Concor has become an important construction partner in South Africa’s wind energy landscape, and recently started work on the Cennergi Holdings and G7 Renewable Energies 140 MW Karreebosch Wind Farm.
Located between the towns of Matjiesfontein and Sutherland, Karreebosch Wind Farm is sited predominantly in the Northern Cape, and will generate energy for a private off-taker. At the heart of the project will be 25 turbines at a height of 100 m with blades over 84 m long. Interestingly, the site of this project is adjacent to the Roggeveld Wind Farm which Concor completed about five years ago.
According to Stephan Venter, Contracts Director at Concor, the early work included establishing the necessary infrastructure on site, given the remote and rugged terrain. This will include access roads, site offices and a batching plant for the large volumes of concrete required for each turbine’s foundation.
Concor has begun constructing about 45 km of internal access roads, allowing the transportation of components and equipment to each turbine site. Roughly 200,000 m3 of blasting are likely to be required for the roads, foundations and other work. The G5 and G7 material for layer works will be produced on site, using jaw and cone crushers feeding a screen.
“The roads need to support the long heavy trucks used to transport turbine components,” says Venter. “For instance, the long turbine blades require the road design to provide sufficient turning radii as well as the right K-values to avoid trucks striking their undercarriage on uneven surfaces.”
In the early phases, Concor conducted detailed ground-line surveys and geotechnical investigations to understand the terrain. This helps in planning cut and fill operations, where material is removed from some areas and used to build up others – creating level roads that can support heavy loads. Geotechnical work was critical to the integrity of the turbine foundations, and a specialised drilling company extracted 25 m cores at each turbine position. This established the founding conditions, allowing the foundation design to include the optimal bolt length for securing the turbine tower.
“The bolt length provides the key element around which we can finalise the design of the steel and concrete aspects of each foundation,” he says. “This is vital preparation as the early stage designs can only rely on conventional geotechnical maps which provide high level data rather than a detailed insight.”
He points out that the foundation design must also align with the loading documents that are specific to each turbine supplier. The planned lifespan of these critical foundations tends to be longer than the 20 year off-take agreement, allowing for contract extensions that could see the turbines operating for up to 25 years.
The concrete foundations for each turbine measure over 20 m in diameter and are between 4 to 5 m deep, requiring about 600 m3 of concrete per foundation. The whole project will consume in the region of 25,000 m3 of concrete and 2,200 tons of reinforcing steel.
“We do our own concrete mix designs and will produce the readymix ourselves using aggregate from our Tweedside tillite quarry 70 km from site,” he explains. “Material will be transported to our own batch plant on site, allowing us to cover most of the concrete value chain ourselves.”
The concrete turbine foundations will be poured continuously to ensure even curing, followed by thermal management to prevent cracking from temperature changes. Water for the batching plant will come from two carefully managed boreholes, which feed a 1,5 million litre water storage facilities on site.
Among the environmental factors that the construction work will have to accommodate is the site’s proximity to the world famous astronomical observatory at Sutherland. This requires any night-time work to be conducted without creating light pollution that would affect visibility at the observatory.
“The concrete pouring cycle on each foundation lasts about 24 hours, which means that some tasks need to be done when it is dark,” he says. “We will take precautions by using special bulbs, for instance, and ensure that we only shine light downwards onto our work and not directly into the sky.”
