Turbine manufacturers typically include a requirement for foundations to be in full contact with the supporting materials during normal power production. The no gapping specification is primarily to ensure that the foundation reliably provides the minimum stiffness and thus operate within the design frequency envelope, and to limit potential for cyclic degradation of the supporting materials.

As turbine sizes and demands on the foundation continue to increase, foundation designers are looking at options to optimize designs.  This includes relaxing the no gapping requirement while ensuring that foundation stiffness requirements are met. Such approaches are supported by contemporary industry standards, provided that robust assessing of soil cyclic degradation potential is performed. An advancement in the standard of practice is necessary and will inevitably be required to reliably evaluate the potential risks due to foundation gapping. Cyclic soil testing during geotechnical investigations will be a necessity for on-shore wind farms for reliable evaluation of foundations designed to incorporate gapping.

This paper presents a discussion of no gapping criteria typically employed in wind turbine foundation design standards, potential impact on foundation stiffness and cyclic degradation of the supporting soils, and design considerations for assessing potential risks due to gapping.