大直径钢管桩水平载荷试验研究

文章首次对直径为2800mm 的超大直径钢管桩开展了单桩水平静载试验,研究分析了超大直径钢管桩的桩侧水平抗力、桩身截面弯矩、桩身水平位移与桩基水平荷载之间的变化曲线,得到了不同水平位移时桩周地基反力模量的变化规律,为超大直径桩基水平承载力设计提供了现场试验数据。     

水平荷载作用下砂土中非柔性桩的p-y曲线修正

中国近海风机大多采用长径比10～20范围内的非柔性桩基础,而现有规范方法主要针对长径比大于20的柔性桩,对我国风机基础的适用性一直存在争议。本文利用有限元法研究砂土中非柔性桩的水平受荷响应,主要关注容许范围内的桩基倾斜,因此采用硬化土小应变本构模型描述砂土的应力应变关系。建立钢管桩-土三维有限元模型进行变参数分析,探究土体相对密实度、桩基直径对初始地基反力模量的影响;讨论了正切双曲线函数和双曲线函数描述土体弹簧刚度(即p-y曲线)的合理性;最终提出了适用于砂土中非柔性桩的修正p-y曲线表达式,并通过与三种不同砂土相对密实度与桩基组合工况下的有限元结果对比,验证了修正p-y公式的合理性。结果表明：土体相对密实度、桩基直径与初始地基反力模量均呈正相关;双曲线函数更适合描述非柔性桩的p-y曲线;修正后的p-y公式提高了水平荷载作用下非柔性桩响应的预测精度。     

p-y曲线对成层土体中大直径单桩的适用性研究

API规范推荐的p-y曲线是由均质土体得到的,并未考虑土层间相互作用,Georgiadis基于柔性桩提出了等效深度法修正p-y曲线,把均质土p-y曲线延伸到了成层土体中。为了研究p-y曲线和等效深度法对于大直径单桩在成层土体中的适用性,取4种典型地质条件:成层砂土、砂土-黏土-砂土、成层黏土和黏土-砂土-黏土,通过L-PILE软件计算了6 m直径单桩基础的p-y曲线、桩顶水平荷载-位移曲线、桩身位移和弯矩。并与ABAQUS建立的单桩基础三维有限元模型计算结果进行比较。结果表明等效深度法对于成层砂土影响不大;对于成层黏土影响较大;对于中间为软弱土层的成层土体,在荷载较大时影响显著,等效深度法计算结果更加接近FEM结果。在成层土体中,p-y曲线应用于大直径单桩对于砂土高估了初始刚度,低估了极限抗力;对于黏土则低估了初始刚度和极限抗力。     

黄河水下三角洲快速沉积粉土层桩基p-y曲线试验研究

利用室内水槽模拟黄河三角洲快速沉积环境,并进行了黄河水下三角洲快速沉积粉土层中单桩水平承载特性模型试验,获得桩身弯矩、位移,以此得到适用于快速沉积粉土层中的桩基p y曲线,与API规范的p-y曲线相比,发现浅层土极限土抗力要大于黏土与砂土,而深层土极限土抗力则介于两者之间。并进行多组试验,讨论了桩的不同埋置方式对p-y曲线的影响,发现贯入桩p-y曲线与预埋桩p-y曲线相比具有较小的初始刚度,以及较大的极限土抗力;同时,相较于预埋桩,贯入桩对浅层土极限土抗力的影响更为显著。     

黏土中海上风电水平受荷大直径单桩设计方法的思考

文章回顾了目前风电大直径单桩基础水平受荷静力响应分析的p-y 曲线规范方法,结合现有成果探讨了该方法会高估桩侧初始刚度并低估极限承载力的原因。为解决其不足,文章介绍了一种基于土体应力应变关系的p-y 曲线方法,它不但能较为正确地反映桩侧刚度,还能跟踪桩周土体的平均塑性应变的累积。在此基础上探讨了桩基在循环疲劳状态下的规范方法,由于其无法精确反映循环次数和幅值对于桩侧刚度弱化的影响,因此,进一步介绍了基于静力p-y 骨干曲线的滞回曲线构造方法,最后,基于上述分析方法,提出了一些大直径单桩优化设计的建议。     

