Air Flow through Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 467

Panel

Air pressure source
Vacuum source

38.2

Quick connect Quick connect

Digital
readout

Transducer

51.7

Vent

u b
(bottom)

Hollow disks

Totalizer

2176

Soil sample

LCD
readout

1.76

u b u t

u t
(top)

Cell u c

Triaxial cell

Flowmeter

Figure 9.13 Permeability apparatus used to measure air coefficients of permeability (after Ba-Te,
2005).

9.8 DIRECT MEASUREMENT OF AIR DIFFUSION
THROUGH WATER

Long periods of time are often required for the laboratory
testing of unsaturated soils. These long time periods allow
air to diffuse through water. Although a high-air-entry disk is
placed between the soil specimen and the measuring system
to resist the passage of free air, air can diffuse through the
water within the ceramic disk (Bishop and Donald, 1961).
The diffused air accumulates beneath the disk and intro-
duces an error in either the pore-water pressure measure-
ment associated with undrained tests or the water volume
change measurement associated with drained tests. Air dif-
fusion is a common and important problem which must be
given serious consideration when testing unsaturated soils
(Bishop, 1969).

1 mm

1 mm

2 mm

2 mm

70 mm

2.5 mm

66 mm

62 mm

9.8.1 Mechanism of Air Diffusion through
High-Air-Entry Disks
Undrained shear strength tests with pore-water pressure mea-
surements can be performed by maintaining the water phase
as a closed system. However, air can diffuse through the high-
air-entry disk and come out of solution in the “compartment”
below the disk, as illustrated in Fig. 9.16a. Diffused air dis-
places the water in the compartment (Fig. 9.16b), and water is
forced upward through the disk back into the soil specimen.

Figure 9.14 High-flow disk placed at top and bottom of air per-
meability soil specimen (after Ba-Te, 2005).

absolute air pressure increased. The water coefficient of
permeability was also measured and found to range between
1 . 3 10 − 4 m/s. Intrinsic permeability val-

ues computed from the measured air and water coefficients
of permeability were found to be comparable.

10 − 4

× and 1 . 4 ×