A unified constitutive model for describing the undrained response of soft rock material is developed. The elastic-plastic response of stress and strain is formulated by a bounding surface plasticity within the framework of critical state. The concept of critical state is exploited in the model to separate brittle and ductile deformation regions. It also represents the residual state of porous rock which is approached at large shear deformation at constant deviatoric stress. The continuum damage evolution model is established in a general irrecoverable thermodynamics framework. The damage evolution law, based on the tensile principle stress, takes into account confining pressure and plastic hardening parameter. The coupling effect of plastic flow and damage evolution is achieved via combining the evolution laws of plasticity and damage. This model is applied to Diatomaceous Mud Rock under a wide range of confining pressure. The characteristic features of elastic-plastic damage in soft rock are captured appropriately in all cases considered.

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