Soil column tests and numerical simulations of moisture-gas coupled flow in a loess cover

The earthen final covers based on the water storage and release principle have a promising application in the landfills in arid and semi-arid areas. Most of the previous studies concentrate on the rainwater infiltration and ignore the landfill gas migration in the soil covers. In this paper, landfill gas migration is simulated by supplying the gas mixture of 50% methane and 50% carbon dioxide at the bottom of a loess soil column. A water ponding of 3 cm is applied at the top of the soil column to investigate the influence of rainwater infiltration on the distribution of gas pressure and composition in the loess cover. On the basis of the experimental study, the Air/W module of Geo-Studio commercial software is used to simulate the moisture-gas coupled flow in the loess soil cover. The research findings are as follows: the simulated rainwater infiltration results in a decrease in the gas permeability of the upper soil layer. The gas pressure firstly increases to a "breakthrough" value, and then decreases to a "stable" value. The gas breakthrough value is approximate to the air entry value of the upper soil. The numerical software is able to simulate the water infiltration process and capture the gas pressure change trend. However, there is difference in the magnitude of gas pressure between the simulated results and the measurements because the Air/W module cannot make a simulation for the complicated physical processes of multi-component landfill gas in the soil column. Some suggestions are proposed for more accurate simulations of moisture-gas coupled flow of landfills in earthen final covers.