# Safety Analysis of Secondary Lining of Yulinzi Tunnel Based on Field Monitoring
## 6 Conclusions  
Based on the field investigation of the Yulinzi Tunnel Project and the monitoring results of the secondary lining deformation and internal force and numerical simulation of the tunnel, this paper discusses and analyzes the strain change law, long-term stress situation, and safety of the loess tunnel lining. The main conclusions are as follows:  
1. Based on field monitoring, strain change data of tunnel lining reinforcement over 9 months were obtained. The strain of the secondary lining reinforcement generally showed a development process of rapid increase in the early stage and slow growth in the later stage. However, the strain change curve in the arch waist and arch foot areas was more complex due to the influence of factors such as the bearing of the early structural self-weight, concrete shrinkage creep, formwork trolley load, and stress concentration;
2. Based on the basic assumptions in \"Structural Design Principles\", a unified calculation Equation for the axial force and bending moment of concrete under different eccentric compression states was established;
3. The axial force and bending moment of the lining section were asymmetrically distributed in space, and the axial force and bending moment of the lower structure of the secondary lining were much larger than those of the upper structure. The main reason is that the disturbance caused by the excavation of the left and right holes caused the inconsistency of the loose range of the surrounding rock on both sides of the section and the smaller lateral pressure coefficient of the surrounding rock leads to a large difference between the vertical load and horizontal load of the surrounding rock. The development of axial force and bending moment showed a trend of increasing with time, which was basically consistent with the change rule of lining reinforcement strain, and overall showed a \"fast first, slow later\" characteristic;
4. The final safety factors of the vault D1 and arch foot D7 were 2.49 and 2.37, respectively, close to the critical value of the safety factor, which were the dangerous areas in the secondary lining section. The development of the safety factor over time is in good agreement with the change rule of the exponential function. The correlation coefficients of the regression analysis results of the seven measuring points were all above 0.99, verifying the reliability of using the exponential function to predict the future safety factor of the tunnel. The predicted safety factor values of the two dangerous areas at the vault and arch foot were both greater than the critical value of the safety factor, which can reasonably predict that the Yulinzi Tunnel will still be in a safe state in the future.
5. The deformation and internal force changes of the secondary lining of the tunnel in the next 100 years were obtained through numerical simulation. In the initial stage of force, the secondary lining experienced significant sinking of the arch crown, rising of the arch springing, and extrusion deformation in the horizontal direction. Over time, the vertical direction of the structure showed an overall sinking trend, with a settlement of 51 mm over 100 years. The time-dependent characteristics of the stress of the structure and the time-dependent characteristics of the calculated safety factor measured in the field are \"first fast then slow, and finally tend to be stable\", which conforms to the change law of the Exponential function.  
Conceptualization, Y.W.; methodology, Y.W.; validation, L.H.; formal analysis, G.Z.; investigation, W.L. and L.H.; resources, W.L. and L.H.; data curation, G.Z.; writing--original draft preparation, G.Z.; writing--review and editing, W.L. and L.H.; visualization, L.H.; project administration, L.H.; funding acquisition, L.H. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Science and Technology Project of Hubei Provincial Department of Transportation (2020-186-2-6).  
Not applicable.