Rock strength and deformation characteristics under different unloading rates

The excavation of rock mass engineering is essentially the unloading process of rock mass, and different unloading rates will significantly affect the strength and deformation characteristics of rock mass, which is of great significance to the safety and stability analysis of rock mass engineering. For modeling various unloading stress paths in the rock mass excavation, the ordinary triaxial compression test, constant axial pressure unloading confining pressure test, loading axial pressure and unloading confining pressure test, and constant principal stress difference unloading confining pressure test were conducted in this research. The effects of different unloading rates on the mechanical properties of rock mass were analyzed. The main conclusions are as follows: (1) Rock shows typical brittle failure characteristics under different unloading paths and different unloading rates, when the confining pressure decreases to a certain value, the rock sample suddenly fails, the axial pressure drops sharply, and the circumferential strain ε₃ increases significantly. (2) If the confining pressure unloading rate is high, when the rock sample is near failure, the deformation modulus E decreases linearly almost at a 90° angle, and Poisson’s ratio μ increases linearly almost at a 90° angle. If the unloading rate is low, the decreasing/increasing trend of E and μ is relatively slower. This shows that the brittle failure of rock sample is more obvious with the increasing unloading rate of confining pressure. (3) The stress state of rock samples under the three unloading paths are basically below the strength envelope of ordinary triaxial compression Mogi-Coulomb, which shows that the rock sample under confining pressure unloading is more easily damaged than that under normal triaxial compression.