Failure mechanism of spandrel under the excavation and unloading condition, at the upstream of underground powerhouse, Shuangjiangkou hydropower station

The failure mechanism of surrounding rocks in underground caverns is one of the most important issues in the field of underground engineering. Aiming at the problems of cracking and large-scale collapse of surrounding rock caused by excavation and unloading under the high stress-strength ratio of the underground powerhouse of Shuangjiangkou hydropower station under construction, the damage evolution process and failure mechanism are studied by combining the field investigation, micro-seismic monitoring, theoretical analysis, numerical simulation, etc. The results show that the failure of spandrel at the upstream of the auxiliary powerhouse is induced by the intense unloading under high geostress. Under the long-time construction suspending without any supporting, the tensile microcracks in the surrounding rock accumulate continuously, and the deterioration of surrounding rock strengthens. The intact rock is splitted into the columnar structure by the tensile fracture under two times of unloading experiments. The yield and instability of the pillar rocks under the approximately uniaxial compression are the main reasons of collapse. The tensile stress zone caused by unloading of surrounding rocks attains to a maximum depth of 7.1 m. and the adverse stress state of the right-angle free surface gradually formed during the excavation provides conditions for the failure. The results can provide references for the construction of similar underground engineering. © 2020, Science Press. All right reserved.