To Anchor or not to Anchor… That Is the Question

Anchored wind turbine foundations work beautifully when they do, but when they don’t, they end up as a nightmare. That’s why “to anchor or not to anchor” is a fundamental question that has to be thoroughly evaluated when contemplating an anchored wind turbine foundation. In this post, we discuss this efficient wind turbine foundation system and why it should be reserved to “appropriate” geotechnical settings.

When we speak of anchored wind turbine foundations, we are typically referring to a foundation cap that is anchored to bedrock. Hence, they are commonly called rock-anchored foundations. The anchors are commonly post-tensioned and are designed as active anchors; i.e., anchors that must retain their pre-tension during wind turbine operation and even during extreme events. This foundation system is highly efficient compared to the classical gravity base (or inverted T) spread foundation. For the “appropriate” geotechnical setting and when sized properly, a rock anchored foundation works like a dream.

What do we mean by “appropriate” geotechnical setting? Below are four pictures of bedrock cores ranging from very high quality to highly weathered bedrock. The “appropriate” label belongs to very high and high quality bedrock. For weathered and highly weathered bedrock, extreme care and in-depth assessments, through lab tests and field load testing, are in order to ensure that creep remains under control and that the anchors retain their pre-tension loads and “active” status. Alternatively, the anchors can be designed as passive anchors (not post-tensioned) but this decision would result in designs that part with the efficiency of anchored foundations.

Over the past two decades, as wind turbines became larger and larger, the gravity base foundation system also grew in size to keep up with higher turbine loads, and so did the pressure to come up with alternate foundation systems that can help contain foundation quantities. Under such pressure, the “anchor or not to anchor” question was extended to soil-anchored foundations and found proprietary systems that use post-tensioned soil anchors. Of course, the issue of anchor creep is doubly more critical for such systems. The primary guidance for the design of prestressed earth anchors is “PTI DC35.1-14: Recommendations for Prestressed Rock and Soil Anchors” by the Post-Tensioning Institute (PTI). However, designers must keep in mind that when PTI developed this guidance, the authors did not envision a wind turbine with a 160-meter rotor spinning at 130-meter hub height. That was not the “structure” they had in mind. They prepared that guidance for static soil or rock anchors used to stabilize the side of an excavation, the face of a tunnel or the side of a mountain. Hence, this guidance should be adapted to the characteristics of wind turbine loads (highly cyclic and dynamic) and the foundation performance expectations.

In conclusion, anchored wind turbine foundations have a place and can be highly efficient. However, the “anchor or not to anchor” question has to be carefully evaluated based on site specific geotechnical conditions and methods that go beyond PTI’s recommendations. We welcome readers’ comments.

The “Structure” We’re Dealing with!

In conclusion, anchored wind turbine foundations have a place and can be highly efficient. However, the “anchor or not to anchor” question has to be carefully evaluated based on site specific geotechnical conditions and methods that go beyond PTI’s recommendations. We welcome readers’ comments at the website or on LinkedIn.