# Safety Analysis of Secondary Lining of Yulinzi Tunnel Based on Field Monitoring
## 4 Analysis of Test Results  
### Change the Law of Secondary Lining Strain  
After the monitoring was completed, the monitoring results showed that the strain growth curves at different positions of the secondary lining structure were quite different. To highlight the internal force variation characteristics of the arch, arch shoulder, arch foot, and inverted arch of the tunnel, the monitoring data were sorted out and analyzed in three areas  
(1) In terms of tension and compression, the comprehensive analysis of Figures 9 and 10, and Table 2 shows that the outer side of the vault area was under compression and the inner side was under tension, and N1 had a maximum tensile strain of 488 \(\mu\epsilon\) during the monitoring period. The outer side of the arch shoulder area was always in a state of compression, and the inner side was in a state of tension at the initial stage of monitoring, and quickly changed to a state of compression as time passed. The outer side of the arched waist area was under compression at the initial stage of monitoring, and then gradually transformed into tension over time, while the inner side was always under compression. The tension and compression conditions of the left and right measuring points at the arch foot were different. Measuring point 7 at the arch foot was under compression both inside and outside at the initial stage of monitoring, and in the later stage of monitoring, measuring point 7 was under tension on the outside and compression on the inside. During the monitoring period, the maximum compressive strain of 657 \(\mu\epsilon\) was observed at N7, and point 8 was compressed both medially and laterally during the monitoring period. According to the data, the stress of the steel bar at the W8 position remained unchanged, and it was judged that the sensor was damaged. The reason for this may have been the stress concentration at the arch foot position. Its pressure value was too large; therefore, the sensor here was damaged. The cross-section of the arch shoulder area was under full-section compression, which can give full play to the compressive capacity of the concrete. Compared with other areas, this area is safer and played a role in load transmission in the overall secondary lining structure. During the monitoring of the position of the arch waist and arch toe of the tunnel, the final stress is in a state of compression on the inside and tension on the outside, and the value of the compressive strain on the inside was very large. The results showed that the lower structure of the tunnel was subjected to a large vertical load transmitted by the upper part, but because the lateral pressure of the surrounding rock on the tunnel structure was small, the tunnel structure was in a state of extrusion deformation at the side wall. Thus, the outer reinforcement shows tensile stress. (2) In terms of strain changes, the strain of the outer side of the vault W1 was always in a state of development with a gradually decreasing growth rate. In the first three months of monitoring, the strain value reached 84% of the final monitoring value. However, within the monitoring time range, the strain curve had not stabilized, indicating that the surrounding rock pressure at the vault position was still developing. The strain on the inner side of the vault, N1, exhibited three developmental stages over time: the early load-bearing stage