Non-linear, stress- and strain-dependent behavior of soft rocks under cyclic triaxial conditions

Four saturated Miocene soft rocks of Japan i.e. Ohya tuff, Yokohama siltstone, Kobe mudstone and sandstone were tested under consolidated-undrained, cyclic loading conditions. The deformation behavior, pore pressure changes and strength mobilization, and 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, to estimate Young's modulus and to study the deformation behavior of these soft rocks. Deformation modulus was found to depend on plastic straining according to an exponential equation, whose coefficients in turn, depend on confining pressure. In addition, it also depends on the deviatoric stress levels causing hysteresis to occur. Patterns of pore pressure changes and associated failure modes were interpreted in terms of the initiation/propagation of micro- and macro-cracks inside the specimens. Characteristic stress levels were found based on which the "true" rock strength components - cohesion and internal friction angle were determined and their mobilization with respect to stress and strain levels was analyzed using a new interpretation method. Volume change tendencies were inferred from pore pressure changes whose rate was related to the contraction/dilation behavior of soft rocks. On that basis the dilation angle was estimated and its variation and stress- and strain-dependence was analyzed. A simple equation was introduced to characterize the variation of both internal friction and dilation angles with plastic straining, whereas their stress-dependence was studied through the variation of fit coefficients with confining pressure. (C) 1998 Elsevier Science Ltd. All rights reserved.