PORE PRESSURE CHANGES AND STRENGTH MOBILIZATION OF SOFT ROCKS IN CONSOLIDATED-UNDRAINED CYCLIC LOADING TRIAXIAL TESTS

Changes in the pore pressure during CU (Consolidated-Undrained) cyclic loading triaxial tests on fully saturated specimens of four Miocene s oft rocks, i.e. Ohya tuff, Yokohama siltstone, Kobe mudstone and sands tone ave analysed. As a result, three modes of failure, i.e. brittle, brittle-ductile and ductile can be recognized, and other characteristi c stress levels e.g. crack initiation stress, plastic flow/gliding str ess, dilation stress, etc. identified. The mobilization of the strengt h of these rocks in terms of cohesion and internal friction angle is t hen discussed. A procedure for the estimation of the rock cohesion str ength is first introduced on the basis of the modified Griffith's fail ure criterion. The determination of the mobilized friction angle at an y state of stress during the tests follows. (C) 1997 Elsevier Science Ltd.