David Annan Member Name
Senior Geotechnical Engineer
Wind farms are one of the fastest growing technologies for renewable energy generation around the world, with a rapidly decreasing lifetime cost of energy. Still, the upfront investment required to get a wind farm off the ground and into operation is enormous. Strong foundations are one of the greatest cost components in a wind farm besides the towers themselves, so developers will naturally be seeking optimised design and cost efficiencies wherever possible to achieve technical and economic feasibility.
Increased productivity means greater profitability, so developers are selecting taller turbines with longer blades to maximise energy generation. The taller the tower, the greater the loads on the foundation, so the foundation needs to be stronger to avoid excessive movement of the tower. With heights reaching as much as 250 metres, the stability of these towers depends on their foundations to transmit into the ground the immense wind forces that buffet the tower, generator and blades.
Even though the base of a turbine is often hidden from view, it is critical for keeping the turbine upright and in place. A foundation failure, while not common, can have a catastrophic impact. Even without a full topple, any tilting, rocking, sliding or subsidence of the turbine, or cracking of the foundation, will have major ramifications for operation and maintenance.
This makes it critical to find the right design for a supporting structure that is not only optimised from a cost perspective but can also do its important job well.
Steps to success
What steps need to be taken to manage risks and stand the best chance of success? It’s all about finding the right combination of appropriate turbine siting and foundation design – but there’s no simple off-the-shelf, one-size-fits-all solution.
The design of the foundation will depend on the type and size of turbine being supported as well as the ground conditions – which can vary not only across the whole wind farm site, but also within the footprint of an individual turbine. The design will also need to factor in the costs and risks associated with various design options and materials.
Putting in the groundwork to get this right as early as possible in the design process (and long before construction!) will contribute to the project’s overall feasibility, as even a small variation in this expensive component could be very important in a project with narrow margins for success.
1. Understanding the ground conditions of the site
Understanding the ground conditions is the first critical step towards achieving an efficient, site-specific footing design.
Ideally, no wind farm would be built on a site with poor ground conditions. However, there are so many factors at play in siting (e.g. wind resource, land ownership, planning regulations, environmental and social considerations) that this may not always be possible to avoid.
A thorough ground investigation consisting of laboratory testing of soil and rock samples as well as on-site testing (building on preliminary desktop review of potential concerns and historical land uses) will uncover the geotechnical risks of the site, such as soft soils, landslides, collapsible soils, seismic activity and liquefaction risk, groundwater issues and the presence of any acid-forming rock or acid sulfate soils.
Ground conditions may vary significantly across a large wind farm site. If these conditions aren’t anticipated or uncovered early enough, and are only discovered during construction, changes will be expensive and will set back construction schedules.
2. Designing to suit the site and the project budget
For any given turbine location, there may be several viable alternative foundation designs. A major consideration will be cost. Which option will be both appropriate to the site conditions and cost-effective to build? Investing a little extra time and money at this stage may pay for itself many times over, given the cumulative effect of potentially being able to use the same foundation design for many turbines across a site.
Foundations need to be designed to keep the load balanced against the ground’s bearing capacity and stiffness. The geotechnical design brings together the analysis of the ground conditions with the calculation of the loads on the foundation, and then determines the foundation dimensions and weight needed to minimize the chance of a failure of the foundation. The strength, compressibility and stiffness of the soil or rock under the turbine will impact the size and depth of the footing needed and will also affect the performance of the foundation.
On sites with very stiff cohesive soils, dense granular soils or weathered rock, the natural first consideration is a shallow ‘gravity pad’ foundation, since it is usually the quickest, and therefore often the cheapest, to construct. Most onshore turbines have these shallow foundations which rely on the weight of a large reinforced concrete base set into the ground and covered by soil, much like the buried base of a wine glass.
On weak soils, shallow foundations run greater risks of overturning, sliding, settlement and tilting. This means that deep foundations (such as pilings or piers) may seem a more appropriate choice to support wind turbines by transferring the load down to the stronger, deeper layers. Yet deep foundations will come with a much higher cost and longer construction time. Ground improvement methods or replacement of the subgrade may be other options to allow shallow foundations to be feasible.
Where the foundation is to be on high-strength rock, which is reasonably consistent at depth, a popular choice is a rock-anchored foundation, which resists overturning or lift-off due to tensioned anchors extending into the rock below a small pad. Additional investigation may be needed to support the adoption of anchored footings as the applied bearing pressure can be up to 10 times higher than an equivalent gravity footing.
While anchored and piled footings may be interesting to design and to talk about, in reality — particularly in Australia — a gravity footing will most often be the appropriate choice from the perspectives of cost and the construction program.
3. Collaborating for success
As in most things, the best chance of success comes from good communication and collaboration. It’s ideal if structural engineers and geotechnical engineers can work together in an iterative and flexible process that responds not only to the known conditions, but also considers the project drivers, from a design point of view as well as in terms of cost and constructability implications. (For example, there may be specific project constraints, such as procurement times for steel and availability of quality rock for concrete aggregate, which could impact on the program and the cost.)
When geotechnical and structural engineers collaborate throughout the design process, there’s a greater sharing of risk, which lessens the need for conservative (and expensive) ‘over-design’. The design process is also likely to be more efficient, with less risk of re-work.
An efficient and optimised design can very quickly become inappropriate if good construction practices are not followed. Cleanliness of the footing excavation surface (removal of debris) will be critical to the long-term performance of the turbines. During construction, it is essential that the geotechnical designer observes the footing bases to confirm that conditions match those anticipated from the geotechnical investigation and to provide advice should conditions differ.
With a thorough understanding of the ground conditions, a site-appropriate foundation design and a collaborative approach among the many roles in the project team, a new wind farm development is likely to be on a solid footing for long-term safety, robustness and profitability.