## 4 On-Site Monitoring of Disaster Prevention and Response Measures for Water-Rich Loess Tunnels
### On-Site Monitoring  
To verify the effectiveness and rationality of the construction measures and accumulate regional engineering experience, on-site measurements were conducted on the mechanical behavior patterns of the surrounding rock and support, including the moisture content of the surrounding rock, convergence deformation of the surrounding rock, contact pressure between the surrounding rock and support, and the stress characteristics of the composite lining support structure during the construction process of the tunnel passing through the soft-plastic to hard-plastic loses strata. The left line of the tunnel entrance test section (ZK81+445, ZK81+460) was selected as the monitoring section, with a tunnel depth of 55-65 m. According to the purpose and content of the monitoring and measurement, the detailed layout of sensors at each measurement point on the monitoring section is shown in Figure 6, and the on-site installation images of the sensors are presented in Figure 7.  
In the monitoring section, monitoring sensors are arranged at intervals of 45\({}^{\circ}\) clockwise from the vault, with measurement point locations numbered as 1, 2, 3, 7, 8. The arch foot and the bottom of the inverted arch are numbered as 4, 5, 6. Multi-point displacement meters for deep surrounding rock are installed at the vault, right arch waist, and right-side wall,  
Figure 6: Schematic diagram of sensor layout types for each measuring point on the monitoring section.  
Figure 7: The on-site installation images of the sensors.  
numbered as SW1-3 (SW represents deep displacement, Measurement range: 0-100 mm; Sensor accuracy: \(\pm\)0.1%F.S). Soil moisture meters are installed across the full section of the surrounding rock, numbered as SF1-8 (SF represents moisture; Measurement range: 0-100%; Sensor accuracy: \(\pm\)0.1%F.S). Surface strain gauges are installed on the inner and outer sections of the primary support steel arch frame, numbered as CGJ1-8 (CGJ) represents the primary support arch frame, N for inside the arch frame, W for outside the arch frame; Measurement range: 0-3000\(\omega\)\(\omega\); Sensor accuracy: \(\pm\)0.25%F.S). Earth pressure cells are installed on the contact surfaces between the surrounding rock and the primary support, as well as between the primary support and the secondary support, numbered as WC1-8 (WC represents contact between surrounding rock and primary support; Measurement range: 0-300 kPa; Sensor accuracy: \(\pm\)0.25%F.S) and CE1-8 (CE represents contact between primary support and secondary lining). Additionally, a laser distance meter and a convergence ruler are used to monitor and measure the convergence deformation of the surrounding rock at measurement points 1, 2, 3, 7 and 8.  
Based on the construction characteristics of the \"micro-bench\" method for loess tunnels and similar engineering experience, the monitoring frequency is controlled by considering the convergence rate of the surrounding rock deformation and the distance between the monitoring section and the excavation face. The monitoring work commenced in March 2020 and has been continuously carried out to ensure the safety of the construction site and the tunnel during its operational period. The determined monitoring frequencies for the monitoring sections are shown in Table 5. The data acquisition for the remaining sensors is all conducted using a multi-channel remote automated monitoring and acquisition system, with the data acquisition frequency set at every 2 h.  
To reduce data errors, the average values of the two monitoring sections are used as the basis for analysis when analyzing the moisture content of the surrounding rock, internal displacement of the surrounding rock, and stress on the lining support. The convergence data from the ZK81+445 section are used as the basis for analyzing the convergence deformation around the tunnel.