Rock Mass Damage Evolution and Mechanisms of Deformation and Failure in the Intersecting Cavern Complex of a Large Underground Hydropower Powerhouse

Perturbation propagation and cumulative damage to the rock surrounding adjacent caverns pose major challenges to construction safety during the excavation of large underground powerhouses. This study investigates these processes at a hydropower project in southwestern China. An ESG high-precision microseismic monitoring system was deployed to examine the spatiotemporal evolution of microseismicity, fracture mechanisms, and their relationship with rock mass damage in adjacent busbar tunnels during excavation of Lifts VI and VII of the main powerhouse. Comparisons between microseismic and conventional monitoring revealed the spatiotemporal relationships among microseismic activity, accelerating deformation, and declining anchor cable loads around the busbar tunnels. Numerical simulations of cavern excavation, considering configurations with and without busbar tunnels, were conducted to assess how the free surfaces of the busbar tunnels affect the surrounding stress and displacement fields, and the numerical results were compared with both monitoring datasets to corroborate the findings. The results show that (1) microseismic activity evolves in response to excavation intensity through three stages: steady accumulation, accelerated activity, and decay; (2) microseismic events cluster at the intersections between the main powerhouse and busbar tunnels, and as excavation progresses the fracture mechanisms remain dominated by tensile and mixed tensile–shear events with no significant change, while the concurrent increase in energy and moment magnitude is confined to the stage of macroscopic damage; (3) temporal variations in microseismic activity rate track changes in rock deformation rate and anchor cable load, while microseismic clusters coincide spatially with zones of rock deformation and failure and declining anchor cable loads; and (4) the free surfaces of the busbar tunnels alter the stress and displacement distributions at cavern intersections. Predicted displacement concentrations, zones of high radiated seismic energy density, and observed damage locations agree well spatially, jointly indicating where rock deformation and failure are concentrated. These findings can provide a basis for excavation control and support design at similar cavern intersections.