In seismic regions, many underground structures are inevitably partially embedded in liquefiable sites, which may cause complex seismic response mechanisms due to the varying distribution of liquefiable soil layers. This study investigates dynamic interaction between underground structures and liquefiable soils employing three-dimensional nonlinear finite element models. The seismic response of both standard and connection sections of the subway station-tunnel of underground structures in liquefiable sites is evaluated to reveal the seismic response patterns of the soil-structure system under different liquefiable soil distribution forms. The results revealed that compared to homogeneous liquefiable sites, liquefiable interlayer sites can cause greater seismic damage to underground structures, potentially leading to failure along the entire length of the subway station. Therefore, the post-earthquake failure modes of the structure and site should be comprehensively considered based on the site layers distribution characteristics.

The connection between subway stations and tunnels in subway systems is a critical consideration in the design of underground transportation systems. Expansion joints may be introduced between the station and tunnel to reduce the stress and deformation transmitted to the structure and mitigate the potential structural damage. However, adverse conditions such as large deformations in liquefiable sites and extreme earthquakes can severely impact the integrity of this connection. This study employs three-dimensional finite element numerical models of dynamic soil-structure interaction in liquefiable sites to investigate the seismic response of the subway station-tunnel connection structure under different distributions of liquefied soil layers and considering various structural connection methods. The results demonstrated that subway station-tunnel structure placed in liquefied interlayer sites experiences greater seismic damage compared to structures with their upper parts embedded in homogeneous liquefiable sites. In addition, using expansion joints between the station and tunnel can indeed reduce the seismic stresses and deformations transmitted to the structure, which can mitigate the extent and severity of its damage. However, the expansion joint can lead to misalignment between the subway station and the tunnel. The findings provide theoretical references for seismic design and disaster mitigation measures for subway structures in liquefiable sites.