## 4 Calculation Result
### Prototype Structure
#### 4.1.1 Influence of Weak Interlayer Thickness on the Seismic Response of Underground Structures  
It is assumed that the shear wave velocity of the weak interlayer is 125 m/s, and the thickness of the weak interlayer passing through the middle of the underground structure is set to 1 m, 2 m, 3 m, and 4 m, respectively. Four different soil thickness conditions and one condition without a weak interlayer are considered to study the influence of the weak interlayer thickness on the seismic response of the underground structure. Due to space limitations, only the displacement values of the side wall, the upper middle column, and the lower middle column for the homogeneous site and the weak interlayer with a thickness of 4 m under the Chi-chi earthquake are shown here, as presented in Figure 14. From the figure, it can be observed that when the weak interlayer passes through the middle of the underground structure, the maximum displacement of the side  
Figure 13: Monitoring points at the structure. wall of the underground structure is 10.2 mm, which is approximately 171.3% higher than that of the homogeneous site. The maximum displacement of the upper middle column is 4.11 mm, which is increased by approximately 158.5%. The maximum displacement of the lower middle column is 5.1 mm, which is increased by approximately 147.6%. This indicates that the weak interlayer has a negative amplification effect on the deformation of the underground structure. Figure 14: Displacement values of key structural members under the Chi-chi earthquake. (**a**) Displacement value of the side wall. (**b**) Displacement value of the upper middle column. (**c**) Displacement value of the lower middle column. The displacement values of the side walls, upper middle columns, and lower middle columns under various working conditions for three different ground motions are further extracted, as shown in Figure 15. From the figure, it can be observed that the displacement values under the weak interlayer site are greater than those under the homogeneous site. With the increase in the thickness of the weak interlayer, the displacement values of the side wall, the upper middle column, and the lower middle column gradually increase. In Figure 16, the maximum shear force and bending moment at the bottom of the middle column and the bottom of the lower middle column are extracted. From the calculation results, it can be seen that in the case of weak interlayer crossing, the internal force at the bottom of the middle column is greater than that of the homogeneous site, and with the increase in the thickness of the weak interlayer, the internal force at the bottom of the middle column also increases. Figure 15: Maximum displacement of structure in different cases. (**a**) Maximum displacement of the side wall. (**b**) Maximum displacement of the upper middle column. (**c**) Maximum displacement of the lower middle column. Figure 16: Internal force value at the bottom of the central column in different cases. (**a**) Maximum shear at the base of the upper middle column. (**b**) Maximum bending moment at the bottom of the upper middle column. (**c**) Maximum shear at the base of the lower middle column. (**d**) Maximum bending moment at the bottom of the lower middle column. 1.2 Influence of the Strength of a Weak Interlayer on the Seismic Response of the Underground Structure  
The thickness of the weak interlayer is 3 m, and the shear wave velocity of the weak interlayer is assumed to be 150 m/s, 125 m/s, 100 m/s, and 75 m/s, respectively. Four working conditions with varying shear wave velocities in different soil layers and one condition without a weak interlayer crossing are set up to study the influence of the weak degree of the weak interlayer on the seismic response of the underground structure. Figure 17 shows the maximum displacement of the side wall, upper middle column, and lower middle column for the homogeneous site and the weak interlayer with a shear wave velocity of 75 m/s under the Chi-chi earthquake.