A wind turbine’s foundation is a critical structural element solely designed to ensure stability over a tower’s lifetime, through transferal and spreading of loads acting on the foundation into the ground.
Designing efficient foundations is crucial for not only economic reasons, but also structural safety, operational control, and generator performance.
Between 25 and 33 per cent of a wind project’s cost is from balance of plant, which includes wind turbine foundations, roads and platforms within the wind farm, medium voltage lines for connecting the turbines, as well as substations and transmission lines.
The foundation has the highest cost within the balance of plant scope – only turbines cost more across an entire project – and foundation cost has high variability, making up about 16 to 33 per cent of the balance of plant or between 4 and 10 per cent of the wind project’s total cost.
Research published last year noted that minimising wind energy’s costs and maximising its sustainability were required to improve its competitiveness, offering a practical approach to assess the sustainability of wind turbine generator foundations from a three-dimensional, holistic perspective.
The main goal of the study was to analyse the lifecycle impacts of a shallow foundation design and compare three different concrete alternatives: conventional concrete, concrete with 66 to 80 per cent of blast furnace slags, and concrete with 20 per cent fly ash.
Using a multi-criteria decision-making model, the study found that concrete with blast furnace slag was the top-ranked sustainable alternative, followed by conventional concrete and then fly ash in third place.
Furnace slag outperformed both fly ash and conventional concrete in terms of environmental impacts, using three primary indicators (human health, ecosystem quality, and resources).
Conventional concrete proved 2.09 per cent more economical than blast furnace slag and 7.26 per cent more cost-effective than fly ash.
The resulting methodology also enables the quantification of sustainability rather than just its qualification, and can be used for design optimisation, such as geometry and materials, from a sustainable perspective.
The researchers observed that the ideal economic design may not coincide with the optimal environmental design, highlighting the need to consider both dimensions simultaneously.
They added: “The social dimension is essential and has the potential to influence the design when it is integrated with both the environmental and economic dimensions.”
EVOLVING DESIGN OF WIND TOWER FOUNDATIONS
There are three distinguishing factors of a wind turbine foundation from other structures, with the core factor being the eccentricity of the loads due to the prevailing load being the tower’s bending moment.
The others are the dynamic behaviour of the structure and its influence on loads through the minimum dynamic rotational stiffness requirement to the soil-foundation interaction, and gapping control requirements to ensure the operation of the foundation will not be compromised due to repeated gapping cycles and potential soil degradation.
Initially, wind towers were constructed using a square-shaped steel ring design. However, as their heights increased, the designs evolved to circular anchor cages, which are now the most commonly used load transfer system for tubular steel towers.
When using a circular foundation, the shear forces decrease from the centre to the outer part of the foundation, and the conical and circular shape of the tower is more economical.
In some countries, such as Australia or the United States, octagonal-shaped foundations are also commonly used, despite circular designs being more competitive.
The price difference can often be quite marginal, and the implementation of steel reinforcements avoids circular rebars, making it easier and quicker.
Octagonal-shaped foundations can also be built with smaller diameters, reducing the concrete volume required.
Other foundation designs used for more niche applications include Patrick & Henderson foundations, also known as tensionless pier, as well as braced, precast, and rock-anchored foundations.
Loads acting on a wind tower foundation consist of dead load from the tower and its nacelle and rotor blades, as a vertical force acting on the foundation, as well as the wind creating substantial loads from the height of the tower and the bending occurring at its base.
For this reason, wind towers typically have a form of hollow truncated cone made of high-quality steel and a base that connects to an in-situ foundation through an interface embedded in concrete.
Interfaces have a multitude of forms and can vary widely – examples include a giant steel pipe with a flange embedded in the concrete foundation, or a “bolt cage” where several long bolts are embedded in concrete.
The foundation can also be divided into two subgroups – spread or piled – with both needing a connecting interface.
Spread foundations use the large area of a big plate to spread loads to the ground, often with a cylindrical geometry or a square prism, and the construction material is almost always reinforced concrete.
Areas with strong or stiff soil that do not give large settlements are more suitable for spread foundations, rather than clays, silty clays, fillings, organic soils, or other soils with low modulus of elasticity and/or strength.
SOIL CONSIDERATIONS CRUCIAL TO FOUNDATION DESIGN
A newly published review of advancements in the modelling and design of wind tower foundations highlighted the importance of considering soil-structure interaction (SSI) for an accurate representation of the overall structural response.
The authors said: “In the design and enhancement of wind structures, critical factors such as displacements and stresses must be considered when evaluating the interactions between the structure and the soil.
“Integrating these design considerations into the improvement of foundations is essential to ensure the safety and efficiency of these structures, meeting the growing demands of the industry and promoting sustainability in wind energy generation.”
Soil properties relevant to foundation design include stiffness, shear strength, permeability, and the soil’s homogeneity.
These can be adjusted using different methods, categorised as either compaction and densification techniques or methods of soil reinforcement through the introduction of additional material.
Compaction or preloading techniques enable the soil to reach consolidation, reducing settlements, while the same effect can be achieved with the vibrating method.
Soil compaction occurs when it is subjected to vibrations, either from the surface through the release of heavy weights or by deep vibrations from vibrating machines.
Reinforcing the ground by adding material can be achieved through several methods, including permeation grouting, jet grouting, and lime and cement column techniques.
