Lessons learnt - Raising an earthfill dam : From the initial design to the dam-raising works - Ganguise Dam (France)

Economic reasons or evolving use of water resources may call for an earthfill storage dam to be built in two separate stages. In such cases, the idea is to design the dam structure so that can be raised during a second phase of construction with the least possible impact on its operation. A look at the Ganguise Dam (a zoned earth fill dam in the South of France) reveals the initial design that paved the way for the two phases of construction, the design of the dam raising works 20 years later, and the unanticipated issues encountered during the implementation phase. The first phase included certain technical choices to allow the future raising of the dam without emptying the reservoir: appropriate core and drainage device inclines. After operating the dam for an initial phase of 20 years, the dam-raising design studies were carried out; its height was to be increased by 6 meters to provide 20 Mm3 additional storage. It thus became obvious from the analysis of the initial dam s behaviour that it was necessary to adjust the raising works design. Considering the lessons learnt when the dam was initially built, it was therefore necessary to modify the downstream structures as well as to impose certain limits during the construction works: a maximum speed for building up the earthfill and special monitoring to check dam safety.