An Image Analyzing Method for Quantitative Evaluation of Crack Spacing in Tunnel Faces
Hayato TOBE#+, Yasuyuki MIYAJIMA, Daisuke FUKUSHIMA
Kajima Corporation, Japan

Appropriate construction of tunnel requires quantitative and immediate evaluation of properties of the face. The properties consist of several factors, such as strength of bedrock, weathering grade, direction of cracks, and spacing of cracks. Traditionally, the evaluation of these factors has been performed mainly by visual observation by civil engineers. The results of the evaluation have been often not quantitative. In recent years, advances in information technology have led to the development of techniques for quantitatively evaluating the strength of bedrock and the weathering grade rapidly. Quantitative measurement of the direction and the spacing of cracks has become possible by applying laser surveying and photogrammetry. However, these techniques are often insufficient for the purpose of immediately measuring the direction and spacing of cracks on site because it takes more than ten minutes for analysis. Therefore, we have developed a new method to measure the crack direction and spacing of bedrock quickly and quantitatively by image analysis. This method measures the spacing of cracks by the following four procedures: at first, cracks of the tunnel face are extracted from the photograph of the tunnel face based on the difference in brightness; secondly, the image of cracks is divided into small meshes; thirdly, the dominant direction of cracks is detected in each mesh; and finally, the cracks in the mesh are scanned in the direction orthogonal to the dominant direction and spacing of cracks are measured. We applied this method to an actual tunnel face and were able to measure the crack spacing of the face in approximately ten seconds.