The work and the technique: Legadadi Dam, Ethiopia

At the beginning of 1965, when the Town Hall of Addis Ababa began looking for new water supply sources, a feasibility study done by Salini Costruttori, later merged into the Webuild Group, identified the Legadadi basin as the construction site to be used to build the dam that would regulate the water flow of the Sendafà River, an affluent of the Legadadi. The search carried out by Salini Costruttori, differently from other studies, allowed establishing the economic feasibility, the estimated optimal regulation capacity of 40 million cubic metres of water, and the possibility of being able to distribute water to all the lower part of the city that made up 75% of users.

Works allowed building a dam system made of a main concrete dam and a secondary rockfill dam under it. This was a quite rare solution specifically studied to meet the extreme excavation depth, and the difficult geological conditions with ash, tuffs and argillified basalts. The main dam has a maximum height of 44 m, and a crest length of 412 m. The secondary dam, on the other hand, has a maximum height of 25 m and a crest length of 590 m.
Great attention was placed on the dam’s spillway, through specific hydraulic tests on lab models, to define a shape that could dissipate the high amount of dynamic energy of the water jet, therefore reducing the riverbed erosion.
The waters of the Legadadi basin are then made drinkable by a plant that carries out pre-chlorination, sedimentation, filtering and chlorination operations, and subsequently channelled towards the city through an aqueduct with a 30 km-long steel pipe.