Rock-anchored WTG foundations

Onshore wind turbine foundations need to resist large overturning moments caused by wind loads on the turbine structure. While traditional spread foundations rely on their own weight to counter these forces, rock-anchored foundations take a smarter, more efficient approach.

When are rock-anchored foundations feasible?

Not every site is suitable for rock-anchored foundations. These foundations require a competent bedrock at and below the foundation depth.

Many sites have relatively shallow bedrock, but not all are appropriate. Often, the rock is too weathered and fractured, sometimes so much that the material behaves more like a soil than a rock. This can compromise its load-bearing capacity and make it unsuitable for anchoring.

However, when the conditions are right, rock-anchored foundations are a great solution, saving significant amounts of concrete and reinforcing steel.

How rock anchors work

Rock-anchored foundations use post-tensioned tendons to tie down the concrete cap to the bedrock. These tendons create compressive forces, mobilizing the surrounding bedrock mass to provide the required stability.

This tiedown effect significantly reduces the need for large massive concrete volumes to counterbalance the uplift forces. You can check this article comparing rock-anchored and shallow foundations for more details.

Well, technically, one could argue that rock-anchored foundations aren't so small, given that an important portion of the bedrock becomes effectively part of the foundation.

What's more, the bedrock sometimes has greater strength than the concrete of the foundation itself.

That's why I like to say we get most of the foundation for free, we're just paying for the concrete cap and the tendons.

Stress evolution in the bedrock: a case study

The animation below shows how the vertical stress (in kPa) evolves within the bedrock of a rock-anchored foundation.

As the wind speeds increase, so do the loads on the turbine blades, resulting in larger overturning moments at the foundation. The small concrete cap tends to tilt in the direction of the wind, but the post-tensioned tendons and the mobilized bedrock help resist these forces, ensuring the stability of the system.

Why to choose a rock-anchored foundation?

WTG rock-anchored foundations are a cost-effective solution that can reduce in half or even more the concrete volume. But not only the concrete volume is smaller, the required reinforcing steel quantity decreases considerably as well.

Even better, the savings grow with larger turbines, making this design ideal for high-capacity WTGs.

However, designing these foundations requires meticulous analysis to ensure their stability and long-term durability.

If you need a foundation in your rocky wind project, get in touch 📧 for an optimized rock-anchored foundation that can drastically reduce your BOP costs.