Seismic response characteristics of subway station structures in liquefiable sites

Under strong earthquakes, subway station structures situated in potentially liquefiable soils may experience complex seismic response scheme when the structure base slab is embedded in non-liquefiable soils while the sidewalls and top slab are buried in liquefiable soils (hereinafter referred to as a “liquefiable sites”). This paper investigates the dynamic seismic responses of multi-story underground structures in liquefiable sites employing an advanced three-dimensional nonlinear finite element model. The results indicate that the seismic response of underground structures is primarily determined by the soil displacement and the soil-structure stiffness ratio. In addition, the seismic response of the soil-underground structure system is strongly influenced by the distinct characteristics of the input ground motion. Seismic input motions rich in low-frequency components are more likely to cause saturated sand layers to liquefy and are likely to trigger more pronounced flow deformations, leading to severe damage to underground structures. Generally, liquefied soils are prone to significant horizontal displacements (i.e., lateral spreading); however, due to the reduced soil-structure stiffness ratio, the soil ability to induce shear deformation in the structure is diminished, and hence the structure does not undergo large horizontal deformations same as that of the soil. Additionally, structures may exhibit a slight tendency to uplift after the earthquake. These observations can inform the seismic design of underground structures in liquefiable sites.