# Seismic Response of Shield Tunnel with Double-Layer Lining
## 6 Conclusions  
Under the influence of strong earthquakes, shield tunnels are at risk of structural damage. The use of a double-layer lining structure helps to improve their seismic performance. In this research, using the Shiziyang Shield Tunnel as a case study, the numerical simulation method is employed to investigate the seismic response characteristics of a shield tunnel with overlapped double-layer lining. The following conclusions are drawn:  
1. The tunnel structure and surrounding stratum exhibit an increasing trend in acceleration amplification factors from the invert to the crown. Due to the intense interaction between the surrounding stratum and segmental lining, their acceleration amplification factors at the corresponding characteristic positions are nearly identical.
2. The stresses in the segmental lining are greater than those in the secondary lining at all locations, with peak stresses occurring in the right springing. The stress in the segmental lining at this location is 2.61 MPa, while it is 0.87 MPa in the secondary lining, indicating an approximately threefold difference. This suggests that the segment remains the primary load-bearing structure under seismic loads.
3. As the stiffness of the segmental lining decreases, its bending moment gradually decreases, while the bending moment in the secondary lining remains almost unchanged. The bending moment in the segmental lining consistently exceeds that in the secondary lining. When the segmental lining stiffness decreases from 100% to 60%, the maximum positive bending moment ratios decrease from 4.77 to 2.86, a reduction of 40.04%, indicating a significant change in load distribution under seismic loads. Additionally, the axial force in the segmental lining does not change significantly, but the axial force in the secondary lining decreases markedly.
4. With increasing burial depth, the bending moment of the tunnel structure initially increases and then decreases. When the burial depths are 0.5D, 2D, and 5.0D, the ratios of the maximum positive bending moment between the segmental lining and secondary lining initially decrease and then increase, which are 7.56, 4.78, and 7.70, respectively. The segmental lining remains the primary load-bearing structure in the shield tunnel with double-layer lining.
5. Increasing the segmental lining stiffness in shield tunnels is beneficial for enhancing seismic performance. Additionally, seismic internal forces in the shield tunnel reach their maximum value at a burial depth of 2D. Therefore, while ensuring structural safety, efforts should be made to maximize the burial depth of the tunnel (>2D).  
**Author Contributions:** Conceptualization, X.G. and Q.C.; software, X.G.; writing--original draft preparation, X.G.; writing--review and editing, Q.C.; project administration, X.G.; funding acquisition, X.G. and Q.C. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was funded by the National Natural Science Foundation of China (Grant numbers 52308400 and 52378342) and the Funds of Scientific and Technological Plan of Fujian Province (Grant number 2022Y4015).  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The raw data supporting the conclusions of this article will be made available by the correspondence author on request.  
**Acknowledgments:** We thank [PERSON] (China Construction Yipin Investment and Development Co., Ltd.) and [PERSON] (Xiamen Metro Construction Co., Ltd., Xiamen, China) for their assistance in engineering background and design.  
**Conflicts of Interest:** The authors declare no conflicts of interest.