An enhanced bounding surface plasticity model for soft rock subjected to drained and undrained condition

A bounding surface model in general stress space is formulated for soft rock subjected to drained and undrained condition. Specifically, a UYC bounding surface is defined based on unified yield criterion by introducing bounding surface plasticity, and it can degenerate to a bounding surface adopting Mohr-Coulomb criterion or Twin-shear criterion; a UYC loading surface that is similar to the UYC bounding surface is defined; a mapping rule in general stress space is given to consider the plastic strain inside the UYC bounding surface; a modified hardening parameter and a modified stress-dilatancy is proposed to capture the residual strength and volumetric strain of soft rock, respectively. The comparison between model prediction and experimental results shows that the constitutive model well predicts the mechanical behaviours of soft rocks subjected to drained and undrained compression condition. Besides, the proposed model overcomes the limitation that the plastic strain of soft rock can’t be well captured by the traditional constitutive models based on unified strength theory.