Longitudinal seismic response characteristics of shield tunnel structures in liquefiable soils

When shield tunnels are located in liquefiable soil deposits, the progressive accumulation of soil strains and the build-up and dissipation of the excess pore water pressure during seismic loading affect the seismic response of the tunnel. This study develops a fully coupled solid-fluid effective stress analysis model using the OpenSees computational platform to examine the seismic response of long-extended shield tunnels. Two advanced bounding surface plasticity constitutive models for liquefiable and non-liquefiable soils are utilized to simulate the soil nonlinear behaviors under undrained conditions. Nonlinear spring models are used to simulate the mechanical behavior of longitudinal joints in shield tunnels. The developed model is employed to evaluate the longitudinal seismic response characteristics of shield tunnel structures under three different ground motions with different frequency contents, considering the soil nonlinearity and hysteretic characteristics. The results reveal that the dynamic interaction between the saturated soil and the tunnel structure in liquefiable soil deposits significantly increases the liquefaction potential of the soil around the tunnel. Consequently, the seismic response of tunnels in liquefiable soils is more pronounced than in non-liquefiable soil deposits. The ground motion spectral characteristics greatly influence the seismic response of long-extended tunnel structures. Low-frequency ground motions have detrimental effects on the tunnel mechanical behaviors. Furthermore, it is observed that the segment rings experience greater internal forces when subjected to compression, compared to tensile loading due to the much higher compressive stiffness of the concrete compared to the tensile stiffness of the connecting bolts. Variability in the spatial distribution of the axial force within the segment rings highlights the influence of the soil nonlinear mechanical properties along the tunnel longitudinal axis.