An investigation of failure modes and failure criteria of rock in complex stress states

Rock in engineering and geological environments is usually in complex stress states. Based on many experimental results for rock under different loading conditions, and combined with failure modes found in previous studies, stress triaxiality is introduced to describe different stress states, and the relationship between failure modes and stress triaxiality is analysed in detail. For a given kind of rock, with decreasing stress triaxiality the failure mechanism changes from tension fracture to local shear failure and general shear failure. Two demarcation points of stress triaxiality exist: between tension fracture and local shear failure, and between local shear failure and general shear failure. The controlling parameters that dominate tension fracture, local shear failure, and general shear failure are different. Using a reasonable assumption based on the experimental results, a failure criterion corresponding to three distinct failure modes is presented. This failure criterion is defined by seven material parameters. The accuracy and applicability of the proposed failure criterion are examined using experimental data derived for sandstone in this study and published experimental data for rock salt and marble under conventional triaxial compression tests. The data used covers a wide range of stress triaxiality and various failure mechanisms. The predictions from the proposed failure criterion agree well with the experimental data.