Evolution of mechanical parameters of Shuangjiangkou granite under different loading cycles and stress paths

Surrounding rocks at different locations are generally subjected to different stress paths during the process of deep hard rock excavation. In this study, to reveal the mechanical parameters of deep surrounding rock under different stress paths, a new cyclic loading and unloading test method for controlled true triaxial loading and unloading and principal stress direction interchange were proposed, and the evolution of mechanical parameters of Shuangjiangkou granite under different stress paths were studied, including the deformation modulus, elastic deformation increment ratios, fracture degree, cohesion and internal friction angle. Additionally, the new stress path coefficient was defined to characterize different stress paths, and the functional relationships among the stress path coefficient, rock fracture degree difference coefficient, cohesion and internal friction angle were obtained. The results show that during the true triaxial cyclic loading and unloading process, the deformation modulus and cohesion gradually decreases, while the internal friction angle gradually increases with increasing equivalent crack strain. The stress path coefficient is exponentially related to the rock fracture degree difference coefficient. As the stress path coefficient increases, the degrees of cohesion weakening and internal friction angle strengthening decreased linearly. During cyclic loading and unloading under true triaxial principal stress direction interchange, the direction of crack development changes, and the deformation modulus increased, while the cohesion strengths and internal friction angle decreases slightly, indicating that the principal stress direction interchange has a strengthening effect on surrounding rocks. Finally, the influences of the principal stress interchange direction on the stabilities of deep engineering excavation projects are discussed.