Environmental Engineering Reference
In-Depth Information
θ
s
50
Inflection point
40
θ
i
(
ψ
i
,
θ
i
)
θ
i
Slope =
30
(
ψ
p
− ψ
i
)
20
10
0
ψ
p
ψ
i
0
20
40
60
80
100
Soil suction, kPa
Figure 5.26
Graphical solution for estimation of three fitting parameter
(a
f
,n
f
,
and
m
f
)
(from
Fredlund and Xing, 1994).
The slope
s
of the tangent line can be calculated as
In Eq. 5.48, volumetric water content
θ
becomes equal to
the saturated volumetric water content
θ
s
when soil suction
is zero. On the other hand, when volumetric water content
θ
becomes zero, soil suction goes to infinity. It is also pos-
sible to use the degree of saturation for curve fitting, since
the degree of saturation varies from 0 to 100%. Gravimetric
water content
w
can be similarly normalized for curve fit-
ting. Three plots are shown for the same silt soil in Fig. 5.28
using different variables to represent the water content of the
soil (i.e., degree of saturation, volumetric water content, and
gravimetric water content). It should be noted that the above
comparisons can be made because the silt soil is a relatively
low volume change soil.
Experimental data has shown that the suction of a soil
reaches a maximum value of approximately 10
6
kPa at zero
θ
i
ψ
p
−
s
=
(5.52)
ψ
i
where:
ψ
p
=
suction at intercept of the tangent line with the zero
water content line (Fig. 5.26) and
ψ
i
=
suction at inflection point on the SWCC.
Small values of
m
f
result in a moderate slope in the high-
suction range, and large values of
n
f
produce a sharp curva-
ture near the air-entry value (see Fig. 5.25). An example of
a best-fit curve to the experimental data for a silt soil from
Brooks and Corey (1964) is shown in Fig. 5.27.
100
Best-fit curve
Experimental data
80
60
40
Void ratio: 0.862
Initial water content: 46.3%
Total density: 2.60 mg/m
3
a
= 67.32
n
= 7.322
m
= 0.499
20
0
0
100
200
300
Soil suction, kPa
Figure 5.27
Best-fit SWCC fitted to experimental data on a silt soil (data from Brooks and
Corey, 1964).
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