Relaxation Characteristics and Failure Mechanism of Steeply Dipping Columnar Jointed Rock Caverns

The columnar jointed rock mass of Baihetan Hydropower Station has special column structure characteristics, and the unloading relaxation damage is serious after the excavation of the cavern. Through the on-site failure investigation, displacement and relaxation depth monitoring of the columnar jointed rock mass of the 4# diversion tunnel of Baihetan Hydropower Station, it is found that the columnar joint cavern presents typical asymmetric failure characteristics, and the relaxation depth of different parts is quite different. The failure modes of the left and right side walls are affected by the inclination angle, showing dump failure and slip failure, respectively. The discrete element numerical simulation results show that the surface surrounding rock of the cavern is mainly caused by inter-column joint tension failure, and the inner surrounding rock is mainly caused by shear failure. The different deformation and failure characteristics of the left and right sidewalls are the result of the combined effect of the cylinder inclination and in-situ stress. Finally, the influence of different columnar joint inclination angles on the deformation and failure characteristics of the cavern is analyzed. The relaxation depths of different parts of the cavern under different inclination angles are significantly different. The prestressed bolt design should be carried out according to the depth of the plastic zone and the inclination angle of the cylinder in different parts of the cavern.