A creep-fatigue model of rock salt and its application to the deformation analysis of CAES salt caverns

The rock surrounding salt caverns used in a compressed air energy storage (CAES) system experiences the combined creep and fatigue load during an operation period, which includes the injection, idle and production stages. To ensure the safety of the CAES salt caverns over the course of the service life, investigating the mechanical response of rock salt under creep-fatigue load is critical. In this study, creep-fatigue tests of rock salt under different conditions were performed in a stress-controlled pattern. The salt axial strain under creep-fatigue load includes initial, steady and accelerated phases. The axial strain presents a continuous increment during the high stress holding time. A fatigue damage body was established to describe the effect of cyclic load on deterioration of mechanical behavior of salt rock. A novel creep-fatigue model of rock salt is proposed by connecting the elastic body, Kelvin model, fatigue damage body and nonlinear viscoplastic body in series. The test data were used to validate the established model and they correlate well with the model data. Subsequently, the proposed model was implemented in the FLAC3D software and a creep-fatigue simulation test of a cylindrical specimen wase performed. The simulation test presents a similar variation trend compared with the test data regarding the axial strain. Finally, a three-dimensional geotechnical model was proposed to analyze the deformation of CAES salt caverns during operation and the proposed law was used as the creep model of rock salt during the simulation. The simulation results between the proposed creep-fatigue model and static Norton power model are further compared.