Kassa dam, a rock fill dam with a central impervious soil core, measuring 90m in height,487m in crest length and 4,480,000 m3 in volume, is the upper reservoir dam in the Okukiyotsu pumping-up power station with generating output of 1,000 MW in Niigata prefecture, the mainland of Japan. The Owner of the dam is Electric Power Development Co.,Ltd.

The foundation ground of the left bank of this dam are not only consisted of sound rocks but also of unconsolidated volcanic mud flow with very cracky lava. Therefore a part of the base of the impervious soil core directly rests on volcanic mud flow. In accordance with this exceptional foundation, we had two serious problems in design and construction which were the foundation treatment at volcanic deposits by grouting as well as securing bearing (Figure in full paper) Figure 1. Plan of dam leakage measuring weir capacity of mud flow of foundation. The plan and cross section are indicated Fig. 1and Fig. 2,respectively. The dam is located in the midst of a region in northern Japan which is covered with deep snow in winter. Accordingly, its construction was forced to be discontinued entirely during the winter season every year. With all this unfavorable condition, the dam construction started in 1972 and the reservoir filling was undertaken in June 1977.

When the reservoir level reached at approximately two-thirds of the maximum depth of the reservoir, an unusual Leakage from the dam foundation began to increase in proportion to the reservoir level. It was decided immediately to execute additional grouting to the grout curtain. However, it was difficult to detect accurately the defective part of grout curtain from the ground water level and ground water pressure by means of the equipments previously prepared. Because of the wide range grout curtain, keeping the reservoir level at about low water level of the reservoir, stepwise investigations were carried out with exploration holes from the grouting gallery and spillway tunnel. And finally the leakage path and the defective part in the grout curtain were found out.

This paper presents a instructive

(Figure in full paper) experience of the foundation treatment by grouting on a dam of which the geological conditions are complicated.


General geology of the damsite is shown in Fig. 3. The dacite which is the bedrock of this damsite, is overlain on the left bank by thick volcanic products, such as mud flow, andesite lava, etc. The volcanic mud flow is composed of volcanic gravels and boulders, sand, and tufferceous silt, and the degree of compactness of this deposits is not uniform in places due to the difference of depositing conditions and grain size composition. The permeability coefficients of the volcanic mud flow were generally 10−3-10−4cm/sec in natural condition. The andesite lava with plenty of cracks and joints caused mainly of cooling of the lava were exposed as lf an accumulation of flat rock blocks at the excavated surface of the dam foundation, and its permeability was extremely high in its original state.

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