Real-Time Detection of Sudden Displacement of Tunnel Face Surface by High-Speed Laser Scanner

In mountain tunnel excavation work, numerous accidents resulting from rock mass falls from a part of the face surface have been reported. It is known that the face surface suddenly displaces by a few millimeters or a few centimeters just before collapse. Detecting such small displacements in real-time enables urgent evacuation of construction workers to their safety. However, it is very difficult for workers to find these displacements visually. In this paper, we propose a new approach for the real-time detection of sudden small displacements using highspeed 3D laser scanner. In this work, we attempted the use of the Velodyne HDL-32e scanner for the detection of displacements. Displacements can be simply detected by comparing time-series scans. To compute displacements accurately, a depth image is created, which is called base frame, by the cylindrical expansion of sequential scans. At each pixel, the averaged distance to the target is computed. Then for each consecutive frame, the differences of the distance to the base frame are calculated. Compared to previous works using multiple laser displacement sensors or terrestrial laser scanners, our approach has several advantages such as the scanning area is large enough for capturing the entire face surface, and the installation of physical targets on the surface is not required. In addition, the scanning frequency of HDL-32e is high up to 20 Hz, and is thus suitable for real-time detection. It is reported that HDL-32e contains time-varying nature, i.e., the range measurement results for static objects vary in the long scanning periods. Specific calibration techniques have been proposed for this problem. However, these techniques require large planar or cylindrical objects in the measured scene. Since such objects do not exist at tunnel construction sites, and carrying such objects to the site is not practical, we skipped the calibration process and carried out various experiments. First, to evaluate the repeatability of the scanner, we scanned a flat wall in the scene multiple times and evaluated the variations of the points. As a result, variation was small and the average of the standard deviations of distances computed at each pixel was 4.3mm. The face surface mainly consisted of rock and clay. To imitate the real face surface, we prepared equipment which can be slid within 1mm accuracy by screw and mounted rocks and clay on it. We scanned this target before and after sliding from 20m distance and compared the difference. The averaged error of the detection accuracy of sliding was -1.96mm, and its standard deviation was 2.79mm when scanned from the front. Finally, we scanned the static target from the fixed position in the long periods (15 hours), and evaluated the difference of the measurement distances. Although the distances vary in the early stage of the scanning (within 1 hour), the static condition of the target was accurately captured by our approach, i.e., the differences of the consecutive frames were small enough. These results show some potentials of the proposed approach for use in real tunnel construction site.