Support structure for tunneling in squeezing ground and secondary lining design

摘要

The Iiyama tunnel is a large-scale tunnel about 22 km long on the Hokuriku Shinkansen Line at the boundary between Nagano and Niigata prefectures. Since the Nabetate-yama tunnel on the Hokuhoku Line, where large squeezing earth pressures were induced, is located near this project, similar geological conditions were expected to exist at the Iiyama tunneling site. Squeezing ground was encountered as predicted from the initial stage of excavation. We dealt with this difficulty by increasing the support rigidity and closing the section earlier. It was impossible however to completely control tunnel deformation, making it impossible to ensure the specified section in some places. As a result, multiple support was adopted to continue excavation in the squeezing ground. This paper discusses the support design for replacement of supports and the concept of secondary lining.