This paper presents the outline of weak rocks in Japan. There are many kinds of weak rocks. However, from the point of view of the origin and/or distinctive physical properties, they may be classified into (1) sedimentary, (2) weathered, (3) low welded-pyroclastic and (4) fractured Rocks.

In this paper the distribution and geological background of (1), (2) and (3) which have the mechanical properties as "soft rock" and their typical engineering properties are briefly described in section 1, 2 and 3 respectively.

The geological structure of Japan composed of island arcs is complicated. Japan Islands covered an area of about 370 × 103 square killometers 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. At present, the geological phenomena (earthquake, volcanism etc.) are active and many active faults are found in this land.

In order to clear the geological environment of Japan, the following figures are illustrated at the end of the paper; the geological structure of Japan (Fig. Appendix 1), the distribution of pre-Neogene sediments and regional metamorophic rocks (Fig. Appendix 2) and the distribution of Quaternary sediments (Fig. Appendix 3).

Sedimentary rocks of Neogene Tertiary and geological structures

The distribution of Neogene Tertiary sedimentary rocks in Japan is shown in Fig. 1Al. Sedimentary rocks of the Neogene Tertiary period are largely divided into green tuffs and non-green tuffs. Areas where the strata contain green tuffs are called the 'green tuff regions'. Green tuff was effused from underwater volcanoes during the Miocene period and is one of the most outstanding rocks typically found in northeast Japan where recent orogeny, thought to be still progressing, is the most active. The green tuff regions cover over half of Japan's surface area and the thickness can reach up to several thousand metres. Non-green tuffs are common in the Miocene- Pliocene sedimentary rocks lying on top of the green tuffs and in the sedimentary rocks on the Pacific side of Japan.

The sedimentary rocks of Neogene Tertiary have relatively simple geological structures, but in the Uetsu fold zone in north-east Japan the anticlines are still upheaving and synclines subsiding, in other words' it is an active fold zone. The rate of rise and fall is about Imm/year. The geological cross-sections for this area are shown in Fig. 1.2 and 1.3. The former shows the geological structure of an entire sedimentary basin and the latter shows that of a tunnel. The extent of the folds and faults should be appreciated.

Characteristics of Neogene Tertiary Sedimentary Rocks

Green tuffs have various physical properties ranging from hard gravels to clays with high swelling properties when immersed in water. Sometimes altered clay, that is solfatalic clay, has been produced by hydrothermal alteration. Solfatalic clay contains large amounts of natrium montmorillonite; and distinctly expands with water. This clay frequently found in Qualernary volcanic areas too. On the other hand, non-green tuffs are mainly sandstones and mudstones,

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