DAMS
supply level by 2.5 m – thus enlarging the storage volume by 2.5 million m³. The solution centred on a sophisticated hydraulic design in the form of a novel, non-linear spillway in the shape of a duckbill. Although duckbill or bathtub spillways are not unique, they are rare worldwide. “The design is relatively new in the engineering world, and as far as we know is not being used in this way in South Africa,” says Lionel Daniels, acting director: Civil Engineering Services, George Municipality. It was extensively researched by Zutari and tested by the Department of Water and Sanitation’s Hydraulic Laboratory in Pretoria, with its shape designed to slow down water flow using basic physics principles. “The project showcased how relatively small, well-engineered and optimised adjustments can provide a more resilient water supply system without compromising on dam safety. Furthermore, the expansion of existing water supply resources is preferable to the development of new sites, as it limits the environmental impact to an already impacted site. Despite the novel nature of the engineered solution, this unique and innovative project had a low capital cost and a small, estimated maintenance cost,” explains Denys. Amended design Zutari used its industry expertise to amend the hydraulic design to ensure that the weir would behave in a safe and predictable manner. The shape of the duckbill causes flow on opposite ends of the overflow flow to collide within the duckbill, causing an upwelling, also known as flow bulking, which breaks the flow’s momentum and causes it to lose its energy. The flow from the rounded upstream end of the spillway is not so opposed and causes this upwelling of flow to move toward the exit of the spillway at rapid velocity. This increase in velocity reduces the water level to a lower elevation such that it can safely pass under the bridge over the spillway. The final duckbill spillway design resulted in a total spillway length of 80 m, with a maximum discharge capacity of 570 m³/s at a freeboard of 4.1 m.
Refurbishment of NALUBAALE DAM Multidisciplinary engineering consulting company GIBB was awarded the contract to design and oversee the construction of Nalubaale Dam in Uganda. Despite disruptions related to the pandemic, construction deadlines were met, high technical standards were achieved, and the project budget was not exceeded.
T
ocated three kilometres downstream from the source of the Nile River in Jinja, Uganda, the Nalubaale hydropower station’s first turbine unit was commissioned in 1954. Cracks were first noticed in the powerhouse structure in 1964. These were caused by an alkaliaggregate reaction (AAR). Cracks had also developed in the main dam wall and leakages from the reservoir were observed against the downstream face. All repair attempts before 2020 had a limited service life as long as the concrete kept on swelling due to AAR. Louiza van Vuuren, civil engineer, GIBB, says Eskom Uganda decided to implement a new grouting programme to improve the structural and functional integrity of the dam structure. The grouting approach used by GIBB was based on the GIN (grouting intensity number) method. The acceptable pressures and volumes of grout injected were derived from stability calculations to avoid hydro-jacking of the dam’s concrete during grouting. “The specifications stipulated that the pressures used for grout injection should be controlled with a pressure transducer, which is fitted to a grout pump equipped with an automatic data acquisition system capable of measuring, displaying, and recording data in real time,” explains Van Vuuren.
Commencing work The contractor mobilised to site in December 2020, initially commencing with the drilling and grouting works on a trial section of the dam. This was to confirm or modify the grouting method (boundary curve, boreholes spacing, stop criteria); the grout mix design and grouting materials; as well as the equipment such as pumps, mixers, sensors and automated control devices, among others. Primary holes were inspected with a borehole imaging device to establish typical crack elevations, directions and widths, and to inspect the mass concrete condition. This led to the detection of large cracks of up to 17 mm wide. Crest levelling surveys of the concrete blocks were also conducted to detect any evidence of hydro-jacking. Due to the grout trial section taking longer than originally anticipated, the project experienced a slow start, compelling the contractor to increase the number of employees on-site – as well as drilling equipment – to catch up with the planned schedule. Van Vuuren says the practical completion of the drilling and grouting works was achieved before the original completion date of the contract and within the original budget. J A N /F E B 2022
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