# Safety Analysis of Secondary Lining of Yulinzi Tunnel Based on Field Monitoring
## 4 Analysis of Test Results
### Change the Law of Secondary Lining Strain
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Position**} & \multirow{2}{*}{**Measure Point**} & **Tension and Compression** & **Tension and Compression** & \multirow{2}{*}{**Final Monitoring Value (\(\mu\epsilon\))**} \\  & & **Situation (Initial Results)** & & **Situation (Final Results)** \\ \hline \multirow{2}{*}{Vault} & W1 & Compression & Compression & 301 \\  & N1 & Tension & Tension & 488 \\ \hline \multirow{4}{*}{Arch Shoulder} & W4 & Compression & Compression & 63 \\  & W11 & Compression & Compression & 68 \\  & N4 & Compression & Compression & 288 \\  & N11 & Compression & Compression & 201 \\ \hline \multirow{4}{*}{Arch Waist} & W5 & Compression & Tension & 70 \\  & W10 & Compression & Tension & 140 \\ \cline{1-1}  & N5 & Tension & Compression & 599 \\ \cline{1-1}  & N10 & Tension & Compression & 353 \\ \hline \multirow{4}{*}{Arch Foot} & W7 & Compression & Tension & 57 \\ \cline{1-1}  & W8 & Tension & Compression & 15 \\ \cline{1-1}  & N7 & Compression & Compression & 657 \\ \cline{1-1}  & N8 & Compression & Compression & 459 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Measuring point strain situation and final monitoring value. (1 d-50 d), the rapid development stage (50 d-90 d), and the slow development stage (90 d-). In the early load-bearing stage, the strain value of N1 is very small, and the growth rate changes little over time. As time progresses, the strain curve undergoes a sudden change, entering the rapid development stage. As can be seen in Figure 5, the strain value at the point of sudden change when N1 enters the rapid growth stage is approximately 50 \(\mu\epsilon\). In the later stages of monitoring, the strain curve enters the slow development stage, with the strain value gradually increasing over time, but the rate of increase is very small. According to the obvious three-stage development of the steel bar strain curve in the tension area of the vault position, it can be known that the concrete and steel bars in the secondary lining tension area in the early bearing stage are under tension together, and the tensile strain of the steel bar is approximately equal to the tensile strain of the concrete at this time, the strain growth at this stage is relatively slow; as the external load gradually increases, the tensile strain also gradually increases. When the tensile strain value of the steel bar exceeds the ultimate tensile strain of the concrete, the concrete will crack, and the concrete in the tension zone will no longer be Under tensile stress, the sudden change point of the curve is the ultimate tensile strain of concrete. After the strain enters the rapid development stage, the stress in the tension area is borne by the steel bars, and the height of concrete cracks becomes larger and larger; after entering the slow growth stage, the surrounding rock pressure on the vault position will increase slowly, and the strain of the steel bars in the tension area will increase It also gradually decreases, and the cracking of concrete will not develop further. The change in the strain value of the outer side of the arch shoulder W4 and W11 was characterized by rapid growth in the early load-bearing stage. The rapid growth stage lasted for about a month, and then it tended to stabilize. The final strain value of the outer rebar remained around 65 \(\mu\epsilon\), which was very small compared to the stress values of N4 and N11. The rate of strain increases on the inner side of the arch shoulder and gradually decreased with the passage of time, and the strain could reach 75% of the maximum value in the first 90 days of monitoring. The strain value tended to stabilize in the later stage of monitoring. According to the strain values of the inner and outer steel bars, it can be seen that the stress state of the arch shoulder area section was in full section compression, which could fully utilize the compressive capacity of concrete.