A New Method and Instrument for Measuring In Situ Gas Diffusion Coefficient and Gas Coefficient of Permeability of Unsaturated Soil

Abstract

A new method and instrument were developed for in situ measurement of the gas diffusion coefficient (Dg) and gas coefficient of permeability (kg) of unsaturated soil. Air containing tracer gas of volumetric concentration of 5%–10% was continuously pumped into a point source at a constant flux rate (q0) until approaching steady state. Then, the pseudo-steady-state tracer gas concentration (Cr0) and gas gauge pressure (ΔPr0) were recorded at a distance (r0) away from the point source. The Dg and kg were determined through the Cr0−q0 and ΔPr0−q0 relationships, respectively. Soil column tests (silty sand) and field tests (sandy lean clay) were conducted to validate the new method. The in situ measured Dg and kg were comparable with those of undisturbed specimens measured by a two-chamber apparatus. The in situ measurement of Dg and kg ranged from 2×10−8 m2 s−1 to 4×10−6 m2 s−1 and 1.9×10−8 ms−1 to 3.9×10−7 ms−1, respectively. The measurement accuracy increased at deeper depth, due to reduced boundary effect at soil surface. The measurement of in situ Dg was more susceptible to the boundary effects than that of kg at shallower depth (i.e., 20 cm deep) in general. The field test results showed that the ratio of in situ Dg (or kg) to that measured by the element tests was generally between 1.2 and 3.3 at or below depth of 0.30 m, due to the in situ heterogeneity of soil.