Seismic response of soil-shield tunnel systems in sandwiched liquefiable soil deposits

Abstract

Long-extended underground structures such as shield-driven tunnels may be constructed through sandwiched liquefiable soil profiles, which can compromise their integrity during strong earthquakes. In this study, the influence of sandwiched liquefiable soil layer on the seismic response of shield tunnels under ground motions with different frequency contents is evaluated through nonlinear numerical simulations. Six soil-shield tunnel finite element models with different sandwiched liquefiable soil profiles are developed using the open-source software framework OpenSees. The solid–fluid fully coupled dynamic effective stress analysis method is adopted for the nonlinear seismic analysis of liquefiable soil. The numerical results demonstrate that the frequency contents of earthquake motion and the spatial location of the sandwiched liquefiable soil profile have great influence on the dynamic responses of the segmental tunnel. In all cases studied, the bottom half of the tunnel embedded in the liquefiable soil layer experienced the most unfavorable condition for the seismic performance. The deformations and internal forces of the segmental joints around the tunnel foot are remarkably higher than at other joints under the same ground motion intensity. Moreover, the Kobe motion with the low-frequency content would significantly amplify the seismic responses of both the site and the segmental tunnel compared to the Chi-Chi motion with the high-frequency content in different sandwiched liquefiable soil deposits. It is also found that the racking ratio of the non-liquefiable soil layer is greater than that of the liquefiable soil layer, which means that the surrounding soil with large shear deformation pushes the underground structure leading to larger adverse lateral deformation of the structure.