海上风电嵌岩桩水平抗力影响因素研究

桩基础是我国海上风电工程中应用最为广泛的基础形式,其中嵌岩桩因其施工难度大,承载力高备受关注。与其他类型的桩基础不同,嵌岩桩的水平承载力不仅受到围岩强度的影响,更与其成桩质量与灌浆材料的强度相关。采用有限元方法分析了嵌岩深度、桩基直径与壁厚、桩身倾斜度等多种因素对嵌岩桩水平承载力的影响,提出了确定嵌岩桩水平极限抗力的标准。研究表明：桩与围岩间的灌浆环会先于桩身发生破坏,因此可将灌浆环受拉破坏作为判断嵌岩桩达到水平极限承载力的标准;桩身倾斜度对嵌岩桩的水平极限承载力影响较大,直径和壁厚的增加,均能提高桩基的水平承载力。     

无粘性土中桩基水平非线性地基反力系数研究

非线性地基反力系数法是分析近海桩基水平大变形性状的有效途径。针对我国近海广泛分布的粉土和砂土地基,建立了无粘性土中桩基非线性地基反力系数与桩基径向土压力之间的关系,据此基于物理模型试验和现场试验结果获得了粉土和砂土地基反力系数与桩径和桩身变位之间的非线性关系,为桩基水平受力大变形分析提供了简单有效的方法,适用于不同直径桩基的计算,得到了桩基物理模型试验和现场试验结果的验证。     

可液化土p-y曲线模型折减方法的研究进展

对于海上风力发电机等海上构筑物的桩基础,在地震或波浪荷载作用下,由于超孔隙水压力的产生,地基土对桩身的水平抗力降低,如果按照正常情况下的p-y曲线进行设计,结果偏危险,因此设计时常对p-y曲线进行折减。本文阐述了可液化土p-y曲线折减方法的研究进展,着重概括了可液化土p-y曲线孔压折减系数的研究方法与成果,并将不同学者的研究成果进行对比,分析了不同研究方法对所得结果的影响。针对国内外学者对该问题的研究存在的不足,本文提出了若干思考和展望,认为应用p-y曲线折减的方法研究可液化土-桩相互作用问题是可行的。     

水平循环荷载作用下桩基p-y曲线模型研究进展

p-y 曲线法常用于水平静力受荷桩的分析，但海洋工程中的桩基除静力外还承受波、流等循环荷载作用，将静力 p-y曲线直接用于循环荷载下桩基的设计研究时，往往会产生误差。本文梳理了水平循环荷载作用下 p-y 曲线模型的研究成果， 按照得到循环 p-y 曲线的不同方式，将其分为总体调整法和参数修正法。在参数修正法中，根据修正时考虑的因素不同，分为考虑荷载特性的修正和考虑桩土相互作用的修正。最后通过对循环荷载作用下 p-y 曲线研究现状的总结对比，评价了不同方法的特点，讨论了当前研究存在的问题并给出了建议。     

海上风机超大直径单桩竖向抗压承载力理论计算及实测方法对比评价

超大直径单桩基础在应用过程中,存在钢管桩沉桩的选锤、防溜桩等关键问题.为解决以上问题,研究超大直径单桩的竖向抗压承载力是有必要的.分别利用《码头结构设计规范》规定的设计做法、圆孔扩张理论法、UWA-05方法、GRLWEAP模型方法对超大直径钢管桩的竖向抗压承载力进行了分析计算并结合工程实例进行比较,发现现行规范规定的钢管桩承载力计算公式对于超大直径钢管桩承载力的估计偏于保守,高应变检测法对于超大直径单桩基础承载力的评估同样偏于保守.     

Numerical investigations of soil plugging effect inside large-diameter,open-ended wind turbine monopiles driven by vibratory hammers

Abstract

This study established a Couple Eulerian–Lagrange model to simulate monopile vibratory penetration for the investigation of soil plugging effect during high-frequency penetration of monopiles for wind turbine. Simulation analysis is focused particularly on soil plugging effect of a large diameter monopile during vibratory penetration into sand, clay, or layered soil. The results of the numerical simulation show that soil plugging effect is unlikely to occur during monopile penetration into the clay soil, while partial soil plugging may occur during the sand penetration. Penetration resistance at the pile toe is transferred to the radial stress around the pile wall. At a critical point penetration process, internal shaft friction becomes larger than external shaft friction. Moreover, radial pressure factors increase during partial soil plugging effect. For layered soil, the topsoil not only has great influence on the soil plugging effect, but also affects shaft friction in the subsoil during monopile penetration.     

