Environmental Engineering Reference
In-Depth Information
10
−
08
10
−
09
10
−
10
van Genuchten SWCC +
k
a
(
S
e
);
S
r
= 38.9%
Test data from Moldrup
et al. (2003)
van Genuchten SWCC +
k
a
(
S
w
);
S
r
= 38.9%
van Genuchten SWCC +
k
a
(
S
w
or
S
e
);
S
r
= 0%
10
−
11
10
−
12
10
6
0.1
1
10
100
1000
10,000
100,000
Soil suction (kPa)
Figure 9.30
Calculated and measured air coefficient-of-permeability functions for soil tested
by Moldrup et al., (2003) using van Genuchten (1980) SWCC model.
when using the van Genuchten's (1980) SWCC equation,
Eq. 9.85 provides a better fit of the data. Setting the resid-
ual volumetric water content to zero appears to improve its
performance in the high-suction range.
The air coefficient of permeability appears to reach the
dry air permeability
k
d
when soil suction reaches 10
6
kPa.
It should be noted that the usage of Eqs. 9.84 and 9.87
can result in the calculation of negative numbers. Setting
the residual water content at zero avoids this difficulty.
Figures 9.31 and 9.32 also show that setting the residual
water content to zero provides an air permeability function
that is close to the combination of Eqs. 9.85 and 9.87. In
summary, the Fredlund and Xing (1994) SWCC equation
appears to provide the closest fit of the air permeability data
over the entire soil suction range.
9.9.5 Air Storage Function
The volumetric air content of a soil is, in essence, the inverse
of the SWCC defined using volumetric water content. Volu-
metric air content versus soil suction (i.e., the SACC) defines
the amount of air in a soil at any particular soil suction. The
slope of the SACC defines the rate of change of the volume
of air in the soil with respect to a change in soil suction and
can be described using the symbol
m
2
:
dθ
a
dψ
m
2
=
(9.88)
where:
θ
a
=
volumetric air content in the soil and
ψ
=
soil suction.
10
−
04
10
−
05
10
−
06
10
−
07
10
−
08
Fredlund & Xing SWCC +
k
a
;
S
r
= 6.8%
Test data from Samingan
et al. (2003)
Fredlund & Xing SWCC +
k
a
;
S
r
= 0%
10
−
09
10
−
10
10
−
11
10
−
12
10
6
0.1
1
10
100
1000
10,000
100,000
Soil suction (kPa)
Figure 9.31
Calculated and measured air coefficient-of-permeability functions for soil tested
by Samingan et al., (2003) using Fredlund and Xing (1994) SWCC model.
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