Two-Phase Flow Modeling of Leachate Injection Effects on Stability of Bioreactor Landfill Slopes

摘要

In bioreactor landfills, the leachate is recirculated using leachate recirculation systems (LRS) under pressure to enhance municipal solid waste (MSW) degradation and reduce overall settlement period. Pressurized leachate injection leads to higher moisture distribution, which results in faster degradation of MSW. However, the high injection pressure in recirculation system near side slopes can generate excess pore fluid pressures and reduce effective shear strength of MSW, leading to instability of bioreactor landfill slopes. In this study, a numerical two-phase flow modeling is used to predict the moisture distribution, generation and distribution of pore-water and pore-gas pressures, and their impacts on stability of bioreactor landfill slopes in heterogeneous and anisotropic waste conditions at a high injection pressure. Two-phase flow model is preferred because landfill leachate and gas exist simultaneously in unsaturated MSW pores, and the model enables the realistic generation and distribution of moisture and pore pressures through porous MSW. Strength reduction technique was employed to perform slope stability analyses as it takes into account of the transient and spatially varying pore water and gas pressures. The model results were validated based on the published studies using single-phase flow modeling and slope stability analysis under simplified conditions (e.g., homogeneous MSW). This study then simulated the effects of heterogeneous and anisotropic waste conditions by incorporating different waste layers of varying unit weight and saturated hydraulic conductivity. The unsaturated hydraulic properties of MSW with waste dry unit weight of 7.8 kN/m3 were taken based on laboratory studies. Overall, it was concluded that high injection pressure in recirculation systems near side slope and heterogeneous-anisotropic waste properties can greatly affect the stability of bioreactor landfill slope, and therefore, the effects of pore water and gas pressures need to be assessed for stability analyses of bioreactor landfill.