The committee on Rock Mechanics, Japanese Society for Civil Engineers, has been discussing 'soft rock engineering in Japan' and presented several papers concerning dam, tunnel, bridge and slope constructions to International Symposium on Weak Rock, Tokyo 1981. This is the summarized paper based on References of Okamoto et al (1981), Iida et al (1981), Working Group on Tunneling-JSCE (1981), Working Group on Bridge Foundations-JSCE (1981) and Kikuchi et al (1981), which can convey general view on engineering experiences with weak rocks in Japan. From the point of view to the origin and/or distinctive physical properties, weak rocks in Japan may be classified into (1) sedimentary, (2) weathered, (3) low welded-pyroclastic and (4) fractured ones. Their typical characteristics are summarized in the following section. Concerning dam foundations on soft rock, several hundreds have already been constructed and, at present, nearly one hundred are under construction or at planning stage. These dams, almost without exception, have some problems such as site selection, material selection for fill-type and concrete dams, design of dam and its foundation, and construction procedures, requiring special ingeneous and cautious measures to cope with the local geotechnical difficult situations. In tunneling, for the decade of 1970s, the number of tunnels in soft rock either completed or under construction was at least 160, including Seikan Tunnel of 53.9 km long, Nakayama Tunnel of 14.8 km long and so on. Most of these tunnels are distributed in the northern half of Mainland, consisting 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. Accompanying the development of such major traffic networks as super-express railways, free ways, and trans-strait bridges, many long span bridges have been constructed on weak rock. It has become necessary to carry out surveys and tests on the strength and deformation characteristics of weak rock. In Japan a standard survey and testing method for weak rock as bridge foundation has not yet been established. In this paper, case studies, concerning five bridges built on weak rock such as Tertiary sedimentary rock, weathered granite, and pyroclastic rock are mentioned. As a special case, the elevated bridge of ShinKobe station, which has been built on an active fault, is included. Landslides and slope failures are frequently attributable to environmental conditions and additional human interferences to ground. Respective experiences in different fields may have some disagreements more or less due to various geological and mechanical conditions, situations, interaction between rock in-situ and man-made structure, highly developed techniques, and so on. The author tried to stand on the neutral position in this paper.
The geological structure of Japan composed of island arcs is complicated. Japan Islands covered an area of about 370,000 square kilometers had several times of orogenic movement in geological history. The amount of upheaval and subsidence of ground occurred since Quaternary period is estimated about ±1,500 m.