Wind turbine foundations: How to save on time, money and materials

As the average size of onshore wind turbines continues to grow, so too are the foundations required… but worry not, there’s a solution available that can help reduce material and excavation costs

birds eye view

Wind turbines are getting larger, 6MW or more is fairly standard for onshore wind turbines these days – and they’re only going to get bigger. Larger turbines, means larger foundations: diameters of 25-30m are no longer uncommon for shallow foundations. And obviously, the larger the turbine, the higher the construction costs for materials and labour. 

So, inventive cost saving solutions and expertise are in high demand. One such solution is Soft-Spot®, which optimises the use of all the resources required for foundation construction – using up to 15% less concrete and 10% less steel than traditional construction methods. It also takes up less space than a standard shallow foundation and cuts down on time, material and excavation costs. 

Katja Wunschel, CEO of RWE renewables, described the solution which has been used in their Rea Unificado wind farm in Spain as “ground-breaking work”. 

Space and cost savings

Soft-Spot® was developed by the French company CTE Wind Civil Engineering SA – an international engineering company specialising in the design and calculation of onshore foundations for wind turbines. The patented Soft-Spot® solution optimises the existing and well-functioning system of flat foundations by integrating structural engineering practices. Put simply, a soft layer of expanded polystyrene (EPS) is placed under the centre of the foundation so that the forces from the wind turbine are dissipated radially into the ground rather than over the entire surface. 

The first project using the Soft-Spot® design was implemented in a wind farm with 60 Vestas V136 turbines in Thailand. Each turbine had a hub height of 157m. Originally, a shallow foundation with 1,192 cubic metres of reinforced concrete and a diameter of 27.4m was planned. The issue was not with the volumes themselves, but the space available. Excavation was not allowed to exceed 25m, but by using a soft layer under the foundation, the diameter could be reduced to 24.5m; and the use of concrete and steel were reduced by 18% and 9% respectively. 

An important prerequisite for this solution is relatively good soil with a load-bearing capacity of at least 135 kilopascals. Because the foundation is ring-shaped, the forces on the outside are higher, while the stresses in the centre are zero. This solution is particularly useful for soils that are buoyant due to high groundwater levels. To ensure stability, foundations in this type of soil are usually made larger and heavier to resist buoyancy forces and to avoid gapping. 

Better for the environment

To date, 1500 WTGs worldwide have benefited from CTE Wind's Soft-Spot® solution and it has been certified by DNV to ensure its reliability. The solution has been used in sites including France, Finland, Brazil, Spain, Scotland, Sweden, Poland, Thailand and even in the Netherlands, where pile foundations are usually used due to the concentrated ground pressure of the turbine.

Arjan Louvewerse of H4a Windenergie B.V., who was a project manager for a Netherlands-based project where Soft-Spot® was utilised, says: “We had a good feeling about the design right from the start. The philosophy of reducing costs with Soft-Spot® was also a good point of discussion with our end customer.”

“CTE Wind is proud of the trust placed in us by our customers," says CTE Wind CEO Alexander Martin. “Our design offers savings in materials and works. And the fewer materials you use, the less carbon emission there will be." Indeed, the use of Soft-Spot® foundations enables the increased demands of ever larger wind turbines to be met, providing significant resource and cost savings alongside a significant reduction in CO2 emissions.

CTE Wind Civil Engineering is an international engineering consulting firm specialising in the design and analysis of onshore wind turbine foundations. CTE Wind has been active in the wind energy sector since 2003. As of 2022, CTE Wind has designed, value-engineered or peer-reviewed foundations for more than 25,000 turbines in 75 countries. CTE Wind is headquartered in France and has offices in Brazil, Poland, Portugal, Spain, USA and Vietnam.

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