Deformation and failure mechanism analyses for the surrounding rock mass in a large cylindrical tailrace surge chamber

The surrounding rock mass stability is the key to the construction of underground powerhouses in hydropower stations. However, high in situ stress and complex geological conditions pose substantial challenges to the construction safety and stability of large underground powerhouses, which can lead to deformation and failure of the surrounding rock mass. This paper considers tailrace surge chamber #8 of the Baihetan Hydropower Station as a case study, which is one of the largest cylindrical tailrace surge chambers in the world. In this case, the shotcrete on the vertical wall cracked and fell off after tailrace surge chamber #8 was excavated and systematically supported. Simultaneously, large deformation of the surrounding rock mass occurred, and some anchor cables failed at the cracking area. By site monitoring and field surveys, the deformation and failure characteristics are described. Based on numerical simulations, the stress evolution induced by excavation is studied, and the mechanism of deformation and failure is analyzed. The results indicate that the high in situ stress, interlayer shear zone, and brittle surrounding rock mass were the main reasons. Under high in situ stress, compressive stress was concentrated near the vertical wall after the excavation. In the presence of interlayer shear zone C 4 , the stress concentration was enhanced and caused the surrounding rock mass to be subjected to high tangential stress, which led to the large deformation and failure.