Lateral Behavior And Caculation Method For Monopile Based On Modified Strain Wedge

As one of the important fields of renewable energy development and utilization,offshore wind turbine has become a research hotspot in the development of the global wind turbine industry.Foundation is an important part of ensuring the safe operation of offshore wind turbine,and its construction cost accounts for about 1/3 of the whole project cost.The monopile foundation has the advantages of simple structure,clear force,fast construction and relatively low cost,which is widely used in offshore wind farms at home and abroad,accounting for up to 80%.However,the environment of offshore wind power is complex,which is subjected to lateral loads such as wind,wave and water flow during its use,and face problems such as combined loads effect,scour effect and soft soil foundation problem.Scholars at home and abroad have done a lot of research on the calculation method and performance of laterally loaded monopile foundation,but there are still some shortcomings.At present,the theoretical calculation method of p-y curve of laterally loaded pile foundation is experimental and empirical dependent.There is still lack of uniform calculation method for the response of rigid pile foundation,especially under combined loads.Many studies have focused on the scour mechanism and the prediction of scour-hole dimensions,but research on the bearing capacity of pile foundations after scour is rare.Due to the characteristics of the soft clay seabed and the scour around pile foundation,the soft soil should be strengthened in order to improve the bearing capacity of the laterally loaded pile foundation.However,there are few reports on the response of the laterally loaded pile foundations after reinforcement.In this paper,aiming at the shortcomings of research of laterally loaded pile foundation,theoretical analysis,numerical calculation and centrifugal test are combined to systematically study laterally loaded pile foundation of offshore wind turbine under normal environment,scour environment and reinforcement environment.The main research contents and results are as follows:(1)This paper establishes an approximate circular cone strain wedge model and proposes an analytical method that can effectively connect practice with theory for the behavior of laterally loaded flexible piles with both free-head and fixed-head conditions in various types of soil based on the real shape of the strain wedge.The modulus of the soil foundation reaction is calculated along the whole pile length based on the finite-difference method,which can calculate the height of the strain wedge directly and can truly reflect the nonlinear deformation characteristics of the pile foundation under lateral loading.The analytical method of the curved strain wedge model,in which the Duncan-Chang model is used to describe the stress-strain relationship of the soil and the relationship between the shear strain and the horizontal strain is improved on the basis of the Mohr circle of strain,is applicable to cohesive soil and cohesionless soil.The proposed method is verified by some case studies,including field measurement experiments for various types of soil and finite-element analysis.Comparisons show that the agreement between the results from the curved strain wedge method and those from the field tests and the finite element analysis is generally satisfactory.(2)An analytical method is first proposed for the response of laterally loaded rigid piles under lateral load based on the curved strain wedge model.The influence of the resistance force and bending moment at the pile bottom and the resistance bending moment caused by the pile deflection are all considered.The effect of the vertical load on the lateral response of the monopile foundation is then analyzed according to the practical fact that the piles are always subjected to combined loads.The effect of P-Δ and the effect of the vertical skin friction on the pile side on the pile foundation caused by the vertical load are considered.Several field tests are used to verify the reasonableness and accuracy of the proposed method.Finally,the influence of different vertical load levels on the response of pile foundation under horizontal load is analyzed.The results show that the vertical load has a favorable effect on the lateral response of the pile in cohesionless soil and cohesive soil under darained condition,especially when the lateral load is larger,the lateral displacement of pile foundation decreases obviously.However,the vertical load has almost no effect on the lateral response of the pile in cohesive soil under undarained condition.And the bending moment(or loading height)of pile top has little effect on the reduction rate of lateral displacement at ground caused by vertical load.(3)This paper proposes a general solution for laterally loaded piles(flexible and rigid)with both free head and fixed head based on the curved strain wedge model under the condition of scour.The rationality and accuracy of the proposed analytical method are verified by two different case studies.The effect of the scour dimension on the response of piles with different pile head conditions and pile types is investigated.The results show that the pile head condition,pile type and scour dimension all have a great influence on the pile response,and should all be carefully considered.Under the same conditions of other parameters,the pile with fixed pile head is less susceptible to the scour dimension than the pile with free head,and the flexible pile is less susceptible to the scour dimension than the rigid pile.(4)Combined with the actual engineering geological conditions and the foundation reinforcement form,the centrifugal model test is used to study the lateral bearing characteristics of large diameter steel pipe pile foundation before and after reinforcement.The proportion of bearing capacity improvement of large diameter monopile foundation after reinforcement is analyzed.The change law of internal force and displacement of large diameter monopile foundation before and after reinforcement is compared.In addition,the soil parameters of centrifugal model test are determined through laboratory tests.According to the relevant parameters of pile foundation before reinforcement,the displacement and internal force of pile foundation are calculated by using the proposed modified curved strain wedge model.And the results are compared with those of centrifugal model test,which further verifies the rationality of the curved strain wedge model.The research above can provide theoretical support for the analysis of the mechanical performance of offshore wind turbine pile foundation under complex loads.