Construction of the Seikan undersea tunnel—II. execution of the most difficult sections

Abstract This paper describes the three main problems associated with the difficult sections of the work which were encountered and for which solutions were found during construction of the Seikan Tunnel: (1) the excavation of squeezing ground; (2) water inflow accidents; and (3) the excavations of unconsolidated zones. Squeezing ground, which appeared in the F-10 fault with an earth pressure as high as 2 MPa, was excavated employing spring line drifts and the short bench method with steel pipe supports filled with reinforcing hoop-shaped steel bars and grouted with a high-strenth mortar. The most serious water inflow incident (maximum rate of 70 m 3 /min.) occurred in the service tunnel on the Hokkaido side in May 1976, causing inundation of 3 km of the service tunnel and 1.5 km of the main tunnel. By correctly judging the situation and making strenuous efforts over the next 5.4 months, the remedial work—which included water drainage—was safely completed. There were no fatalities or serious injuries resulting from the accident. The unconsolidated soft sand layer of the Kuromatsunai Formation, which existed in the middle of the Tsugaru Straits, was excavated without any water inflow accidents. This was accomplished with the aide of water-seal grouting and by carrying out construction procedures with care. Although different measures were required for each problem, the difficulties were overcome by careful investigation and construction management.