Construction World December 2021

to that original ly tendered. Cof fer dams had to be constructed at unplanned t imes to take account of the shi f ted river flow. Unfortunately, thi s increased the t ime for complet ion and the overal l cost . Nevertheless , throughout al l the unplanned di srupt ions , stringent qual i ty control was maintained, both by the Engineer and the Contractor. Concrete batch records , cube test resul ts , pre-concrete inspect ions and hydro mechanical instal lat ion inspect ions and the l ike, were wel l documented, eas i ly obtainable and constant ly archived for easy reference. The dam was bui l t in a deep val ley; implying that the dam’s sur face area i s relat ively smal l for the volume of water held, making i t ideal ly pos i t ioned to reduce the ef fect of evaporat ion in the dr y cl imate experienced in Namibia. The Neckartal Dam reser voi r wi l l have a sur face area of approximately 42 km 2 , a perimeter of 295 km and ful l supply volume of 853 Mm³ . Once commi ss ioned, the dam wi l l contribute s igni ficant ly to the sustainable economic development of the //Karas Region. The dam wi l l yield water for i rrigat ing farms for crop cul t ivat ion 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 wi l l generate hydropower for the i rrigat ion scheme, as demand requi res . The total volume of concrete requi red to complete the construct ion of the Neckartal Dam was just over 1 000 000 m 3 . Interest ingly, only 65 kg of cement was used per cubic metre of concrete. Thi s low volume of cement i s one of the characteri st ics of us ing rol ler compacted concrete (RCC) that makes i t such an economical ly sound choice for the construct ion of a dam wal l of thi s magni tude. At peak product ion, construct ion had taken place 24 hours a day, seven days a week . On complet ion, thi s mass ive mul t i -bi l l ion Namibian Dol lar dam structure, s i tuated in the Fi sh River, wi l l be the largest dam in Namibia, wi th nearly three t imes the volume of Hardap Dam (current ly the biggest reser voi r in the countr y) . 

diameter, f rom where i t wi l l be routed to the i rrigat ion areas for agricul tural use as needed. The Dam was bui l t us ing the Rol ler Compacted Concrete (RCC) construct ion technique and i s 78,5 metres high wi th a crest length of 518 metres . The dam has a storage volume of 857 mi l l ion cubic metres extending over a 39 km 2 sur face area at i ts ful l supply level . To construct the project , over 1,1 mi l l ion manhours were ut i l i sed. In the period f rom 2018 to 2020, there where zero workdays lost due to no injuries , which was a record-breaking feat in terms of Heal th and Safety for the contractor. In Keetmanshoop and in the surrounding areas the Project created a total of 5 500 jobs ; 3 000 workers were hi red di rect ly and 2 500 indi rect ly wi th 65% of these workers coming f rom the //Karas Region. Two phys ical models of the dam wal l structure were studied during the des ign stage of the dam. These models were constructed to scales of 1:60 and 1:120, wi th the latter model including the upstream topography. The ini t ial aim was to invest igate the hydraul ic behaviour of the stepped RCC spi l lway and to determine the ef ficiency of the energy di ss ipat ion structures s i tuated downstream. The study revealed that for the recommended des ign di scharge (RDD) flood event , a sub-atmospheric pressure region occurred downstream of the spi l lway crest and was further accentuated for larger flows . These resul ts were in contradict ion wi th known l i terature which predicts hydrostat ic pressure to be present during the RDD (USACE, 1992) . The phys ical models showed that the ef fect of three-dimens ional flow, cur vature of the dam wal l and the asymmetric approach channel could not be neglected when des igning the Ogee crested spi l lway. The resul ts were subsequent ly integrated for the development of s igni ficant improvements to the spi l lway layout , as wel l as the energy di ss ipat ion structures . Flow in the Fi sh River i s seasonal wi th the wet season f rom December to May, in good years . As the contract was awarded later than envi saged, the contractor had to re-program hi s work compared

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20 th Best Project Awards 2021