海上风电楔形单桩基础竖向承载力特性分析

为提高基础利用率增加海上风电设施的可行性，对楔形单桩基础竖向承载力特性进行研究分析。采用PLAXIS 3D 有限元软件建立楔形单桩基础模型，从桩侧摩阻力、桩侧法应力及土体位移对比分析楔形单桩基础与等截面单桩竖向承载特性差异，并探讨内摩擦角、楔角及楔高对承载力的影响。研究表明：楔形单桩基础竖向承载力高于等截面单桩基础，且承载力随着楔角、楔高的增大而增大，提高率最大达24.786%。倾斜侧壁的引入改变了桩侧摩阻力的传递规律；倾斜侧壁挤密桩周土体，桩侧摩阻力与法向应力增大，从而有效提高单桩基础的竖向承载力。研究成果可为今后海上风电单桩基础截面型式的设计提供参考。     

Analysis and Design of Monopile Foundations for Offshore Wind-Turbine Structures

Offshore wind turbines (OWTs) are generally supported by large-diameter monopiles, with the combination of axial forces, lateral forces, bending moments, and torsional moments generated by the OWT structure and various environmental factors resisted by earth pressures mobilized in the soil foundation. The lateral loading on the monopile foundation is essentially cyclic in nature and typically of low amplitude. This state-of-the-art review paper presents details on the geometric design, nominal size, and structural and environmental loading for existing and planned OWT structures supported by monopile foundations. Pertinent ocean-environment loading conditions, including methods of calculation using site-specific data, are described along with wave particle kinematics, focusing on correlations between the loading frequency and natural vibration frequency of the OWT structure. Existing methods for modeling soil under cyclic loading are reviewed, focusing in particular on strain accumulation models that consider pile–soil interaction under cyclic lateral loading. Inherent limitations/shortcomings of these models for the analysis and design of existing and planned OWT monopile foundations are discussed. A design example of an OWT support structure having a monopile foundation system is presented. Target areas for further research by the wind-energy sector, which would facilitate the development of improved analyses/design methods for offshore monopiles, are identified.     

海上风电大直径加翼桩水平承载性能研究

为改善海上风电大直径钢管桩的水平承载性能,基于ABAQUS有限元软件对单桩改进形式的加翼桩结构进行了系统研究,计算分析了软黏土地基中加翼桩在水平荷载作用下桩身弯矩、应力、位移、桩身泥面处倾斜率和极限承载力,研究了加翼桩面积、形状、埋深和刚度等翼板参数对加翼桩水平承载性能的影响规律,根据加翼桩的桩-土作用机理,参考现行规范模式提出适用于软黏土地基大直径钢管桩的P-Y曲线。研究结果表明,加翼桩通过在泥面处设置翼板可降低桩基泥面处倾斜率50%、提高桩基极限承载力60%以上,加翼桩水平承载性能明显优于单桩。     

海上风电单桩基础土相互作用特性影响因素分析

采用有限元软件ABAQUS建立了海上风电单桩基础与土相互作用数值计算模型,将波浪、洋流及风荷载等效成双向对称循环荷载,研究了水平循环荷载作用下不同因素对桩身水平位移、剪力和弯矩的影响规律。研究表明,随着循环荷载比的增加,桩身位移零点和桩身剪力反弯点沿埋深逐渐下移,桩身弯矩最大值点位于浅层土体;不同荷载频率时桩身位移在零点以上变化较大,桩身弯矩随着频率的增加逐渐增大;单向循环荷载作用下桩身位移最大,双向对称循环荷载作用下桩身位移最小;壁厚较小时对桩身水平位移影响较大;在位移零点之上范围内可以考虑设计"上厚下薄"的钢管桩,以减小桩身水平位移;不同桩壁厚时桩身剪力曲线在埋深约6D处出现交点,且泥面处桩身弯矩变化不明显。     

