Failure of rock mass induced by unloading near excavation face: a true triaxial test

The sudden release of stress near the excavation face during tunnelling in hard rock under high geostress conditions can trigger severe failures such as collapse and rockburst. However, conventional uniaxial and triaxial tests are inadequate in replicating the complex unloading conditions encountered in situ. This study employs a true triaxial unloading system to simulate various stress paths and investigate the deformation behaviour and failure mechanisms of granite under unloading-dominated conditions. The results show that specimens primarily fail due to strong dilatancy along the unloading surface, with combined shear, tension, and splitting failure modes. Higher axial stress leads to brittle splitting failure concentrated in the specimen’s central zone, whereas lower axial stress induces tensile and slabbing failure near the unloading boundary. Mechanical parameter evolution and energy dissipation patterns reveal that increased confinement enhances energy storage capacity while intensifying brittle behaviour upon unloading. Furthermore, three classical rock failure criteria are evaluated. The Mogi–Coulomb criterion demonstrates the best predictive capability for unloading-induced strength under true triaxial stress paths. These findings provide theoretical and practical insights for failure prediction and support design in deep underground excavation projects.