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.