The failure index of hard rock under high geostress conditions

Yield and failure are the important content of materials´ mechanical property and two different stages in the deformation process of materials which can be gained only by experiment. Based on systematic mechanical property experiment, especially true triaxial experimental which can simulate to the complicated evolvement of stress load which produced by excavating and support underground engineering under high stress conditions, a new Three Shear yield criteria is proposed for hard rock. According to the distance between a point in stress space and yield plan and the plastic deformation ability of materials, the failure index is established which can be used to tensile brittle failure, shear brittle failure and shear ductile failure. The limit value of effective plastic strain is proposed according to the experiment results. The research result can be used in the safety and stability analysis of hard rock underground engineering under high geo-stress conditions.