Characteristics of Rock Deformation and Fractures in an Underground Powerhouse Caverns

摘要

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.