# 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
#### 5.2.1 Structural Bending Moment  
The maximum bending moments of the segmental lining and secondary lining at different burial depths are illustrated in Figures 14 and 15, respectively. Figures 14 and 15 show that the distribution characteristics of bending moments for segmental lining and secondary lining are generally consistent under different burial depths, with larger values concentrated in the conjugate 45\({}^{\circ}\) regions and relatively smaller values elsewhere. The maximum positive bending moments for both the segmental lining and secondary lining occur in the springing, while the maximum negative bending moments occur in the spandrel. The maximum bending moments under each burial depth are summarized in Figure 16.  
Figure 13: Variation curve of the maximum axial force under segment stiffness degradation. Figure 16 indicates that, as the burial depth increases, the bending moments initially increase and then decrease. For the segmental lining, the maximum positive bending moment is always greater than the maximum negative bending moment. When the tunnel burial depth increases from 0.5D to 2.0D, the maximum positive bending moment of the segmental lining increases from 112.52 kN\(\cdot\)m to 165.31 kN\(\cdot\)m, an increase of 46.9%, and the maximum negative bending moment increases from 97.67 kN\(\cdot\)m to 144.27 kN\(\cdot\)m, an increase of 47.7%, both showing a consistent rate of increase. By contrast, as the tunnel  
Figure 16: Variation curve of the maximum bending moments under different tunnel burial depths: (**a**) Segmental lining; (**b**) Secondary lining. Figure 14: Maximum bending moment of segment (Different burial depths. Unit: kN\(\cdot\)m). Figure 15: Maximum bending moment of secondary lining (Different burial depths. Unit: kN\(\cdot\)m). burial depth increases from 2D to 5D, the bending moments of the segmental lining gradually decrease. The maximum positive bending moment decreases from 165.31 kN\(\cdot\)m to 143.58 kN\(\cdot\)m, a decrease of 13.14%, and the maximum negative bending moment decreases from 144.27 kN\(\cdot\)m to 115.43 kN\(\cdot\)m, a decrease of 19.99%. For the secondary lining, the maximum positive bending moment also consistently exceeds the maximum negative bending moment. When the tunnel burial depth increases from 0.5D to 2.0D, the maximum positive bending moment increases from 14.87 kN\(\cdot\)m to 34.59 kN\(\cdot\)m, an increase of 132.5%, and the maximum negative bending moment increases from 13.18 kN\(\cdot\)m to 30.35 kN\(\cdot\)m, an increase of 130.3%. When the tunnel burial depth increases from 2D to 5D, the maximum positive bending moment gradually decreases from 34.59 kN\(\cdot\)m to 18.66 kN\(\cdot\)m, a decrease of 46.07%, and the maximum negative bending moment decreases from 30.35 kN\(\cdot\)m to 15.78 kN\(\cdot\)m, a decrease of 48%. As the tunnel burial depth increases, the ratio of the maximum bending moments between the segmental lining and secondary lining initially decreases and then increases. When the burial depths are 0.5D, 2D, and 5.0D, the ratios of the maximum positive bending moments are 7.56, 4.78, and 7.70, respectively, and the ratios of the maximum negative bending moment are 7.41, 4.75, and 7.31, respectively. Based on the above analysis, it is evident that the maximum bending moments of the segmental lining are consistently greater than those of the secondary lining, and this pattern remains unchanged regardless of the changes in the burial depth. The segmental lining continues to serve as the primary load-bearing structure in the shield tunnel with double-layer lining. Furthermore, with the gradual increase in burial depth, the structural bending moments initially increase and then decrease, with the burial depth of 2D as the critical point, while the bending moment distribution ratio between the segmental lining and secondary lining initially decreases and then increases. The burial depth of 2D can be considered as the critical depth between shallow and deep burial depth.