多向荷载作用下层状地基刚性桩筏基础分析

提出一种多向荷载作用下层状地基中刚性桩筏基础的计算方法。基于剪切位移法,采用传递矩阵形式分析了竖向荷载下桩顶面-桩顶面相互作用;引入修正桩侧地基模量,采用有限差分法分析了水平荷载下桩顶面-桩顶面相互作用;基于层状弹性半空间理论,分析了多向荷载下桩顶面-土表面、土表面-桩顶面、土表面-土表面的相互作用关系。建立了桩土体系柔度矩阵,得到了多向荷载下层状地基中刚性桩筏基础的受力和变形的关系以及桩的内力和变形沿桩身分布规律。通过与有限元对比,验证了该方法的合理性和修正地基模量的优越性,并对多向荷载作用下的桩筏基础进行了计算分析,计算结果表明,水平力将会引起桩筏基础的倾斜。     

陆上风轮机梁板基础受力特性简化分析

提出一种多向荷载同时作用下的刚性桩筏基础简化计算方法,基于Mindlin解,分析桩顶面-桩顶面、桩顶面-土表面、土表面-土表面的相互作用关系。推导多向荷载下桩土体系柔度矩阵,得到刚性桩筏基础的受力和变形的关系,通过与有限元对比证明了文中方法的正确性,在此基础上对耦合荷载作用下的风机基础的受力和变形特性进行分析。研究表明,耦合荷载作用下风机基础将会产生竖向不均匀沉降,并会引起桩顶轴力的的巨大的差异。     

Analysis of piled raft with nodular pile subjected to vertical load using a Winkler model approach

An investigation is made to present analytical solutions provided by a Winkler model approach for analysis of piled rafts with nodular pile subjected to vertical loads in nonhomogeneous soils. The vertical stiffness coefficient along a piled raft with the nodular pile in nonhomogeneous soils is derived from the displacement given by the Mindlin solution for elastic continuum analysis. The vertical stiffness coefficients for the bases of the raft and the nodular part in the nodular pile in a soil are expressed by the Muki solution for the 3‐D elastic analysis. The relationship between settlement and vertical load on the pile base is presented considering the Mindlin solution and the equivalent thickness in the equivalent elastic method. The interaction factor between the shaft of the nodular pile and the soil is expressed taking into account the Mindlin solution and the equivalent elastic modulus. The relationship between settlement and vertical load for a piled raft with the nodular pile in nonhomogeneous soils is obtained by using the recurrence equation of influence factors of the pile for each layer. The percentage of each load carried by both nodular pile and raft subjected to vertical load is represented through the vertical influence factors proposed here. Comparison of the results calculated by the present method for piled rafts with nodular piles in nonhomogeneous soils has shown good agreement with those obtained from the finite element method and a field test. Copyright © 2016 John Wiley & Sons, Ltd.     

Design method of piled-raft foundations under vertical load considering interaction effects

The paper has proposed a design method considering interaction effects for a piled raft foundation. In this method, the raft is considered as a plate supported by a group of piles and soil. The ultimate load capacity of the pile group is taken into account in calculating the settlement when the foundation is subjected to a large vertical external load. In addition, this method supports estimation of the nonlinear behaviour of the piled raft foundation by considering the nonlinear behaviour of the piles.A step-by-step procedure to apply the proposed method to calculate the settlement and distribution of the bending moment of the piled raft foundation is introduced. To verify the reliability of the proposed method, models of a 16-pile raft and a 9-pile raft with different pile lengths embedded in homogeneous silica sand were tested in a centrifuge and comparisons were made between the results of the proposed method, the results of centrifuge tests, and those of Plaxis 3D. Good agreement between centrifuge modelling and the proposed method is demonstrated, thus showing the potential of the proposed method.     

