黄土覆盖层水–气耦合运移土柱试验及数值模拟

基于水分储存与释放机理的土质覆盖层在干旱和半干旱地区垃圾填埋场具有较好的应用前景。前人研究土质覆盖层大多侧重于其雨水存储及防渗性能,忽略了填埋气在覆盖层中的运移及其影响。通过在黄土土柱底部通入甲烷和二氧化碳等比例混合气体模拟填埋气在覆盖层运移,在土柱顶部施加3 cm常水头模拟雨水入渗对黄土覆盖层中气压分布以及各气体组分分布的影响。基于试验结果,利用商业软件Geo-Studio中的Air/W模块对黄土覆盖层水–气耦合运移进行了数值模拟与分析。研究结果表明:模拟雨水入渗降低了上层黄土的导气系数,导致土柱中气压显著增加,呈现出先增加至突破值然后回落至稳定值;气压突破值约等于上部土体进气值;数值模拟能较好地模拟水分入渗过程并捕捉到气压变化趋势,但由于其无法模拟多组分填埋气在土柱中的复杂物理过程,导致气压的模拟结果与实测值存在差距。本文为更准确模拟分析土质覆盖层中水与填埋气耦合运移过程提供了一些建议。

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.