# Real-Time Tunnel Deformation Monitoring Technology Based on Laser and Machine Vision
## 4 Test Results and Analysis
### Field Test  
To verify the stability and accuracy of the monitoring system, the system was taken to the Xingyuan Town Tunnel in Heilongjiang province for a field test. As shown in Figure 24, the tunneling environment was adverse and filled with dust, darkness and sudden vibration during construction. Tunnel trolley, as shown in Figure 24, ran between initial support and secondary lining; this may block the laser transmitting path, so that the laser transmitter module was fixed on the earth close to the side face; therefore, the proposed monitoring system had no conflict with construction as shown in Figure 24.  
As shown in Figure 24,d, the dust is heavy, this may decrease the laser spot energy received by the image acquisition module, so that the laser transmitter module used 500 mw high power lasers with an optical lens to achieve better long-distance transmission effect. In addition, the distance from the laser transmitter module to the measured points was varying from 30 m to 80 m, and the laser spot energy decreased with increasing distance, so that a laser energy transmission test with difference receiving distances was conducted in the dusty tunnel to detect the image quality acquired by image acquisition module. The test data are shown as Table 6  
Figure 23: Test sketch map. 1. reference monitoring image acquisition module A; 2. reference monitoring image acquisition module B; 3. image acquisition module I; 4. image acquisition module II. (\(L_{1}=40\) m, \(L_{2}=46\) m, \(L_{3}=35\) m, \(L_{4}=38\) m). \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Devices** & \multicolumn{2}{c}{**Reference Monitoring Image Acquisition Modules**} & \multicolumn{2}{c}{**Image Acquisition Modules**} \\ \hline Number & 1 & 2 & 3 & 4 \\ Initial laser spot coordinates & (92.02, 48.13) & (69.27, 66.21) & (80.42, 60.30) & (73.89, 58.62) \\ Final laser spot coordinates & (97.18, 48.15) & (73.36, 66.18) & (62.22, 60.37) & (55.12, 58.70) \\ Horizontal displacement before compensation & 5.16 & 4.09 & 18.20 & 18.71 \\ Horizontal displacement after compensation & — & — & 0.09 & 0.14 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Laser transmitter module feasibility test data (unit: mm). According to Table 6, laser spot energy decreased with increasing receiving distance, but the laser spot could still be obtained by image acquisition module, and the gray value of the laser spot met measurement requirements; this indicates that the proposed system is not susceptible to dusty and dark environment. The laser spot image obtained in 70 m receiving distance is shown in Figure 25a, and the high power laser with optical lens is shown in Figure 25b. In order to verify the stability and accuracy of the monitoring system, a field test was conducted in the tunneling environment. The image acquisition module was fixed on the edge of initial support of the tunnel, as shown in Figure 26a. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Receiving** & **Gray Value of** & **Receiving** & **Gray Value of** & **Receiving** & **Gray Value of** \\
**Distance (m)** & **Laser Spot** & **Distance (m)** & **Laser Spot** & **Distance (m)** & **Laser Spot** \\ \hline
20 & 255 & 45 & 246 & 70 & 185 \\
25 & 255 & 50 & 233 & 75 & 171 \\
30 & 255 & 55 & 221 & 80 & 169 \\
35 & 255 & 60 & 210 & 85 & 155 \\
40 & 255 & 65 & 196 & 90 & 139 \\ \hline \end{tabular}
\end{table} 
Table 6: Laser energy transmission test data. Figure 24: Tunnel environment during construction. (**a**) Tunnel construction trolley; (**b**) dusty and dark tunneling environment; (**c**) deformation monitoring during construction; (**d**) tunnel face environment. Figure 25: (**a**) Laser spot image in 70 m receiving distance; (**b**) high power laser with optical lens.