A number of experimental studies have been done regarding the mechanism of tunnel face failure. Most of the studies are based on the observation of failure mechanisms using experimental models of two- or three-dimensional planes, and the three-dimensional behavior of face failure is not well studied yet. The authors have investigated the failure zone behind a face quantitatively by using X-ray computed tomography (CT) to visualize face failure on a three-dimensional basis through a tunnel pull-out model test. As a result, failure zone development at the time of face failure could be described elaborately. The quantitative findings include that arching effect occurs when the overburden ratio is 2D (two time the outer diameter of the pull-cut core) or more and that the volume of the failure zone behind the face is independent of the overburden ratio.
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