A nonlinear elastoplastic subloading surface model for rock under cyclic loading-unloading with application to compressed air energy storage cavern operation

A novel nonlinear elastoplastic subloading surface model (NEPS) is proposed for rock subjected to cyclic loading–unloading under general triaxial stress states. The model employs a modified generalized Zhang-Zhu (GZZ) criterion to describe three-dimensional nonlinear strength, introduces a plastic internal variable to characterize the evolution of rock mechanical properties with plastic deformation, and establishes a generalized subloading relation for the three-dimensional modified GZZ yield surface that evolves with the plastic state, thereby accounting for plastic evolution and loading history during stress reversal. Calibration against conventional triaxial cyclic loading–unloading tests and independent predictions under conventional and true triaxial cyclic conditions show that the NEPS model can reasonably reproduce the principal features of cyclic stress–strain response and residual-deformation accumulation. The model is further applied to the analysis of the mechanical response of compressed air energy storage (CAES) caverns during operation. The results show that the plastic state induced by excavation is carried over into the operation stage, and that the surrounding rock continues to adjust progressively with cycling even within a fixed internal pressure range. Expanding the operating pressure range intensifies irreversible deformation and promotes the outward development of plastic deformation in the surrounding rock. Interestingly, even when the operating internal pressure returns to the same level, the surrounding rock may exhibit different mechanical states, depending on the preceding cyclic process and the extrema of internal pressure previously experienced. The findings provide new insights into the evaluation of the operational state of CAES caverns.