## 4 Numerical Simulation Analysis
### Soil Properties in the Seismic Response of the Underground Substation
#### 4.2.1 Displacement Responses  
The relative displacements under 0.20 g are shown in Figure 19. The layer drifts and layer drift angles under 0.20 g are shown in Table 7.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Number** & **Name of Soil** & **M1** & **M2** & **M3** \\ \hline
1 & Plain fill & 5.0 & 10.0 & 15.0 \\
2 & New loess & 12.0 & 24.0 & 36.0 \\
3 & Silty clay & 15.0 & 30.0 & 45.0 \\
4 & Pebble bed & 55.0 & 110.0 & 165.0 \\
5 & Silty clay & 15.0 & 30.0 & 45.0 \\ \hline \hline \end{tabular}
\end{table}
Table 6: Soil elasticity modulus (MPa).  
Figure 18: The peak internal forces of surface-6 under El-Centro motion.  
(1) From Figure 19, it can be inferred that the displacements of the side wall decrease with the increasing of the elastic modulus of soil and the horizontal drift diminishes mostly at the top of the side wall, and the horizontal relative displacement in the Y-direction decreases by about 73.9% in Lanzhou motion. The reason is that, with the increasing of the soil elastic modulus, the structure constraint increases and structural deformation decreases, which usually lead to the diminishing of horizontal drift.  
(2) From Table 7, the drift angle is inversely proportional to the elastic modulus of surrounding soil. With the increase in the soil elastic modulus, the drift angle of each layer significantly reduces;  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & \multirow{2}{*}{**Section**} & \multicolumn{3}{c}{**El-Centro Motion**} & \multicolumn{3}{c}{**Lanzhou Motion**} \\ \cline{3-8}  & & **M1** & **M2** & **M3** & **M1** & **M2** & **M3** \\ \hline \multirow{4}{*}{Floor 3} & layer drift in the X-direction & 1.76 & 1.26 & 0.8 & 1.77 & 1.15 & 1.04 \\  & layer drift angle in the X-direction & 1/2898 & 1/4048 & 1/6375 & 1/2881 & 1/4435 & 1/4904 \\  & layer drift in the Y-direction & 4.03 & 2.37 & 1.91 & 3.14 & 1.83 & 0.76 \\  & layer drift angle in the Y-direction & 1/1266 & 1/2025 & 1/2513 & 1/1529 & 1/2623 & 1/6316 \\ \hline \multirow{4}{*}{Floor 2} & layer drift in the X-direction & 1.65 & 1.23 & 0.86 & 1.65 & 1.12 & 0.78 \\  & layer drift angle in the X-direction & 1/2909 & 1/3902 & 1/5581 & 1/2909 & 1/4286 & 1/6154 \\  & layer drift in the Y-direction & 3.81 & 2.34 & 1.85 & 2.98 & 1.76 & 0.68 \\  & layer drift angle in the Y-direction & 1/1260 & 1/2051 & 1/2595 & 1/1611 & 1/2727 & 1/7059 \\ \hline \multirow{4}{*}{Floor 1} & layer drift in the X-direction & 1.79 & 1.36 & 0.96 & 1.77 & 1.23 & 1.23 \\  & layer drift angle in the X-direction & 1/2682 & 1/3529 & 1/5000 & 1/2712 & 1/3902 & 1/3902 \\ \cline{1-1}  & layer drift in the Y-direction & 3.83 & 2.36 & 1.87 & 2.99 & 1.75 & 0.93 \\ \cline{1-1}  & layer drift angle in the Y-direction & 1/1253 & 1/2034 & 1/2567 & 1/1605 & 1/2743 & 1/5161 \\ \hline \hline \end{tabular}
\end{table}
Table 7: Layer drifts (mm) and layer drift angles (rad).  
Figure 19: Relative horizontal displacement of the side wall.  
the minimum value decreases sharply from 1/1611 to 1/7059. Therefore, the soil elastic modulus has a very important influence on the deformation of the underground substation.