Energy evolution and damage mechanisms of sandstone under true triaxial loading and unloading along different principal-stress paths

To elucidate the energy evolution and damage mechanisms of deep roadway surrounding rock subjected to principal-stress loading–unloading, conventional true-triaxial compression tests, graded loading–unloading tests along different principal-stress directions, and single-cycle loading–unloading tests were conducted on sandstone using an in-house-developed TAWZ-5000/3000 computer-controlled true-triaxial rock testing system. Based on the observed three-dimensional strain coupling, an energy-calculation framework incorporating deformation and work contributions in all three principal-stress directions was developed. Damage evolution under the different principal-stress paths was further evaluated using a dissipated-energy-based damage variable and acoustic emission (AE) monitoring. The results showed that loading–unloading along a single principal-stress direction induced strain variations not only in the cycling direction but also in the other two directions, demonstrating pronounced three-dimensional deformation coupling. Distinct hysteresis loops developed in the stress–strain curves, and their areas increased nonlinearly with increasing loading–unloading level. Under maximum-principal-stress cycling, the dissipated-energy ratio increased progressively, reflecting sustained accumulation of irreversible deformation and damage under increasing σ 1 . Under intermediate-principal-stress cycling, the elastic-energy ratio increased progressively while the dissipated-energy ratio remained relatively low, indicating a strong capacity for elastic energy storage. Under minimum-principal-stress cycling, dissipated energy increased at an accelerated rate, accompanied by the most pronounced post-test macroscopic cracking, indicating that repeated unloading under reduced confinement markedly promotes deterioration of rock integrity. These findings provide new insights into the path-dependent energy evolution and damage mechanisms of deep rocks subjected to excavation-induced disturbances and contribute to the stability assessment and control of deep roadway surrounding rock.