Appropriate construction of tunnel requires accurate determination of crack distribution, rock strength, and weathering grade. Tunnel face evaluations are commonly based on subjective visual inspections, the results of which are likely to vary from person to person. While quantitative methods based on laser measurements or photographic surveys to accurately determine crack distributions are available to eliminate these variations, these methods are time-consuming. To achieve the fast, simple, and consistent determination of crack distributions on tunnel faces, we developed a quantitative analytical method based on image analysis. The method divides the tunnel face into several areas. An image of each area is rotated, and the directions of the major cracks are read numerically. We found that applying this method at an actual tunnelling site identified the principal crack directions and determined crack intervals in about one minute.
Appropriate construction of tunnel requires accurate tunnel face evaluations. With conventional evaluation methods, a tunnel face is assigned scores for various parameters, including rock strength, weathering grade, and distribution of major cracks based on preestablished criteria. The results of this approach are inconsistent and likely to vary from person to person. To eliminate these variations, we devise a quantitative analytical method that does not depend solely on subjective visual inspections.
Rock strength and weathering grade can be assessed via simple quantitative analysis due to recent technical developments. Rock strength is determined quantitatively by an in situ test like the point load test. Weathering grade is determined quantitatively by a process that combines color analysis and X-ray diffraction analysis (Tobe et al., 2014).
Regarding crack distribution, the strike and dip of a given crack can be identified by laser measurements or photographic surveys (Ishihama et al., 2016). This method is highly accurate, but the associated measurement and analysis require considerable time. The measurements require suspending excavation work, with repercussions for construction timetables. With this approach, cracks are analyzed one by one to determine the principal direction, based on an image of a tunnel face form obtained from a survey. It takes several tens of minutes to several hours to complete this analysis. If the goal were simply to identify crack directions and intervals in a manner consistent from inspector to inspector, this approach would be excessive.
Based on our investigation of fast, simple ways to quantitatively determine crack distributions on tunnel faces that do not affect construction timetables and eliminate inconsistency of results from different individuals, we devised a quantitative method based on image analysis of photographs of tunnel faces.