Symmetry and Coupled Effects of Intermediate Principal Stress and Unloading Kinetics on Energy Dissipation and Fracture Behavior of Sandstone

Excavation unloading in deep rock masses involves a transition from symmetric states of energy storage to asymmetric energy dissipation, in which variations in intermediate principal stress (σ2) play a critical role. To investigate these symmetry-breaking mechanisms, controlled-rate true triaxial unloading experiments were performed on sandstone using a miniature creep-coupled testing system. During unloading of σ3 at 0.1–0.3 MPa/s, the evolution of elastic, dissipated, and plastic energies was quantitatively evaluated. The results reveal pronounced asymmetric energy responses governed by both σ2 and the unloading rate. Dissipated energy dominates the entire unloading process, while elastic energy exhibits a non-monotonic trend with increasing σ2—first rising due to enhanced confinement and then decreasing as premature failure occurs. Higher unloading rates significantly accelerate total, elastic, and dissipated energy conversion and intensify post-peak brittleness. A new metric, plastically released energy, is proposed to quantify the asymmetric energy release from peak to residual state after failure. Its dependence on σ2 is strongly non-monotonic, increasing under moderate σ2 but decreasing when σ2 is sufficiently high to trigger failure during unloading. This behavior captures the essential symmetry-breaking transition between elastic energy accumulation and irreversible plastic dissipation. These findings demonstrate that true triaxial unloading induces energy evolution patterns far from symmetry, controlled jointly by σ2 and unloading kinetics. The established correlations between σ2, unloading rate, and plastically released energy enrich the theoretical framework of energy-based symmetry in rock mechanics and offer insights for evaluating excavation-induced instability in deep underground engineering.