ABSTRACT
Air permeability of soil (and other porous materials) is the coefficient, ka, governing convective transmission of air through soil under an applied total pressure gradient. The
theory for the flow of air through soil is based on Darcy’s law, which states that the
velocity of a fluid flowing through a porous column is directly proportional to the pressure
difference and inversely proportional to the length of the column. This has been investi-
gated thoroughly for both isotropic and anisotropic media (e.g., Maasland and Kirkham 1955;
Corey 1986). Importantly, large pores and wide cracks contribute most to air permeability
because the volumetric flow of air through a single pore varies as the fourth power of the pore
radius. The air permeability coefficient, ka, has units of m 2 and is also known as the intrinsic
permeability to air (Reeve 1953). It can be derived from Darcy’s law (for laminar flow of
liquids) using simple assumptions about isothermal, nonturbulent flow of a viscous gas
(Kirkham 1946).
