Construction World December 2018

Architects

WINNER and SPECIAL MENTION AfriSam Innovation Award for Sustainable Construction

PROJECT INFORMATION • Company entering: Earthworld Architects • Client: REH Group • Project value: R4,5-million (building cost) • Start date: October 2015 • End date: November 2016 • Main Contractor: Eigenbau • Architect: Earthworld Architects • Principal agent: Aurecon • Consulting Engineer: Aurecon

B efore this scheme was implemented, the river was a relatively small stream with a base flow of approximately 150 ℓ /s. Now it has an average flow of over 24 500 ℓ /s throughout the year. By 2025 this will increase even further with the implementation of the second phase of the Lesotho Highland Water Scheme which has already started. Several structures were constructed in 2000 to mitigate the erosion caused along the river channel by such increased flow. The Botterkloof Dam is one of these structures, located approximately 1,5 km from the tunnel outfall. The Stortemelk plant was retrofitted on the left bank of this existing dam and effectively bypasses it. The plant sits on a narrow strip of land between the Botterkloof Dam and Farrels Dam. This meant that it had to be carefully designed and constructed to fit in the space between these two dams. The design had to be low visual impact and consider space limitations as well as the environmental concerns. The site is situated right next to the river and also in a privately-owned conservancy. This posed a couple of interesting design and construction challenges. It was tricky to get the necessary permissions, permits and land all year round. This is because the river is fed by the nearby Lesotho Highland Water Scheme that conveys water from the Katse and Mohale Dams through the transfer tunnel, via the Muela hydropower station and Muela Dam in Lesotho and into the Ash River. From here its journey continues to the Liebensbergvlei River, the Wilge River and into the Vaal Dam from where water is pumped to Gauteng for domestic and industrial use. STORTEMELK HYDROPOWER PLANT Stortemelk's turbines and generator harness kinetic energy from water, of which there is plenty

rights for the project. Botterkloof Dam belongs to the Department of Water and Sanitation but financed through the Trans-Caledon Tunnel Authority, while the Boston A Dam belongs to a local farmer. The power station itself is located on a piece of land purchased from the farmer, while the intake is on a servitude belonging to the Department of Water Affairs and Sanitation. Furthermore, local geotechnical conditions created some constraints. The site is situated on mostly mud stone and sand stone sedimentary rock which does not offer strong bearing capacity and deteriorates rapidly once exposed. Construction also entailed doing excavations in the dam (intake) and in the river (tailwater). In both cases, this meant digging excavations up to 8 m below the water level. This had to be done without affecting the stability and operations of either of these dams. Due to the space limitations on site, careful planning and getting the right contractors on board was also key. The solution was a shallow but wide intake, followed by a square concrete penstock leading to the hydropower station. The project contributed to the local economy through procurement of over R10,5-million during the construction process. Almost half of the total staff complement working on the project (including the design and architectural team) also came from local communities. During its 20-year power purchase agreement term, the project will contribute 1% of its revenue generated to a local charity as part of an economic development initiative. Construction on a project like this takes about 18 months, but the plant itself will be around for decades. The plant is designed to allow for a complete unstaffed operation. This is a relatively new approach in small hydro in Southern Africa, where most plants still have full-time operators on site. The building designed suits the plant. Both will be working hard in the landscape, and the design approach reflects that. The plant is compact, yet designed to ease operation and maintenance work. It comprises a simple, reinforced concrete structure, cast against the excavated rock. The design of the building above ground had to be compatible with the natural beauty of Clarens. 

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