Fracture behavior of jointed rock under excavation unloading: laboratory testing
摘要
In deep-buried tunnel construction, the instability issues caused by excavation under high-stress conditions have been challenging. In this study, a series of true triaxial single-sided unloading tests were performed on jointed sandstone specimens with different joint inclinations to explore the fracture behavior of jointed rock at different burial depths under excavation unloading. The results show that the specimen under Loading Path I (unloading first and then loading) exhibits an unloading rebound phenomenon and a distinct yielding stage, reflecting a ductile failure. The specimen under Loading Path II (loading first and then unloading) shows a higher brittleness and greater susceptibility to failure, without an obvious yield stage. In the specimen under Loading Path I, the macroscopic failure forms a V-shaped notch due to buckling tensile cracks, whereas the failure of specimen under Loading Path II resembles the rockburst phenomenon with far-field tensile cracks. Additionally, the increased burial depth increases the yield and peak stresses of specimen and suppresses large-scale cracks but intensifies the crack propagation in specimen upon unloading, resulting in a more severe failure. The joint inclination affects the crack propagation and fragmentation characteristics of specimen, with the greatest influence occurring at 30°–45°. At joint inclinations of 0°–30°, the failure of specimen is dominated by shear cracks initiated from joint tips. At joint inclinations of 45°–60°, the failure of specimen involves mixed tensile-shear cracks, while at the joint inclination of 90°, the tensile cracks parallel to the joint dominate.