Abstract
The Seikan Tunnel is a super-long undersea tunnel which passes under the Western Passage of the Tsugaru Strait, an area subject to rapid tidal currents and an extremely changeable climate which have presented difficult conditions for geological survey works on site. The tunnel has a total length of 53.850km. (with an undersea length of 23.300 km.)
Utilizing the best marine geological survey methods at our disposal our survey was conducted both before and during excavation of the tunnel. Great labor and toil were required for this task. In the tunnel itself, we have undertaken long horizontal exploratory borings to investigate the geologic conditions before proceeding with the excavation of each new section. After twenty years since the start of the excavation, the pilot tunnel was broken through and completed on January 27, 1983.
At present, the construction works are focusing upon the remaining section of 2 km of the main Tunnel. Consequently, the outline of the geologic structure and characteristics of the undersea portion has become clear as a result of the marine and underground investigation hitherto carried out. Here the authors have put together the geologic outline of the so-called “Green Tuff” area, particularly those geologic conditions which have posed the most difficulty for grouting and excavation work.
This report will be presented in four serials as follows:
1. The outline of geological survey and geology.
2. Stratigraphy and unconformity.
3. Fault and seepage water.
4. Pyroclastic dyke and slightly cemented sandy strata.
