Numerical Simulation Of Seepage Control And Methane Oxidation Performance Of A Three-Layer Capillary Barrier Soil Cover

As the"skin"of municipal solid waste landfill,the main function of final soil cover is to prevent moisture from entering the solid waste body and generating leachate,and to avoid the disorderly release of landfill gas.In this thesis,a biochar modified three-layer capillary barrier was proposed based on the two-layer cover system recommended in the Geotechnical Specification for Sanitary Landfill CJJ176-2012.The new three-layer soil cover has a better methane oxidation capacity due to the enhanced microbial activity as a result of the addition of biochar.A numerical model of water-air-heat coupled transport considering methane oxidation was developed using the finite element software COMSOL Multiphysics,and the model was calibrated using the available laboratory test data.Parametric studies were conducted using the validated numerical model.A one-dimensional model was used to investigate the effects of coarse-grained soil layer,initial water content of fine-grained soil layer,landfill gas production rate,and rainfall intensity on water-gas transport and methane oxidation within the three-layer capillary barrier cover.A two-dimensional model was used to investigate the effects of slope angle on methane oxidation and the spatial variation of water-gas-heat transport within the three-layer cover.The main conclusions are as follows:(1)The larger the average particle size of the coarse-grained soil layer material,the smaller the corresponding water entry valueψ_w,and the higher the water content of the fine-grained soil layer when the infiltrated water breakthroughs the capillary barrier interface.Using the fine and gravel as the fine and coarse-grained layer material,respectively,it is found that the water content of the fine-grained layer at the breakthrough point is close to its saturation.(2)Increasing the initial water content of the fine-grained soil layer reduces the duration of the capillary barrier effect;increasing the rainfall intensity and landfill gas flux reduces the time for reaching the peak methane emission rate,the steady state methane emission rate is closer to the landfill gas influx rate,a reduction in the extent of the methane oxidation zone at the top of soil cover and the efficiency of methane oxidation.(3)The peak methane oxidation efficiency of the soil cover under rainfall conditions analyzed using the steady-state method is 1.6 times the theoretical value determined by the proposed numerical model.The methane oxidation efficiency calculated using the indirect transient method,which takes into account the effect of transient changes in methane storage within the soil cover,is close to the theoretical value determined by the numerical model.(4)The water redistribution is not sufficient for a gentle slope.When the slope angle increases from 5°to 18°,significant water redistribution occurs in the top biochar-modified silt and the middle fine-grained soil layers under the action of two-dimensional seepage flow.For the steeper slope angle,methane oxidation occurs mainly in the downslope of the soil cover due to the change in water content distribution.