As regards darns on soft rock foundations in Japan, several hundred have already been Constructed and, at present, nearly one hundred are under construction or at planning stage.
These darns, almost without exception, have some problems such as site selection, material selection for fill-type and Concrete darns, design of darns including foundations, and construction procedures" requiring special ingenios and cautious measures to cope with the local geotechnical difficult situations.
After outlining the geological features of soft rocks in Japan, this paper briefly describes the present status of dam construction on such foundations with special reference to relevant geotechnical problems.
Soft rocks, which are of low strength due to loose texture, low induration or poor cementation, are broadly grouped tentatively by the authors of this paper as follows:
Sedimentary rocks of the Neogene Tertiary
Sedimentary rocks in non-Green Tuff region
Sedimentary, rocks in Green Tuff region
Low welded volcanic rocks of the Quaternary
Weathered granite These groupings' are convenient to elucidate the problems arising from the dam construction on soft rock foundations.
Before entering into the details of individual geotechnical problems concerning soft rocks mentioned above, it would be informative to give an outline of the geological features of Japan formed during and after Neogene Tertiary with special reference to soft rocks.
Neogene deposits are distributed along seacoast almost all over Japan, especially of wide occurrence in the Northeast Japan as shown in Fig. 1.
(Figure in full paper)
The regions occupied by Neogene rocks can be broadly subdivided, as mentioned before, into two groups: non-Green Tuff and Green Tuff regions.
The latter region, which has geotechnical features somewhat different from the former, occupies Northeast Japan (including the Fossa Magna) the sea of Japan side in Southwest Japan and eastern Hokkaido as shown in Fig. 2.
In this region, submarine volcanic activity was violent and extensive from early to middle Miocene epoch, hence the formations up to the middle Miocene are mainly composed of volcanic rocks and pyroclastic rocks such as lava, tuff and tuff breccias.
(Figure in full paper)
As this region underwent subsidence from late Oligocene or early Miocene and remained under deep sea in the latter half of Miocene, upheaval to land taking place from the end of Pliocene to the beginning of Pleistocene, most of the formations are the products of submarine eruptions. Of which, lava is relatively hard, and tuff and tuff breccia of early Miocene are competent enough to support loads exerted by arch dams.
Arch dams so far constructed on such rock foundations are Muromaki, Susobana, Hoheikyo, Kawaji and Okumiomote dams.As the volcanic activity was somewhat calm and local in the latter half of the Miocene, the deposits in this age are mostly mudstone and sandstone with some intercalations of volcanic and pyroclastic rocks.
The seimentary rocks of this age are generally poorly indurated and, hence, low in strength.
