Deformation and failure mechanism of surrounding rock mass in a large-scale underground powerhouse under high in-situ stress

The surrounding rock mass stability of a large-scale underground powerhouse is the key issue among construction process. This paper takes the left bank underground powerhouse of the Baihetan Hydropower Station as a case study, which is subjected to high in-situ stress and complex geological conditions. Site monitoring, field survey and numerical simulation were conducted to study the deformation and failure mechanisms of surrounding rock mass. Main patterns of deformation and failure were identified, mechanisms were analyzed, and countermeasures were proposed. Results indicate that there are two patterns of deformation and failure, the stress-dominated type and structure-dominated type. Deformation and failure at different locations are mainly influenced by in-situ stress and rock mass structure. Since the direction of the in situ maximum principal stress gently inclines to the upstream side and intersects with the powerhouse axis at a large angle, compressive stress is concentrated at the upstream spandrel and downstream sidewall foot during excavation. The stress concentration results in spalling, fracture and shotcrete cracking, belonging to the stress-dominated failure, and the countermeasures are timely systematic support and prestressed bolt. At the downstream spandrel and sidewalls, unloading deformation is the main pattern and prestressed anchor cable at right time is recommended. The structure-dominated large deformation occurs at the arch and downstream sidewall under the impact of shear zones, and rock mass grouting and supplementary prestressed anchor cable are effective measures. The results can provide references for the failure mechanism analysis of surrounding rock mass and construction dynamic optimization of large-scale underground powerhouse.