The deformation and fracture characteristics of a large underground caverns with high in-situ stress are analyzed and correlated with its geology, data from monitoring and geophysical surveying, and construction process. Comparing with low to medium in-situ stress, under high in-situ stress and low strength-to-stress ratio, the deformation of rock mass is no longer controlled by the "opening displacement" of major structure planes, the proportion of deformation induced by rock fractures increases. The deformation magnitude and EDZ depth are relatively large. The EDZ depth of sidewall is 9–15 m in generally, some is greater than 15 m. The deformation magnitude increases with the extending of EDZ depth, the development of rock mass deformation with time should be understood as the progressive failure of rockmass, which is different with normal rheologic deformation behavior of soft rock. Cracks at the downstream arch of powerhouse chamber and transformer chamber are caused by the unloading splitting and outward buckling failure of layered rock mass under high stress, which leads to the increasing of measured displacement at the downstream arch abutment. Cracks at the upstream sidewall of transformer chamber are tensional and are caused by the compression fracture of rock mass due to stress concentration.
In China, a number of built or being building hydropower stations are distributed in the southwest mountain area. As the tectonic activity in Southwest China is active, high in-situ stress, complicated geological structures and relatively poor rock mass become the key problems encountered during the construction of the caverns. Due to the complexity of the underground powerhouse caverns, the uncertainty of geological conditions and the variability of the construction process, meanwhile affected by the limitations of existing numerical methods, the numerical modeling results and the practical deformation characteristics of the underground caverns often have a certain disparity.
