(Figure in full paper)
The Seikan Tunnel which links Honshu with Hokkaido has a total length of 53.85 km. Under the seabed, three tunnels - the main tunnel (which accommodates double-track Shinkansen), the pilot tunnel, and the service tunnel - are being excavated. The pilot tunnel precedes the other two tunnels to enable thorough study of geology to be made and to review the construction method, etc. The service tunnel is excavated in parallel with the main tunnel to study geology, process seepage, etc., and at the same time, to shorten the construction period by branching out a gallery for connection to increase the number of working faces in the main tunnel.
In January 1964, excavation of the pilot tunnel was started from the Hokkaido side. Seventeen years have passed since then, and as of April 1981, it still has 2600 m to go before it is completed.
The summary of geological features known to date are;
There is a wide distribution of strata called "green tuff" along the Seikan Tunnel route, Consisting chiefly of Neogene Miocene green tuff varieties. These strata contain mudstone sandwiching beds of tuff rocks developed by volcanic activities; occasionally, the thickness reaches scores of meters. The Fukuyama, Kun-nui, Yakumo and Kuromatsunai strata belong to the green tuff arranged in the historical order. There are many faults among these strata. In particular, faults F-10 and F-2 greatly influenced construction.
(Figure in full paper)
The tunnel construction near the F-10 fault zone was disturbed by a huge earth pressure and insufficient bearing strength of the ground. It took as many as four years and several months to complete after the excavation was started in February 1975.
The geology near the F-10 fault zone is chiefly featured by the Kun-nui stratum. The Fukuyama Stratum is partially mixed in along the F-10 fault. The rock in the Kun-nui stratum is chiefly composed of sand tuff, followed by coarse tuff and brown mudstone. The Fukuyama stratum consists of andesite-tuff and welded tuff. As the F-10 fault, transverse faults (fault diagonally crossing the tunnel) and Seeping Fault (Seeping Fault refers to the fault that resulted in the seeping accident) are located among them, this leads to the weakening of the ground.
When the earth pressure was measured in the upper half of 'the main tunnel near the F-10 fault, results shown in Fig. 6 were obtained. The earth pressure of 200 t/m2 obtained by the measurement is the record pressure measured to date. The increase rate of the ground pressure is very high;
A short bench engineering method that develops a circular cross-section using steel pipe supports filled with high- strength mortar was first tried in breaking through the F-10 fault. Because of many problems, the excavation had to be discontinued about 200 m this way of the F-10 fault and the engineering method had to be reviewed in full.