Failure of Rock mass Induced by Unloading Near Excavation Face: A True Triaxial Test

In tunneling through hard rock under high geostress, sudden stress release near the excavation face can lead to collapse or rockburst. Traditional uniaxial and triaxial tests do not accurately simulate the unloading conditions near the excavation face. This paper employs true triaxial unloading tests across various stress paths to study the stress-strain behavior and failure characteristics of rocks. The results show that specimens primarily fail due to strong dilatancy along the unloading surface, with combined shear, tension, and splitting failure modes. Increased axial pressure leads to brittle shear and splitting failure in the rock's center, while decreased axial pressure results in tensile plate fracture near the unloading surface. Additionally, three classic rock failure criteria are discussed, with the Mogi-Coulomb criterion showing the best alignment with strength data during unloading in true triaxial tests.