Measurements of Drying and Wetting Gas Diffusion Coefficients and Gas Permeability of Unsaturated Soils Using a New Flexible-Wall Device

Wetting–drying cycles have a significant impact on the gas diffusion coefficient ($D_p$) and gas permeability ($k_a$) of unsaturated soils. The soil volume change during wetting–drying cycles limits the application of the traditional rigid-wall device for measuring $k_a$ and $D_p$, due to the gas preferential flow along the interface between soil and the rigid-wall container. Although flexible-wall devices for measuring $k_a$ are available, no such device exists for measuring $D_p$. Thus, the effects of wetting–drying cycles on $D_p$ remain unclear, particularly for fine-grained soil. The present study developed a flexible-wall device to investigate the effects of a wetting–drying cycle on the $D_p$ and $k_a$ of unsaturated soils. Both the flexible- and rigid-wall devices were adopted to measure $k_a$ and $D_p$ of three soil types, including fine sand, silt and kaolin. The rigid-wall device could overestimate $k_a$ by up to approximately one order of magnitude, whereas it overestimated $D_p$ by approximately 2–3 times. Regardless of $k_a$ and $D_p$, the difference in measurements between the rigid- and flexible-wall devices became more significant at a lower water content and along the drying path because of the gas preferential flow caused by soil shrinkage in the rigid-wall device. Accordingly, the kaolin exhibited the largest difference in $k_a$ and $D_p$ as measured by the flexible- and rigid-wall devices because it had the largest clay minerals and the finest particle size, resulting in the largest volume shrinkage. The $k_a$ and $D_p$ measured by the flexible-wall device along the drying path were generally larger than those along the wetting path, probably because of entrapped gas in the soil caused by water spray during wetting.