performance of the two Pham and Fredlund (2008)
equations can be illustrated using published SWCC results
on several soils. The selected SWCC results are for
(i) Regina clay (Fredlund, 1964), (ii) Jossigny silt (Fleureau
et al., 1995), (iii) kaolin (Fleureau et al., 2004), (iv) Beaver
Creek sand, (v) processed silt, (vi) Saskatchewan silty
sand, and (vii) Indian Head till (Pham, 2005). All soils
0.6
w
sat
= 50
S
1
= 4
0.5
w
= 12
0.4
a
= 50,000;
b
= 2
a
= 25,000;
b
= 3
a
= 50,000;
b
= 1.5
0.3
0.2
0.1
0
0.1
1
10
100
1,000
10,000
100,000
10
6
Soil suction, kPa
Figure 5.57
Plot of SWCC equation when using different curve-fitting parameters (after Pham,
2005).
Table 5.4 Soil Properties for Four Soils Tested at University of Saskatchewan
Liquid Limit
Plastic Limit
Specific Gravity
Soil
(LL)
(PL)
(G
s
)
Beaver Creek sand
N/A
N/A
2.65
(Bruch, 1993)
Saskatchewan silty
sand
N/A
N/A
2.65
Processed silt
26.8
(Wilson, 1990)
25.4
(Wilson, 1990)
2.70
(Wilson, 1990)
Indian Head till
36.1
16.4
2.73
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