Abstract: To enhance national energy security, recent years have witnessed an increase in state investment directed towards the research and development of compressed air energy storage (CAES) technology. These initiatives are inherently more complex than traditional projects, as the stress relaxation characteristics of geological formations, particularly rocks, are influenced by a multitude of factors. According to the characteristics of CAES engineering, stress relaxation experiments of salt rock under barometric stress coupling were carried out from a new perspective to explore the stress relaxation characteristics of salt rock under different air pressures and axial starting stress conditions. The results show that: (1) The macroscopic behavior of stress relaxation in salt rock is independent of air pressure, and the process can be categorized into three distinct stages: rapid relaxation, decelerated relaxation, and stable relaxation. Notably, the rapid relaxation phase accounts for approximately 50% of the total relaxation, while the stable relaxation phase represents the predominant portion of the entire relaxation process. (2) The stress relaxation behavior of salt rock is significantly influenced by both axial starting stress and air pressure. Specifically, increased axial starting stress and air pressure correlate with heightened initial stress relaxation rates, greater relaxation rates, and an enhanced degree of stress relaxation during the stable relaxation phase. (3) Under cyclic pressure conditions, the stress relaxation of salt rock exhibits a cyclic relaxation pattern, with the relaxation curve being governed by the stress relaxation curve of salt rock at constant pressure, corresponding to its cyclic upper and lower limits. (4) Environmental scanning electron microscopy (ESEM) images of the relaxed salt rock reveal that the formation and propagation of micro-cracks constitute the primary mechanism of internal damage within the salt rock. Post-relaxation, the meso-structural damage observed in the salt rock includes grain debris, intragranular cracks, intergranular cracks, and trans-granular cracks. (5) The improved nonlinear two-element generalized Maxwell relaxation model can better describe the stress relaxation behavior of salt rock under different starting stresses and pressure conditions, and provide a certain reference for the stability assessment of salt cavern compressed air energy storage.

Key words: salt rock, barometric stress coupling, stress relaxation, nonlinear stress relaxation model