A new method and apparatus for measuring in situ air permeability of unsaturated soil

A new apparatus and an interpretation method simplified from existing solutions of spherical air flow in soil were developed to measure in situ air permeability (ka) of unsaturated soil. By pumping air with a controlled volumetric flux (qv) into a spherical aeration bulb and measuring the corresponding pseudo-steady-state air gauge pressure (Δp0), ka was determined based on the slope of a qv–Δp0 relationship. The method was applied to measure ka in a full-scale landfill cover. The ka measurements were verified by finite-element modelling of air flow. The measurements show good agreement with the computed results. Sensitivity analyses reveal that using a smaller bulb radius (r0) would increase the measurement accuracy by minimising the boundary effects associated with atmospheric conditions and contrasting ka between neighbouring soils. Yet, the use of a small bulb would result in more prominent effects of preferential flow and hence substantial overestimation of ka. For a small bulb size (r0 = 0.01 m), the measurement accuracy of ka could be within 15% (i.e., 2.4 × 10−13 to 6.3 × 10−13 m2) of the true values when the ratio of vertical-to-horizontal ka is between 0.4 and 1.