Residual Shear Strength of a Residual Soil of Granulite

This paper discusses the residual shear strength of a granulite residual soil that was involved in a large landslide in Brazil. The residual strength was investigated because of the great mobility achieved by the soil after failure. The experimental program comprised physical and mineralogical characterization and residual shear strength measurements. Direct shear tests show a progressive breakage of soil structure. In the tests the soil showed brittleness under the higher normal stresses. The residual friction angle measured in direct shear test is about 12.0 degrees, remarkably smaller than 25.0 degrees measured in peak. For ring shear tests a linear failure envelope indicates a friction angle of 7.7 degrees. The mobilization of the residual condition imposes a severe fall in shear strength and may explain the mobility presented by the soil during the reported landslide. Particle size analysis indicated that the material becomes finer after more energetic sample preparation, but the shearing did not cause significant particle disintegration. Most of the published correlations between the residual friction angle and clay fraction or plasticity index do not apply to this material.