Nant de Drance: 1700 tonnes of concrete immersed in Lac d'Emosson

Construction work on the Nant de Drance pumped storage power station is progressing well. That goes for the water catchment, too: On 22 September 2011 the first of two 1700-tonne intake/discharge structures was installed – by a procedure never before used in Switzerland.

Masterful technical feat for pumped storage power station

The intake/discharge unit is designed to guarantee optimal flow and keep the water catchment free from stones and gravel. Like everything else associated with the Nant de Drance project, the intake/discharge units of the water catchment boast impressive dimensions: 22.5 meters wide, 10 meters high, 22 meters long and around 1,700 tonnes in weight. Time and temperature factors were the main reasons behind the engineers' decision to build the gigantic concrete component by the shore; because the entrance to the pressure tunnel on the lake bed is only accessible between April and May, when the reservoir drops to its lowest annual level. This short window of time at temperatures still in the low winter range did not permit on-site installation.

120 meters down to the lake bed within 24 hours The intake/discharge structure was therefore built on a platform on the south-western shore of the reservoir, following which the construction was filled with air and dragged around 930 meters over Lac d'Emosson by means of an enormous pontoon. From there the gigantic component was lowered centimetre by centimetre until, around 24 hours later, it reached its destination 120 meters below the surface of the lake. This technical feat is based on the principle of the diving bell, and is a first in Switzerland. The procedure has never before been used to install such a construction under water.