A 14.5 km long tunnel in bedrock constitutes an essential part of the heat transmission line by which district heat will be transmitted starting in 1982 from Naantali Power Plant to consumers in the region. This report examines the financial and technical factors which led to the choice of the tunnel alternative, as well as the tunnel design including investigations and studies related to it. The report examines also the environmental harm and risk factors caused by rock excavation and the use of tunnel in a population center. The concluding part of the report deals with the time schedule, mode of implementation and cost of project.
Imatran Voima Oy's Naantali Power Plant, 3 × 133 MW, constructed in 1958–1978 is situated on the border of a densily populated region comprising two coastal towns Naantali and Raisio and one coastal city Turku. There are 200 000 residents in this region. Until recent years the buildings have taken care of their own heating with their own boilers with the exception of small area heating systems. Steep escalation in the prices of oil - the fuel primarily used - has made the transfer to district heat and to concentrated production of heat more economical in large production units using cheaper fuel. After 1982 the major part of the district heat required by the Turku area will be produced with coal at Naantali Power Plant. Because of this the condensing units with a capacity of 133 MW will be altered to produce district heat. Owing to improved efficiency one unit will produce in addition to 90 MW of electric energy also about 170 MW of district heat. However, the construction of the heat transmission system with its stand-by and peak heat centres form the major part of the project as far as costs are concerned. In order to take care of the construction of the district heat line and heat transmission to consumers at a maximum distance of 25km, the municipalities in the region and Imatran Voima Oy have established a limited company by name of Turun Seudun Kauko Lampö Oy.
The terrain in the Turku region on the shore of the Baltic Sea comprises rocky hills and between them valleys filled with soft clays. It was found that the varying and difficult foundation conditions would increase the cost of an ordinary district heat channel to be sunken in the ground owing to great need for piling and supports in the excavation. Likewise placing large approx. 1.4 × 2.4 m DN 800 district heat culvert in the narrow street areas of the old city would have caused expensive alteration works in the existing municipal pipelines and cables as well as great difficulties in traffic arrangements. It would have been impossible to avoid lowering of the ground water level in sections where the ground water level is high. Even crossing of water courses, with railways and highways would have been cost-increasing details.