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Abstract

Palos Verdes Peninsula contains large landslides in areas that are otherwise desirable for residential development. The landslides can be developed providing the factor of safety is at least 1.5 or can be raised to 1.5 during development. The greatest uncertainty in the calculated factor of safety is the residual shear strength of bentonite that forms the bases of slides. The use of too high a shear strength inflates the calculated factor of safety and can result in landslide failure after development. Tests by various investigators yield a wide range of residual shear strengths for bentonite samples from Palos Verdes Peninsula. Most residual friction angles (4φr) range between 6° and 14° with one lower value (3.5°) and several higher values reported. Residual cohesions (Cr) range mostly between 0 and 36 kPa (0 to 750 lb/ft2). Data for various bentonite samples from Palos Verdes Peninsula indicate they have similar compositions and Atterberg limits. Calcium montmorillonite is the principal clay mineral; the liquid limit is generally between 80% and 110% with the plasticity index between 40% and 70%. The data suggest that much of the reported variation in residual shear strength is the result of differences in sample preparation, testing methods, and interpretation of results rather than true differences in strength. Bulk samples of bentonite from the base of the Portuguese Bend landslide were remolded and tested by conventional direct shear, long sample direct shear, and ring shear devices to determine the effect of testing method on residual shear strength measurements. The three devices gave similar results—for conventional direct shear, φr = 6.9° and Cr = 33.0 kPa (690 lb/ft2); for long sample shear, tyr = 6.8° and Cr = 23.8 kPa (497 lb/ft2); for ring shear, tyr = 6.7° and Cr = 7.2 kPa (150 lb/ft2). Back calculations indicate the ring shear results most closely simulate the residual strength along the base of the Portuguese Bend landslide. For all those devices, the residual shear envelope is a curve whose slope reaches an asymptote near a confining pressure of 200 kPa (approximately 4,000 lb/ft2). Reported φr angles are commonly too high because samples were not tested at sufficient confining pressure to define the asymptote and a best fit straight line was drawn through the data points.

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