Pore pressure changes and strength mobilization of soft rocks in consolidated-undrained cyclic loading triaxial tests

Abstract

Changes in the pore pressure during CU (Consolidated-Undrained) cyclic loading triaxial tests on fully saturated specimens of four Miocene soft rocks, i.e. Ohya tuff, Yokohama siltstone, Kobe mudstone and sandstone are analyzed. As a result, three modes of failure, i.e. brittle, brittle-ductile and ductile can be recognized, and other characteristic stress levels e.g. crack initiation stress, plastic flow/gliding stress, dilation stress, etc. identified. The mobilization of the strength of these rocks in terms of cohesion and internal friction angle is then discussed. A procedure for the estimation of the rock cohesion strength is first introduced on the basis of the modified Griffith's failure criterion. The determination of the mobilized friction angle at any state of stress during the tests follows.