Mechanical parameter evolutions and deterioration constitutive model for ductile-brittle failure of surrounding rock in high-stress underground engineering

The deep surrounding rock is usually in the true triaxial stress state, and previous constitutive models based on the understanding of uniaxial and conventional triaxial test results have difficulty characterizing the degradation and fracture process of rock ductile-brittle failure under true triaxial stress state. Therefore, this study conducted a series of true triaxial tests to obtain the understanding of the ductile-brittle behaviour of rock, and then combined the test results and the Mogi-Coulomb strength criterion, and proposed calculation methods for the elastic modulus E, cohesion c and internal friction angle u and the evolution functions of E, c and u of rock under true triaxial stresses. With the decreasing of the minimum principal stress r3 or increasing of the intermediate principal stress r2, the marble post-peak stress drop rate gradually increases, the ductility gradually weakens, and the brittleness significantly strengthens. The calculation method and evolution function of rock E, c and u under true triaxial stress were proposed. E decreased at first and then tended to remain stable with the increasing of equivalent plastic strain increment dep. c and u slowly increased at first and then rapidly decreased. With a method of parameter degradation rate to realize post-peak stress drop rate to reflect the ductile-brittle characteristics, a new three-dimensional ductile-brittle deterioration mechanical model (3DBDM) was established. The proposed model can accurately characterize the influence of r2 and r3 on mechanical parameters, the ductile-brittle behaviour of rock under true triaxial stresses, and the asymmetric failure characteristics of surrounding rock after excavation of deep underground engineering. The proposed model can be reduced to elastic-perfectly plastic, elastic-brittle, cohesion weakening friction strengthening (CWFS), Mohr-Coulomb, and Drucker-Prager models.