Coupled Damage-Plasticity Modelling of Saturated Shale under Undrained Condition

Abstract

Shale is classified as a poroelastic material as the shale matrix has comparable stiffness with its constituent solid grains. Also, shales at large depth are often fully saturated. Due to their low permeability, which can be in the nano-Darcy range, shales remain undrained during typical loading conditions. Additionally, shales are quasi-brittle materials which can exhibit very pronounced peak shear stress followed by pronounced strain softening during shearing. All these unique aspects of shale behavior must be considered in developing a model that predicts the change of the pore pressure, the effective stress response and the undrained shear strength of shales during loading. This study presents a coupled plastic-damage constitutive model which considers poroelastic effect with the objective to predict the mechanical behavior of saturated shales under undrained condition. The proposed model couples the plastic mechanism, which is formulated on the effective stress space, and the damage mechanism which is responsible for the elasticity degradation and the post-peak softening of shale. Validation of the proposed model against triaxial est results demonstrates that the proposed model can predict the stress-strain behavior, the effective stress path and the pore pressure evolution of shale specimen during shearing.