Seismic triggering of catastrophic failures on shear surfaces in saturated cohesionless soils

Abstract

This paper is concerned with an analysis of the seismic performance of infinite slopes in undrained conditions. The material assumed on the sliding surface is a loose saturated sand susceptible to a gradual loss in undrained shear strength after failure with the progress of unidirectional shear displacement. The undrained monotonic and cyclic shear behavior of this sand was investigated through an experimental study based on ring shear tests, with initial stresses corresponding to the static conditions on the sliding surface of the analyzed slopes. These tests provide the experimental framework for a modified sliding block method to estimate the earthquake-induced undrained shear displacements for conditions of no shear stress reversals on the sliding surface. The proposed estimation procedure incorporates the shearing resistance obtained from undrained monotonic ring shear tests to approximate the undrained yield resistance at a certain displacement during an earthquake. The term catastrophic failure is used in this study to define the accelerated motion of a potential sliding soil mass due to the static driving shear stress exceeding the reduced undrained yield resistance of the soil on the shear surface. The critical displacement necessary to trigger a catastrophic failure on the shear surface under seismic conditions was derived based on the shear resistance – shear displacement curve obtained under monotonic loading conditions. Using the shear resistance – shear displacement data from undrained monotonic ring shear tests and several processed horizontal earthquake accelerograms, the minimum peak earthquake acceleration necessary to cause a catastrophic shear failure under various seismic waveforms was estimated for conditions of no shear stress reversals on the sliding surface.Key words: earthquakes, slopes, critical shear displacement, sand, ring shear tests, undrained shear strength.