Slope Stability of Bioreactor Landfill with Leachate Recirculation Using Horizontal Trench System

A numerical two-phase flow modeling was performed for leachate recirculation in bioreactor landfills using horizontal trench (HT) system to evaluate the moisture distribution, pore water and capillary pressures generation, and the resulting impacts on the stability of a simplified bioreactor landfill. A staggered geometric configuration of HT system with known horizontal and vertical spacing was modeled based on the results from previous study. The lateral distance of HT system from side slope (setback distance) was varied in the range of 15-30 m, as recommended by USEPA. The waste was considered as heterogeneous and anisotropic with varied unit weight and saturated hydraulic conductivity. Leachate was injected at high injection pressure, both continuously and intermittently. Unsaturated hydraulic properties of MSW were based on the van Genutchen model. It was found that staggered HT system, located near side slope, could significantly affect the moisture distribution and lead to landfill slope failure under continuous injection as a result of excessive pore fluid pressures as compared to the HT systems, located relatively far away from side slope. However, intermittent injection sequencing with injection in alternate HT layers proved effective in maintaining stable slope without generating excess pressures, and resulting in an optimal environment for biodegradation.