Application and validation of profile–image method for measuring deformation of tunnel wall

The deformation of a tunnel is one of the major phenomena that indicate the safety of a tunnel under operation. Yet, the deformation of tunnel profiles so far cannot be effectively obtained, as confined by the long time required when using conventional geodetic surveying and the insufficient precision provided by other devices. The profile–image method, by which the profile of a tunnel can be determined by laser-lit profile in an image is tested in this study to validate its applicability. The methodology, including superimposing of images, and the results of validations are presented. The results indicate that the profile–image method has a precision of 4 mm for medium size tunnel. Meanwhile, the factors affecting the precision of measurements are highlighted, including distortion of lens, number of calibration points, types of lasers, and contrast of profile to the background. The influences of these factors are also presented.