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.

In order to investigate the energy characteristics of Jinping marble during progressive damage, triaxial cyclic loading and unloading tests on marble samples under five different confining pressures were carried out. The evolution of elastic modulus, the rule of energy evolution and distribution, the effect of confining pressure on energy evolution, and the characteristics of progressive damage evolution of marble under cyclic loading and unloading were analyzed. Furthermore, the energy failure criteria of rock strength based on energy consumption ratio and energy consumption difference were systematically discussed. The results show that both the loading and unloading elastic moduli of the marble are monotonically degraded during the whole deformation and failure process from the initial yielding to the residual stage. The deviation of elastic strain energy calculation with the assumption of constant modulus of elasticity should be avoided. The increase of confining pressure can not only effectively enhance marble's energy absorption and storage efficiency, but also strengthen marble's energy absorption and storage limit. It can also restrain marble's energy dissipation efficiency and the increase amplitude of dissipation energy. The ratio of elastic energy decreases and the ratio of dissipative energy increases from pre-peak to post-peak, and the ratio of residual energy tends to be stable. The residual elastic energy and the residual dissipated energy increase approximately linearly with the increase of confining pressure, while the maximum elastic energy and the maximum dissipated energy increase nonlinearly with the increase of confining pressure. The modified damage evolution equation considering initial damage and residual bearing capacity can well capture the variation characteristics of damage variable based on dissipative energy under different confining pressures. The criterion of energy consumption ratio based on the definition of energy relative proportion can be regarded as the criterion of failure of marble material. While the criterion of energy consumption difference based on the definition of absolute difference of energy can be regarded as the criterion of failure of marble structure.

To clarify the plastic law and hardening characteristics of deformation and failure of staggered zone under high in-situ stress unloading conditions with excavation in underground engineering, a series of triaxial tests under different confining pressures and different loading and unloading stress paths was conducted. The plastic deformation laws of the staggered zone under different loading and unloading stress paths were deeply investigated. Based on the results of the experiments, the dependency of stress path of internal variables such as the equivalent plastic work, the equivalent plastic strain and the plastic volumetric strain as hardening parameters were further explored in the stress space. Meanwhile, a modified formula for hardening parameters with the independency of stress path was proposed. The results showed that, 1) The effect of stress path was significant on the deformation properties of staggered zone under high in-situ stress. The high initial confining pressure would inhibit the development of the circumferential plastic deformation of samples with the staggered zone under the same unloading stress path. And the plastic volumetric residual strain (6%) of samples with the staggered zone under the stress path of unloading axial pressure and confining pressure was significantly greater than that of other unloading stress paths (2%−4%) under the same initial confining pressure. The promoting effects of different stress paths on the plastic volumetric deformation of the staggered zone were as follows. The promoting effect of the stress path of unloading axial pressure and unloading confining pressure was the strongest, that of the stress path of constant axial pressure and unloading confining pressure was the second stronger, and that of the stress path of loading axial pressure and unloading confining pressure was the weakest. 2) There were a certain of the dependency of stress path of its internal variables such as plastic volumetric strain, equivalent plastic strain and equivalent plastic work during the deformation and failure process of the staggered zone in different unloading stress paths. Therefore, it is not accurate to directly take any of the above state parameters as the hardening parameters and assume that it is independent of the stress paths in the elastic-plastic analysis of staggered zone. Therefore, a modified method of the equivalent plastic work was proposed. It was found that when the parameter n_s reflecting the material properties of staggered zone was equal to −0.4, the modified equivalent plastic work had the obvious independency of stress path, which was more appropriate for describing the unloading plastic strain hardening characteristics of the staggered zone under high in-situ stress unloading conditions. The plastic mechanical characteristics of the staggered zone revealed in this paper can further deepen the understanding of deformation and failure of the staggered zone under high in-situ stress unloading conditions, and provide a theoretical basis for the analysis of the failure mechanism and support control of the staggered zone in practical engineering.