Mechanical behavior of soft rocks under triaxial cyclic loading conditions

Abstract

Consolidated-undrained, cyclic triaxial tests were performed on four saturated Miocene soft rocks of Japan namely Ohya tuff, Yokohama siltstone, Kobe mudstone and sandstone. Deformation behavior, pore pressure changes and strength mobilization as well as contraction/dilation characteristics of these rocks were studied with emphasis on the non-linearity, stress- and strain-dependence of their mechanical properties. A procedure was introduced to locate the yield point, estimate Young's modulus and study the deformation behavior of these soft rocks. Patterns of pore pressure changes and associated modes of failure were studied. Characteristic stress levels were found, based on which the “true” rock strength components were determined and their mobilization with respect to stress- and strain levels was analyzed. Volume change tendencies were inferred from pore pressure changes. The rate of such changes was related to the contraction/dilation behavior of soft rocks, from which the dilation angle was estimated and its stress- and strain-dependence was studied. The findings were applied to the FEM simulation of triaxial tests on-and of excavation in/on soft rocks.