Chapter 12 Undersea tunnel-effect of drainage and grouting

The Seikan Tunnel, the world's longest undersea tunnel, had to be constructed under the condition of severely high groundwater pressure, namely as high as 2.4 MPa, because it passes 240 m below sea level in the deepest portion. According to prior extensive geological surveying and the construction of two inclined shafts, it became evident that the undersea tunnel could be excavated by making proper use of the cut-off method by grouting. As of early 1970, however, just after the inclined shaft of the main island section was completed, some theoretical problems related to the undersea tunnel construction remained unsolved. These problems consisted of the effect of making mechanical improvements to the ground by grouting (including the cut-off efficiency by grouting), the optimum extent of the grouting, and the effect of drainage. This study addresses the important subjects related to drainage and grouting during tunneling. In addition, theoretical studies on the effect of drainage by drain holes and the optimum extent of grouting for tunneling under the prevailing condition of high water pressure are presented in some detail. In other words, by applying a Mohr-Coulomb type of elasto-plastic model, the mechanical interaction between the surrounding ground and the water pressure is theoretically investigated in order to clarify the effect of drainage and to determine the optimum extent of the grouted zone in undersea tunnel construction projects. This study was carried out since the first author participated in the JSCE (Japanese Society of Civil Engineers) Geotechnical Committee on the Seikan Tunnel from 1971 to 1985.