The Geological Outline of the Undersea Portion of the Seikan Tunnel

Abstract

There are many faults in area of Kunnui Formation, but few in Kuromatsunai Formation.
It seems to the reason of the geological age and of which Kunnui Formation is mainly underlain by solid character's rock (tuff, lapilli tuff, and tuff breccia) and Kuromatsunai Formation is fluid quality (mudstone).
The biggest fault is F. 10 fault which appeared in about 33km. The thrust length seems to about five hundred meters, because Fukuyama Formation is in fault contact with Kunnui Formation (Kn 4).
About one kilometer of Kunnui Formation (tuff and mudstone, Kn 2-Kn 4) near the fault is squeezing ground, and the maximum earth pressure amount to 2.0MPa.
Excavating of main tunnel carried out by the circular short bench (with spring line drift) method after elavolate grouting.
On the other hand, during the construction there were four accidents of water inrushing in the area of fault zone.
Every restoration work took about five to twelve months.
In the fault zone, as advance boring came to deadlock, investigation for proving was carried out by short drilling (coring and noncoring) from working face and side well.
To prevent water inrushing, new technologies of measuring must be developed to grasp the process of seepaging and loosening of the ground. This is very important task for maintenance of tunnel.
Meanwhile, chemical quality of seepage water is almost near that of sea water and is alteration sea-water type by exchange of ion, and partially is hot-spring type.