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.

Neckartal 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, 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

required for the dam’s construction; however, the key logistical challenge was supplying this material,” says Stables.

That’s because the dam site is situated approximately 1 000 km from the nearest cement factory, located at Otavi in northern Namibia, and 1 200 km from the closest fly ash sources in South Africa’s highveld region.

With high transport costs and large quantities of cement involved, an innovative approach to the RCC mix designs was needed. “A key objective was to reduce the cementitious content and in turn the overall project cost. This was achieved using the latest developments in RCC technology and factoring in the arid temperature conditions,” says Stables.

From an aggregates perspective, an intensive geotechnical investigation identified a suitable source of dolerite approximately 10 km from the dam site, which further supported the preference for an RCC gravity dam.

The dam wall curves along a 500 m radius. The river section is very wide, and the left bank is much steeper than the right. The substantial spillway requirement was provided over the RCC structure by using an ogee spillway and a stepped downstream spillway with an apron for energy dissipation

The dam outlets release water, via an intricate design of pipework, through two Francis turbines, to an abstraction weir and pumping station located 13 km downstream to provide water for future irrigation

Perfecting the mix design

The mixes and the aggregate specifications were carefully developed with the aid of an internationally recognised RCC expert to ensure an economic solution. The low cementitious mixes, containing only 85 kg and 110 kg of cementitious materials, impressively achieved a compressive strength of 26 MPa at 365 days and a density of 2 650 kg/m3 .

Placement temperature

Thermal analyses conducted during the early phase of the design indicated that the maximum RCC placement temperature should not exceed 28°C to avoid thermal cracking in the dam structure, with a maximum joint spacing of 20 m. This was 5°C higher than the maximum temperature usually specified for RCC by the Department of Water and Sanitation in South Africa. To achieve this, the contractor imported and constructed a 3 MW cooling plant capable of cooling both the coarse aggregate and the mixing water. The cooling plant consisted of two chiller units, four air blast units and four insulated aggregate silos through which the cold air was blown. During the course of the project, the average temperature for placed RCC was 22°C.

Condition monitoring

Following an extensive construction

With the ambient temperature over 30°C almost every day in summer and quite regularly over 40°C, the contractor had to run a 3 MW cooling plant continuously in summer to meet the specified placing temperature. The average temperature for placed RCC was 22°C programme, the completed works saw the Neckartal Dam filling rapidly for the first time in January 2021. To date, the structure is performing well with minimal leakage along the construction joints.

Going forward, the dam’s performance will be monitored by nearly 300 electronic instruments to measure the behaviour of the structure. There are 52 long-base strain gauges to monitor the induced joint openings, and 100 piezometers in the foundation to monitor the hydrostatic pressure under the dam. In addition, there are temperature gauges, multiple head extensometers, tilt meters, 3D joint movement gauges, survey targets, V-notch gauges and water level recorders.

“Instrument readings are recorded every few minutes and all are downloaded to a dedicated computer, which can be accessed remotely over the internet. Consequently, the employer and the design team can continuously monitor the dam,” adds Stables.

Once the 5 000 ha of land have been put under irrigation, approximately 4 000 direct and indirect jobs are expected to be created. Farmers in the region will be able to take advantage of the unique climatic conditions to produce table grapes, dates and lucerne, in the process positively contributing to Namibia’s micro- and macro-economic growth.