In a conventional tunneling under large overburden and accordingly high ground stress conditions, tunnels often suffer from large ground deformation especially when the ground strength and stiffness are insufficient. In Japan, stiff and heavy support systems with ring closure are often constructed as close to the excavation face as possible for overcoming such difficult conditions. One of the heaviest systems is, so called, “double-layer support system”. In Austria and Switzerland, on the other hand, “ductile support system”, which flexibly follows the large ground deformation, has been developed for 20 years and commonly used as a support system for squeezing ground. Instead of the double-layer system, the ductile method could be used and could be economical since it doesn't require the second support layers resulting in less construction materials and procedures. This study aims to investigate the applicability of the ductile support system and its cost advantage against the double-layer support system in squeezing ground conditions in Japan. In detail, given the poor ground conditions in two tunnels where the double-layer support systems have been actually applied, the support systems with both design concepts are studied by means of Convergence Confinement Method (CCM). As a result, it is found that the ductile support system with several yielding elements can ensure enough stability. In addition, the ductile support system could reduce the material cost by more than 30 % in those cases.
A conventional tunneling method, in which ground surrounding the tunnel is supported by shotcrete and rock bolts, is called NATM in Japan and in some Alps neighboring countries. However, the design and construction concept of support system seems to be quite different in squeezing ground conditions.
Japanese standard specifications for tunneling describes that stiffer support members than usual should be installed in such difficult conditions, and they should be closed with the invert as close to the face as possible (JSCE, 2007). These measures are recognized as the method preventing ground from excessive loosening of ground around the tunnel and thus restraining ground deformation. Due to early ring closure, the tunnel displacement could be constrained, but the support system might be overloaded leading to buckling of steel support and/or cracks of shotcrete lining. To ensure the tunnel safety in such unfavorable member failures, “double-layer support system” has been applied in recent tunneling projects in Japan where first layer with shotcrete and steel support is overwrapped by the second layer.