# Safety Analysis of Secondary Lining of Yulinzi Tunnel Based on Field Monitoring
## 3 Data Monitoring
### Monitoring Equipment  
#### 3.2.1 Smart String-Type Steel Stress Meter  
The sensor used in this measurement was the smart string-type steel stress meter JMZX-416 AT, which is mainly used for measuring the steel stress inside reinforced concrete structures. The measuring range of this stress meter was \(\pm\)200 MPa, with an accuracy of 1% FS. (1) Accuracy Calibration Test of Steel Bar Stress Meter  
As a method for monitoring the stress of steel bars in tunnel lining, the steel bar meter needs to use standard calibration instruments to calibrate its measurement accuracy to ensure the accuracy of the stress monitoring results of secondary lining steel bars. The accuracy calibration test device mainly consisted of two parts, which were the steel bar meter loading device and the data acquisition device. The loading device of the steel bar meter had passed the accuracy testing standard. In this experiment, the tensile test of the steel bar stress meter was carried out by the loading device of the steel bar meter. Figure 3: Lateral arrangement of sensors. Figure 2: Longitudinal arrangement of sensors. The data acquisition device adopted JMZX-3001L comprehensive tester, and the overall installation effect diagram is shown in Figure 4.  
The steel bar stress meter number adopted the number of the measuring point arrangement, and the steel bar stress meter to be tested was fixed on the loading device through the bolts at both ends. First, the stress meter needed to be preloaded before the test starts, and the preload load was 50 MPa. After preloading, it was necessary to test the frequency when the load is 0 of the strain meter, and then load it in five levels within the range of 200 MPa, namely, 0 MPa, 50 MPa, 100 MPa, 150 MPa, and 200 MPa. Two loading tests for each steel bar stress meter were performed, increasing the load step by step, and the data was read after each level of load was applied for 15 s. The comprehensive tester could obtain the frequency value of the steel string under different loads of the steel strain meter, and the stress value could be converted through the frequency. The relationship between the frequency of the strain meter and the stress is shown in Equation (1):  
\[P=KK_{0}\Big{(}f_{i}^{2}-f_{0}^{2}\Big{)} \tag{1}\]  
where \(P\) is the pressure; \(K\) is the calibration factor; \(K_{0}\) takes the value 0.00071186; \(f_{i}\) is the measurement frequency; \(f_{0}\) is the frequency when the load is 0. The stress value data obtained by converting the actual frequency conversion of the inner and outer steel stress meters of the seven representative measuring points are summarized in Table 1. From Table 1, it can be seen that within the measuring range, the maximum error of the steel stress meter does not exceed 0.7 MPa, which meets the requirement of 1% FS Accuracy requirements, therefore the instrument is reliable in measuring steel stress. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Sensor Number** & **Test Number** & **0 (MPa)** & **50 (MPa)** & **100 (MPa)** & **150 (MPa)** & **200 (MPa)** & **K** \\ \hline N1 & 1 & 0.0000 & 49.9234 & 100.0677 & 149.9911 & 200.1354 & \\  & 2 & 0.0000 & 49.7025 & 100.2886 & 150.2120 & 199.9145 & \\ W1 & 1 & 0.0000 & 50.0346 & 100.0692 & 150.1038 & 200.1384 & \\  & 2 & 0.0000 & 50.0346 & 100.2858 & 150.1038 & 199.9218 & \\  & 1 & 0.0000 & 49.9968 & 99.9936 & 149.9904 & 199.9872 & \\ N4 & 2 & 0.0000 & 49.9968 & 100.3964 & 149.9904 & 199.7640 & 0.2232 \\  & 1 & 0.0000 & 50.0000 & 100.0000 & 150.0000 & 200.0000 & 0.20000 \\ W4 & 2 & 0.0000 & 50.0000 & 100.2000 & 150.0000 & 199.8000 & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Sensor measured data and error statistics. Figure 4: Layout of test equipment. (2) Installation of measuring equipment  
As shown in Figure 5, the installation for this measurement involved welding the two ends of the steel stress gauge onto the main reinforcement steel bar being measured, and data transmission was performed through the tail transmission line.