【作者】 张帆；

【导师】 戴国亮；

【作者基本信息】 东南大学 ， 土木工程， 2022， 博士

【摘要】 海上风电作为可再生能源开发利用的重要领域之一,已成为全球风电行业发展的研究热点。海上风电基础是保障海上风电安全运行的重要组成部分,其建设成本约占整个工程成本的1/3。单桩基础具有结构简单,受力明确,施工快捷,造价相对较低等优点,在国内外海上风电场中广泛应用,其占比多达80%。然而海上风电所处环境复杂,使用期间会承受风荷载、波浪荷载、水流荷载等水平荷载作用,将会面临着组合荷载效应、冲刷效应以及软土地基问题。针对水平受荷单桩基础计算方法及承载性能,国内外学者已经进行了大量的研究,但仍存在一定的不足:目前主流的水平受荷桩基理论p-y曲线计算方法具有试验和经验依赖性;对于刚性桩基的响应,特别是在水平荷载、竖向荷载以及弯矩组合荷载作用下的响应,还没有统一的计算方法;对单桩基础冲刷机理和冲刷坑尺寸预测的研究较多,但对冲刷效应下单桩基础承载特性研究较少;针对我国近海岸海床地质浅层土体为深厚软弱黏土以及桩基周围土体面临冲刷的特点,为了提高水平受荷桩基承载性能,需对浅层软弱土体进行加固,而对于浅层土体加固后桩基水平受荷性能的研究鲜有报道。本文针对以上关于水平受荷单桩基础研究的不足,采用理论分析、数值计算以及离心模型试验相结合的研究方法,分别对在正常环境、冲刷环境以及加固环境条件下水平受荷单桩基础进行了系统研究。主要研究内容和成果如下:(1)基于水平受荷单桩基础周边土体应变楔实际形状,提出了水平受荷柔性单桩基础修正圆锥应变楔模型计算方法,该方法既适用于桩顶自由桩基又适用于桩顶固定桩基。首先,根据水平受荷单桩基础随深度的非线性变形,计算各深度处地基反力模量,这样可直接计算得出土体应变楔的高度,获得单桩基础在较大荷载作用下,受荷深度范围内桩基真正的双向变形(与加载方向相同的变形和与加载方向相反的变形)。其次,根据邓肯-张模型以及改进的土体剪切应变与水平应变之间的关系,使得该应变楔模型计算方法可以应用于黏性土和无黏性土。最后,通过现场试验和有限元分析结果,验证了修正圆锥应变楔模型计算方法的可靠性。(2)基于柔性单桩基础修正圆锥应变楔模型,提出了一种在水平荷载及弯矩组合荷载作用下,可考虑由桩基转角引起的桩基与土体之间的竖向摩擦力、桩底阻力和桩底阻抗弯矩的刚性桩基解析方法。同时考虑竖向荷载作用下的P-Δ效应和由竖向荷载引起的竖向摩擦力对水平受荷桩基响应的影响,提出了一种可以考虑竖向荷载对桩基水平受荷响应影响的解析方法。通过现场试验分别验证了这两种解析方法的可靠性,分析结果与现场试验结果吻合较好。最后分析了不同竖向荷载大小以及加载高度对桩基水平受荷响应的影响,研究结果表明,在排水加载的无黏性土和排水加载的黏性土中,竖向荷载对桩基水平受荷响应具有有利影响,特别是在水平荷载较大时,桩基水平位移减小较为明显;而在不排水加载的黏性土中,竖向荷载对桩基水平受荷响应几乎没有影响;桩顶弯矩的大小(或加载高度)对由竖向荷载引起的地面处水平位移的减小率影响不大。(3)基于修正圆锥应变楔计算模型,提出了一种在冲刷条件下,能同时考虑冲刷坑尺寸和土体应力历史变化的水平受荷单桩计算方法,该方法既可以适用于具有不同桩顶约束条件(桩顶自由和桩顶固定)的桩基又可以适用于不同类型桩基(柔性桩基和刚性桩基)。通过两个算例对该方法进行了验证,算例结果与计算结果吻合较好。结合冲刷坑尺寸和土体应力历史的影响,讨论了在不同冲刷坑尺寸下,桩顶约束条件和桩基类型对水平受荷桩基响应的影响。结果表明,在分析冲刷对水平受荷桩基响应的影响时,应充分考虑冲刷坑尺寸、桩顶约束条件和桩基类型等参数。在其他参数相同的情况下,桩顶自由桩基比桩顶固定桩基更容易受到冲刷坑尺寸的影响,柔性桩基比刚性桩基更容易受到冲刷坑尺寸的影响。(4)结合实际工程地质条件和地基加固形式,通过离心模型试验对多层地基加固前后大直径钢管桩水平承载特性进行研究,分析加固后大直径单桩基础承载性能提升比例,对比加固前后大直径单桩基础内力与位移变化规律。另外,通过室内试验确定离心模型试验土体参数,根据加固前桩基离心模型试验相关参数,采用提出的修正圆锥应变楔模型对桩基位移和内力进行计算,并将计算结果与离心模型试验结果进行对比分析,进一步验证了修正圆锥应变楔模型的合理性,从而为海上风电桩基在复杂地质条件下的受力性能分析提供一定的理论支持。更多还原

【Abstract】 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.更多还原