Electric Power Development Company (EPDC) has been proceeding a pumping up project at the south border of the north east district of main land of Japan (Fig-1). Ohuchi dam is the upper reservoir dam which is located at the Onogawa creak, a tributary of Agano river. And Ohkawa dam which is built at the main stream will serve as a multipurpose dam including the role of lower reservoir.

(Figure in full paper)

Ohuchi dam is a rockfill dam with center impervious core of 102 m in height, 340 m in crest length, the upstream slope of which is 1 to 3.0, that of downstream 1 to 2.5 respectively. The lower portion of both slope will be protected by additional embankment for the weighting zone (Fig-2). The project area on the whole is at so called the green tuff zone where the miocenic sedimentary rocks and tuff mainly distribute. The site of Ohuchi dam belongs to one of the tuff distributed zone. The tuff at the dam-site bed is mainly coarse grained, contains green lens shaped patches, and partially contains isolated sandstones and siltstones.

Over the bedrock of tuff along the stream and lower portion of the left bank, non cemented quartarnary deposits difined as mud flow lay themselves thickly and widely. They were believed to come from the mud flow accompanied by the collapse at the upstream area of the stream and left bank small valley. The deposits are considered to be multilayered.

The maximum thickness of the deposit near the river bed reaches to almost 30 m. It contains rough flakes of tuff and clayey, silty materials of the same origins (Fig-3).

(Figure in full paper)


It is clear that the bedrock of the tuff is fully enough or satisfactory to the foundation of a rockfill dam even it is weathered or partially affected by the fault. However, the suitability of the overlaying deposits for the foundation is not clear, but expected to be possible because its huge thickness which will cause big amount of excavation if it may necessarily be cut away.

EPDC has already experienced the same problems through the construction works of Numappara dam and Kassa dam, and proved that some of soft or non cemented deposits could be available for rockfill dam foundations.

As is the same with those cases, the purposes of the investigations in this project can be considered as follows.

  1. To know the bearing, resistance, consolidation properties and strength characteristics to verify the suitability for the dam foundation

  2. To obtain the data to determine design criteria

  3. To establish a suitable method of quality control test at the field works

  4. As a special study, to obtain the necessary data for the dynamic analysis on overall structures of dam and foundation.

The test items were so various that some of them were rockmechanical tests and others were soil mechanical tests because the deposits are highly moistured, widely grained soily materials.

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