Abstract
Although the stability of the working-face is one of the most important issues during the excavation of a tunnel, the mechanism of working-face collapses and their causes have not been sufficiently discussed. This situation may lead to improper countermeasures against working-face collapse. For example, in projects such as tunnel excavation with only a thin overburden, improper measures may greatly affect the surface structures, resulting in significant societal loss. This paper presents a case-study of a large box excavation and traction project right under a railway, examines the collapse mechanism of the working face, and discusses the role of face bolts as one of the countermeasure works. It was conclusively confirmed that a large-scale collapse is highly likely to happen at the working face, resulting from an equivalent strain of geo-materials induced by tensile stress during excavation, and that the installation of face bolts would limit tensile stress and effectively prevent a large-scale collapse of the working face.
