Experimental Study on Mechanical Properties of Deeply Buried Granite During Layered Excavation of Large Underground Caverns

The progressive brittle failure of high-stress hard surrounding rock masses caused by layered excavation severely threatens the stability of large underground caverns. We summarize the stress path in the potential failure zone of the Shuangjiangkou underground powerhouse during layered excavation, and then design the multistep unloading confining pressure test and the cyclic loading–unloading test with multistep unloading confining pressure. The deformation, strength, acoustic emission, and failure characteristics of granite, as well as the evolution of critical stress thresholds and mechanical parameters are analyzed based on experimental results under different stress paths. The peak strength of the rock significantly depends on the stress path and the amount of unloading confining pressure. The rock failure process under multistep unloading confining pressure is divided into stable and unstable crack growth stages, with the failure mode shifting from tensile cracks to mixed tensile–shear cracks. The evolution modes of critical stress thresholds, cohesion, and internal friction angle with amount of unloading confining pressure differ significantly from that with the equal plastic strain. The former is nearly linear, while the latter is exponentially nonlinear. The evolution of the deformation modulus, cohesion, and internal friction angle confirms the rationality of the cohesion-weakening–friction-strengthening model with degraded deformation modulus for the stability analysis of surrounding rock masses under layered excavation. The experimental methods for obtaining the evolution of rock mechanical parameters using a single specimen under unloading confining pressure are also discussed. The results imply that the layered excavation-induced nonlinear deterioration of the deformation modulus and strength parameters of high-stress hard surrounding rock masses should be considered in the stability analysis of large underground caverns. Further, the timely compensation of the excavation-induced confining pressure loss is critical to ensure the stability of high-stress hard surrounding rock masses under layered excavation.