The barrier layer in the final cover system should have the dual property of preventing the infiltration of precipitation and stopping the LFG migration into the atmosphere or the areas surrounding the landfills. Compacted soil has been used as a barrier layer material due to its low hydraulic permeability. Most of modern landfills employ a composite barrier layer systems consisting of compacted soil and underlying geomembrane. However, the effectiveness of the compacted soil as a gas barrier has hardly been investigated while its hydraulic performance has been extensively studied.

  The main objective of this study is to assess the effectiveness of the compacted soil as a gas barrier in the final cover system. The specific objectives of this study are: (1) to investigate the effect of compaction condition on the gas permeability; (2) to examine the effect of gas slippage and soil-water interaction on the defference between gas and hydraulic permeabilities; and (3) to investigate the effect of soil properties on the relationship between gas and hydraulic permeabilities.

  A series of gas permeability tests were conducted on natural and artificial soils compacted at various molding moisture contents in order to investigate the effect of molding moisture content on the gas permeability. The gas permeabilities of wet compaction soils were smaller than those of dry compaction soils over the similar gas-filled porosity. The lower gas permeability at the similar gas-filled porosity is attributed to the fact that soil compacted wet of optimum had the smaller pore sizes and the more tortuous paths.

  In compacted soil specimens, measured gas permeability was about two or three orders of magnitude greater than hydraulic permeability. The defference between gas and hydraulic permeabilities of the compacted soils can be explained by gas slippage, which makes gas more permeable, and by soil-water interaction, which impedes water flow. For all the tested soils, the ratio of gas permeability to intrinsic gas permeability ranged from 10.2 to 1.52, which indicates that gas slip enhanced the gas permeability only 2 to 52% of its intrinsic gas permeability. Therefore, it can be concluded that the difference between gas and hydraulic permeabilities is mainly due to soil-water interaction. The effect of soil properties on the gas and hydraulic permeabilities was investigated using sand-clay (i.e., kaolinite and bentonite) mixtures. For the compacted sand-kaolinite mixture at the optimum moisture content, the gas permeability decreased as kaolinte content increased from 10 to 60%. The values of hydraulic permeability also decreased as kaolinite content increased up to 40%, however they were relatively constant when kaolinite constant exceeded 40%. The gas permeability of compacted sand-bentonite mixture was significantly less than that of the sand-kaolinite mixture because of cracks induced by desiccation. However, hydraulic permeability of the sand-bentonite mixture having high bentonite content was maintained to be low because cracks may be swelled and healed on rewetting.

Keywords : compacted soil, gas permeability, gas slippage, hydraulic permeability, molding moisture content, soil-water interaction