A Variable-Parameter Creep Damage Model Incorporating the Effects of Loading Frequency for Rock Salt and Its Application in a Bedded Storage Cavern

Laboratory tests were conducted to assess the effects of the loading frequency on the time-dependent behavior and damage properties of rock salt under confining stress states. Axial two-stage irreversible deformation based on the loci of the minimum load of each cycle was observed, and this observation was similar to the result of conventional creep tests under static loads. The unloading modulus decreased exponentially with respect to time, and the damage variable was represented in terms of the decay of the material stiffness. To account for the effects of the loading frequency on the time-dependent degradation of rock salt, a unified damage evolution equation was formulated based on the experimental results. A creep damage model of rock salt was proposed by introducing non-stationary modular components into the Burgers viscoelastic model. Numerical simulation was performed using the newly developed model to evaluate the stability and serviceability of a storage cavern in a bedded salt formation under various loading scenarios. The simulated results indicate that a lower injection-withdrawal frequency results in a greater volume convergence rate and a wider dilatancy region of the storage cavern. Additionally, the stress concentration and dilatancy of the surrounding rock mass extend much deeper into the mudstone interbeds than into other regions of the cavern.