Geotechnical Design Considerations for Onshore Wind Turbine Shallow Foundations

Electricity generated from wind power continues to grow rapidly as an important part of the energy mix in many regions throughout the world. Advances in technology have led to a rapid increase in wind turbine capacity with the average wind turbine installed in the United States in 2014 being capable of generating about 2 MW of electricity, reflecting an 8-fold increase since 1990. The industry has also seen taller towers being utilized to harness wind resource at higher elevations to take advantage of higher wind speeds and reduced turbulence. Taller towers typically require larger or more robust foundations to support the wind turbine systems and typically translate to higher capital costs for the projects. Balance of Plant (infrastructure and foundation) construction costs make up about 30 percent of capital costs for typical utility scale wind power projects and although there has been a reduction in wind turbine costs over the years, construction costs remain relatively high. Optimized foundation designs contribute to overall reduction in capital costs and would contribute to advancement of renewable energy generation. Geotechnical design is an essential component for the design, planning and construction execution of utility scale wind power developments. Wind turbine foundations have specific design requirements which have to be taken into account to ensure reliable operation of the turbines over a typical design life of 20 to 25 years. It is also important for the geotechnical and structural design aspects to be adequately synchronized in order to develop efficient designs taking into account site specific geotechnical conditions and potential constraints. This paper presents a discussion of important considerations for geotechnical investigations and design for utility scale wind turbine shallow foundations including global stability, bearing capacity, differential settlement, rotational stiffness and cyclic degradation considerations. The paper includes detailed discussion of geotechnical design requirements and design approaches which can be utilized to optimize geotechnical aspects of wind turbine foundations. Merits and limitations of typical approaches such as allowable stress design compared to limit state design for bearing capacity will be discussed.