Damage evaluation of surrounding rocks subjected to dynamic internal unloading and external loading

Dynamic disasters in deep underground engineering are frequently triggered by the interaction between the pre-existing excavation damaged zone (EDZ) and external disturbances such as blasting or earthquakes. Current studies on the dynamic response of tunnels have mainly focused on wave scattering around openings without considering EDZ, whereas the cumulative effects of excavation-induced damage and subsequent dynamic loading remain insufficiently investigated. Therefore, this study formulated a theoretical and numerical framework to investigate the dynamic response of tunnels considering the cumulative damage induced by transient unloading and subsequent external dynamic loading. An analytical solution for the dynamic response considering the EDZ was derived using integral transformation and the wave function expansion method. Subsequently, a numerical simulation scheme utilizing the restart technique and a specific elasto-plastic damage model was implemented in LS-DYNA to reproduce the progressive failure process. The results indicated that the depth of the EDZ increased with the initial stress σ 0 thereby significantly affecting the stress distribution around the tunnel and the evolution of the dynamic damage zone (DDZ). Under dynamic disturbances, the near-field EDZ undergoes a stress drop due to softening, forcing the high-stress concentration to migrate to the deeper intact rock mass. This mechanism of unloading and stress transfer reveals the dual role of the EDZ. While it acts as a flexible layer that dissipates wave energy via scattering and damping, its degradation simultaneously drives peak stress migration into the deeper rock mass, thereby expanding the damage zone and increasing the risk of rockbursts.