Seismic Fragility and Performance Analysis of Shallow Underground Subway Station Structures in Different Silty Clay Interlayer Sites

Aiming at the influence of silty clay interlayer on the seismic performance of the shallow underground subway station structure, this paper designed five soft sites with different burial depths of silty clay interlayer and a nonlinear dynamic time-history analysis finite-element model of soil–structure interaction was developed by taking a two-story subway station structure as a prototype. Twelve natural ground motions were selected for incremental dynamic analysis. The results showed that the presence of silty clay interlayer in the lateral foundation of an underground structure was detrimental to the seismic performance of the subway station structure, which increased the lateral deformation and seismic damage to the structure, especially when the interlayer was located in the lower portion of the lateral foundation. However, when the interlayer was located in the bottom foundation of the underground structure, the interlayer acted as a natural seismic isolation layer, which reduced the deformation and damage to the structure to a certain extent. The closer the interlayer to the structure, the more significant the interstory drift ratio (IDR) reduction. The seismic fragility curves were developed based on peak ground acceleration IDR. The effect of the interlayer on the probability of damage to the structure would gradually appear with the improvement of the seismic performance level (SPL). When the SPL exceeded SPL2, the probability of damage was greatly increased when the interlayer was located in the middle or lower portion of the lateral foundation. When the SPL exceeded SPL3, the probability of damage was greatly reduced when the interlayer was located in the bottom foundation of the underground structure.