Dynamic Response of a Deep Rock Mass under Transient Unloading: A Scale Model Experiment and Analytical Study

To investigate the influence of the unloading rate on the mechanical response of deep surrounding rock, a self-developed confining pressure loading device equipped with a pendulum hammer is designed and utilized in this study to conduct 1:100 scaled model experiments. The findings revealed that the final equilibrium state of axial strain exceeded its transient peak, whereas tangential strain exhibited the opposite trend, with a particularly pronounced transient overload at the cavity wall. As the propagation distance increased, the transient overload of axial strain gradually intensified, while that of tangential strain diminished, reaching its maximum at the cavity wall. Theoretical analysis further demonstrated that higher unloading rates significantly amplified the peak stress overload and the increment of the principal stress difference in the cavity wall and adjacent regions. The peak tangential stress overload was observed to be greatest near the cavity wall. The principal stress difference increment exhibited three stages with distance: (1) a sharp decline near the cavity wall, (2) an increase within 1–3 cavity diameters due to axial stress peak overload, and (3) a gradual decrease beyond 3 diameters, with the affected region expanding as unloading rates increased. The axial stress was most sensitive to unloading rates at 1.5 cavity diameters from the wall, while tangential stress showed maximum sensitivity at the wall. Peak stress overload exhibited a linear logarithmic relationship with the unloading rate. Additionally, unloading rates significantly affected the local stress distribution and determined the spatial extent of transient unloading effects.