Experimental Research And Numerical Analysis On Mechanical Response Of Offshore Wind Generator Foudation Soil

Although extremely strict requirements for deformation behaviors of offshore windgenerator foundation structures and adjacent soils are controlled in their design and use,the knowledge about the actual deformation laws are still very limited. In thisdissertation a new test apparatus was first developed and a series of experiments andnumerical analysis were then made for the purpose of investigating the uniquedeformation behavior of offshore wind power foudation soil. The main achievementsare summarized as follows.1. A new large-scale computer-controlled compression-torsional multi-axialloading apparatus has been successfully designed and developed. This apparatusintegrates many geotechnical testing functions including cyclic triaxial, cyclic torsionalshear, cyclic true triaxial and cyclic plane strain testing. It has been used to test thedeformation response of offshore wind power foudation soil.2. Through3D FEM numerical analysis using two different soil constitutivemodels, the stress-paths and stress variations in the foundation soil, induced by windsand waves, were systematically investigated for a typical offshore wind generator withsingle pile foundation. Three-dimensional cyclic rotation of the principal stress axeswithin a limited rotating angle is found to take place always in the foundation soiladjacent to the foundation structure of an offshore wind generator. In particular it isshown to be become a two-dimensional cyclic rotation of principal stress axes when twodirections of the wind load and the wave propagation keep the same. The stress pathsbecome more complex when the frequencies of those two cyclic loads are not the same.No signicant difference in the above-mentioned stress paths and variations is shown bycomparing the results of the numerical analysis using the different constitutive models.3. The basic stress-strain characteristics and related micro-physical mechanisms forsaturated Toyoura sand under repetitive cyclic rotation of principal stress axes within alimited rotating angle were investigated based on a series of drained tests and2D DEMnumerical simulation in which the magnitudes of all three principal stresses keptconstant. Effects of the rotating angle α and the initial direction angle α0of the majorprinciple stress are investigated. As a result, the following main experimental facts andfindings are confirmed by making a comparison with the test results of the monotonicrotation of the principal stress axes.1) The development of each strain component issignificantly influenced by the two angles α and α0, and it shows a graduallystrain-hardening tendency with the increasing cycles.2) The volumetric strain displays a tendency from the contraction in the first few cycles to the expansion in the later, whichis obviously governed by the rotating angle α. The occurrence of this phenomenon isattributed to the lateral expansion in the intermediate principal stress direction induceddue to the cyclic oscillating and dislocation of the sand particles within the plane wherethe major and minor principal stresses act.3) The shear stress-strain behaviors andrelated deviatoric strain paths are both controlled by the two angles α and α0. Thedouble shear strain amplitude is significantly smaller than that when the princial stressaxes rotate monotonically along one direction.4) The direction of the shear strainincrement depends only on the loading and unloading of the total shear stress beingcomposed of the initial and cyclic shear stresses, which is quite different from that ofthe monotonic rotation of the principal stress axes.5) The macro deformation of thesand as a granular material is mainly dependent on slippage of the particles, while noobvious developments in both the particle deflection and the anisotropic degree areshown.