Effect of Fast Shearing on the Residual Shear Strengths Measured Along Pre-Existing Shear Surfaces in Kaolinite

Abstract

Several studies have been performed to evaluate the shear-rate dependency of the residual shear strength for clayey soils. From these studies, two general hypotheses have been proposed to explain the changes in shear strength that are observed at varying shear displacement rates: (1) changes in shear strength can be attributed to changes in the effective normal stress that are caused by development of shear-induced pore water pressures, and (2) changes in shear strength can be attributed to changes in the "mode of shearing". In order to illustrate the effect of the shear displacement rate on measured shear strengths, slow and fast ring shear tests were performed using the Bromhead ring shear device, on pre-sheared discontinuities in kaolinite. In the Bromhead ring shear device, it is difficult to measure the induced pore water pressure directly; consequently, this paper focuses on the effect of changes in the "mode of shearing". A Scanning Electron Microscope (SEM) imaging technique was used in this study, with magnifications in the range of 25X to more than 100000X. Comparisons between the image results and measured shear strengths are performed, and the relative contribution of the shear mode to the measured shear strengths is discussed.