Damage evaluation of rock salt under multilevel cyclic loading with constant stress intervals using AE monitoring and CT scanning

When salt caverns are used for the medium of underground energy storage, the surrounding rock experiences cyclic loading due to periodical injection-production. The cyclic load contains constant stress intervals and varied stress amplitudes, which threatens the stability of the salt caverns. In this study, multilevel cyclic loading with constant stress intervals on rock salt were conducted under uniaxial compression conditions. Acoustic emission (AE) monitoring and post-test computed tomography (CT) scanning were used to evaluate the damage evolution during the loading process. Results show that the density of the hysteresis loops in overall stress-strain curves present two-stage alternating sparse and dense loops during the loading. The cumulative AE count/energy increases with increasing the number of cycles and loading stage, and the growth rate for a single stage gradually becomes constant. The AE signals were divided into six kinds according to frequency-amplitude distribution. The presence of stress intervals increases the number of cracks and the median scale cracks account for about 90% of the total number of cracks. By analyzing the CT images, the crack morphology of the salt sample without hold time shows several inclined main cracks with numerous branch microcracks while that with the hold time of 20 s shows multiple branch cracks and numerous microcracks. To quantitatively evaluate damage evolution during the loading process, a damage variable is defined based on the relative variation of AE parameters. The damage accumulation for a single loading stage presents a two-phase pattern, and the damage degree during the accelerated deformation accounts for approximately half of the total damage. This study can provide guidance for designing the operation parameters when salt caverns are used for the storage of natural gas, air and CO2.