Deformation and failure characteristics of top arch surrounding rock of super large underground caverns

Due to the effect of the internal staggered zones at the left bank of Baihetan Hydropower Station, the destruction phenomenon of the surrounding rock at the top arch between the southern wall and the units No. 3 is obvious. The deformation evolution mechanisms, spatial distribution patterns and failure modes of the surrounding rock are analyzed based on the geological conditions, geophysical prospecting, monitoring data and constrution documents. The results show that the circumferential shear deformation and failure mode of tunnel is characterized by the “negative growth” of displacement, and the “bending breakage” as well as “drum collapse” failure phenomenon occurs mainly in the acceleration phase of shear deformation. According to the stress-time curves, the variations of “step type” stress of bolts keep in step with the “stress-controling type” deformation of the surrounding rock at the corresponding position, and gradually decay with the increase of the depth. The variations of stress of “tension-compression” type bolts are mainly affected by the local control of structural planes. The variations of stress of “stress steep drop” type bolts are associated with the incompatible deformation of anchorage system. This study may provide important supports for future reinforcement designs.