The Pannecière Dam, with its multiple arches and buttresses, is located on the Yonne river in the heart of the Morvan National Park. At 49 metres high and over 350 metres long, the structure, which came into service in the early 50s, required several types of repair as well as strengthening against the risk of earthquakes.
The dam was suffering from:
-a loss of monolithism due to water circulating through a network of cracks resulting from post-war construction methods and concrete formulation;
-slight swelling due to alkali-aggregate reaction in the sealing mortar (gunite) on the upstream side;
-undesirable rotation and collapsing movements in the buttresses on the downstream side.
To rectify these various problems and set up the rehabilitation site, the 80 million cubic metres of water in Panneciere Lake had to be drained away.
Significant amounts of work carried out on both sides of the dam
Freyssinet's work was sequenced by location:
-On the central arches, counter-arches a metre thick, six metres high and with a volume of 150 m3 each were constructed to improve the distribution of loads and prevent rotation at the base of the arch;
-On the skew arches, located at the ends of the dam, filler blocks with a volume of over 1,000 m3 each were cast in order to bring the foundations of each arch level with each other and thus create a flat base for the counter-arches, which were built in the same way as for the central arches.Long de 350 mètres et d'une capacité de 80 millions de mètres cubes, le barrage de Pannecière nécessitait un renforcement pour être conforme aux nouvelles réglementations sismiques. Freyssinet a réalisé de nombreux travaux, en s'appuyant sur ses solutions de réparation Foreva®. Son intervention a porté notamment sur la stabilisation et le renforcement de l'ensemble des voûtes, la dépollution des parements amiantés et la réparation des bétons de la structure. En outre, Freyssinet a renforcé la voûte abritant une usine EDF.
In addition, on the downstream side, the structure's earthquake resistance was improved by the installation of braces, prefabricated on site, between each buttress.
26 braces, each14 metreslong and weighing 20 tonnes, were installed in two rows inside the dam's arches, with the aim of both stabilising and stiffening the structure in the event of an earthquake by preventing the buttresses from oscillating.
-Arch E, through which the hydroelectric plant passes, could not be strengthened on the downstream side, so a different solution was found. Using climbing formwork, a reinforced concrete reinforcement was cast over 1/3 of the upstream wall of the dam;
-In addition, Freyssinet subsidiary MTS removed the asbestos from the upstream facing of the dam by sanding the paint coating with suction at source, thus avoiding the need for a containment system to be installed on the wall (13,700 m2).
-After the asbestos had been removed, 2,800 m2 of the original gunite, Freyssinet repaired the 13 arches with shotcrete.
The teams had to deal with some very violent weather conditions between November and February (site under water, temperatures dropping below -10°C...). Despite the difficult climatic conditions, Freyssinet's work on the Pannecière Dam has come to an end and the dam was refilled at the end of December 2012.