The deep excavation unloading stress path and initial high stress difference ( \(\sigma_{2}^{0} - \sigma_{3}^{0}\) ) are distinctive features that differentiate deep engineering from shallow engineering. To investigate the mechanism of deep hard rock failure induced by initial stress differences under excavation unloading stress paths, a series of typical true triaxial unloading tests are conducted using a true triaxial experimental system. The results show that as the initial stress difference increases, there is a gradual enhancement in both the strength, brittleness and ultimate energy storage capacity of the rock. Additionally, the direction of rock fracture propagation gradually aligns with σ2, leading to an increase in the macroscopic failure angle of the rock. Moreover, the roughness of the rock fracture surface decreases, while transgranular and tensile fractures become more prevalent, often exhibiting sudden, high-energy fracture events. Building upon the non-uniform stress distribution characteristics induced by the initial stress difference, a principle governing the differential development of rock fractures is proposed. This principle underscores the pivotal role of the initial stress difference in triggering spalling of the surrounding rock parallel to the sidewall. Furthermore, a functional relationship between the initial stress difference and the propensity for rockburst events is established. These findings hold significant implications for understanding the failure mechanisms of surrounding rock in deep hard rock engineering excavations and for evaluating the risks associated with spalling and rockburst hazards.
