## 5 Conclusions  
In this paper, a 3D refined numerical model is established to analyze and determine the optimal segment thickness for the shield tunnel with an inner diameter of 5.9 m. A full-scale test is carried out to study the bearing performance of the shield tunnel under excavation unloading conditions. The main conclusion are as follows:  
(1) Increasing the segment thickness to a certain degree can improve the structural stiffness and deformation resistance of the shield tunnel. When the thickness increases from 350 to 400 mm, the tunnel deformation decreases significantly. However, once the segment thickness continues to increase, the convergence deformation of the structure changes little. (2) Under the excavation unloading condition, the bending moment transfer effect between the middle and upper rings continues to increase. The internal force of the tunnel structure increases significantly at positions of \(0^{\circ}\), \(55^{\circ}\), \(125^{\circ}\), \(190^{\circ}\) and \(270^{\circ}\) as the surrounding load decreases. Compared to the design load stage, the maximum bending moment and axial force increase by 36% and 74.1%, respectively, in the final failure stage. (3) At the excavation unloading stage, the dislocations at the \(90^{\circ}\) and \(180^{\circ}\) sections exhibit a nonlinear increase, while the dislocations at the \(0^{\circ}\) and \(270^{\circ}\) sections still maintain a relatively small growth rate. (4) There was no bolt yield during the entire unloading process, indicating that the excessive strength of the bolt is not fully utilized in the entire life cycle of the large inner-diameter tunnel structure. (5) In the final failure stage, due to the loss of the load-bearing capacity of the tunnel and the effect of the collaborative constraints between the upper and lower rings, the internal force of the middle ring of the tunnel decreases slightly. (6) The failure mechanism of the shield tunnel can be described as follows: in the early stage of the external load, the shield tunnel will display joint openings and dislocations; as the load increases, cracks gradually appear near the tunnel joint in different directions; in the ultimate load stage, the shield tunnel will lose the load-bearing capacity, leading to falling blocks at the top and bottom of the tunnel. (7) The optimal thickness of the shield tunnel is closely related to the tunnel structure profile and the surrounding geological environment. It is necessary to consider the inner diameter of the shield tunnel, the strength of the tunnel segment, the types of soil layers around the tunnel, and the distance from the surrounding construction. (8) This paper discussed the bearing performance of the large inner diameter shield tunnels under symmetric unloading conditions. In practical situations, the external load applied to shield tunnels is not completely symmetrical due to the excavation of foundation pits or adjacent tunnel constructions. In the future, research can be focused on the bearing performance of the large inner-diameter shield tunnels under asymmetric unloading conditions. Figure 39: Photos of the cracks on the inter surface of the tunnel segment at different angles. **Author Contributions:** Conceptualization, X.Z. ; methodology, X.Z. ; software, C.L. ([PERSON]); validation, C.L. ([PERSON]); formal analysis, X.Z. ; investigation, C.L. ([PERSON]); resources, C.L. ([PERSON]); data curation, C.L. ([PERSON]); writing--original draft preparation, X.Z. ; writing--review and editing, C.L. ([PERSON]); visualization, C.L. ([PERSON]); supervision, X.Z. ; project administration, C.L. ([PERSON]); funding acquisition, X.Z. 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 number 52208443, China Postdoctoral Science Foundation grant number 2022M722958, Natural Science Foundation of Shanghai grant number 222R1466500, and Key R&D Program of Zhejiang grant number 2023C03182.