Exponential true triaxial strength criteria for rock

The rock strength is the premise and foundation for stability evaluation of rock engineering and optimization design of structures. Based on the true triaxial test results, the evolution of rock strength are analyzed: (1) The rock strength increases with the minimum principal stress, however, its increasing amplitude gradually decreases and trends to be zero; (2) The ratio of generalized compression to the extension strength firstly increases then decreases, and finally tends to be one with the increasing minimum principal stress. The envelope of strength criteria on deviatoric plane varies from approximate triangle to circular; (3) During the process of the intermediate principal stress increasing from the minimum principal stress to the maximum one, the rock strength firstly increases and then decreases. The exponential function with three parameters satisfies the basic characteristics of rock strength on the meridian plane. And the Lode dependence functions of L_WW, L_MN and L_YMH well reflect the effect of the intermediate principal stress, and unconditionally satisfy the smooth, continuous and convex requirements. Then the exponential true triaxial strength criteria are proposed by combining the extension and compression meridian curves with the selected lode dependence functions. The influences of strength parameters and space characteristics are further analyzed. Finally, fourteen types of rock true triaxial test data are employed to validate the proposed criteria. The results show that the proposed exponential true triaxial strength criteria are of good accuray in predicting rock strength, and can well reflect different types of rock ranged from soft to hard.