Seismic Stability Analysis of Expanded MSW Landfill Considering Composite Failure Surface in Translational Mode

Vertical expansion in existing landfills is accomplished using an engineering berm at the toe of the landfill, which is a better alternative to horizontal expansion and new landfill construction from economic and environmental points of view. In this study, the pseudostatic limit equilibrium seismic stability analysis of a side-hill type landfill that was expanded using an engineered berm, which considers a composite failure surface, was performed. The waste heterogeneity was incorporated by considering the unit weight variation with depth. Two failure conditions were considered for the expansion of municipal solid waste (MSW) landfills: (1) under berm failure (when the waste mass slides with the berm); and (2) over berm failure (when the waste mass slides over the back slope of the berm). Based on the geometrical parameters, different landfill geometric models were considered in this study. Closed-form solutions for the average factor of safety (FS) and yield acceleration coefficient were developed. The expanded landfill could safely resist a horizontal seismic acceleration of 0.18g under the most critical situation. When the back slope of the berm became steeper than 3.85H:1V, the failure condition shifted from over to under berm. The location of the potential failure surface depended on the shear strength parameters of the waste and liner material. In addition, the failure surface shifts from passing within the landfill to along the liner with a change in the internal friction angle of the waste material from 30˚ to 35˚, which facilitates material selection for liners. The maximum height to which the volume of landfill could be safely increased was 110 m.