Gas and liquid permeability in the variably saturated compacted loess used as an earthen final cover material in landfills

Abstract

Landfill final covers are crucial to mitigating both the emission of landfill gases and surface water infiltration into solid wastes. Loess that is widely distributed in Northwest China has been transformed into earthen final cover (EFC), although not yet on a large scale. There remains a need for laboratory bench-testing in assessing the performance of this loess as an EFC material, particularly with respect to gas and liquid permeability. To evaluate gas and liquid permeability in the variably saturated loess at the laboratory scale, loess specimens with various resultant dry densities and post-compaction water contents were prepared. Laboratory tests were performed including constant gas pressure-gradient tests and wetting tests in a step-wise fashion to measure gas and liquid permeability as well as water retention behaviours. Large differences were found between intrinsic permeability to gas and liquid, and an empirical power relationship was proposed to characterize the measured intrinsic permeability to gas and liquid relative to the void ratio. The data of relative gas permeability as a function of liquid saturation plotted nearly along a single curve for all dry densities; this was also the case for the relative liquid permeability. Based on the regression results of water retention curves, the measured relative gas permeability values were well described by the van Genuchten-Mulaem (vG-M) model, and a certain difference between the relative liquid permeability evaluated by the vG-M model and the modified Childs and Collis-George equation. An unique relationship was found between the ratio of liquid to gas permeability and liquid saturation for all dry densities, in a range of residual liquid to gas saturation. An empirical model was proposed to efficiently describe gas and liquid permeability of the compacted loess at various liquid saturations and dry densities as well as the data of the loam from another experimental program.

Keywords: Dry density; Earthen final cover; Gas permeability; Liquid permeability; Liquid saturation; Loess.