Publication: Construction World
Issue: February 2019
Upon completion, the Neckartal Dam will be the largest dam in Namibia, with a full supply volume of 853 Mm3, exceeding the volume of the existing largest dam, Hardap Dam by a factor of nearly three. Projects of this magnitude need innovative construction technology in order to be executed successfully, Neckartal more so due to its desolate location in the arid climate of southern Namibia.
The dam is built in a deep valley, implying that the dam’s surface area is relatively small for the volume of water held, making it ideally positioned to reduce the effect of evaporation in the dry climate experienced in Namibia. The Neckartal Dam reservoir will have a surface area of approximately 42 km2, a perimeter of 295 km and full supply volume of 853 Mm³.
Once commissioned, the dam will contribute significantly to the sustainable economic development of the //Karas Region. The dam will yield water for irrigating farms for crop cultivation covering up to 1 960 ha during Phase 1 which may be extended up to 5 000 ha in the future, employing several hundred people in the process. Two turbines at the main dam will generate hydropower for the irrigation scheme, as demand requires.
The project consists of a 76,5 m-high roller compacted concrete (RCC) gravity arch main dam; a 10 m-high RCC abstraction weir (both with an uncontrolled Ogee crest); a 2,1 m³/s pump station; a 9 km-long, 1,1 m diameter pipeline; and an HDPE-lined embankment balancing dam to hold some 90 000 m³ of water.
Innovation to decrease construction time
Making use of a continuous uniform double curved shape (the Ogee spillway) for the dam spillway inherently posed some difficulties for construction. Innovation was required to decrease construction time while maintaining the accuracy of the profile, avoiding honeycombing and blowhole formation of the finished Ogee structure.
For the main dam, conventional construction methods (using sliding formwork or guide rails) needed a re-think in order to reduce the duration time to construct the Ogee crest. The construction of the crest was executed using controlled permeability formwork. This technique reduced the construction duration by a factor of more than two when compared to conventional construction techniques.
To aid the site engineers’ supervision (developing, monitoring and quantifying different construction activities), unmanned aerial vehicles (UAVs) were introduced. UAVs were used to develop accurate three-dimensional models from their photographic surveys which also were used for construction progress monitoring of the project. In addition, accurate surveys, project monitoring, material quantity measurements, Building Information Modelling integration and the sharing of insights around the construction site, were carried out with the aid of the UAVs. The Project is one of the first dam construction projects in the SADC region to apply this technology.
Contributing to a decrease in construction time, an improved productivity in the site laboratory was achieved by concentrating materials testing on the final RCC product rather than the individual constituents. The RCC mix design sieve analysis envelopes were based on the required upper and lower limits of the specification for each particular material ingredient.
As a production and quality control measure, this new material sieve analysis and moisture content test procedure was developed in the laboratory to correlate between designed RCC and produced RCC. This new laboratory technique drastically reduced the testing time when compared to testing all the different aggregates, sieve analyses and moisture contents. In future projects, flakiness and elongation test methods may be incorporated into this testing procedure during RCC construction. This innovative procedure allowed a quick assessment of the final RCC product.
The total volume of concrete required to complete the construction of the Neckartal Dam was just over 1 000 000 m3. Interestingly, only 65 kg of cement was used per cubic metre of concrete. This low volume of cement is one of the characteristics of using roller compacted concrete that makes it such an economically sound choice for the construction of a dam wall of this magnitude. At peak production, construction had taken place 24 hours a day, seven days a week.
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