Energy evolution and failure mechanism of tunnel dynamic unloading in deep rock mass abounding in closable minor joints

Discontinuities, ranging from micro to macro, are prevalent in natural rocks, especially deep brittle rock masses subjected to high in-situ stress. Previous studies on the unloading of deep tunnels have focused on large-scale discontinuities and have paid little attention to ubiquitous minor joints. These compactable joints affect static performance (strength and deformation characteristics) and dynamic response (stress wave concentration and dispersion). Through modelling, calibration, and validation, the jointed rock mass model in this study can demonstrate nonlinear deformation behaviour under static compression and increased wave velocity with increasing confining pressure under dynamic loading. Then, the Fourier transform method is utilised to solve the elastic theory solution of exponential unloading, and the excavation relaxation method is employed to simulate the tunnel unloading process in Particle Flow Code. The accuracy of the simulation process and measurement are analysed by comparing the theoretical solutions and the simulation results. The dynamic stress, failure pattern, and energy evolution of tunnel unloading in jointed rock masses are analysed from three influencing factors: unloading rate, lateral pressure coefficient, and dip angle distribution. Results indicate that joints are dynamic disturbance amplifiers near the tunnel and reducers at greater distances; additionally, the increase in the unloading rate will intensify these effects. Initial joint characteristics and in-situ stress determine the distribution of open joints and, thus, the wavefront shape. When most joints are closed under in-situ stress with low lateral coefficients, more severe tensile failure occurs in the direction perpendicular to the joint dip angle; conversely, when these joints are open, more severe shear damage will occur along the direction of the joint. The kinetic energy core shifts marginally from the direction of maximal principal stress towards the vertical of the joint dip angle, indicating that existing projects in this direction will be disturbed later but severely. Therefore, compactable minor joints should be considered to comprehend the dynamic response of tunnel unloading and to evaluate surrounding rock disturbance and failure.