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.