Shear strength characterization and back analysis for a landslide complex

Future phases of landfill expansion at a municipal solid waste landfill in Southern California required the characterization and mitigation of a 16-hectare complex of coalescing pre-existing landslides in sedimentary bedrock. Traditional back-analysis of the entire slide mass yielded unreasonably low values of slide plane shear strength. A hybrid approach, which combined laboratory testing for site-specific validation and application of empirical correlations, with back analysis of a retrogressive failure geologic model, resulted in a realistic evaluation of slide plane shear strength. A limit equilibrium model developed to simulate the progressive nature of the failure, correctly predicted the geometry of each stage of the failure accompanied by a corresponding factor of safety at or less than unity, thereby validating both the progressive nature of the failure and the shear strength parameters. Numerical analysis using FLAC was an additional verification of the progressive failure.