Air Flow through Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 466

collected in a graduated burette that had an air-oil interface.
The air flow was measured under atmospheric pressure con-
ditions by adjusting the legs of the U-tube and maintaining
the air-oil interfaces at the center elevation of the specimen.
The air coefficient of permeability for Sasumua clay was
measured at various confining pressures using the triaxial
permeameter. A steady-state air flow was maintained dur-
ing measurements of the coefficient of permeability. Two
specimens were compacted at an optimum water content of
60.5%. The Sasumua clay had high clay content and was
compacted at standard AASHTO compaction. Values for
the air coefficient of permeability were obtained by apply-
ing Darcy's law to the test results. The results are plotted in
Fig. 9.11 as a function of the confining pressure σ . Decreas-
ing air permeability values were observed as the confining
pressure was increased or the matric suction was decreased.
Air and water permeability tests have been measured on
Backwater boulder clay at increasing values of net confin-
ing pressure (σ u a ) from 35 to 1000 kPa and at decreas-

ing matric suction values from 94 to 14 kPa. Several sets
of air and water permeability measurements are shown in
Fig. 9.12. The results showed decreasing air coefficients of
permeability and increasing water coefficients of permeabil-
ity as the soil specimen was compressed. The matric suction
in the soil decreased as the confining pressure was increased.

−

Figure 9.12 Water and air coefficients of permeability measured
on Backwater boulder clay using triaxial permeameter cell (from
Barden and Pavlakis, 1971).

9.7.3 Triaxial Permeameter Cell for Measurement
of High Air Coefficients of Permeability
Ba-Te (2005) used a triaxial cell to measure the air perme-
ability of soils under the application of both positive and
negative air pressures. The laboratory tests were performed
on fine and uniform crushed silica sand (i.e., Leighton
Buzzard sand), with grain sizes ranging between 0.09 and
0.15 mm. A flowmeter with a maximum air flow rate of 2000
(standard) cm 3 /min was used. The test procedure was similar
to that used for the measurement of water coefficient of
permeability.
The air permeability apparatus consists of three main
parts: the pressure source, the triaxial cell, and the flowmeter
(Fig. 9.13). Pressure transducers were also used to measure
the air pressure at the top and bottom of the soil specimen.
The platens placed at the top and bottom of the soil
specimen were specially designed to ensure that there was no
impedance to air flow through the soil. Details of the platen
design are shown in Fig. 9.14. The holes in the stainless steel
plate were 0.075mm in diameter. The air permeability of the
top and bottom disks was at least two orders of magnitude
greater than that of the soil. The mass flowmeter (i.e.,
GFM air flow meter) had a range from 0 to 2000 (standard)
cm 3 /min with an accuracy of

1.5% of full scale.
The soil specimens were 70mm in diameter and 60mm
in height and were confined using a flexible rubber mem-
brane. The cell pressure used to confine the soil specimen
was 140 kPa.
Test results on dry Leighton Buzzard sand ranged from
2 . 0

±

Figure 9.11 Air coefficients of permeability for two specimens of
Sasumua clay measured using a triaxial permeameter (after Matyas,
1987).

10 − 6 m/s, as shown in Fig. 9.15. The
air coefficient of permeability was found to increase as the

10 − 6 × to 9 . 0 ×