Abstract: In studying the deformation and failure mechanism associated with tunneling, it is very important to consider the transient excavation-induced unloading effect and analyze the dynamic response of surrounding rock. Based on the dynamic unloading model of a circular tunnel under hydrostatic pressure, a procedure for simulating dynamic unloading response of tunnel excavation is developed by using Fish language in FLAC3D. A numerical example is analyzed and the results are compared to the theoretical results, showing the validity to the proposed procedure. It is shown that the stress and volumetric strain of the surrounding rock vary in a wavy manner and the proposed procedure can be effectively used to simulate the dynamic unloading response of the surrounding rock. Based on the calculated damage range using the Griffith strength criterion, it is found that surrounding rock damage area is larger than that of the static analysis with considering the dynamic unloading effect. The proposed procedure is applied to analyze the dynamic response of the surrounding rock under different unloading conditions including unloading time, unloading path, tunnel shape, tunnel diameter and ground stress and so on. It is shown that as the unloading time decreases, the unloading path increases in a way gradually to drastically, and the tunnel diameter increases, and that with the increase of geostress and its difference, the difference between the maximum and minimum principal stresses increases. The proposed instant unloading analysis procedure can provide new insights into the deformation and failure mechanism of the surrounding rock of deep buried tunnels.

Key words: tunnel excavation, transient unloading, dynamic response, surrounding rock characteristics, numerical simulation