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 grouting 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 2m deep and in every 1m2 were 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 depo sits 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.
The Nishikawa landslide in Kôchi prefecture is one of representive landslides occured in the Mikabu green rocks. In order to investigata the mechanism of this landslide, geophysical investigations have been carried out. In this paper, the results of seismic prospecting are discussed. The relation between rock facies and P wave velosities measured is as follows: 1st layer: 0.3km/s, 5m thick, soil 2nd layer (1): 1.0-1.5km/s, 20m thick, soft, clastic sediment and talus deposit 2nd layer (2): 2.2km/s, 15m thick, soft clastic sediment and weathering part of basement rock 3rd layer: 2.7-3.2km/s, 50m thick, weathering part of basement rock (black schist of Sambagawa southern marginal belt (2.7km/s) and phillitic green rock of the Mikabu green rocks (3.2km/s)) 4th layer: 4.0km/s, basement rock The landslide in this area is closely connented with the weathering Mikabu green rocks, because the head of the landslide exists in the Mikabu green rocks and the sliding part occures in the clastic sediments consisting mainly of the weathering Mikabu green rocks. We must give attention to that it is quite within the bounds of possibillity that the 3rd layer flows plastically for a long time giving a great infuluence on the 1st and 2nd layers and as its result this landslide has occured.
This paper deals with experi mental method for estimating the rock pressure in pilot tunnels by seismic prospecting. By the seismic prospecting, the author measures the distribution of P Wave velocities in pilot tunnels and estimates the rock pressure from the degree of the decreace of P Wave Velocities in the rock and geological conditions. Still more, he estimates the rock pressure in tunnels by the experimental method of K Terzaghi.