The geological composition of Japan's mountain regions consists of a basement of Paleozoic strata and Plutonic rocks, overlayed by Neogene Tertiary formations which include large quantities of ejecta from volcanoes active from the Tertiary to the Holocene periods. This basement is overlayed by distributed formations after the Tertiary, which compose the terrace and the hill regions by the rim of the plains. Volcanic pyroclastics are widely distributed on the bases of volcanoes.
Formerly, those hill regions which are composed of soft rocks have been detoured around, or passed through by cuttings. However, with the upgrading of routes and in order to avoid the densely populated areas of the plain rims, recent years have seen a large increase in the number of tunnels constructed in these areas.
The findings of the Working Group on Tunnelling in Soft Rocks (see Fig. 1) indicate that, for the 10 year period from 1971 to 1980, the number of tunnels in soft rock either completed or under construction in Japan was at least 160. This figure includes such long tunnels as the Seikan Tunnel (53.9km) or the Nakayama Tunnel (14.8km). Moreover, tunnels On the Shinkansen network account for more than half the total number.
Design and construction of tunnels in Soft rock meets with various problems and requires a high degree of investigative work. For instance, there is the effect of massive plastic earth pressure encountered in tunnels with large heights of overburden. There are also such matters to be dealt with as the liquification of rock under high water pressure, or the surface and internal displacement occuring in rocks with low heights of overburden.
The suitability of mechanical excavation for preliminary surveys under such conditions must also be established. The subject of this paper, then, is the current state of surveying, design and construction in soft rock, as well as such problems as are encountered in each of these undertakings.
Tunnels differ from other structures in that, because of their siting, geological surveys of a highly precise nature are difficult to carry out. For this reason, survey and test related to tunnels is carried out intermittently from the design stage to the construction stage, with a degree of accuracy which is intended to increase gradually. At the planning stage, for instance, the most favorable route is chosen and geological feasibility surveys are carried out in order to avoid later large-scale changes in the tunnel's construction costs or progress schedule. At the construction stage however, feasibility surverys and measurements of foreward estimates and geological properties are conducted so that design and construction will be undertaken most suitably with regard to the actual conditions. Table 1 is the summary of a tunnel's geological survey.
A special feature of surveying at the design stage is that consists principally of seismic prospecting of the region by the refractive method. Rock classification by seismic rate is also very common.