Drainage conditions around monopiles in sand

Large diameter monopiles are typical foundation solutions for offshore wind turbines. In design of the monopile foundations in sand, it is necessary to understand the drainage conditions of the foundation soil under the design loading conditions as the soil performance (strength and stiffness) is highly dependent on the drainage conditions. This paper presents a numerical investigation into this issue, with a purpose to develop a simple design criterion for assessing the soil drainage conditions around a monopile in sand. It is found that for typical monopile foundations in sand, the drainage condition during a single load cycle is generally expected to be undrained. However, the current state-of-practice uses p-y springs derived for drained soil responses for monopile design. The impact of this discrepancy on monopile foundation design was evaluated and found to be insignificant due to the relatively low level of loading as compared to the capacity of the soil.     

Driving energy losses for constant diameter and tapered submerged monopiles

Large monopiles are used as foundations for offshore wind turbines and are generally designed with a tapered section or conical shape. Some loss of driving energy is expected to occur during installation of these structures due to the submerged section of the tapered monopile. The current literature on this subject is limited and indicates rather large losses compared to field observations.A numerical model of the monopile–water–soil system was set up in the general-purpose finite element package Abaqus. By simulating the hammer impact and the resulting stress wave propagation through the monopile and water, the energy losses to be expected can be calculated accurately. The model was verified against independent finite element analyses and experimental data.A parametric study was performed and the effect of hammer characteristics, submerged monopile length and monopile geometry on the driving energy losses were quantified. The results enable a simple relationship between the energy losses and the monopile geometry to be proposed which increases linearly with pile diameter, taper angle, and submerged length. The losses are typically on the order of 0.15–0.3% per metre submerged length for large tapered monopiles.     

软粘土中插入式大直径薄壁圆筒结构的一种极限状态分析模型

根据插入式大直径薄壁圆筒结构的工作特点建立了其合理的破坏机制，以此为基础建立了受横向荷载作用的插入式大直径薄壁圆筒结构的上限法极限分析模型，从中可以得到作用于插入式大直径薄壁圆筒结构上的横向极限荷载近似的上限解；并根据上限法模型计算结果，提出了计算受横向荷载作用的插入式大圆筒结构侧壁极限抗力的简化修正模型，利用此修正模型，基于极限平衡法，可以求解作用于插入式大直径薄壁圆筒结构上的横向极限荷载。     

Un- and reloading stiffness of monopile foundations in sand

The eigenfrequency of offshore wind turbine structures is a crucial design parameter, since it determines the dynamic behavior of the structure and with that the fatigue loads for the structural design. For offshore wind turbines founded on monopiles, the rotational stiffness of the monopile-soil system for un- and reloading states strongly affects the eigenfrequency. A numerical model for the calculation of the monopile’s behavior under un- and reloading is established and validated by back-calculation of model and field tests. With this model, a parametric study is conducted in which pile geometry, soil parameters and load conditions are varied. It is shown that of course the rotational stiffness varies with mean load and magnitude of the considered un- and reloading span, but that for most relevant load situations the initial rotational stiffness of the monopile system, i.e. the initial slope of the moment-rotation curve for monotonic loading, gives a good estimate of the actual stiffness. Comparisons of different p–y approaches show that the ordinary API approach considerably underestimates the initial stiffness, whereas the recently developed ‘Thieken’ approach and also the ‘Kallehave’ approach give a much better prediction and thus might be used in the design of monopiles in sand.     

海底管道沉箱基础的复合承载力包络面研究

当负压沉箱被用作深水管汇或管道终端基础时,其长径比常介于1～2之间,而目前的沉箱复合承载力包络面表达式大都针对长径比不超过1的情况,少数覆盖长径比大于1的研究又不适用于土体表层强度非零的情况。采用有限元方法,模拟竖向力、水平力和弯矩共同作用下沉箱基础的响应,采用Probe加载模式获得沉箱的复合承载力包络面。进行大量变动参数分析,针对长径比为1～2的沉箱,讨论了长径比和土体强度分布对单向承载力和包络面的影响,并给出了预测沉箱复合承载力的归一化表达式。