Evolution mechanism of unloading mechanical characteristics of high-stress rock under action of intermediate principal stress

To investigate the controlling effect of the intermediate principal stress (σ₂) on the mechanical behavior and failure mechanism of deep high-stress granite under roadway excavation unloading conditions to provide a theoretical basis for the stability control of surrounding rock in deep engineering projects. A self-developed true triaxial disturbance unloading test system was employed to conduct single-face unloading tests on intact coarse-grained granite specimens (200 mm × 100 mm × 100 mm) collected from Anhui Province. Three σ₂ levels (10/20/30 MPa) were set to simulate the stress path of σ₃ unloading to zero during excavation. The response laws of peak strength, deformation evolution, and failure morphology were systematically analyzed. The peak strength exhibited a monotonically increasing trend with the rise of σ₂ (σ₁ increased from 177.70 MPa to 217.32 MPa). During the unloading process, axial strain continuously increased, and lateral expansion was significant. When σ₂ = 20 MPa, the compressive strain in the intermediate principal stress direction reached its minimum, indicating the existence of a critical constraint state. The failure mode evolved from localized spalling to through-going slab-like splitting, and the energy accumulation–release mechanism under high σ₂ conditions dominated rockburst-type instability. The intermediate principal stress enhances rock strength by strengthening lateral constraints; however, high σ₂ intensifies the risk of energy release during unloading. Optimizing the stress path to approach the critical constraint state can balance the contradiction between strength and deformation.