# Study on the Influence of Deep Soil Liquefaction on the Seismic Response of Subway Stations
## 1 Introduction
_The Code for Seismic Design of Buildings_[19] stipulates that if liquefied soil layers within a 20 m sub-surface range are identified, corresponding judgment criteria and mitigation measures will be provided based on the liquefaction level of the soil. However, no recommendations are offered for liquefied soil layers situated beyond a depth of 20 m. This lack of guidance leaves designers uncertain about how to address liquefied soil layers exceeding this depth. In practical engineering within the Beijing area, many subway stations have base plates buried more than 20 m deep, such as the Xidan and Wangfujing stations on Subway Line 1; the Xizhimen and Fuxingmen stations on Subway Line 2; and the Chongwenmen and Dongdan stations on Subway Line 5. Should the embedding depth of a subway station's base slab reach 20 m, and should there be saturated sandy soil beneath, the possibility of liquefaction under seismic activity arises. The extent of such liquefaction and its impact on the subway station structure is not well-documented. _The Code for Seismic Design of Urban Rail Transit Structures_[20] indicates that, for station structures with base slabs embedded deeper than 20 m, it is necessary to conduct dedicated research on the liquefaction of deep soil layers (soil layers deeper than 20 m). Currently, research has been conducted by scholars regarding the consideration of liquefied soil layers below 20 m in depth. [PERSON] et al. [21] investigatedthe impact of the depth of liquefied soil layers on the seismic response of piles through numerical simulation methods. The effect of structure-soil-structure interactions on seismic responses was studied by [PERSON] [22] using the finite difference software FLAC 5.0, which involved coupled hydrodynamic analysis. The issue of bearing capacity degradation caused by liquefaction in shallow foundations on two-layer soil profiles was explored through the use of numerical analysis by [PERSON] [23]. By utilizing the characteristics of natural liquefied soil, [PERSON] [24] isolated seismic protection structures and reduced the seismic impact on superstructures by partially repairing the surface soil, which then acted as a natural base isolation system. [PERSON] [25] analyzed the seismic response of shallow foundations on liquefied soil which had been improved by deep soil mixing (DSM) via numerical simulations. The study examined the influence of the diameter and depth of DSM columns and the spacing and diameter of DSM column groups on the bearing capacity and settlement of shallow foundations, thus offering new insights for practical engineering applications. The study aimed to investigate the seismic performance of subway stations under specific geological conditions, particularly when soil layers with potential liquefaction hazards exist beneath the station slabs. The significance of this issue lies in the safe operation of the subway system and the reliability of urban infrastructures. In light of this, the research focused on the specific case of a subway project in the Beijing area, where the complex geological environment and high population density both make safety requirements for subterranean structures exceedingly stringent. In the process of conducting research, a geological model of the study subject was initially established based on field surveys and geological exploration results. This model encompasses the physical and mechanical properties of the soil layers, as well as the distribution of liquefiable strata. Given the approximate 20 m depth of the subway station's base slab, potential issues arising from soil liquefaction include an increase in buoyancy and variations in lateral earth pressure, all of which could impact the structural safety and functionality of the underground structure. The method of numerical simulation was selected for seismic response analysis, permitting the performance evaluation of the structure under various hypothetical conditions without disrupting actual engineering operations.