# Seismic Response of Shield Tunnel with Double-Layer Lining
## 5 Influence of Calculation Parameters on Dynamic Response of Double-Layer Lining Shield Tunnel
### Tunnel Burial Depth
#### 5.2.2 Structural Axial Force  
The maximum axial forces of the segmental lining and secondary lining under different burial depths are illustrated in Figure 17.  
Figure 17 shows that as the tunnel burial depth increases, the axial force distribution pattern of both the segmental lining and secondary lining remains largely unchanged, with higher axial forces in the springing and spandrel regions.  
The maximum axial forces for each condition are summarized in Figure 18.  
Figure 17: Maximum axial force of tunnel structure (Different burial depths. Unit: kN).  
Both the segmental lining and secondary lining exhibit a trend where the maximum axial forces initially increase and then decrease with increasing burial depth, while the minimum axial forces show no significant change. For the segmental lining, the maximum axial force occurs in the right springing. When the tunnel burial depth increases from 0.5D to 2.0D, the axial force at the right arch foot increases from 627.99 kN to 1147.33 kN, an increase of 82.69%. However, as the tunnel burial depth increases from 2.0D to 5.0D, the axial force in the right springing decreases from 1147.33 kN to 837.54 kN, a reduction of 25.25%.  
For the secondary lining, when the tunnel burial depth increases from 0.5D to 2.0D, the maximum axial force increases from 371.99 kN to 540.17 kN, an increase of 45.2%. Conversely, when the tunnel burial depth increases from 2.0D to 5.0D, the maximum axial force decreases from 540.17 kN to 447.16 kN, a reduction of 17.2%.  
The burial depth of 2D can be considered as the critical depth between shallow and deep burial depth. When the tunnel is in a shallow burial depth state, the overlying stratum has a significant influence on the seismic internal forces of the tunnel, and the internal forces continue to increase with the increasing burial depth. When the tunnel is in a deep burial depth state, it is securely confined by the surrounding stratum, making it relatively safer, and the internal forces on the tunnel continue to decrease with the increasing burial depth. The segmental lining and secondary lining undergo uncoordinated deformation under different burial depths, resulting in differences in the axial force distribution between them. The variation of axial forces in the segmental lining is more significant with increasing burial depth.  
The ratios of the maximum axial forces between the segmental lining and secondary lining for each condition are summarized in Figure 19.  
Figure 19: The ratios of the maximum axial forces between the segmental and secondary lining.  
Figure 18: Variation curve of the maximum axial force under different burial depths.  
Figure 18 shows that as the tunnel burial depth gradually increases, the ratio of the maximum axial forces between the segmental lining and secondary lining exhibits a trend of initially increasing and then decreasing. When the burial depth increases from 0.5D to 2.0D, the ratios rapidly increase from 1.69 to 2.21. However, when the burial depth increases from 2.0D to 3.0D, the ratios rapidly decrease from 2.21 to 2.03, and it gradually decreases to 1.87 as the burial depth increases to 5.0D. It can be seen that in shallow burial depth conditions (<2D), with increasing burial depth, the axial force on the segmental lining gradually increases in comparison to the secondary lining, eventually reaching more than twice the axial force of the secondary lining. In deep burial depth conditions (>2D), as the burial depth increases, the axial force on the segmental lining decreases gradually in comparison to the secondary lining but always remains at least 1.5 times greater than the secondary lining, indicating that the axial force on the segmental lining consistently remains dominant.  
In summary, with the increase in tunnel burial depth, the uncoordinated deformation between the segmental lining and secondary lining leads to significant changes in the axial force distribution ratio.