| Helical piles were utilized as the foundation of lighthouse and other offshore structures over one hundred years ago.In recent decades,the helical pile is getting attention from engineers in offshore wind turbine area and is being treated as a kind of novel environmental pile foundation for offshore wind turbines,because of its advantages,such as the fast installation speed,the portable construction equipment,noiseless installation process.As the foundation of offshore wind turbine,the installation,uplift capacity during working period and recycle are very important to the design and the construction of helical pile.However,the research on the mechanistic of these problems mentioned above is relatively scarce.Based on the three key factors mentioned above,this thesis carried a systematic study to understand more about them with the help of model test,centrifuge test and theoretical analysis.The main research contents and innovative research results are as follow:(1)The large-scale model tests for the installation process of single-plate helical piles are carried out in a reinforced concrete sand tank with a dimension of 4m width,4m length and 2m height.The influence of the installation speed of helical pile,the out diameter and the pitch of helical plate in fine silica sand are analysed.The model test results show: the installation torque increases with the raise of the out diameter of helical plate when the installations speeds are same;the compression required for installation has direct influence on the installation torque,the more compression the bigger installation torque;there are two different kinds of installation speed,the fast and the slow,and installation speed influences the compression directly,then it influences the installation torque indirectly;the pitch of helical plate has little influence on installation torque and compression required for installation when the helical piles are installed under slow installation speed,however,the compression required for installation and the installation torque both increase with the raise of the pitch of helical plate when the helical piles are installed under fast installation speed.(2)By combing the screw drive theory and traditional force analysis model of static pressure pile,the helical plate is transferred into a sloping plate with thickness.Then a theoretical model is proposed according to the force analysis about the sloping plate and the central shaft.This theoretical model considers the influence of the size of helical plate,soil parameters and the compression required for installation.The results got from this theoretical model have an extremely good fit with the corresponding model test results,this comparative analysis also verify the accuracy of this theoretical model.(3)With the help of Particle Image Velocimetry(PIV)technique,a serious of centrifuge tests about the uplift behaviour of single-plate and multi-plate helical piles are conducted in fine silica sand.The influence of the relative density of sand,embedment of plate and the spacing between adjacent helical plates on uplift capacity and the corresponding failure mechanism are analysed.The test results show: For single-plate helical pile,the uplift capacity factor and the influence area of failure mechanism increase with the raise of the relative density of sand and embedment firstly,then the uplift capacity factor remain unchanged when the embedment reaches some threshold embedment.For multi-plate helical pile in dense silica sand,the adjacent plates have influence on each other when the spacing between helical plates is smaller than six times of the out diameter of helical plate,the ultimate uplift resistance of multiplate helical pile is smaller than the total uplift resistance of one-plate helical piles with corresponding embedment.When the maximum embedment of plate is equal to 12 diameter of plate and the spacing between the adjacent plates is bigger than six times of the out diameter of plate,the adjacent plates have no influence on each other.(4)Based on M-C yield criterion,a theoretical calculation model,which could consider the influence of installation process,is proposed to predicted the uplift capacity of single-plate helical pile which is shallow embedded in dense sand.A serious of large-scale model test about the uplift capacity of single-plate helical piles are conducted in reinforced concrete sand tank with a dimension of 4m width,4m length and 2m height.The test results verify the accuracy of this theoretical calculation model and confirm the value of the parameter in this theoretical calculation model,(5)By combing the screw drive theory and traditional force analysis model of pile pulling,the helical plate is transferring into a sloping plate with thickness.Based on the force analysis of sloping plate and central shaft,a theoretical model is proposed to calculate the recycle torque of single-plate helical piles in sand.Then a serious of largescale model tests were done to verify this theoretical model.This theoretical model not only considers the influence of soil parameters and the shape of helical pile on the recycle torque,but also considers the influence of vertical force on helical pile.The good fit between test results and theoretical results verifies the accuracy of this theoretical model.(6)Selecting a wind farm in China.On the premise that the environmental load and the geological conditions are known,the jacket foundation is selected as the supporting part of the wind turbine,and the single-plate helical pile is selected as the pile foundation in the case.The corresponding design calculation is carried out based on the results of this paper. |
