Sabaishigawa Dam, a 37 meter high concrete gravity dam, is the first such dam to be built in Japan on soft rock whose compressive strength is less than 100 kg/cm2. It is now seven years since this dam was completed in 1973.

This paper will first of all discuss the mechanical properties of the dam's foundation rock, as obtained from both laboratory and in-situ tests, and will then briefly describe the design and consturction work.

In conclusion, the necessary precautions to be taken in constructing a concrete dam on such soft rock will be explained in reference to the behavior of the dam since filling.


Many dams are necessary in a country like Japan, whose restricted and mountainous land gives rise to short, fast-flowing rivers that flow like waterfalls with the consequent need for flood control and the development of water resources. In such conditions, the danger of dams being overtopped is so great that the most suitable type is a concrete gravity dam. However, in order to build this kind of dam safely and economically, certain specific geological requirements must be fulfilled. Soft rock is one of the most difficult Soft to build a dam on.

Soft rock formed after the Neogene period typically lacks strength,is highly deformable and is in addition extremely sensitive to variation in the moisture content. Because of all these causes, it has been considered very difficult to build a dam on Such rock. Fill-type dams, however, are not always economical as an alternative in such cases, in addition to the danger of overtopping, it is often difficult to find materials for the embankment around the site, and the same problem as with concrete dams occurs in construction of the outlet works on soft rock.

However, the great efforts made in studying how to build a concrete gravity dam on such rock were finally crowned with success with the completion of Sabaishigawa Dam in 1973. This paper deals with the construction of this dam and the necessary considerations in constructing a 'concrete gravity dam on soft and weak rock.

The foundation strata of this dam belong to the so-called Uonuma Group formed from the end of the Pliocene to the beginning of the Pleisticene Quartenary, and consist of rather recent sandstone, conglomerate, and mudstone. The sandstone and conglomerate are almost unconsolidated, and the mudstone is only consolidated to a limited extent. These rocks have a tendency to turn to mud and collapse when subjected to repeated drying and wetting, but not to the extent seen in regions of Green Tuff. Beneath the Uonuma Group lie sedimentary rocks' of the Neogene Tertiary period which are deposited regularly, but the rock quality changes gradually and divisions between the various strata are not clearly marked. The strata incline downstream at an angle of about 20° around the dam-site due to folding, and for this reason the strata get older in the upstream direction.

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