Application and validation of simple image-mosaic technology for interpreting cracks on tunnel lining

As a simple, portable method that uses inexpensive tools for structural safety inspections of tunnels, this study presents a novel image-mosaic technology that generates layouts of tunnel wall surfaces. Geometric distortion in camera images of 3D tunnel surfaces is rectified through equations related to projection characteristics and the bilinear interpolation method. Successive taken images with at least four reference points in each one are then resampled to a 2D coordinate for the layout of tunnel surfaces. Test results for a laboratory-scale tunnel demonstrate that the spatial accuracy of the proposed image-mosaic technology for a mosaicked layout from vertical images is approximately 1.6° with sufficient image resolution for detecting lining cracks with widths exceeding 0.45 mm. Field application to a full-size, operational tunnel indicates that the layout of tunnel wall surfaces by the proposed technology is sufficient for identifying lining crack distributions and detecting temporal variations in cracks.