Seismic fragility assessment of circular metro tunnels in loess deposit

The deterministic method is always adopted for the seismic resistance of loess tunnels. Considering the randomness of ground motions, the seismic fragility assessment of a shallowly buried circular tunnel in the loess region was carried out under pseudo-static conditions with seismic loading in the transversal direction. The displacements calculated by one-dimensional analysis were applied to obtain the seismic responses of the tunnel. Then the maximum damage index was output by a compiled Python program. The optimality of intensity measures was briefly discussed through the testing criteria of correlation, efficiency, practicality, and proficiency. Fragility curves were generated based on the fragility function in terms of peak ground acceleration (PGA) and peak ground velocity (PGV) to evaluate the tunnel’s seismic performance. The results show that the vulnerable parts that shift with different PGAs in conjugate directions are particularly prone to suffering damage. And PGA and PGV are identified as the appropriate indices for predicting the probability of tunnels in various damage states. Void behind the arch dome can increase the fragility of tunnels, and the tunnels embedded in softer sites become more vulnerable to seismic damage.