Study on mechanical response of surrounding rock under excavation unloading in deeply buried underground caverns with high geostress

In recent years, large-scale hydropower projects in China have mostly been constructed in the high mountain and canyon regions of the southwest. These projects are characterized by significant elevation differences, strong unloading effects, and high in-situ stresses, which frequently lead to deformation and failure of the surrounding rock in underground caverns. Taking the under-construction RM Hydropower Station on the Lancang River as a case study, this paper systematically analyzes the basic characteristics and patterns of deformation and failure in the surrounding rock of the diversion tunnel, based on geological data and test results. Numerical simulation was employed to study the deformation, stress evolution, and plastic zone distribution characteristics of the surrounding rock during layered excavation, and the deformation and failure mechanism induced by excavation unloading was further analyzed. The results showed that the deformation and failure of the surrounding rock were mainly concentrated in the arch shoulder on the riverside, which was attributed to the combined action of the structural plane and high in-situ stress. During the layered excavation process, significant stress concentration occurred in the surrounding rock of the arch shoulder. The maximum principal stress reached approximately 70 MPa, the failure depth was less than 50 cm, and the failure zone was characterized by a "large range and shallow depth". The research results can provide technical reference for the excavation and support design of deep buried large underground powerhouses in similar regions.