往复荷载下层状地基柔性筏板-群桩共同作用分析

提出一种层状地基中柔性筏板-群桩共同作用分析方法,探讨筏板刚度对桩筏基础沉降的影响,并成功预测了往复荷载下桩筏基础的长期沉降。筏板刚度采用Mindlin板理论的有限单元法分析;桩-土体系的刚度矩阵中,桩顶面-桩顶面、桩顶面-土表面以及土表面-土表面的相互作用分析采用层状剪切位移法借助层状地基的Burmister位移解求得。基于层状地基中柔性筏板-群桩的沉降计算方法以及往复荷载下土体压缩模量的衰减特性得到了桩筏基础的长期沉降预测方法。与已有文献方法和离心模型试验结果的对比分析表明,柔性筏板-群桩共同作用方法得到的沉降值具有较高的精度。     

考虑上部结构的桩筏基础非线性共同作用分析

当前虽然已有考虑桩筏非线性的设计,但仍无人在此基础上,考虑上部结构。因此考虑上部结构,进一步认识其与桩筏基础非线性共同作用机理,优化桩筏基础设计,具有重要的现实意义。本文以子结构法凝聚上部结构的荷载及刚度,以平面壳体单元模拟筏板,按有限层法模拟桩土之间的弹性相互作用,用广义剪切位移法模拟桩的非线性工作性状,建立了一种考虑上部结构共同作用的桩筏基础非线性分析方法,并编制了分析程序。通过实例分析,探讨了上部结构与桩筏基础非线性共同作用的机理,研究了合理布桩方式,探讨了以差异沉降为目标的优化设计的可能途径。     

Finite element simulations of the behavior of piled‐raft foundations using a multiphase model

Finite element simulations of the behavior of a piled raft foundation have been carried out using a multiphase model conceived as an improved homogenization approach. According to this model, the ground reinforced by a group of piles is treated as a homogeneous continuous medium. In this approach, no specific interface elements are necessary to account for the mechanical interaction between the piles and the ground: this interaction is described by means of two scalar parameters, one stiffness parameter and one which can easily be derived from the maximum ground‐pile friction. The implementation of the model into a finite element code provides an efficient tool for the analysis of the influence of the pile number or length on the settlement and bearing capacity of a square piled raft foundation and of the way the total applied load is shared between the raft and the piles. Results are compared with a standard 3D finite element analysis. The comparison highlights the fact that the proposed approach remains to be improved to account for tip resistance. Copyright © 2012 John Wiley & Sons, Ltd.     

Non-linear soil–structure interaction in disconnected piled raft foundations

Disconnected piled raft foundations are characterised by no structural connection between the upper raft and the underlying piles, mostly playing the role of settlement-reducers. The resulting raft–pile gap is usually filled with a granular interlayer, through which the loads from the superstructure are transferred to the piles.In this paper, the complex interaction mechanisms involving the foundational components (raft, piles and soil) are numerically investigated by means of 3D finite elements analyses, accounting for soil non-linearity. The main features of the soil–structure interaction mechanisms under purely vertical external loads are explored over a realistic range of raft–soil gaps for different pile configurations, in which the number of piles – i.e. their spacing – is varied. Special attention is also devoted to the structural response of the piles in terms of axial and bending internal stress resultants. In particular, while disconnection beneficially affects the structural pile response, increasing the raft–pile gap tends to reduce the overall settlement/stiffness efficiencies.The numerical results being presented are in substantial agreement with the outcomes from literature small-scale experiments and suggest a number of relevant theoretical inferences.     

竖向荷载作用下桩筏基础可视化模型试验研究

进行了群桩基础可视化模型试验。在试验过程中对各级竖向荷载作用下群桩基础桩、土的变形采用高清晰数码相机拍摄了照片。通过对照片进行后处理分析,得到了桩、土的位移场,也对桩身轴力也进行了量测,在该基础上研究了群桩基础的变形性状和破坏模式,重点分析对比了桩间距和桩长对桩端土体沉降以及桩身侧摩阻力的影响。试验发现,桩距是影响桩土相对滑移量的主要因素,桩距越大,桩身与土的相对滑移量就越大,桩端刺入量也越大。在柔性筏基下,随桩距增加中桩的桩土相对滑移量可能会大于边桩。桩端刺入量是大桩距桩基础主要的沉降构成,以桩端刺入量为研究对象建立一套大桩距基础新的沉降计算理论,似乎值得进一步研究。桩顶向上刺入（可通过设置褥垫层、桩顶预留净空或设置可压缩垫块来实现）有利于桩间土的压密,减小桩端刺入量,甚至改变破坏模式。     

Vertical harmonic vibration of piled raft foundations in layered soils

This paper develops a method to analyze the piled raft foundation under vertical harmonic load. This method takes into account the interactions among the piles, soil, and raft. The responses of the piles and raft are formulated as a series of equations in a suitable way and that of layered soils is simulated with the use of the analytical layer‐element method. Then, according to the equilibrium and continuity conditions at the piles–soil–raft interface, solutions for the piled raft systems are obtained and further demonstrated to be correct through comparing with the existing results. Finally, some examples are given to investigate the influence of the raft, pile length‐diameter ratio, and layering on the response of the piled raft foundations. Copyright © 2017 John Wiley & Sons, Ltd.     

