Seismic vulnerability of circular tunnels in soft soil deposits: The case of Shanghai metropolitan system

Abstract

This paper aims at the vulnerability assessment of circular tunnels in soft soil deposits of the metropolitan subway system of Shanghai, accounting for the effects of soil-structure-interaction, local soil conditions and tunnel burial depth. The study focuses on the seismic vulnerability in the transversal direction. A suite of twelve earthquake records is selected and scaled from 0.1 g to 1.0 g to perform non-linear incremental dynamic analysis (IDA), and thus to assess the tunnel response under increasing levels of ground shaking intensity. A visco-elasto-plastic model with Mohr-Coulomb yield criterion is adopted to capture the non-linear behaviour of soil deposits under the seismic excitation. Fragility curves are constructed as a function of free-field peak ground acceleration (PGA) and peak ground velocity (PGV) considering the main sources of uncertainties, with the damage measure (DM) being defined in terms of acting over capacity bending moment ratio of the tunnel lining. The derived fragility curves are compared with existing empirical and analytical ones. Furthermore, vulnerability curves are generated from the derived fragility curves. The significant role of local soil conditions and tunnel burial depth in the vulnerability of tunnels in soft and deep alluvial deposits is highlighted. The proposed fragility and vulnerability functions may be particularly used for the risk analysis of circular tunnels in soft deposits (Vs30 < 200 m/s) exposed to seismically-induced transient ground deformations in the transverse direction.