Loading-unloading responses of non-persistent flawed granite subjected to cyclic disturbances

During geotechnical construction, flawed rock masses experience dynamic cyclic disturbances, leading to cumulative deformation and progressive damage. Consequently, elucidating the fracture mechanisms under cyclic loading is crucial for ensuring the safety and prolonged operation of deep underground engineering. This study investigated the mechanical responses of the surrounding rock at different locations by conducting triaxial tests on flawed granite using three distinct cyclic loading and unloading paths. Based on the maximum tangential stress criterion, a fracture mechanics model for open flaws was developed to analyze the intrinsic influence of confining pressure and flaw inclination on crack initiation behavior. The results indicate that graded unloading of confining pressure significantly weakens the flawed rock mass, reducing its peak stress to only 77.5% of that observed under constant confining pressure. Conversely, flawed rock masses exhibit a substantial increase in bearing capacity under increasing graded cyclic loading, achieving a peak stress 19.3% higher than that under cyclic disturbance loading. At a constant confining pressure of 40 MPa, the type of disturbance loading has no significant effect on the failure mode. The flawed granite specimens form a nearly V-shaped shear failure zone along the open flaw. However, confining pressure unloading induced a more complex shear-tensile composite failure mode in the specimens. The crack initiation angle increases nonlinearly with confining pressure, but decreases gradually as the flaw inclination angle ( β ) increases. These findings provide valuable insights for the safe construction of deep underground engineering.