Deformation and failure analyses of large underground caverns during construction of the Houziyan Hydropower Station, Southwest China

During the excavation process of the underground powerhouse caverns at the Houziyan Hydropower Station, some failure phenomena (including split, spalling, ballooning and cracking of the shotcrete layer) occurred and posed a great threat to the safety of the structure, construction equipment and people's lives. Monitoring data show that the surrounding rock mass deformation at several locations (more than 17% of the total monitoring points) is larger than 50 mm after the sixth step of excavation, which is beyond the average deformation of completed underground main powerhouse caverns under similar scale and in-situ stress conditions in China. The field investigations, monitoring data and numerical simulations indicate that high second principal stress with a direction sub-vertical to the axis of the main powerhouse has a remarkable effect on the deformation and failure of the underground powerhouse. The failure types along the depth of the high sidewall at the main powerhouse can be identified as peeling off at the surface and slabbing and spalling at the near surface and deeper tension fracture failure. The failure at the high sidewall of the main powerhouse has obvious time effects. Expanding the deep fractures, tension and sliding of weak structural planes is highly correlated with the increase of the deformations, and the high second principal stress leads to an increase of the dynamic unloading effects and unloading relaxation degree, which lead to a higher occurrence of crack expansion and deep fractures. Meanwhile, the relatively high second principal stress exacerbated the stress concentration at the upstream spandrel and downstream arch foot of the main powerhouse and led to serious ballooning and cracking of the shotcrete layer and split failure of the surrounding rock mass.