DAMS, RESERVOIRS & STORAGE
Neckartal Dam sets a new standard for RCC innovation The early stages of construction. The full supply level of the dam was fixed at RL 787.5 masl, with the mean foundation level in the spillway area at RL 718 masl. Some parts of the dam foundation needed to be lower, at RL 714 masl
Strategically positioned to support socio-economic growth, and specifically farming, Namibia’s Neckartal Dam is one of the most significant concrete structures built in the last decade within Southern Africa. Engineered by Knight Piésold Consulting, it ranks as the region’s eighth largest dam by storage volume.
N
eckartal Dam is located on the Fish River, a major tributary of the Orange River, with the primary objective of supplying bulk water to a planned 5 000 ha irrigation scheme located 40 km south-west of Keetmanshoop. This is a region much in need of water. Daytime temperatures often exceed 40°C during the summer, with a mean annual precipitation of less than 150 mm. Based on factors that include the local geology and remote location within Namibia’s Karas region, Neckartal was designed and constructed as a roller compacted concrete (RCC) dam. It measures 78.5 m in height,
22
IMIESA July 2022
with a crest length of 518 m and a gross storage capacity of 857 million m3, extending over a 40 km2 surface area at its full supply level. “The curved layout of the dam improves the structural stability, increases the length of the spillway, and enhances the dam aesthetics,” explains David Stables, principal project leader at Knight Piésold Consulting, and project manager for the dam’s development. Some 844 000 m3 of RCC and approximately 110 000 m3 of conventional concrete was used to build the dam. Three different types of RCC were used, namely a higher cementitious content of RCC to
create an impermeable upstream face, and two lower cementitious RCC mixes for the body of the dam. Construction started in September 2013 on behalf of the Namibian Ministry of Agriculture, Water and Forestry, which subsequently became the Ministry of Agriculture, Water and Land Reform in 2020. The contractor was Salini, which became Salini Impregilo in 2014 and, later, Webuild in 2020.
Material challenges One of the construction options presented in the past was a rockfill dam; however, there are no substantial clay sources available in the vicinity of the dam location. To construct a rockfill dam successfully would have required a reinforced concrete face, or an asphaltic core to provide a watertight structure, which was not practically achievable. The best approach was therefore a concrete gravity dam, more specifically an RCC structure. “Based on the engineered design, approximately 116 000 tonnes of cement were
