## 3 Stratum-Foundation Pit-Station-Track Interaction Model  
### Establishment of the Finite Element model  
In this paper, the new and existing subway structures are optimized. The basic assumptions are as follows:  
(a) The retaining structure of the new foundation pit and the adjacent existing stations and retaining structures are all linear elastic structures. (b) Soil is an ideal elastic-plastic material with isotropy and mean value. The stratum is simplified by the model. The influence of groundwater is not considered in the model, and the initial in situ stress of the model only considers the self-weight of the soil and ignores the structural stress of the soil. (c) The whole excavation process of the foundation pit is simulated through the construction sequence. In the excavation process, only the change in the spatial displacement of part of the foundation pit is considered, and its time effect is not considered. (d) Due to the large excavation area of the foundation pit and many model element nodes, some structures of the model are simplified to improve the calculation speed. According to the principle of equivalent stiffness, the bored pile structure of the new foundation pit is replaced by the elastic plate element structure, and the track rail is replaced by the elastic beam. According to the foundation pit design, station design drawings, geological survey reports and other relevant data, the overall stratum-foundation pit-station-track interaction model is established using the Midas GTS NX three-dimensional finite element software. The size of the model is determined according to the location relationship between the new foundation pit and the adjacent subway station, the influence range of foundation pit excavation and the relevant theory of surface settlement caused by the foundation pit. The size of the model is 280 m long (along the direction of the station), 200 m wide (along the transverse direction of the station) and 70 m down from the ground surface. The overall model diagram is shown in Figure 18.  
The model has 802,309 units and 1,403,010 nodes, including five layers of stratum soil, soil within the excavation range of the foundation pit in layers and blocks, a grouting equivalent layer, a foundation pit support system structure, station and tunnel interval soil and a tunnel and track structure system. The soil within the excavation range of the foundation pit is divided into four parts longitudinally along the foundation pit, and the parts are divided into nine layers from top to bottom. The grouting equivalent layer is the layer below 6 m between the foundation pit and the station, and its depth range is 20 m. The foundation pit support system structure includes an enclosure pile, temporary steel  
Figure 17: Left track level. lattice column, temporary concrete column, three concrete truss internal supports, three waist beams and one anchor cable. The structures of the station and tunnel are obtained by extracting the soil of the station and interval tunnel and giving the structural attributes of the station and tunnel. The track bed is simulated by a solid element. A schematic diagram of the model position relationship between the new foundation pit and the existing station is shown in Figure 19.  
The new model simplifies the stratum soil according to the stratum parameters, uses 3D solid elements for simulation and endows different parameter attributes to different soil layers in the process of establishing the grid. The soil constitution adopts the Drucker-Prager yield criterion, referred to as the D-P criterion, considering the effect of hydrostatic pressure and reflecting the real yield and failure state of soil and thus eventually ensuring the speed and accuracy of calculation to analyze the deformation near the foundation pit better. Figure 19: Schematic diagram of the model position relationship. Figure 18: Overall model diagram. The model mainly considers the self-weight conditions of the soil and structure.