Preliminary study of shear strain-based design approach for onshore wind turbine foundations

Different wind turbine foundation types were analyzed with PLAXIS 3D using a hardening/softening model with a small strain stiffness constitutive model to capture the characteristics of degradation of clays. The study found that even though the conventional design requirements were all satisfied, the shear strains under foundations varied greatly. A literature review regarding the shear strain versus soil degradation was performed, and relevant threshold strains that limit the soils to recoverable behaviors were discussed. A shear strain-based design approach was then proposed and a flowchart for the design process was presented for onshore wind turbine foundations. The proposed design approach simplifies the foundation designs under cyclic loads and relieves the concerns of cyclic degradation and the gapping issue of the foundations subjected to cyclic loads. The analysis results show the shear strains of soils could be effectively controlled within the small-strain threshold by using different structural dimensions, and sometimes together with geotechnical measures. The study found the widely used traditional gravity foundations are governed by conventional design requirements as the shear strains typically are smaller than the threshold level. However, the strain control criterion is dictating deep foundations used for wind turbines.