Effect of voids on the seismic vulnerability of mountain tunnels

Void defects directly affect the dynamic response of tunnels. These defects have been recognised as an important factor that exacerbates seismic damage and the destruction of tunnel linings. Existing studies that consider the tunnel-void dynamic interaction use deterministic analysis methods, but they lack a quantitative analysis based on a probability method, which can reflect the random nature of earthquakes and structures. In the present work, a series of 2D nonlinear dynamic analyses of tunnel seismic response that consider the tunnel-void interaction is carried out by using the incremental dynamic analysis method. The automatic extraction of the resulting data and the calculation of the maximum damage index (DImax) during the seismic response process are performed by a Python program. The fragility curves of the tunnels are obtained with consideration of void defects. The effects of void type, void location, void size, surrounding rock classification and seismic direction on the vulnerability of the tunnels are investigated. Results show that void size and type and surrounding rock condition exert considerable effects on the seismic vulnerability of tunnels. The presence of voids behind linings leads to increased tunnel vulnerability, which presents a nonlinear increase with the increment of void size. The degree of influence of void size on vulnerability varies with void location. Tunnels with voids between surrounding rocks and the primary support are more vulnerable than those with voids between the primary support and the secondary lining. Hence, void type cannot be ignored. The effects of voids on vulnerability become evident when tunnels are embedded in weak surrounding rocks. The vulnerability under the horizontal seismic direction is apparently greater than that under the vertical direction at the same seismic intensity. However, the sensitivities of vulnerability to seismic directions vary with different void locations.