Damage evolution and failure mechanisms of rock under true-triaxial dual-face unloading: implications for deep excavations

To clarify excavation-induced rockburst mechanisms in deep, high-stress rock masses with two newly formed free surfaces, we conducted true-triaxial dual-face loading–unloading simulations using RFPA3D-Parallel coupled with the twin-shear unified strength criterion. Two stress paths were examined: simultaneous dual-face unloading of the minimum principal stress (σ3) and simultaneous dual-face unloading of the intermediate principal stress (σ2), followed by displacement-controlled reloading of the maximum principal stress (σ1) under different post-unloading confinement conditions. The simulations tracked acoustic emission (AE), damage evolution, stress–strain response, and final failure morphology under different initial principal-stress states. Dual-face σ3 unloading produced bilateral, shear-dominated V-shaped failure pits and relatively high post-unloading peak strengths of approximately 295–300 MPa. By contrast, dual-face σ2 unloading generated tensile cracks approximately parallel to the free surfaces, progressive slab splitting, and lower peak strengths under the reduced retained confinement. For both paths, an instantaneous σ1 stress drop occurred after boundary release, and its magnitude was governed primarily by the normal stress acting on the unloaded surfaces. These results demonstrate the directional dependence of dual-free-surface failure and provide a mechanistic basis for AE-based damage assessment, rockburst-risk interpretation, and support design in deep excavations.