# Study on Seismic Reduction Measures of a Diaphrag Wall--Underground Structure System
## 2 Underground Structure-Isolation Layer-Diaphragm Wall System
### Time History Analysis
#### 2.1.1 Model  
A two-story and two-span subway station is selected with a cross-section width of 20.20 m and a height of 13.74 m. The thickness of top plate, middle plate, bottom plate and side wall of the station are 0.8 m, 0.4 m, 0.9 m and 0.7 m, respectively. The size of the central column is 0.8 m \(\times\) 1.0 m. The longitudinal spacing of the middle column is 9 m. In the two-dimensional (2D) finite element analysis, the elastic modulus of the central column is reduced. The central column is equivalent to a continuous longitudinal wall with a thickness of 0.8 m. The thickness of soil layer on the station is 3 m. The buried depth of the bottom plate is 16.74 m. The embedded depth of the diaphragm wall is 25 m and the thickness is 0.8 m. The C35 concrete is used for the station beams, slabs, side walls and diaphragm walls, respectively. The C50 concrete is used for the central column. HRB400 and HPB300 rebar are used for the reinforcement. The cross section and reinforcement arrangement of the main structure are displayed in Figure 2.  
Figure 1: Structural system model of diaphragm wall–segmented isolation layer–subway station. (**a**) System diagram of diaphragm wall–segmented isolation layer–station structure. (**b**) Combination diagram of the rigid isolation layer, flexible isolation layer and diaphragm wall and (**c**) its profile. A 2D finite element model is established by using ABAQUS software, as shown in Figures 3-6. The width and height of the site model are taken as 150 m and 60 m. The concrete plastic damage constitutive model is selected for the main structure of the station and the concrete of the diaphragm wall. The ideal elasto-plastic constitutive model is selected for the reinforcement. The truss element is used for discretization. The reinforcement is embedded into the concrete in the way of built-in area. The bond and slip between the concrete and the reinforcement are ignored. The plane strain element of four nodes is adopted for the station structure, diaphragm wall and surrounding soil. In order to meet the accuracy requirements of numerical simulation, the mesh size of the element shall not exceed 1/6-1/8 minimum wavelength and the soil meshes adjacent to the structure are refined. The mesh size of the main structure and the diaphragm wall unit are about 0.2 m. The mesh size of the soil elements varies from 0.7 m to 2 m. The side wall and the diaphragm wall in the model are connected by surface to surface. The normal direction is set as \"hard\" contact and the tangential direction is set as \"penalty\" contact. The friction coefficient \(\mu\) takes 0.55 [4]. Roller boundaries are set on both side boundaries of the whole model. The width of the model is more than seven times the width of the station, so as to eliminate the influence of boundary effect. The fixed boundary is adopted at the bottom of the model. For the same purpose, the station bottom plate is 43.26 m away from the bottom boundary of the model, which is more than three times the height of the station. Figures 5 and 6 show the 2D integral time-domain finite element analysis model of the soil-diaphragm wall-isolation layer structure under the condition of setting the integral isolation layer and setting the segmented isolation layer, respectively. Figure 3: Finite element model without a diaphragm wall. Figure 2: A cross section of the subway station. The setting conditions for different working condition in the analysis are listed in Table 1. When considering the influence of different section forms, the thickness of the integral isolation layer and segmented isolation layer is 1 m and the height is taken as 13.74 m. The C35 concrete is used as the rigid isolation layer material in the segmented isolation layer. The ideal elastic material with an elastic modulus of 3.15 MPa and a Poisson's ratio of 0.2 is used as the flexible isolation layer. When studying the influence law of the stiffness of the isolation layer materials, the isolation layer materials are in the form of segments.