Seismic vulnerability assessment of rectangular cut-and-cover subway tunnels

•We develop fragility curves of various types of metro box tunnels with respect to PGA, PGV, and PGV/Vs30.•The effect of site profile on the calculated fragility curve is shown to be significant.•The dependence on site characteristics is lowest when PGV/Vs30 is used.•The multi-box tunnels are more susceptible to earthquake damage than single box tunnels. This study develops fragility curves for rectangular cut-and-cover tunnels from nonlinear frame analyses. Fragility curves are generated for single, double, and triple boxes constructed for the subway system. A wide range of site profiles is used to evaluate the effect of soil characteristics on the calculated fragility curve. The fragility curves are developed for minor, moderate, and extensive damage states. The damage indices defined in a previous study as the ratio of the elastic moment demand to the yield moment at the critical section of the tunnel lining was used in the analyses. Fragility curves of the tunnels are generated in terms of surface peak ground acceleration (PGA), peak ground velocity (PGV), and PGV/Vs30 (Vs30 is the time-averaged shear-wave velocity to 30 m depth). The results highlight that the fragility curve is highly sensitive to the site profile, and that PGA based curves result in the largest scatter. The effect of the site profile is significantly reduced when PGV/Vs30 is used, where the fragility curves for all site profiles fall within a narrower band. This is because both the intensity of the ground motion and the soil stiffness is accounted for in the parameter PGV/Vs30. Considering the importance of site amplification characteristics, it is recommended that PGV/Vs30 be used instead of PGA and PGV in the generation of fragility curves of underground structures. Comparisons also demonstrate that multi-box tunnels are more vulnerable to earthquake damage compared with single box tunnels because the seismic demand is always larger.