A parameter-variable creep damage model considering the effects of loading frequency for rock salt and its engineering application in deep storage cavern

Laboratory tests were conducted to assess the effects of loading frequency on the time-dependent behaviour and damage properties of rock salt under confining stress states. An axial two-stage irreversible deformation complied from the loci of minimum load of each cycle was observed, which is similar to that of conventional creep tests under static loads. The unloading modulus was found to decrease negative-exponentially with respect to duration time. The damage variable was represented in terms of the decay of material stiffness. To take into account the effects of loading frequency on time-dependent degradation for rock salt, a unified damage evolution equation was formulated based on the experimental results. A creep damage model for rock salt was proposed by introducing the non-stationary modular components into the Burgers viscoelastic model. Numerical simulation was further performed utilizing the newly developed model to evaluate the stability and serviceability of storage cavern in bedded salt formation under various loading scenarios. The simulated results indicate that a longer injection-withdrawal cycle results in a greater volume convergence rate of storage cavern. The dilatancy region surrounding the storage cavern extends progressively along the radial direction with increasing loading cycle.