Mechanical characteristics of failure and rockburst proneness of fractured granite from Shuangjiangkou hydropower station under triaxial loading and unloading

To investigate the mechanical behaviors of failure and rockburst proneness of fractured rock mass during excavation unloading under high in-situ stress. Triaxial loading and unloading tests of fractured rock samples with different dip angles under different stress paths were conducted, and mechanisms inducing catastrophic effects of deeply buried fractured granite were discussed from the perspectives of mechanical characteristics of failure and energy evolution. The results imply that failure modes of fractured granite under triaxial loading and unloading are mainly dominated by shear failure. Coplanar cracks produced by granite under unloading generally often are composite tensile-shear cracks, which show more significant brittle failure. Under loading and unloading, the energy storage limit Uem of rock first decreases, then increases with the increase of the dip angle of fractures. When the dip angle of prefabricated fractures is closer to the fracture angle of the intact rock mass, the energy storage limit is smaller and shows an increasing linear relationship with the unloading ratio. The rate of change of elastic strain energy in fractured rock under triaxial loading suddenly increases more frequently. The elastic energy release rate δ was defined to evaluate the rockburst proneness of deeply buried fractured granite under loading and unloading. Under unloading, the elastic energy release rate δ of rock is slightly affected by changes of the dip angle; the elastic strain energy in the rock is rapidly released, greatly increasing the rockburst proneness.