## 2 Engineering Background and in Situ Monitoring Methods
### In Situ Monitoring Method of Surrounding Rock Pressure
#### 2.2.1 In Situ Monitoring Scheme  
The detailed scheme is as follows:  
1. Equipment selection. To ensure the effectiveness of the monitoring system, a comprehensive comparison of existing monitoring equipment and methods was conducted, taking into account their applicability, technical feasibility, and compatibility with the specific conditions of the tunnel environment. The selection criteria included factors such as accuracy, durability, and the ability to perform under challenging conditions. A summary of the comparative analysis is presented in Table 1. Considering the geological and environmental conditions of the surrounding rock, including high temperature and water-rich environments, the selected sensors must exhibit high temperature resistance and corrosion resistance, while meeting the demands of long-term monitoring and high accuracy. Therefore, a vibrating string pressure sensor was chosen for this purpose, as it satisfies these critical requirements. The sensor used in this paper is the JMZX-3001 type intelligent vibrating string pressure box of Changsha Jinma Measurement and Control Technology Co., Ltd. in Changsha City, China. The pressure accuracy is \(\pm\)0.1% FS and the temperature accuracy is \(\pm\)0.5 \({}^{\circ}\)C.
2. Sensor layout. The sensor layout design was guided by the following principles: 1. Given the potential blasting disturbances and unloading effects due to the excavation of adjacent tunnels, the entire section of the tunnel may be subjected to negative impacts. It has been shown that the pressure sensor should be evenly distributed in the tunnel section to ensure the accuracy of monitoring the key parts of each section [36]. When [PERSON] [37] carried out the study of large tunnel deformation, the layout scheme of the surrounding rock pressure monitoring points involved arranging eight sensors uniformly in the tunnel section. As a result, eight pressure sensors are evenly distributed along the tunnel section, with each monitoring section arranged in a single row (Figure 3).  
Figure 2: The location plane of the fault and tunnel.  
The positioning of the sensors was determined based on the geological conditions of the surrounding rock encountered during excavation. Pressure sensors are strategically placed at locations corresponding to the most unfavorable structural planes.  
The first set of eight pressure sensors in a single row is arranged within the first borehole, while the second and third sets of pressure sensors are installed sequentially in subsequent boreholes within the same section.  
The excavation sequence of the three tunnels is that the parallel pilot tunnel is used as the first tunnel excavation, the right line starts to follow after the excavation of more than 200 m, and the left line starts to advance after the excavation of more than 150 m. Due to the small clearance between the tunnels, the surrounding rock of each tunnel has different degrees of damage after the excavation of the left line, and the parallel pilot tunnel is the most serious. In order to facilitate the monitoring of the changes in the surrounding rock pressure when the face of the adjacent tunnel is close to and away from the section, the preliminary design is to install the first group of pressure sensors in the parallel pilot tunnel to determine the mileage of the monitoring section. When the two backward tunnels of the left and right lines follow up, the second group and the third group of pressure sensors are installed in turn at the monitoring section. The relative diagram of the installation process and position of the pressure sensor is shown in Figure 4.  
\begin{table}
\begin{tabular}{c c c c c} \hline
\begin{tabular}{c} High humidity \\ environment and \\ long-term stable \\ monitoring. \\ \end{tabular} & Slow dynamic response \\ \hline \end{tabular}
\end{table}
Table 1: Comparison of monitoring equipment and methods [32; 33; 34; 35].