All successful wind projects begin with strong foundations. Larger wind turbines naturally call for taller towers and larger foundations that consume enormous amounts of concrete and rebar. To handle variations in wind turbines, the current state of practice includes spread-footing foundations, “tensionless” pier foundations, and hybrid foundations that use rock anchors and micropiles to resist overturning turbines and compressive loads. A brief review of foundation designs shows the advantage of the hybrid foundation.
Spread-footing foundations are most common. Large gravity concrete pads, hexagonal in shape, use about 400 to 500 cubic yards of concrete and a truckload of rebar. The load transfer from the tower is accomplished with high capacity, post-tensioned threaded bars connected to an embedment plate near the foundation bottom. Bars are sleeved through the foundation to transfer loads generated by the turbine from the tower base all the way down to the embedment ring in the foundation bottom. Enginneers traditionally specify a preload for the anchor bolts that is higher than the tower service loads, thus eliminating fatigue and impact factors. Post-tension bars use clever cold-rolled “knuckle” threads, which are more durable than regular NC (National Course) threaded bars necessary for ease of tower installation.
Hybrid foundations are one way to reduce concrete and rebar, and are becoming more common. In them, a shorter (shallower) and smaller diameter concrete “cap” is constructed with blockouts – recessed holes – to allow drilling through the constructed
foundation. Contractors then drill a small, about 8-in. diameter, hole to a depth dictated by geotechnical reports (typically 30 to 40 ft), place an anchor in the hole, and completely fill it with a grout-a neat cement. Anchor features include a no-bond zone (also called a Free Stressing Zone) at the upper 10 ft of the anchor, so the load transfers deep into the ground. Good practice requires a load test on every anchor, as well as an extended “creep” test, one that evaluates the anchor movement over time, on a sampling of the production anchors. The anchors are then locked off at loads higher than anticipated service loads.
As turbine and tower technologies change, so must foundations. What was common 10 years ago is no longer the standard, and a decade from now the foundations on wind farms will continue to evolve to meet higher loads and stricter design standards. WPE
By: Tom Bird, Vice President, Western Division of Williams Form Engineering Corp.
Filed Under: Components, Construction