Spatiotemporal difference of deformation and failure affected by a large discontinuity in the surrounding rock mass: A case study at the Baihetan underground powerhouse

The deformation and failure of the surrounding rock mass is a key issue during the construction of large-scale underground powerhouse, and large discontinuities are likely to cause this problem in the presence of complex geological structures. This article takes the right bank underground powerhouse of the Baihetan Hydropower Station as a case study. In this case, deformation mutation of the surrounding rock mass occurred in the south section of the main powerhouse, with the maximum deformation reaching 178 mm, and the deformation and failure of different parts showed differences. A comprehensive study integrating field survey, site monitoring, laboratory test and numerical simulation was carried out. By field survey and monitoring, characteristics of deformation and failure are described, and the spatiotemporal difference in deformation is analysed. The stress evolution during excavation is studied based on numerical simulations, the mechanical response of rock is derived through laboratory tests, and the mechanism of spatiotemporal difference is revealed. The results indicate that the main reason for the spatiotemporal difference is the presence of slightly inclining interlayer shear zone C₄. In the south section, the excavation-induced stress concentration at the arch was enhanced due to C₄, with the maximum principal stress exceeding 70 MPa, and the high compressive stress here triggered the deformation mutation of surrounding rock mass. After undergoing a stress path from concentration to unloading, the surrounding rock mass at the downstream sidewall was seriously damaged, and its deformation also mutated under approximately vertical stress. The mutation resulted in the uneven spatial distribution, large increment and time-dependent feature of deformation.