Residual Shear Strengths of Bentonites on Palos Verdes Peninsula, California

Abstract

Several large active and ancient landslides have failed along seams of bentonite located on the Palos Verdes peninsula, California. This is a desirable location for residential development due to its commanding ocean views. Numerous geotechnical reports regarding the residual shear strength of bentonites have been prepared over the years. The shear test procedure used has primarily been the direct shear test, but triaxial and ring shear testing have been performed. In-situ testing of block samples has also been performed on at least two occasions. Data indicates that Palos Verdes bentonite samples that have been tested for shear strength have similar mineralogical properties and similar Atterberg Limits. It would be expected that the residual shear strength values obtained would be similar. However, the actual residual shear strength values measured by various researchers vary widely. Direct shear and ring shear tests on samples of remolded bentonite performed for a study by the first author indicate that testing methods can be a significant source of variation in the measured residual shear strength. The residual shear strength determined by various methods was used in slope stability analyses of the Abalone Cove landslide, for which the slide geometry and groundwater conditions at the time of movement are known. The analyses indicate that the residual shear strength of the bentonite determined by ring shear testing resulted in the best fit as determined by a factor of safety of 1.0.