The effects of stiffness anisotropy on elastic solutions of circular foundations under complex loading

Current wind turbine (WT) foundation codes and design approaches rely on rather conservative geotechnical assumptions to aid the simplification of design processes. Moreover, the industry often treats WT foundations as an extension of the turbine itself, using catalog approaches for design; which can lead to over-design and cost increases. Existing design codes incorporate uncoupled analytical elastic solutions that can be used for preliminary designs under idealized soil conditions such as linear elasticity and isotropy. However, most natural soil deposits exhibit some degree of stiffness anisotropy and stiffness degradation with cyclic strains. This paper describes the results from a series of analyses performed to obtain coupled elastic solutions of shallow and embedded foundations founded on anisotropic soils with linear increase of stiffness with depth under complex loading. The effect of the domain dimensions was examined and a domain width of 50 times the circular foundation diameter was found to be sufficiently accurate for the finite element analysis. Correction factors were derived to account for the effects of soil vertical non-homogeneity and foundation embedment. The results show that the vertical stiffness rate of change affects the vertical, horizontal and moment stiffness, but does not affect the coupling between horizontal and moment responses. The effect of soil stiffness anisotropy on foundation responses was investigated by an application of the stiffness equations. The comparison shows that the stiffness anisotropy can considerably affect the vertical and horizontal translations of the foundation but has a limited influence on the rocking behavior.