A wind turbine generator (WTG) structure is typically supported on a large octagonal (at the base) mass of concrete and steel rebar reinforcement that supports overturning, rotational stiffness, and bearing capacity stability requirements. The overturning stability and maximum bearing pressure of the system vary as a function of wind speed and direction. The highly eccentric loading conditions (i.e., large ratio of overturning moments to vertical gravity loads) lead to an uneven pressure distribution that is assumed as a uniform soil pressure distribution over an oval-shaped effective area offset from the center of the foundation by the system eccentricity. However, this assumed distribution is mechanically incorrect. Current WTG foundation theory relies on elastic halfspace models that have been developed for other purposes, with little physical evidence to support the model in a WTG foundation system. Because of this, field verification of dynamic shear modulus values and pressure distribution assumptions is very relevant to establishing mechanistically correct WTG foundation responses, along with dynamic stress-strain relationships versus depth correlations (relevant for stiffness and settlement analyses).
The University of Wisconsin-Madison is working with Heartland Community College (Illinois) on instrumenting a WTG foundation that will shortly be put into service, with emphasis on evaluating the dynamic forces and the foundation soil response to in-service wind action. The monitoring will measure dynamic force parameters (magnitudes and periods), monitor foundation soil responses, and analyze and develop recommendations for a mechanically validated, dynamic foundation response. The results of the proposed research program will be leveraged across multiple educational environments through outreach and teaching activities. The project will provide an opportunity to engage undergraduate and graduate students with the construction, analysis, and operation of a renewable energy system. Experimental results and analyses will be incorporated into the PI?s nationally recognized and attended continuing engineering education short courses in wind energy including Wind Turbine Foundation and Tower System Design and Wind Energy Civil Balance-of-Plant Design. In addition, results will be directly used in the PI's first of its kind, for-credit design course in the UW-Madison College of Engineering - CEE639, Wind Energy Site Design and Construction.
Billions of dollars are invested each year in the U.S. to support the siting, design and construction of wind energy parks. The addition of these new power facilities is becoming more difficult as economic constraints increase and the civil, structural and geotechnical conditions for sites become ever more complex and multi-faceted. With tower heights approaching 100 m and above, design criteria for access roads, crane pads, collector systems and turbine foundations are rapidly changing to meet the needs of wind facilities in all types of wind resource areas and topography. Many foundation types are used to support wind turbine generators (WTG) with substantial hub height (> 80 m), vertical loads (1,800–3,000 kN), and moments (25,000 to 80,000 kN•m). Shallow turbine foundations are typically reinforced concrete (140–500 m3), octagonal in shape, contain 125-360 kN of rebar and cost from $100,000 to $250,000. The minimum diameter is usually dictated by foundation stiffness or maximum allowable edge pressure. This large octagonal (at the base) mass of concrete and steel rebar reinforcement that supports overturning, rotational stiffness, and bearing capacity stability requirements. The overturning stability and maximum bearing pressure of the system vary as a function of wind speed and direction. The highly eccentric loading conditions (i.e., large ratio of overturning moments to vertical gravity loads) lead to an uneven pressure distribution that is assumed as a uniform soil pressure distribution over an oval-shaped effective area offset from the center of the foundation by the system eccentricity. Because of this, field verification of dynamic shear modulus values and pressure distribution assumptions is highly relevant to establishing mechanistically correct WTG foundation responses, along with dynamic stress-strain relationships versus depth correlations (relevant for stiffness and settlement analyses). The University of Wisconsin-Madison (UW-Madison) worked with Heartland Community College (Normal, Illinois) to instrument a WTG foundation that was recently put into service, with emphasis on evaluating the dynamic forces and the foundation soil response to in-service wind action. The instrumentation system measured dynamic force parameters (magnitudes and periods) and foundation soil responses such that a mechanically validated, dynamic foundation response could be determined. According to field measurements, WTG foundation soil deformation and pressure distribution ranged from 0.0005 mm to 0.02 mm and 20 kPa to 110 kPa, respectively. The soil deformation profile and pressure distribution are not uniform and depend on depth below the foundation, offset from the centerline, and eccentricity of the dynamic load. The results of the research program was leveraged across multiple educational environments through outreach and teaching activities. The project provided an opportunity to engage undergraduate and graduate students with the construction, analysis, and operation of a renewable energy system. Experimental results and analyses have been incorporated into nationally recognized and attended continuing engineering education short courses in wind energy that are annually offered at UW-Madison including Wind Turbine Foundation and Tower System Design and Wind Energy Civil Balance-of-Plant Design. In addition, results from this project were incorporated into a for-credit design course in the UW-Madison College of Engineering – CEE639, Wind Energy Site Design and Construction.
