Agenda - Planning Commission - 08/06/2009

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Northern Power lOOkW turbine -Entegrity Wind Systems SOkW turbine in Quinter, Kansas 11. Soil Studies: Wind turbine foundations face forces that are not unlike those seen in light- pole or flag-pole foundations, but for . the same height, wind turbine towers generally experience higher forces at the very top of the tower. The wind turbine foundation therefore plays an important role in an installation, as does the soil in which it is installed. As standard practice, manufacturers engineer foundations for "worst-case" scenarios by assessing soil conditions based onU.S. Geological Survey soil maps, core samples, studies from the National Resource Conservation Service, or other resources. For "abnormal" soils consisting of rock, gravel, sand, peat or water-saturated earth/muck, etc., a manufacturer or local professional engineer conducts a tailored, project- specific soil review and often designs a custom foundation for the site. The installer always confirms with the manufacturer or an independent professional engineer that the site's soil conditions meet minimum standards as specified by the tower manufacturer, and that the tower is designed to local engineering standards (which vary by jurisdiction). For decades this has effectively ensured safe, durable installations while maintaining their affordability. These built-in safeguards make formal site soil analyses unnecessary for residential-scale (20kW or smaller) turbines and avoid the need for special studies and designs that can exceed the cost of the foundation itself. For example, a foundation designed for a "worst case" scenario might require 50% more concrete than that for another, similar structure. Assuming this fortified foundation also costs 50% more, an installation for a residential turbine normally costing, say, $4,000, . would increase to $6,000. This is expensive, but a relatively small share of the total cost of the system. However, a foundation 50% more robust (and expensive) for a turbine larger than 20kW, where the foundation might normall,Y cost $30,000, would now cost $45,000 - an increase of $15,000. At this point, a professional engineer's "wet stamp" based off a site-specific. geotechnical study is now the best economic option for achieving the same, safe ends. The best practice would be to require an engineer's wet stClmp and a soil analysis for turbines greater than 20kW, but allow "worst-case" foundations (alr~ady the industry's standard practice) for turbines any smaller. This would ensure quality, secure, and affordable installations.