AWEA WINDPOWER Conference, Houston, June 2008 Page 1 of 14 Wind Turbine Foundation Behavior and Design Considerations Kirk Morgan, Eric Ntambakwa Garrad Hassan America, Inc. ABSTRACT Despite certain aspects of wind turbine foundations which are common industry-wide, there is marked diversity in the understanding of the foundation behavior and application of design codes. This has led to different foundation designs for identical turbines with similar geotechnical conditions. The wind industry continues to grow using larger multi- megawatt turbines. An understanding of the driving considerations for the associated growth of the foundations will permit safe and economical designs while giving due consideration to design codes that may not explicitly address wind turbine foundations. This paper discusses the key design criteria and components inherent to wind turbine foundation design, specifically the spread footing. Potential impacts to these areas from larger turbines and how the engineering/construction community may address them are explored. 1 INTRODUCTION 1.1 Foundation types Wind turbines are founded on a variety of foundation types that for engineering purposes are primarily selected depending on geotechnical conditions and the foundation method of developing resistance to overturning forces. The simplest form of foundation is the spread footing which is essentially a gravity foundation that relies upon soil overburden and concrete to provide sufficient weight to resist overturning of the foundation at extreme wind loads. This type of design is applicable in a broad range of subgrade strengths from soils to rock. Rock socketed and short piers rely primarily upon end bearing and secondarily upon side wall friction and lateral earth bearing pressures. Their strength is derived from subgrades of significant strength at relatively shallow depth. Pile and cap foundations are often found in regions where competent soil or rock is found at much greater depths. Piles transmit loads from the cap to the subgrade via a combination of friction and end bearing and resist lateral loads through lateral earth pressure on the pile. Rock anchor and cap foundations are typically encountered in regions where strong bedrock is reachable at shallow depth. The foundation resists loads through a combination of bearing pressure beneath the cap at the bearing layer and tension in steel bars grouted into boreholes and post-tensioned (rock anchors).