Air Flow through Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 450

CHAPTER 9

Air Flow through Unsaturated Soils

9.1 INTRODUCTION during the construction of tunnels under water and is referred

to as an air cushion. There are certain conditions that must
be met in order for the air pressure translation process to be
successful. For example, the air-entry value of the material
around the tunnels should be greater than the required air
pressure translation.
Ba-Te (2005) studied the effect of pore-air pressure reduc-
tion on the stability of slopes. It was found that even a
relatively small reduction in pore-air pressure can result in
a significant increase in the factor of safety of a slope. The
results were confirmed as part of an analytical study as well
as laboratory experiments utilizing a tilting box of sand soil.
The air in an unsaturated soil consists of two forms: “free
air” and “dissolved air.” Water has the appearance of being
a permeable material with a diffusion coefficient that has the
form of a coefficient of permeability of water with respect to
air flow. The flow of air in and out of solution obeys the laws
of a compressible fluid flowing through a porous medium.
The volume-mass relations for free air and dissolved air
were described in Chapter 2. The axis translation technique
in the laboratory requires that consideration be given to the
effect of air diffusion through water.
There are certain problems where air pressure changes
and temperature changes are interrelated and occur simulta-
neously. Interrelated air flow and heat flow are common to
the processes that take place in solid waste containment (i.e.,
landfills). The same is true for waste materials from mining
operations (e.g., waste rock). Chemical reactions often gen-
erate heat which, in turn, changes air pressure and causes
the flow of air by convection. Air flow and heat flow often
occur simultaneously, and it becomes necessary to consider
the respective processes in either an uncoupled or a coupled
manner. However, it is useful to first understand air flow
as an independent physical process prior to combining the
process with heat flow.

Air behaves as a compressible fluid whose density (or the con-
centration of air molecules per unit volume) changes signifi-
cantly with a change in air pressure or temperature. The effect
of changes in air density due to pressure changes is taken
into consideration in the PDEs that describe air flow. The
effect of changes in air density due to temperature changes is
considered as a mechanism that couples with heat flow.
Air is a gas comprised mainly of oxygen and nitrogen
along with small percentages of other gases and water vapor.
Each component of air behaves as an independent gas, but
for most geotechnical engineering problems it is possible to
consider the combination of gases as a single gas, namely,
air. It is possible to write the constitutive law of air flow
in the form of Fick's law or Darcy's law. It is useful to be
able to use either Fick's law or Darcy's law in geotechni-
cal engineering and be able to convert between the intrinsic
permeability of a soil and the measured hydraulic conductiv-
ity (or coefficient of permeability). The Klinkenberg effect
associated with the absolute pressure at which air flow mea-
surements are made is also described in this chapter.
A basic understanding of the physical concepts of air
flow is presented along with example problems involving air
movement. The need to accommodate air phase compress-
ibility makes the formulation of the air flow PDEs appear
to be more complex than the PDEs for water flow. On the
other hand, the low density of air makes it possible for the
gravity term (i.e., unit weight of air) to be omitted.
The geotechnical engineer does not frequently encounter air
flow problems; however, when air flow problems are encoun-
tered, it is important to understand the differences between
the physics of water flow and the physics of air flow. One of
the applications of air flow involves forcing air through the
soil for the purpose of removing volatile soil contaminants. If
a positive air pressure is used to force air through the soil, the
process is referred to as air sparging. If a negative air pressure
(i.e., a vacuum) is used to produce air flow through the soil,
the process is referred to as soil vapor extraction. Increased
air pressure (i.e., air pressure translation) is sometimes used

9.2 THEORY OF FREE AIR FLOW

Air flow can occur through the mechanisms of convection,
conduction, and radiation. This chapter considers air flow