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Experimental and numerical study of soil-structure interaction subjected to cyclic-dynamic loading : application to offshore wind turbine monopile foundation

Abstract : Offshore wind turbines form one of the principal solutions adopted to ensure the cleanliness, sustainability, and renewability of energy sources. Therefore, this field is growing exponentially, as shown by the increase of the dimensions and capacities of these structures. Besides, the environmental loading acting on these structures equally increases, imposing particular attention on the design of the foundation system. Due to economic and practical reasons, mono-pile foundations are commonly adopted. The form of these mono-piles depends on wind turbines growth, moving to a smaller length-to-diameter ratio and then to more rigid behavior. The behavior of this kind of mono-piles under lateral cyclic loading still involves some knowledge gaps. Recently, several physical models have been developed to fulfill this purpose, leading to propose a conservative design method. Due to the problem complexity and despite the progress achieved in this field, conflicting findings are still encountered, depending on the considered approach to the problem.In the framework of this thesis, two reasonable sets of mono-pile dimensions are proposed in a way to obtain different factors of rigidity. The aim is to examine the cyclic behavior of each mono-pile foundation and then evaluate their performance as a foundation of “DTU 10 MW RWT” in relatively deep waters. An accurate set of scaling laws under laboratory gravity is proposed and adopted in the development of representative scaled models. The challenge is to respect the combination of the soil non-linearity, the soil-structure interaction, and the dynamic response of the system. The obtained scaled model should not only simulate the qualitative behavior of the system but also be able to offer quantitative insight into this behavior. Finite element simulations using “Cesar 3D – LCPC” are carried out to verify the dynamic similarity between the scaled models and the prototype.Conscious of the difficulties and limitations underlying the 1g physical modeling, a part of this study is dedicated to the investigation of sand behavior at different stress levels. This study aims to determine the parameters of stress-dilatancy relation for Fontainebleau (NE34) sand, permitting the determination of the sand state required to simulate the prototype sand state at the laboratory scale. Besides, specific procedures are followed to ensure the tests repeatability at corresponding low-stress levels. Cyclic strain-controlled tests are performed, showing tests repeatability and, therefore, the validity of the testing procedure. Then, cyclic tests with different excitation frequencies are performed, highlighting the dynamic aspect of the system response. The analysis of a typical test shows the capability of the proposed scaled model to fulfill some knowledge gaps. This part ends with the recommendation of a cyclic tests program, aiming at tackling some ambiguous points to assist in the scientific debate in this domain.On another side, a constitutive model is developed to simulate the sand behavior at different stress levels. This model is based on “Hoek-Brown” failure criteria and a suitable hardening function. This model is calibrated against the experimental findings of monotonic, strain-controlled, triaxial tests. Then, this model is implemented in “Cesar 3D – LCPC” finite element program and validated against a parametric study of monotonic laterally loaded rigid mono-pile. A good agreement is obtained between the numerical and experimental findings. This parametric study aims to discuss and evaluate the performance of failure criteria proposed for rigid mono-pile foundations. This discussion ends with the suggestion of a new failure criterion and the recommendation to adopt tilt criteria instead of displacement criteria in case of rigid mono-piles.
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Hussein Bakri. Experimental and numerical study of soil-structure interaction subjected to cyclic-dynamic loading : application to offshore wind turbine monopile foundation. Géotechnique. École des Ponts ParisTech, 2021. English. ⟨NNT : 2021ENPC0030⟩. ⟨tel-03657013⟩

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