In Norway a number of underground openings have been excavated for storing of drinking water. Such rock cavern tanks are safe, invisible and easy to extend and maintain. With reasonable geological conditions they are cheaper than conventional concrete and steel tanks, when the required volume extends approx. 8.000 m3. A close cooperation between the consulting engineer and the engineering geologist is necessary for a successful result. The author's experience from such cooperation are outlined and typical examples of rock cavern tanks are described.

INTRODUCTION

During the last 10 - 15 years a number of underground openings have been excavated in the hard bedrock of Norway either as replacement for or as alternative to open reservoirs or concrete or steel tanks for storage of drinking water. A closed water tank, as is the rock cavern tank, has several advantages compared with the traditional open reservoirs. It is above all easier to keep undesired pollutions under control with a closed tank. In open reservoirs the drinking water is exposed to the influence of sunlight and pollutions from the air. Open reservoirs are also commonly situated in natural or artificial depressions and will therefore have a draining effect on the surrounding landscape. Especially if such reservoirs are placed close to populated areas, there is a danger that polluted surface water or groundwater may be drained into the drinking water. Today the health authorities in Norway will normally not accept open reservoirs for storage of drinking water. New drinking water systems will have to include closed tanks of some kind. Old schemes with open reservoirs will often have to be redesigned and reconstructed.

FUNCTIONS AND LOCATION OF WATER TANKS

The basic function of a water tank is to act as a storage buffer to cover the variations in the consumption and keep the water head stable. This makes it easier to operate the treatment plant and the pumps at constant capacities. It allows smaller dimensions of the main pipe lines and gives stable pressures. In addition the water tanks will act as emergency storage in case of fire or failure in the supply system. Small water tanks are normally single chamber tanks. If the total volume exceeds approx. 10.000 m3, the tanks are often made as double or even multiple chamber tanks. This allows one chamber to be emptied for cleaning and maintenance without interrupting of the water supply. A water tank should be situated at an elevation which gives a suitable water pressure in the consumption area. It is also preferable to locate the tank as close to the consumption area as possible. This is especially important if the capacity of the tank is designed to cover the variations in the daily consumption. Most water tanks in Norway have been freestanding structures made of conventional reinforced concrete or prestressed concrete. When double chambers have been necessary, either two separate structures have been made or two concentric chambers in one structure. To minimize the impact such concrete structures may have on the environment, they have been dug often to some extent into the ground or tried hidden away in other ways.

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