Most of the 18 000 MW of hydro-electric power installed in Norway, come from plants located underground. The course of development from 1950 until today has been from steel-lined shafts in the supply system to unlined shafts and tunnels. This type of design entails significant economic advantages. The tendency has been to build constantly larger units, and power stations with installed capacity of 1 200 MW are now under construction. A total of about 150 km of tunnels are driven annually. Nearly half the cost of the power plants is accounted for by the cost of blasting alone. A description of the arrangement of machinery and equipment is given in the following, and methods of excavating underground power stations and supply systems are discussed. Pelton turbines with a capacity of 300 MW are being installed, and shafts up to 1 000 m in length are driven, using Alimak raise climbers. Necessary geological studies, when using unlined shafts subject to high water pressure, must concentrate on ensuring safety against hydraulic fracturing. High rock stresses occur frequently in connection with Norwegian hydro-electric power plant construction.

INTRODUCTION

99 per cent of all electric power in Norway is hydro-electric power. In 1950, power stations With total installed capacity of approximately 3 GW had been built. By the end of 1979, total installation had increased to approximately 18 GW, with a mean annual production capacity of 85 TWh, or about 20 000 kWh per capita. For the last 30 years, practically all installations have been located underground. 15 out of 18 GW (Fig. 1), are installed in 150 underground power stations, ranging from 10 to 640 MW each. Of the 5–6 GW presently under construction, 99 per cent will be located underground, including two stations of approximately 1 200 MW each. Norwegian power stations are situated in areas under different geological and topographical conditions. Most of them are high-pressure plants with total heads ranging from 200 to 1 000 m. The plants are generally located in solid rock, however, frequently intersected by a number of distinct weakness zones. Topographical extremes - especially in the western part of Norway - also give rise to stability problems, mainly due to high rock stresses.

TRENDS IN DEVELOPING A PROJECT''S POTENTIAL HEAD

It is evident from Fig. 1 that until 1950 most of the power stations in Norway were built in the open. The policy of locating stations underground, was adopted after the second world war. This policy was mainly based on economic considerations, however, increased security against war-like acts, and less maintenance also played a part. Subsequently, importance has also been attached to conservation of the environment. Before long the course of development was such that for most projects, clear economic advantages accrued from locating power stations underground. There are essentially three factors that have caused this outcome: The first is the expansion of the electric power transmission network, which has made it possible to transmit large quantities of electric power over great distances.

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