With improved mechanical performance, the use of reinforced concrete (RC) materials have rapidly increased. Generally, the design integrity of RC structures is always validated via modeling. To date, the finite element (FE) method remains the most commonly used approach for modeling various structural components, both in industry and for research purposes. Several FE models have been developed to study the mechanical behavior of various structural designs. Whereas modeling of nonlinear dynamic behaviors of RC structures is challenging, the accuracy of the process can be improved by accounting for the effects of the soil-structure interaction (SSI), which further complicates the process. This can be attributed to two numerical problems: excessive computation demands and numerical instabilities associated with the dynamic analysis.
Notably, it is impractical to study the mechanical behavior of wind turbine structures in a laboratory setting owing to their large sizes. Over the past decades, the design of wind turbines has significantly evolved to enhance their operational efficiency. Such developments have increased the potential influence of SSI on their structural response. Thus, the effects of SSI and soil-foundation-structure interaction (SFSI) are crucial and must be considered for the safe and robust design of wind turbine structures. Among the available numerical methods for investigating the effects of SSI and SFSI, the direct integrity method has been widely used by several researchers and engineers. However, this approach is simplistic and less effective as it does not approximate the soil domain. Additionally, previous findings established the importance of predicting natural frequency when studying the mechanical responses of wind turbine structures. Therefore, developing of a more accurate approach for simulating SFSI effects while accounting for soil discretization is highly desirable.
On this account, Mr. Dewald Gravett and Professor George Markou from the University of Pretoria developed a new computational algorithm to model and analyze onshore wind turbine structures to investigate the SFSI phenomenon. The three-dimensional (3D) numerical models were constructed using soil material properties and wind turbine structural designs derived from an onsite geotechnical study performed under the WindAfrica project that mainly focuses on investigating wind turbine structures founded on soft and expansive clays. The 3D models were used to investigate the dynamic response of the turbine structures by considering accurate soil discretization: soil superstructure, soil domains and pile foundations, with the use of 8-noded hexahedral elements. The feasibility of the proposed modeling approach was validated against the existing experimental data. Lastly, the inclination and effects of the buttered piles were investigated parametrically. The work is currently published in the journal, Engineering Structures.
Results demonstrated that the proposed models were capable of foreseeing the application of hexahedral elements to discretize the SSI model domains characterized by the modal and pushover analysis of an 80m tall wind turbine VESTA tower founded on different soil profiles. While turbines structures were more likely to fail at the base of the tower due to buckling, the optimum inclination of the battered piles for all the soil profiles was found to be that of 10 degrees. Although the findings contradicted the previous research findings, this approach was more accurate than the existing approaches considering its ability to foresee the use of the more accurate and computational demanding hexahedral elements. In addition, the use of battered piles significantly reduced the soil deformation levels, thus minimizing the chances of developing large strain concentrations that will lead to nonlinearities in the soil domain.
In summary, 3D detailed models were used to investigate SFIS effects on wind turbine structures. The numerical findings provided a better understanding of the different levels by which SFSI influences the mechanical behavior of the turbine structures in different soil profiles. The obtained results agreed with the previous studies, confirming the usefulness of battered piles in improving the mechanical performance of wind turbine structures, whereas the derived optimum pile inclination was found to be that of 10 degrees. Overall, the study established that the piles should be embedded in rocks, when feasible, to improve safe stress transfer to the bedrocks and minimize the deformations and tower displacement. In a statement to Advances in Engineering, Professor George Markou explained that the study provided useful insights to develop state-of-the-art modeling approaches for the robust design of wind turbine structures.
Gravett, D., & Markou, G. (2021). State-of-the-art investigation of wind turbine structures founded on soft clay by considering the soil-foundation-structure interaction phenomenon – Optimization of battered RC piles. Engineering Structures, 235, 112013.