ABSTRACT
A tunnel collapse occurred in a deep fractured rock. The cover depth of the tunnel was over 70m, and its height and width were 7.7m and 11.6m, respectively. The length of tunnel collapse was 14m and total volume of rock fragments flowed into the tunnel was about 600m3. To identify the failure mechanism, detailed investigation including additional rock drilling, theoretical review and numerical back analysis were performed. Vertical and horizontal drilling revealed that the rock was severely fractured with faults, and the area of rock fall developed to the distance of 14.4m from the tunnel crown. It was revealed that the blast excavation disturbed the rock mass and triggered raveling of rock fragments, and finally caused mass rock fall. Measures reinforcing the ground around the tunnel and further excavation method and procedure were proposed.
