Summary

The Tokyo Electric Power Co., Inc. is now in the process of excavating a large cavern--33.5 m wide, 51 m high, and 160 m long--to set a 1,050 MW pumped-storage power plant 400 m under the ground surface. The cross section of the cavern is horseshoe-shaped, the first time that such a shape has been used in Japan. It is planned to redistribute the stress resulting from excavation along the walls. To this end, the supports and lining of the walls are designed to make effective use of the resistance of the surrounding rock. The paper reports on the rock foundation tests, stability analyses using FEM, measurements Plan, and the supporting and lining work carried out for the cavern stability. Zusammenfassung Tokyo Electric Power Company beschaftigt sich augenblickich mit dem Ausschachten einer riesigen Kaverne von 33.5 m Breite, 51 m Höhe und 160 m Lange, die in 400 m Tiefe ein 1,050 MW Pump-speicherkraftwerk aufnehmen soll. Die Kaverne wird einen hufeisenförmigen Querschnitt haben und wird somit die erste dieser Form in Japan sein. Auf diese Weise beabsichtight man die durch die Ausschachtung au der Wandung Umlagerungs-Spannungen gleichmaßig zu verteilen. Die Ausbauwiederstand der neuen Wande werden so konstruiert, daß die Widerstandsfahigkeit des umgebenden Gesteins voll genutzt wird. Die Abhandlung befaßt sich weiterhin mit den Untersuchungen des Gruendungsgesteins, Festigkeitsnachweisen nach dem FFM-Verfahren, Meßprogramm, Ausbau und Auskleidung

Resume

Tokyo Electric Power Company est actuellement en train d''excaver une caverne importante (largeur 33.5 m × hauteur 51 m × longueur 160 m) qui abritera, à 400 m au-dessous de la Surface du sol, une centrale hydroelectrique du type à pompage ayant une puissance de 1,050 MW. sa section transversale en forme de fer à cheval est employee premierement au Japon. Les contraintes occasionnees par l''excavation est envisage de redistribuer dans les parois de la carerne et, à cette fin, les supports et le revêtement de paroi sont etes concu pour incorporer la resistance de la roche environnante. Ce rapport presente les essais sur la roche, l''analyse de la stabilite par FEM, le plan de mesurement et les travaux des supports et du revetement de paroi.

INTRODUCTION

The Tokyo Electric Power Company has been constructing a large-scale pumped-storage power plant with capacity of 1,050 MW since Oct. 1979, which is located in a northern mountain area in Tochigi Prefecture. civil Engineering works come to be a peak stage in 1981. The plant is of the underground type; since its main transformer room and the machine hall are set in the same cavern, the dimensions of the power station cavern -- 33.5 m in breadth, 51 m in height, and 160 m in length -- rank it among the largest in the world. (Figure 1) The horseshoe-shaped cross section of the cavern was adopted for cavern stability and rationality of construction and had never before been used in Japan. At the present time (Dec. 1981), excavation of the station cavern is roughly 75 percent complete.

DESIGN OF THE POWER STATION CAVERN AND THE SUPPORTING AND LINING
2.1 Topography and geology of the station Site

The underground power station is located 400 m below the surface of a ridge extending to the southeast at a slope of 400 from Gassan Mountain, a peak in the eastern reaches of the Nikko Mountain Range. Its horizontal distance from the reservoir is approximately 600 m. The geology of the station site is made up of siliceous sandstone, breccia and hornfels, to which sandstone and slate belonging to the Chichibu paleozoic strata were altered by the influence of intrusion of magma accompanying acid igneous activity during the end of the Mesozoic period or the Paleozoic period. The bedrock at the location of the plant is composed primarily of siliceous ''sandstone, breccia and sandstone-slate in alternating strata approximately 50 m thick of which the strike and dip run EW/50 N.

(Figure 2) Both components of the bedrock are hard and compact; their classification ranges for the most part between CHand B class.

(Figure in full paper)

Hornfles is distributed from nearby the lower portion of the penstock on the mountain side of the plant to the upstream. This bedrock is hard and compact, too. But there is groundwater in some places, and its pressures is about 5 - 10 kg/cm2. There are several small fractured zones roughly 10 to 50 cm in width, surrounding the plant, but there hardly exists a continuous fractured zone that intersects the plant itself.

Joints are found at intervals from 10 to. 40 cm; there are few in the breccia, while in the siliceous sandstone, sandstone- slate and hornfels they are relatively well developed. There is remarkable concentration, however: the strike and dip of the most predominant N 75 W/85 NE, and its percentage on the order of 6%.

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