長野ダム (ロックフィル) の地質と施工
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概要
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The foundation of Nagano Dam which consists of tuffaceous sandstone, phyllitic slate, conglomerate, shale, siliceous sandstone and hornblende porphyrite is very complicated. 78faults were found in the damsite and the vicinity of the powerhouse. The boundaries of the rocks are mostly fault contact. The organic surface soil other than sand, gravel and talus deposit was excavated out for the foundation of the rock─fill zone. Where the foundation rocks for the impervious zone were thought impossible to get watertight by the grouting were also excavated. The foundation of the impervious zone is phyllitic slate on the right bank, while is conglomerate on the river bed or the left bank. In general, the coglomerate was excavated more than the phyllitic slate. There are numerous faults transversely the foundation of the impervious zone, so the permeability test or the grouting test for the said faults were carried out prior to the construction. According to the tests the faults were known to have the watertightness by the performance of grouting, consequently all faults were treated by the grouting other than the concrete replacement. The curtain grouting was arranged in single line on the higher portions of the both banks, while double lines on the river bed portion was done. In order to strengthen the main curtain the blanket grouitng was carried out at the up─and down─stream side of the main curtain where the geologic condition is poor. The depth of the grouting was divided into 15m, 20to 30m and 50to 60m respectively. It was operated firstly from the deeper hole, and then orderly the shallow one based on the geologic informations obtained from the former hole. The groutings with about 2 m deep and in every 1 m^2were carried out on the higher portions of both banks where the geology was poor, especially at the shallow underground portion which the former grouting was not well effected. The average injected amount of cement in the conglomerate was more than in the phyllitic slate. The materials of the impervious zone are the weathering products derived from the talus deposits of sandstone, slate, green phyllite and limestone. For the natural water content was higher than the optimum water content, a drainage trench was digged along the highest portion of the borrow area in order to lessen the water content. The materials of the rock─fill zone are composed of Mesozoic tuff─breccia and sandstone. The amount of dynamite to quarry by the bench cut method was more than by the tunnel blasting method. After the completion of the dam, the leakage and the deformation are so insignificant that it is expected to be very stable.
- 一般社団法人日本応用地質学会の論文