Linear energy storage and dissipation and microscopic damage of brine bearing sandstone under cyclic compressive shear

In underground engineering practice, rock masses are typically in a compressive-shear stress state and are highly prone to shear failure. Particularly during coastal mineral resource exploitation, engineering rock masses undergo chemical erosion by waters rich in ions such as Na + and Cl − . To investigate the energy allocation laws of brine-saturated rock masses during shear failure, this study conducted cyclic compressive-shear loading tests on water-saturated and brine-saturated sandstone. Using the area integration method, the total deformation energy, elastic deformation energy, and dissipated deformation energy during loading and unloading were calculated. The results confirm the applicability of linear energy storage and dissipation laws of both water-saturated and brine-saturated rocks under compressive-shear conditions, with correlation coefficients ( R 2 ) exceeding 0.97. Brine immersion significantly adversely affects the energy storage capacity of sandstone. As the salt concentration increases, the energy storage coefficient ( a ) decreases progressively, reaching a maximum reduction of approximately 7.602% at a concentration of 5.0 mol/L. Based on these linear laws, an energy-based damage variable ( D ) was defined using dissipated energy. This variable was then employed to characterize the damage evolution of sandstone during cyclic compressive-shear loading. Brine immersion leads to increased damage during cyclic compressive-shear loading, particularly at high concentrations where the damage variable rises rapidly with increasing stress. SEM observations further reveal the underlying microscopic mechanisms. Ion exchange and crystallization pressure are found to be the primary drivers of mineral fragmentation and microcrack propagation. This study provides critical theoretical support for assessing the stability of underground structures, such as salt cavern energy storage and subsea tunnels, in brine environments.