Construction on what is believed to be the world’s fourth longest tunnel through hard rock came to a standstill last month when a crater 20m deep, and 70m in diameter opened up above the 22.2km long Iiyama rail tunnel in Japan, injuring three workers.
Newspaper reports said that two cave-ins occurred on the morning of 11 September, which caused earth to flow into the tunnel. The three workers were buried by a third cave-in, as they were piling up sand bags to prevent further inflows.
Ground conditions on the Iiyama tunnel’s alignment are affected by squeezing ground pressure. Construction is being carried out by full-face excavation with a short bench to keep ground-loosening to a minimum. The tunnel also has a fairly large cross-section (estimated at about 150m2) to allow for primary support deformation, and the possible need for secondary support.
In a recent document outlining tunnelling activities in Japan, Kazutoshi Yuyama, senior assistant chief for the client, the Japan Railway Construction Public Corporation, said: “Because it is thought that a large load will not act on the secondary support, only a small allowance has been taken for deformation, and the support is designed as a thin ring shell consisting only of steel supports and shotcrete.”
He also said that geological investigations are predicting the need for multiple support, but “further economy is being sought by saving the provision of secondary support where the primary support does not reach the point of yielding,” he said.
Construction on the Iiyama tunnel started in 2000. It forms part of the Hokuriku Shinkansen railway project, which will connect Tokyo and Osaka via the Japan Sea coast – a distance of about 590km.
