Deformation monitoring of large tunnel wall under construction by digital photogrammetry measurement at Ritto Tunnel

The second Meishin (Tokyo-Kobe) highway that will be the most important to Japanese economy in the 21st century, is under construction. In this highway tunnels are the major structures which go through many mountainous areas. The standard cross-section of the tunnel in this highways is very large(250m )compared to ordinary tunnels and the tunnel width is very wide(18m). The tunnel is first bored by 5m diameter TBM and it is enlarged later. Photogrammetry measurement with a CCD camera is applied to deformation monitoring of large tunnel. Tunnels on construction sites like highways must be carefully and continuously observed for deformation during construction. Although geology is granite, it has many fractures under Tectonic process in Japan. In the tunnel, the fractured zone was detected widely by investigation by TBM pilot tunnel, we needed to check the stability of tunnel wall after enlarge section process. Conventionally proposed monitoring methods include total station. However, none of these are widely employed because of poor workability, insufficient precision and/or high running cost under tunnel work. For this reason, it decided to apply precision photogrammetry and the technique of investigating the tunnel deformation of the whole visually by three dimensions was developed. The reflective target of 121 or more sheets was stuck on tunnel wall, by taking a photograph, the 3-dimensional coordinates of a target were acquired and deformation of tunnel wall was visualized. In this paper, we show an example of the result. Figure 1. Location of Ritto Tunnel. This paper discusses monitoring of tunnel deformations by photgrammetry with a CCD camera. Reflective targets are placed over a tunnel wall, and their object coordinates are measured by a photogrammetric technique. Their displacements are then statistically detected every two epochs of time. Since it is usually hard to deploy long scales at construction sites, an observation equation has been developed to detect target displacements without the need for scales. At every time epoch, object coordinates of targets are independently adjusted by a freenetwork. Parameters for detecting displacements are tested, in which the inconformities of coordinate systems are incorporated in test equations. The strengths of the observation networks are compared by simulation and validated by experiments using a model space on a reduced scale of 1/20. In a typical case, 3mm target displacement over a 20m x 30m tunnel wall are proved to be detectable. 2 MEASUREMENT BY PHTOGRAMMETRY The procedure for digital photogrammetry is generally illustrated in Figure 3. Figure 3. Procedure of measurement In order to measure the displacements, we need to install the targets that can reflect the flashlight of camera as shown in Figure 4. The fundamental mathematical model of digital photogrammetry is an optical triangulation that describes the perspective transformation from two dimensional image coordinates into three dimensional object space coordinates. The ultimate extension of the principles is to adjust many photogrammetric measurements to ground control values in a single solution known as a bundle adjustment. The analytical process is so named because of the many lights rays that pass through each lens position constituting a bundle of rays. Any object point can be determined as the intersection of the corresponding rays from each of many images. Figure 4. Target that we install on the object All parameters describing the perspective transformation process can be also determined without prior knowledge of camera positions and calibration parameters. The computational model of the bundle adjustment is based on the well-known collinearity equations as shown in Figure 5.: ( ) ( ) ( ) ( ) ( ) ( ) 0 33 0 32 0 31 0 13 0 12 0 11 Z Z a Y Y a X X a Z Z a Y Y a X X a c ∆x x − + − + − − + − + − − =