Seismic fragility analysis of tunnel underlain by a soft soil layer considering the uncertainty of the ground motions

Taking the tunnel in the underlying soft soil layer as an example, the dynamic analysis models of tunnel underlain soft soil layer and emerged in the homogeneous stratum are established based on the finite difference software, and nonlinear time-history analyses are carried out. Considering the uncertainty of the input seismic wave, the seismic vulnerability analysis of the typical underlying soft soil tunnel is carried out with the bending moment bearing capacity of the lining section as the damage index. The corresponding structural vulnerability curve is obtained and compared with the vulnerability curve of the homogeneous stratum. The results show that the structural dynamic response of the tunnel in the underlying soft soil layer is significantly higher than the seismic response of the tunnel in the homogeneous stratum, and the maximum bending moment of the tunnel appears at the left arch foot of the tunnel. The amplification effect of peak acceleration in soft soil layer and surface is the most significant, where the maximum acceleration amplification coefficient is 1.4. The peak acceleration of the surface is 1.6 times larger than that of the bedrock. The probability of slight damage in the tunnel under 0.2g peak acceleration is less than 50 % when the peak acceleration of the ground motion is 0.4g, the tunnel has a probability of more than 80 % of slight damage, and more than 30 % of the probability of serious damage, and more than 5 % of the probability of serious damage. The slight damage and moderate damage probabilities can be affected mainly when the tunnel underlain a soft soil layer and the incremental ratio of tunnel underlain soft soil layer is significant in slight damage status and the maximum incremental ratio is 26.5% when the peak acceleration is 0.4g.