An elastoplastic model for soft sedimentary rock considering inherent anisotropy and confining-stress dependency

The inherent anisotropy of soft sedimentary rock is a very important factor that influences the mechanical behavior of the rock. The confining-stress dependency of the shear stress ratio at the critical state, briefly referred to as the confining-stress dependency, is another important factor that should be taken into consideration when discussing the mechanical behavior of the rock. In this paper, based on an elasto-viscoplastic constitutive model for soft sedimentary rock (Zhang et al., 2005), a new model capable of describing both the inherent anisotropy and the confining-stress dependency of soft sedimentary rock is proposed in the framework of generalized stress space, called the tij concept (Nakai and Mihara, 1984), and the subloading yield surface ( Hashiguchi and Ueno, 1977 and Hashiguchi, 1989). In order to describe the confining-stress dependency, an evolution equation for the shear stress ratio at the critical state M⁎ is introduced. A transformed stress, proposed by Boehler and Sawczuk (1977) and first introduced into a constitutive model by Oka et al. (2002), is also adopted into the model. In order to examine the performance of the newly proposed model, triaxial compression tests for soft sedimentary rock under different loading conditions were simulated and the results have been compared with the corresponding test results obtained from the authors and other researches available in the literature. It is found that the model can describe both the inherent anisotropy and the confining-stress dependency of soft sedimentary rock using a set of parameters with a fixed value for a given material.