Kettleman Hills Waste Landfill Slope Failure. II: Stability Analyses

Analyses were made to determine the cause of a stability failure of a 90 ft high, 15 acre hazardous waste landfill in which lateral displacements of up to 35 ft and vertical settlements of up to 14 ft were measured. The failure developed by sliding along interfaces within the composite geosynthetic‐compacted‐clay liner system beneath the waste fill. The shear resistances of the different interfaces in the liner system were determined by direct shear and pullout tests as described in a companion paper (Mitchell et al. 1990). Conventional two‐dimensional (2D) stability analyses of representative cross sections and three‐dimensional (3D) analyses of the overall waste fill and liner configuration are described. Each type of analysis was applied to two cases: (1) The “Probable Minimum Clay/Liner Wetting Case,” in which shear along a wetted HDPE liner/compacted clay interface was assumed to occur only over a small area of the base; and (2) the “Full‐Base‐Wetting Case,” in which the HDPE liner/compacted clay liner interface was assumed to have become “wetted” over the full central base of the fill basin. The 2D stability analyses gave factors of safety of 1.2‐1.25 and 1.1‐1.15 for the minimum wetting case and the full base wetting case, respectively, while the 3D analyses yielded values of 1.08 and 1.01 for these two cases. Uncertainties in the strength parameters and analysis methods lead to a best estimate of the computed factor of safety at the time of failure of 0.85‐1.25.This provides good agreement with the observed field performance, and suggests that the techniques used to evaluate liner‐interface shear strengths and to perform overall stability analyses may be appropriate for future evaluation of other, similar lined waste‐repository fills.