Rock-anchored vs Shallow WTG Foundations

Onshore wind turbine (WTG) foundations are subjected to large overturning moments due to the wind loads acting on the turbine structure. To maintain stability, conventional shallow foundations rely on their own weight to resist these forces, spreading the load over a large surface area of soil.

Rock-anchored foundations, however, take a different, more efficient approach. These foundations use post-tensioned tendons to anchor the concrete foundation to the underlying bedrock. The tendons preload creates compressive forces that tie the foundation down to the bedrock, mobilizing the rock mass itself to provide additional stability. This method significantly reduces the need for large concrete volumes, making the design more cost-effective and sustainable.

Rock-anchored foundation requirements

Unfortunately, not every site is suitable for installing rock-anchored foundations. These foundations require specific ground conditions: the bedrock must be both shallow and strong enough to provide reliable support. Ideally, the bedrock should be strong, fairly homogeneous, non-weathered, and with no major joints. Without these characteristics, the foundation may fail to perform as intended.

In fact, there have been some wind projects designed with rock-anchored foundations where the ground conditions were not appropriate, resulting in costly failures. In one notable example, unforeseen deformations in the ground caused some of the anchors to lose their preload completely, compromising the system's stability. As a result, all the turbines had to be immediately shut down for complex and lengthy repairs, and the project incurred significant additional costs to address the issue.

Ground pressure comparison

The animation below illustrates the increase in ground pressure (in kPa) resulting from the loading of a WTG foundation. The top model represents the shallow foundation, while the bottom model shows the rock-anchored foundation.

In this project, the rock-anchored foundation required less than half the concrete volume of the shallow foundation, leading to substantial material savings.

However, the stress on the ground is roughly an order of magnitude higher for the rock-anchored foundation. By comparison, the increase in ground stress in the shallow foundation is minimal, nearly undetectable, given the color scale used to illustrate the higher stresses in the anchored foundation.

If you want to explore whether a rock-anchored foundation is viable for your wind project, drastically reducing costs, don't hesitate to get in touch 📧.