Three-dimensional analysis of bearing behavior of piled raft on soft clay

The piled raft has proved to be an economical foundation type compared to conventional pile foundations. However, there is a reluctance to consider the use of piled rafts on soft clay because of concerns about excessive settlement and insufficient bearing capacity. Despite these reasons, applications of piled rafts on soft clay have been increased recently. Current analysis methods for piled rafts on soft clay, however, are insufficient, especially for calculating the overall bearing capacity of the piled raft. This study describes the three-dimensional behavior of a piled raft on soft clay based on a numerical study using a 3D finite element method. The analysis includes a pile–soil slip interface model. A series of numerical analyses was performed for various pile lengths and pile configurations for a square raft subjected to vertical loading. Relatively stiff soil properties and different loading types were also used for estimating the bearing behavior of the piled raft. Based on the results, the effect of pile–soil slip on the bearing behavior of a piled raft was investigated. Furthermore, the proportion of load sharing of the raft and piles at the ultimate state and the relationship between the settlement and overall factor of safety was evaluated. The results show that the use of a limited number of piles, strategically located, might improve both bearing capacity and the settlement performance of the raft.     

Analysis of piled raft coefficient and load-settlement on sandy soil

A piled raft foundation is a combined foundation, which is developed to utilize the load-carrying capabilities of both raft and piles. To obtain an optimum piled raft design, it is important to properly evaluate and consider the load-sharing behavior between the raft and piles, which changes according to the settlement level of the piled raft. In this study, 27 three-dimensional finite element models were analyzed to investigate the piled raft coefficient with linear and nonlinear load-settlement behaviors. The length of piles was varied between 10, 15, and 20 m. The spacing between pile centers was varied between 3D, 5D, and 7D, and the pile diameter was kept constant. The number of piles and the distance between the exterior piles and the edge of the raft were maintained at 9 and 1 m, respectively. The sand conditions varied between dense, medium, and loose. The results indicated that the piled raft coefficient increases when the load-settlement curve is linear and decreases when the load-settlement curve is nonlinear. The influence of the incremental increase in pile length on the piled raft coefficient is more pronounced in short piles than in longer piles. The raft thickness has a negligible effect on the piled raft coefficient.     

Integral equation method for analysis of piled rafts with dissimilar piles under vertical loading

In a piled raft, the length and arrangement of piles has a significant effect on the stresses and deflections of the raft. The use of piles with different dimensions and properties below a raft is an innovative concept and can optimize the design of a piled raft. In this study, an integral equation method with a fictitious pile model was adopted to analyze the piled raft foundation with dissimilar piles. The Fredholm integral equations of the second kind were obtained for this problem. The loads shared by piles and subsoil, the load transfer, and the settlement of the piled raft were obtained using numerical calculation. The results from the present method were compared with those in the literature. An optimization technique was introduced to design piled rafts with dissimilar piles. The stiffening effect of piles on the surrounding soil is also discussed as compared the conventional interaction factor approach.     

Study on interaction between dissimilar piles in layered soils

This paper, under the assumption that the pile–pile interaction showed elasticity, analyzed the pile–pile interaction between two dissimilar piles in layered soils using the shear displacement method and taking the sheltering effect into account to modify the conventional interaction factor between two dissimilar piles. The linear stiffness of single pile was adopted herein to analyze the influence of the pile stiffness factor, the pile length factor, and the diameter of shorter pile on the pile–pile interaction factor between two dissimilar piles. The interaction factor between two dissimilar piles drew from the present method was generally in good agreement with the interaction factor computed by Wong and Poulos. However, this paper intends to provide a different but more convenient means of estimating the interaction factor between piles that have dissimilar diameters, lengths and pile tip conditions, and the settlement of dissimilar piles in pile groups. Copyright © 2010 John Wiley & Sons, Ltd.     

