Deformation and failure mechanism of shield tunnels in longitudinally heterogeneous ground under strong earthquakes

Nonuniform soil profiles can induce seismic ground-motion amplification and deformation incompatibility, thereby intensifying the seismic response and altering the deformation patterns of shield tunnels. To address this critical issue, this study takes a cross-river tunnel project in the lower reaches of the Yangtze River as a case study. Considering the dynamic nonlinearity of riverbed soil and the refined assembly of segments, a 3D refined finite element model with a longitudinal length of 500 m was established. The effects of seismic wave characteristics and the shear wave velocity ratio of adjacent soil layers on the failure mechanism and deformation mode of tunnels crossing longitudinally heterogeneous ground were systematically investigated. The results indicate that under seismic loading, the tunnel exhibits significant longitudinal non-uniform deformation, with the peaks of longitudinal joint opening and staggering concentrated on the soft soil side near the strata interface. The tunnel manifests a distinct “S-shaped” bending deformation at the interface, and the zone significantly affected by the abrupt change in strata extends to a range of 5 times the tunnel diameter into the soft soil side. Furthermore, concrete damage in tunnel segments presents asymmetric evolution. In soft-soil sections, damage is mainly concentrated along conjugate directions of the tunnel cross-section, whereas in hard-soil sections, damage is primarily localized near the springline region. The findings provide useful guidance for the anti-seismic design of cross-river tunnels in heterogeneous ground conditions.