The present thesis aims to model and evaluate Soil Structure Interaction (SSI) issues for the dynamic response of onshore wind turbines grounded on a layered soil. The recent growth of wind energy installations has certainly provoked a rapid expansion into regions where the soil conditions may be complex and the seismic risk high. However, there is a lack of knowledge about unfavorable effects due to the interaction between the wind turbine, its foundation and the underlying layered soil. Although recent norms provide guidance for the consideration of SSI effects, these are often neglected, as their modeling is not straightforward. Whenever the soil is taken into account, in fact, it is usually idealized as a homogeneous half space, thus neglecting significant effects due to stratification. In this work, both rigorous and simplified investigation methods for SSI analyses are considered. The coupling between the finite elementmethod (FEM) and the boundary element method (BEM) has been chosen among the rigorous approaches, whereas lumped parameter models (LPM) for the foundation-soil system have been implemented as a simplified alternative. Through benchmarkexamples, important aspects of the soil-structure modeling are highlighted. Moreover, the suitability of the simplified approach has been proved by means of comparisons with the FEM/BEM coupling. The proposed models involve a 3-blade wind turbinegrounded on a layered half space, which has been idealized as a horizontal layer overa homogeneous half space. The numerical investigation is divided into two parts. In the first part, the dynamic response of the structure-soil system is analyzed in frequency domain with the accurate FEM/BEM coupling. A preliminary assessment of the foundation-soil system has been carried out; the focus has subsequently shifted to the whole turbine-soil assemblage. The effects of different parameters have been systematically evaluated, in order to provide a range of values for which the SSI has tobe accounted for. The second part focuses on the transient response of a 5-MW reference turbine subjected to aerodynamic and seismic loads, considering SSI effects. In accordance with international standards, three scenarios have been considered: idling conditions, normal operational state and emergency shutdown. For the seismic excitation, recorded strong motions have been selected for the control motion, which have been modified in order to account for the presence of the horizontal layer. Several simulations have been carried out in order to assess the influence of the SSI on the internal forces and displacements of the wind turbine structure; results are shown and discussed. The final purpose of this study is to provide a reference for practitioners, designers and researchers about SSI effects for layered soils, which helps to gain a deeper understanding of this phenomenon in seismic areas, thus contributing to more reliable performance assessments and costs estimations.
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