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The photo on the left shows a construction crew pouring the concrete for the north turbine foundation at the Gundersen Health System Cashton Greens Wind Project in Cashton, WI, during November of 2011. The photo on the right shows several concrete trucks making delivery to the site.
The foundation is a key component to a wind turbine. It provides stability for the structure and allows it to reach great heights of more than 300 feet in the air. As a general rule, the taller the turbine, the better the wind velocity and power output. Turbine foundations will often have an octagonal shape to transmit forces from the tower more evenly into the soil. Before a foundation is poured a geotechnical survey must be completed to make sure that the soil has the properties to support the immense, dynamic forces from such a tall and heavy structure. The soil is sampled at several depths to make sure it is acceptable for the special traits needed and then a foundation design can be completed. Once the soil survey and foundation design are complete the excavation can begin and then a reinforced steel rod frame is constructed to form the shape of the foundation. In many cases, a vault or chase will be formed into the foundation to allow for buried electrical and communication cables to enter the tower from underground.
Once started, the concrete pour must be completed in one continuous step. This usually involves a large concrete plant in the vicinity and a fleet of concrete trucks making a flurry of trips to the turbine site until all of the foundation concrete has been poured. For several weeks after the pour has been made, concrete certification samples will be tested to verify that the structure is curing properly and will have the proper strength for the next step in the construction phase as well as in its intended use. Once the cure has been validated to the appropriate specifications, the foundation can be backfilled with rock and soil to the surface level.
Foundations are critical to the turbine function and construction schedule. They are also a very costly part of the turbine and prices for steel and concrete can vary with time and location. When considering a wind turbine project, a trade-off should be made between the benefits from additional turbine height in your project’s wind regime and the additional cost of a larger foundation and taller tower. There is not a standard solution so an optimum for each project location should be evaluated. The future sales price of energy, project capital budget, return on investment, and the construction schedule are other factors to consider in this decision.