Air Flow through Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 473

0.20

NCHRP Data

0.15

0.10

0.05

0.00

0 20 40 60 80 100 120

Percent bubbling pressure
(bubbling pressure = 500 kPa)

Figure 9.23 Air diffusion rates through 5-bar ceramic disks (after Padilla et al., 2006a).

3.00

NCHRP Data
NSF Data
Average

2.50

2.00

1.50

1.00

0.50

0.00

0 0 0 0 0 0 0 0 0 0 0
Percent bubbling pressure
(bubbling pressure = 1500 kPa)

Figure 9.24 Air diffusion through 15-bar ceramic disks (after Padilla et al., 2006a).

Equations 9.79 and 9.80 show two different forms for writ-
ing the SACC equation. There is more than one way to define
the amount of air in a soil just as there was more than one
way to define the amount of water in a soil. Some soil-water
characteristic curve (SWCC) empirical equations have been
defined over the entire water content range while others have
been defined between saturated conditions and residual con-
ditions. The use of various SWCC equations is investigated
for the estimation of the air permeability function.
Numerous empirical equations have been used to describe
the SWCC (Brooks and Corey, 1964; van Genuchten 1980;
Fredlund and Xing, 1994). The Fredlund and Xing (1994)
equation for the SWCC is written as follows:

where:

a f ,n f ,m f = fitting parameters for the SWCC,

e = irrational constant equal to 2.71828 for tak-

ing the natural logarithm,

ψ = any soil suction value, and

ψ r = soil suction at residual condition.

Residual suction can vary over a wide range depending
upon the soil type. Residual suction can exceed 3000 kPa
for clay soils and be less than 100 kPa for coarse-grained
soils. The Fredlund and Xing (1994) equation uses three
fitting parameters, namely, a f ,n f , and m f . Equation 9.81
can be combined with Eq. 9.80 to yield an expression for
the SACC. The SACC can be developed for both the drying
and wetting curves.

θ s 1

ln ( 1 + ψ/ψ f )

1

θ(ψ) = −

(ψ/a f ) n f ]

m f
(9.81)

ln ( 1 + 10 6 /ψ r ) { ln[ e + }