# Displacement Monitoring of Subway Tracks and Tunnels According to Adjacent Construction
## 2 Displacement Monitoring of Tunnel Structures  
### Field Measurement  
In this study, tunnel convergence meter sensors were installed 25 m before and after the train direction based on the center of the adjacent excavation work area. Subway tunnel structures may deform owing to adjacent excavation work. Safety is evaluated using the measurement results of a tunnel convergence meter sensor. Before the adjacent excavation work, a tunnel convergence meter sensor is installed in the subway tunnel structure to secure the initial data. Subsequently, the deformation of the tunnel structure owing to adjacent excavation work is measured, and safety is assessed through monitoring until the construction is completed. In this study, long-term monitoring was conducted for 18 months, with data collected at 60 min intervals. Figure 1 shows a full view of the tunnel convergence meter and tracked settlement sensors installed in the subway structure.  
In this study, tunnel convergence meters and track bed settlement sensors were installed in a subway tunnel and track structure, as shown in Figure 1. Table 1 lists the specifications of the installed sensors [14; 16].  
Figure 2 shows the installation locations of the adjacent excavation work area, tunnel convergence meter sensor, and track bed settlement sensor.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Category**} & \multicolumn{2}{c}{**Tunnel Convergence Meter**} & \multicolumn{2}{c}{**Track Bed Settlement**} \\  & \multicolumn{2}{c}{**(Tape Extensometer)**} & \multicolumn{2}{c}{**Monitoring Sensor**} \\ \cline{2-5}  & **Angle** & **Length** & **Angle** & **Length** \\ \hline Measurement range & \(\pm\)60 & 0–50 mm & \(\pm\)15\({}^{\circ}\) & 0–50 mm \\ \hline Applied voltage/total resistance & 5–15 V direct current & 10 k\(\Omega\) & 5–15 V DC & (5 k\(\Omega\)) \(\pm\) 20\% \\ \hline Linearity & 0.05\({}^{\circ}\) & \(\pm\)1\% & \(\pm\)0.1\({}^{\circ}\) & \(\pm\)1\% \\ \hline Sensitivity & 0.001\({}^{\circ}\) & 0.01\% maximum & 0.001\({}^{\circ}\) & 0.001 mm \\ \hline Operating temperature of sensors & \(-\)20\(\rightarrow\)+80 \({}^{\circ}\)C & \(-\)20\(\rightarrow\)+80 \({}^{\circ}\)C & \(-\)30\(\rightarrow\)+65 \({}^{\circ}\)C & \(-\)20\(\rightarrow\)+80 \({}^{\circ}\)C \\ \hline \hline \end{tabular}
\end{table}
Table 1: Sensor specifications.  
Figure 1: Sensor installation section and front view: **(a)** Front view of tunnel convergence meter (TL) sensor installation; **(b)** Front view of track bed settlement (RM) sensor installation.  
As shown in Figure 2, tunnel convergence meters and track bed settlement sensors were installed to monitor subway tunnels and track structures close to the adjacent excavation construction site. The tunnel convergence meter sensor was installed based on a total of three tunnel front sections, from Section A to Section C. A tunnel convergence meter sensor was installed on the tunnel front section to analyze the behavior of the tunnel structure in the vertical and horizontal directions. Sixty-eight track bed settlement sensors were installed in the direction of the train movement at approximately 140 m. To analyze the behavior of the track structure, a track bed settlement sensor was installed to monitor the behavior in the vertical direction along the train length. The 18-month monitoring covered up to Step 7 of the adjacent construction process, as shown in Table 2.  
Figure 2: Sensor installation plan: (**a**) Sensor installation front view; (**b**) Schematic of automated measurement sensor installation plan.  
The site under investigation was under construction, and the analysis was performed using the measurement results up to Step 7 of Table 2.