Theory of Water Flow through Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 342

50

45 Experimental data

Fredlund and Xing (1994)

40

35

30

25

20

15

10

5

0

10 6 0.1 1 10 100 1000 10,000 100,000

Soil suction, kPa

(a)

10 − 4

Experimental data
Fredlund and Xing (1994)
van Genuchten-Burdine (1980)
van Genuchten-Mualem (1980)
Brooks and Corey (1964)
Campbell (1974)
k res

10 − 5
10 − 6

10 − 7

10 − 8

10 − 9

10 − 10

10 − 11
10 − 12
10 − 13

10 − 14

10 − 15

10 6 0.1 1 10 100 1000 10,000 100,000

Soil suction, kPa

(b)

Figure 7.15 Calculation of water permeability functions using various estimation procedures:
(a) Fredlund and Xing (1994) fit to the SWCC; (b) various water permeability functions for silty
sand and their relationship to residual coefficient of water permeability (after Ebrahimi-Birang
et al., 2004).

7.3.9.2 Water Vapor Permeability Functions
Six procedures were used to calculate tortuosity factors for
water vapor flow through soils and the results are shown in
Table 7.3 and in Fig. 7.17. The vapor permeability coeffi-
cient is negligible in the low-suction range where liquid water
flow is dominant. The tortuosity model proposed by Lai et al.
(1976) was selected and results are shown for the sand, silty
sand and, silt soils in Fig. 7.18.
All tortuosity functions produce similar vapor coefficients
of permeability in the high-suction range. The equation by
Lai et al. (1976) produces the highest values for the vapor
permeability coefficient with results presented in Table 7.4.
Figure 7.19 shows the variations of water, vapor, and
overall permeability coefficients for a silty sand soil. The
coefficient of vapor permeability k v has its lowest value as
the soil approaches saturation and begins to increase as air

10 − 5

10 − 6

10 − 7
10 −8
10 − 9
10 − 10
10 − 11
10 − 12
10 − 13
10 − 14
10 − 15
10 − 16
10 − 17

Silt sand Sand
Sandy clay loam Sandy loam
Silty loam

10 20 30 40 50

Soil number

Figure 7.16 Coefficients of water permeability at residual con-
ditions calculated using Fredlund et al. (1994b) estimation of per-
meability function (after Ebrahimi-Birang et al., 2004).