## 3 Installation Methods and Circuit Protection
### Installation Methods of Steel Arch Strain Gauge  
The installation method for the steel arch strain gauge involved the following steps:  
1. The welding fixture was utilized to weld the fixture base with the strain gauge model to the measuring point position, to ensure that the welding orientation could not be misaligned. Subsequently, the fixture was opened, and the strain gauge model was taken out. The upper part of the clamp featured a groove; the function of the groove was to provide a certain space for rotation of the strain gauge, in order to prevent the strain gauge from becoming stuck. The entire fixture was designed to be detachable; when other constructions on the site risked the safety of the sensor, the sensor could be disassembled and the fixture alone could be retained, as shown in Figure 10a. Due to the long welding time on site, the subsequent operations were delayed and the welding was carried out using the pre-welding method of the steel bar processing shed, as shown in Figure 10b. The implementation of this method effectively improved the work efficiency. Figure 9: Installation of fiber optic displacement meter. (**a**) Drilling; (**b**) displacement meter lowering; (**c**) orifice sealing; (**d**) drilling grouting; (**e**) testing; (**f**) connecting. 2. Indoor assembly and line protection was the next priority. First of all, the fiber grating strain gauge was put into the fixture, and adjustments were made using a leveler to ensure that the sensor and the fixture remained level. Fixation was then performed, and the fixation screw was further fixed with an M4 screw. Once the M4 screw was locked, four further screws were required to be locked simultaneously and evenly. Finally, the sensor was temporarily connected to the fiber grating demodulator to detect and record the wavelength information of the sensor in the free state. It was found that when the strain wavelength and the free state were relatively large (more than 200 p.m.), the M4 screw required loosening. Subsequently, it was re-uniformly locked until the strain wavelength change was less than 200 p.m. The installed strain gauge is shown in Figure 11.  
The strain gauge lead hose was connected in series, and the steel wire hose had sufficient flexibility to ensure that the bending radius of the lead was more than 5 cm. At the same time, the hose had a certain rigidity to ensure that the lead wire was not broken when it was pressed. A hole was drilled on the steel arch wing; the diameter of the hole was consistent with the steel wire hose hole, which was used for the steel wire connection between the steel arches, as shown in Figure 12.  
Figure 11: Installed strain gauge. Figure 10: Welding fixture: (**a**) Field welding fixture; (**b**) Indoor welding fixture. In view of the complexity of the geological conditions of the construction tunnel, mineral polyurethane foam was used for secondary plugging protection in order to further protect the lead. Mineral polyurethane foam has good waterproof performance and strength, which could effectively lessen the impact force of shotcrete, the seepage impact force of surrounding rock, and the expansion force of concrete solidification. In order to test whether the material had an effect on the data collected during the foaming process, relevant tests were performed, as shown in Figure 13. The tests showed that the accuracy of the strain gauge data remained consistent following the application of mineral polyurethane foam. The covering effect of the mineral polyurethane foam is shown in Figure 14.  
Because the steel arch was transported by a loader, in order to avoid the phenomenon of lead breakage during transportation, the lead wires wound at both ends of the steel arch were sealed, and the methods of foam filling and foam paper covering were adopted, as shown in Figure 15.  
Figure 14: Mineral polyurethane foam covering effect test. Figure 13: Mineral polyurethane foam and strain gauge compatibility. Figure 11.0 Line protection during transportation. 3.