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Abstract The deformation and fracture characteristics of the surrounding rock mass are analyzed according to the geological conditions，monitoring and geophysical prospecting data，and construction process. The analytical results indicate that under the condition of high in-situ stresses and low strength-to-stress ratio，the deformation of surrounding rock mass is no longer controlled by the opening displacement of major faults，but by the unloading relaxation of the surrounding rock. The deformation amount and relaxation depth are relatively larger. The main relaxation depth of side wall is between 12 and 15 m，some are even greater than 15 m. The deformation amount increases with the extending of relaxation depth，while the relaxation depth extends with the excavation construction and the passage of time. Therefore the time effect of rock mass deformation should be considered as the gradual expansion of rock failure，which is different from normal rheological concept. The cracks at the downstream hance of main powerhouse and main transformer house are caused by the unloading splitting and buckling failure of layered rock mass under high stress；and this rock mass failure leads to the increasing of deformation at the downstream hance. While the crack type at the upstream side wall of transformer house is of tensile failure of shotcrete layer，its causes are the compression fracture of rock mass because of stress focus during the excavation of busbars tunnels. Under the condition of high in-situ stresses，whether the scheme as first total excavation of main powerhouse and main transformer house，then busbars tunnels excavation that can be accepted in the construction of underground cavern group is a significant issue.
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Received: 25 February 2010
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