Centrifuge Model Tests On Seismic Behavior Of Single Pile For Wind Turbines

Located in the joint of the circum-Pacific seismic belt and the Mediterranean-Himalayas seismic belt,our country,China,is one of the countries with the strongest seismicity and the most severe earthquake disasters all over the world.Wind farms that have been built or planned to be built are mainly distributed on the "Three North" and eastern coast.At present,there is a lack of a comprehensive investigation of the dynamic response of wind turbine foundations under the earthquake.The design principle of that is usually adopted based on lateral deformation control method.However,marine environment is characterized for complexity and variability,resulting in the following challenges for earthquake resistant design of wind turbines:as wind turbines are subjected to lateral effects such as wind,waves,currents and other factors constantly for a long period,it is unclear whether these initial lateral effects have an impact on the seismic response of the entire wind turbine system and the surrounding soil when subjected to earthquakes;the soil around the wind turbine foundations is eroded by wave and current incessantly,and local scour pit is easy to form around the foundation,which will reduce the lateral bearing capacity and service life of these foundations;when earthquake happens,how much the local scour pit will affect the dynamic characteristics and seismic response of wind turbine foundations;since earthquakes are composed of foreshocks,main shocks,and aftershocks,and traditional research mainly focuses on the response of the foundation under main shocks,it is unclear how the dynamic response of the single pile foundation for the wind turbine evolves,and deserves further study whether the dynamic characteristics deviate from the design value in the whole earthquake sequence.In this paper,the physical simulating study of the single pile foundation of offshore wind turbine was carried out using ZJU-400 centrifuge shaking table.Based on the principle of centrifuge similarity criterion,a series of centrifugal models similar to the first-order natural frequency of the site were prepared,including dry sand foundation,horizontal saturated sandy foundation,and saturated sandy foundation with local scour pits.Additionally,a flexible lateral environmental load application device was developed for supergravity environment,and three groups of centrifuge tests were carried out.The main research results are as follows:(1)Differences in dynamic response of the offshore wind turbine single pile foundation and the surrounding soil in dry and saturated sandy foundations were compared and analyzed,revealing the impacts of evolution of excess pore pressure on aforesaid dynamic response.In response to the evolution of the dynamic response of the offshore wind turbine single pile foundation during the earthquake sequence,the differences of seismic response between single pile foundation and the soil around were investigated for one seismic sequence.The results show the effects of earthquake history on top acceleration amplification factor,internal force and lateral displacement of the pile,and on the acceleration,excess pore pressure evolution,stress-strain curves and shear modulus attenuation changes of the surrounding soil.The pile-soil interaction has also been studied.(2)The seismic responses with initial lateral load and without that were compared,revealing the internal mechanism of the initial lateral load effect on the development of the lateral displacement of the pile top.The suppressed phenomenon of excess pore pressure of soil in front of pile occurred when subjected to initial lateral load,and the internal relationship between this phenomenon and the stress state of the soil there was analyzed through numerical simulation.The p-y curves of the horizontally loaded pile were derived at static loading and shaking stages,indicating the influence of initial lateral load on pile-soil interaction.(3)As for the local scoured single pile foundation,the differences of seismic response of this foundation with and without local scour pit were carefully compared and analyzed,exploring the internal mechanism of the influence of local erosion on the internal force and deformation of the pile and the dynamic characteristics of the soil around.The dynamic p-y curve is deduced by calculation,and the influence of local scour effect on pile-soil interaction is revealed;(4)Tcl running code related to the dynamic test of triaxial unit has been prepared to evaluate the applicability of multiple yield surface model in simulating the effect of initial static shear stress.The seismic response of the single pile foundation under LEAP with initial lateral load was simulated by the software OpenSees,and was compared with experimental results.Combining experimental results with numerical ones,the p-y curve model was established by introducing the excess pore pressure ratio into the hyperbolic p-y model on the basis of the effective stress principle.With the back-analysis program,the numerical model was verified.