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 (Dp) and gas permeability (ka) of unsaturated soils. The soil volume change during wetting–drying cycles limits the application of the traditional rigid-wall device for measuring ka and Dp, due to the gas preferential flow along the interface between soil and the rigid-wall container. Although flexible-wall devices for measuring ka are available, no such device exists for measuring Dp. Thus, the effects of wetting–drying cycles on Dp 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 Dp and ka of unsaturated soils. Both the flexible- and rigid-wall devices were adopted to measure ka and Dp of three soil types, including fine sand, silt and kaolin. The rigid-wall device could overestimate ka by up to approximately one order of magnitude, whereas it overestimated Dp by approximately 2–3 times. Regardless of ka and Dp, 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 ka and Dp 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 ka and Dp 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.