Evaluation of a final cover slide at a landfill with recirculating leachate

A forensic investigation was conducted to determine the mechanism causing the slide of a final cover on a 4:1 (horizontal: vertical) slope at a municipal solid waste (MSW) landfill where leachate had been recirculated. The slide occurred as a veneer displacement along the interface between the geomembrane (GM) and geosynthetic clay liner (GCL). Site observations suggested that elevated gas pressures were a significant contributor to the failure. Laboratory tests were conducted to determine the shear strength parameters of the GM–GCL interface and the reduction in normal stress required to cause displacement of the GM–GCL interface. Hydraulic conductivity and gas permeability of the GCL were also determined. Slope stability analyses were conducted to determine the gas pressure required to cause a slide and the factor of safety that would have existed if the gas pressures was at zero gage. Good agreement was obtained between gas pressures measured in the field, measurements of the reduction in normal stress required to cause sliding on the GM–GCL interface in a large-scale direct shear test, and the gas pressures corresponding to FS = 1 (imminent sliding) from the slope stability analysis. The findings from this study, and a similar case history, illustrate the importance of managing gas at an acceptable level beneath the cover at MSW landfills.