The work and the technique: Kárahnjúkar Hydroelectric Plant, Iceland
Works on the Kárahnjúkar Hydropower Plant were a titanic, yet ambitious engineering challenge: building a hydropower plant powered by Europe's largest glacier, also channelling two glacial rivers into a underground tunnel to produce power. The project involved the construction of a rockfill dam, a network of tunnels and a hydroelectric plant inside a mountain 1 km deep.
The dam is about 730 meters long at the crest and with a volume of 8.5 million cubic metres. Once the base was built, the reinforcing grid was placed and concrete was poured through a special cementing machine, which was designed and built just for this project, and capable of climbing up the dam's decidedly steep wall.
The main headrace tunnel is the centrepiece of the entire project, as it moves water 40 km from its collection point to where it is transformed into electricity.
Icelandic plateau's feature lava rock, among the world’s hardest. To excavate the tunnel, for the first time in Icelandic history, 3 TBMs working concomitantly were used, 24/7 for 2 years. Since at those latitudes the GPS instruments could not be used, topographers were fundamental to control and make sure that the right trajectory was kept for each of the 3 TBMs, with 2 daily surveys for the entire duration of excavations.
A jumbo was used, a highly precise and fast hydraulic monitoring computerised drill, where the TBM could not go. Its 3 multi-directional arms, are all capable of creating holes in the rock up to 5 metres, for underground explosion purposes.
Crumbling operations to reduce rocks in smaller parts were very important. The conveyor belt, one of the longest in the world, managed to constantly free the excavation areas from the rocks.
The power station was built at 40 km, within a small mountain downstream from the dam's location. It is a strategic location: water from the Jokulsa a Dal River glacier in the Halslòn reservoir is transported through the tunnel to the top of the mountain. It is then dropped into a vertical penstock, transforming the water’s falling into electricity.
Works on the plant were completed in 5 years despite harsh environmental conditions and some problems caused by the rock's instability that required manual rock removals to free the TBMs or the running application of a protective rubber membrane as an additional precaution for the dam's lining.