Environmental Engineering Reference
In-Depth Information
10
−
08
10
−
09
10
−
10
Fredlund & Xing SWCC +
k
a
;
S
r
= 38.9%
Test data from Moldrup
et al. (2003)
Fredlund and Xing SWCC +
k
a
;
S
r
= 0%
10
−
11
10
−
12
10
6
0.1
1
10
100
1000
10,000
100,000
Soil suction (kPa)
Figure 9.32
Calculated and measured air coefficient-of-permeability functions for soil tested
by Moldrup et al., (2003) using Fredlund and Xing (1994) SWCC model.
60
50
a
f
= 100 kPa
n
f
= 0.8
m
f
= 1.54
40
30
% Water
% Air
20
10
0
10
6
0.1
1
10
100
1000
10,000
100,000
Soil suction (kPa)
Figure 9.33
Inverse relationship between SWCC and SACC.
The symbol
m
2
is not a compressibility of the air phase;
rather, it is a symbol that indicates the change in air storage
with respect to the soil suction. The compressibility of the
air phase is defined in terms of the general gas law, as shown
in Chapter 15.
Figure 9.33 shows a plot of both a SWCC and a SACC
illustrating the reciprocal nature of these two curves. Both
curves can be generated using the Fredlund and Xing (1994)
SWCC equation. The initial porosity was assumed to be
50% and the void ratio was assumed to be 0.5. The SWCC
parameters are shown on Fig. 9.33. The assumption is made
that there is no change in void ratio as suction is changed.
The reference for the coefficient of air permeability becomes
the completely dry soil state. The air-entry value (on the
desorption curve) becomes the point at which air can only
diffuse through the water phase of the soil.
9.10 APPLICATIONS TO SATURATED-
UNSATURATED AIR FLOW PROBLEMS
The application section illustrates a few of the concepts asso-
ciated with air flow. Relatively simple air flow problems are
used to illustrate the flow of air through an unsaturated soil.
The conditions defining each problem are described along
with the soil properties needed to solve the problem. All
problems are solved within the context of a boundary value
problem.
9.10.1 Solution to Air Flow Boundary Value Problem
A boundary value problem requires that the following infor-
mation be available to solve the problem at hand. First, the
ground surface geometry and the stratigraphic soil units must
be defined. A decision must be made regarding whether the
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