The recent authorisation by the National Department of Environmental Affairs of the proposal to build a wind farm on the outskirts of Grahamstown has added fuel to a number of debates. One of these is whether or not the geology in the area is stable enough to support these enormous structures.
A geo-technical survey has yet to be done, before construction begins. meanwhile, the debate rages on.
The underlying geology in Grahamstown consists of quartzite, shale and clays of the Cape Supergroup. According to Dr Nick Stavrakis, who specialises in applied geology, the possible presence of clay in the underlying geology could lead to the wind turbines being unstable.
When clay is placed under pressure, especially directed pressure, “the clay liquefies... and starts to flow. There’s no support once it starts to liquefy and basically you have collapse. So wherever the stress is in that foundation, there’ll be movement,” he says.
Stavrakis is not opposed to green power; however, he believes that there are better locations for the eight proposed turbines than the Waainek Road, namely, the flat peneplain area near the substation. This area, according to Stavrakis, has a thick, hard silcrete crust and is fairly barren, making it unsuitable for agriculture.
He also says, “I believe it’s owned by the community. It’s the closest point to an Eskom substation, so you wouldn’t have power losses along the way.”
Palaeontologist, Dr Billy de Klerk, points out that clay deposits, covered by a layer of silcrete, occur mostly on the flat areas above Grahamstown. Underlying the high ground is the Witteberg group of rocks, which consists predominantly of quartzites.
“The high ground here is ideal... they [the Witteberg rocks] are as hard as hell and they’re on the high ground where you want your turbines to be in the first place. The higher your turbine, the more air flow it’s going to pick up,” says de Klerk.
“I think that Grahamstown has actually sited their turbines correctly on the high ground, and that minimises the amount of clay material below any turbine's position. The high ground is resilient, that’s why it’s high, so any of the softer weathered material has been washed away already.”
Stavrakis says, however, that the Witteberg quartzites are structurally disturbed by faults and therefore have weaknesses. He says that, in addition, interbedded with the quartzite are thick layers of soft clay-filled shale.
Even though hard rock would be more suited to the placement of wind turbines, de Klerk, who has done consulting for other wind farm proposals, believes this is not necessarily always the case.
“The foundations have to be 20 by 20 metres, and about five metres down. Then they sink cables into boreholes in various directions, which are then anchored in that concrete block,” he says.
Both Stavrakis and de Klerk point out that a comprehensive geo-technical survey of the area needs to be done before there is certainty about the suitability of the geology to support the turbines.
“It’s complex, because we don’t really know the underlying geology in great detail and apparently no geological or geo-technical study has been done,” says Stavrakis.
De Klerk says, “They haven’t completed their geo-technical survey, so they may not go and put them exactly at those points because of the considerations of what the substrate is going to be. Is it shale or is it quartzite? If it’s shale, they would think twice about the positioning. If it’s quartzite it’s competent and not a problem.”
A geo-technical survey has yet to be done, before construction begins. meanwhile, the debate rages on.
The underlying geology in Grahamstown consists of quartzite, shale and clays of the Cape Supergroup. According to Dr Nick Stavrakis, who specialises in applied geology, the possible presence of clay in the underlying geology could lead to the wind turbines being unstable.
When clay is placed under pressure, especially directed pressure, “the clay liquefies... and starts to flow. There’s no support once it starts to liquefy and basically you have collapse. So wherever the stress is in that foundation, there’ll be movement,” he says.
Stavrakis is not opposed to green power; however, he believes that there are better locations for the eight proposed turbines than the Waainek Road, namely, the flat peneplain area near the substation. This area, according to Stavrakis, has a thick, hard silcrete crust and is fairly barren, making it unsuitable for agriculture.
He also says, “I believe it’s owned by the community. It’s the closest point to an Eskom substation, so you wouldn’t have power losses along the way.”
Palaeontologist, Dr Billy de Klerk, points out that clay deposits, covered by a layer of silcrete, occur mostly on the flat areas above Grahamstown. Underlying the high ground is the Witteberg group of rocks, which consists predominantly of quartzites.
“The high ground here is ideal... they [the Witteberg rocks] are as hard as hell and they’re on the high ground where you want your turbines to be in the first place. The higher your turbine, the more air flow it’s going to pick up,” says de Klerk.
“I think that Grahamstown has actually sited their turbines correctly on the high ground, and that minimises the amount of clay material below any turbine's position. The high ground is resilient, that’s why it’s high, so any of the softer weathered material has been washed away already.”
Stavrakis says, however, that the Witteberg quartzites are structurally disturbed by faults and therefore have weaknesses. He says that, in addition, interbedded with the quartzite are thick layers of soft clay-filled shale.
Even though hard rock would be more suited to the placement of wind turbines, de Klerk, who has done consulting for other wind farm proposals, believes this is not necessarily always the case.
“The foundations have to be 20 by 20 metres, and about five metres down. Then they sink cables into boreholes in various directions, which are then anchored in that concrete block,” he says.
Both Stavrakis and de Klerk point out that a comprehensive geo-technical survey of the area needs to be done before there is certainty about the suitability of the geology to support the turbines.
“It’s complex, because we don’t really know the underlying geology in great detail and apparently no geological or geo-technical study has been done,” says Stavrakis.
De Klerk says, “They haven’t completed their geo-technical survey, so they may not go and put them exactly at those points because of the considerations of what the substrate is going to be. Is it shale or is it quartzite? If it’s shale, they would think twice about the positioning. If it’s quartzite it’s competent and not a problem.”
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| A wide band of soft carbonaceous shale (the dark layer above Stavrakis' head in picture) interbedded in quartzites of Witteberg Group. This structurally-weak zone is on the approximate stratigraphic horizon as the proposed Waainek Wind Farm area and specifically the beds underlying the three turbines closest to the Umaria Umama Monastery. A layer such as this would be hugely problematic for the placement of a heavy wind turbine. Photo by Kerry Holmes |
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| Faulting with clearly-visible subsidence in Witteberg Group rocks in road cutting to the south of Grahamstown. The main fault traces are shown with broken white lines on the photograph. According to Stavrakis, such a position would constitute an unsuitable site for a large wind turbine. Photo by Kodak Express |
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| Well-known palaeontologist, Dr Billy de Klerk, feels the proposed location for the wind farm is probably suitable, as it is on resilient, high ground. Photo by Mike Davies-Coleman |
Published in Grocott's Mail on 15 April 2011.
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