Loading and Dynamic Response Considerations for the Design of Wind Turbine Foundations on South African Soils

Abstract

Wind energy has been selected an appropriate means of diversifying South Africa’s energy mix and improving its electricity capacity in view of growing socio-economic and environmental pressures. However, this comes with engineering challenges, one being designing efficient foundations for wind turbine structures. This discussion was centred on the sources of loading that wind turbines experience and the consequences of this on the geotechnical design of gravity footings. Rotational stiffness of the foundation was shown to have an important effect on the dynamic response of the wind turbine tower, and thus, on the assumptions surrounding the calculation of the natural frequency of the global system. Means of assessing the rotational and lateral stiffness as well as models investigating soil stiffness inclusion in natural frequency assumptions were evaluated in the context of a South African case study, specific to the South Eastern city of Port Elizabeth. Soils of this region were dominated by weathered silty fine sands and varying degrees of pedogenic calcrete, creating unique challenges in design. Soil stiffness effects on natural frequency assumptions were found to be more critical than the minimum stiffness requirements applied by design guidelines and had a notable effect on dynamic amplification for an undamped system.