Analysis of damage characteristics and energy evolution of salt rock under triaxial cyclic loading and unloading

The gas injection, water-soluble excavation, and gas recovery process in the gas storage salt cavern can be regarded as the loading and unloading process of the surrounding rock stress. By conducting triaxial loading and unloading tests on salt rock samples, several findings are presented: 1) The failure formed in the rock sample is a shear failure mode when confining pressure is below 15 MPa, and after confining pressure exceeds 15 MPa, it is bulging in the middle part of sample. The scientific strain is proved to be more suitable for describing the deformation of salt rock sample than the engineering strain. 2) During the unloading-reloading process, the produced stress difference is small at the beginning of unloading, while the rock undergoes more stress loss in the post-peak stage than that in other stages. 3) When the strain increases to reach a specific value, the dissipative energy difference increases with higher confining pressure. 4) Higher confining pressure improves the initial damage degree to the sample. But under different confining pressures, the damage variable has a similar evolution trend, which reaches a steady growth stage with the loading/unloading cycles gradually.