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

A new method and instrument were developed for in situ measurement of the gas diffusion coefficient ($D_g$) and gas coefficient of permeability ($k_g$) 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 ($q_0$) until approaching steady state. Then, the pseudo-steady-state tracer gas concentration ($C_{r0}$) and gas gauge pressure ($\mathrm{\Delta }{P}_{r0}$) were recorded at a distance ($r_0$) away from the point source. The $D_g$ and $k_g$ were determined through the $C_{r0}-q_0$ and $\mathrm{\Delta }{P}_{r0}-{\mathrm{q}}_0$ relationships, respectively. Soil column tests (silty sand) and field tests (sandy lean clay) were conducted to validate the new method. The in situ measured $D_g$ and $k_g$ were comparable with those of undisturbed specimens measured by a two-chamber apparatus. The in situ measurement of $D_g$ and $k_g$ ranged from ${2\times 10}^{-8}{\mathrm{m}}^2{\mathrm{s}}^{-1}$ to ${4\times 10}^{-6}{\mathrm{m}}^2{\mathrm{s}}^{-1}$ and ${1.9\times 10}^{-8}{\mathrm{ms}}^{-1}$ to ${3.9\times 10}^{-7}{\mathrm{ms}}^{-1}$, respectively. The measurement accuracy increased at deeper depth, due to reduced boundary effect at soil surface. The measurement of in situ $D_g$ was more susceptible to the boundary effects than that of $k_g$ at shallower depth (i.e., 20 cm deep) in general. The field test results showed that the ratio of in situ $D_g$ (or $k_g$) 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.