高速铁路沉降控制复合桩基的性状试验研究

基于沉降控制设计理念,无砟轨道京沪高速铁路地基处理采用筏板+垫层+疏桩的方法,形成复合桩基以实现有效减少工后沉降和充分利用地基承载力的优化加固方案。为探索该新方法沉降控制机制,选用CFG桩开展了复合桩基现场试验研究,对复合桩基在高速铁路路基填筑、静置、预压卸载过程中的地基沉降变形、桩和桩间土土压力、筏板顶与底部压力进行了长期观测,分析了路基沉降变形、桩-土应力比和荷载分担比以及筏板的受力随填筑高度和固结时间的变化规律。研究表明：筏板＋垫层＋疏桩联合加固地基方案在初期充分发挥了桩间土承载作用,导致桩与桩间土产生差异沉降;随着垫层的调节作用,筏板可集中发挥桩体的承载能力及显著提高桩顶应力集中程度,地基土沉降主要发生在加固区范围内,从而揭示了复合桩基在路基荷载下的承载机制和变形特性。现场试验结果可为指导高速铁路CFG桩复合桩基设计参数的进一步优化提供试验依据。     

考虑流变与固结效应的桩筏基础-地基共同作用分析

土的流变性与地基固结的综合作用,导致了上部结构与地基变形的时效性,并呈现出明显的非线性,对桩筏基础与地基共同作用的工作机理及其工作性能产生重要影响。为此,采用弹黏塑性流变模型考虑土的流变特性,通过有限元方法数值求解Biot耦合固结方程,对桩筏基础与地基共同作用的时间效应问题进行了非线性数值分析。通过算例计算,对加载后桩筏基础荷载分配和沉降特性及下覆土层中孔隙水压力的扩散和消散规律进行了探讨。研究表明,地基孔隙水压力的增长和消散不仅具有Mandel-Cryer效应,而且依赖于土的流变变形,尤其在排水条件较差时更为明显。因此,在分析桩筏基础内力变形的时效性时必须考虑土的流变性与地基的固结作用的联合效应。     

An approximate numerical analysis of pile–raft interaction

This paper presents an approximate method of numerical analysis of piled–raft foundations in which the raft is modelled as a thin plate and the piles as interacting springs of appropriate stiffness. Allowance is made for the development of limiting pressures below the raft and of the ultimate axial load capacity of the piles. Comparisons between this analysis and existing solutions verify that, despite the approximations involved, the analysis can provide solutions of adequate accuracy for the settlement and pile load distribution within a piled raft. Comparisons are also made with the results of a series of centrifuge tests and with measurements of the performance of a full-scale piled raft. In both cases, the analysis predicts very well the settlement and proportion of load carried by the piles.     

A simplified analysis method for piled raft and pile group foundations with batter piles

A simplified method of numerical analysis has been developed to estimate the deformation and load distribution of piled raft foundations subjected to vertical, lateral, and moment loads, using a hybrid model in which the flexible raft is modelled as thin plates and the piles as elastic beams and the soil is treated as springs. Both the vertical and lateral resistances of the piles as well as the raft base are incorporated into the model. Pile–soil–pile, pile–soil–raft and raft–soil–raft interactions are taken into account based on Mindlin's solutions for both vertical and lateral forces. The validity of the proposed method is verified through comparisons with several existing methods for single piles, pile groups and piled rafts. Workable design charts are given for the estimation of the lateral displacement and the load distribution of piled rafts from the stiffnesses of the raft alone and the pile group alone. Additionally, parametric studies were carried out concerning batter pile foundations. It was found that the use of batter piles can efficiently improve the deformation characteristics of pile foundations subjected to lateral loads. Copyright © 2002 John Wiley & Sons, Ltd.     

An approximate analysis procedure for piled raft foundations

A piled raft foundation comprises both piles and a pile cap that itself transmits load directly to the ground. The aim of such a foundation is to reduce the number of piles compared with a more conventional piled foundation where the bearing effect of the pile cap, or raft, is ignored. This paper describes a ‘hybrid’ approach for the analysis of piled raft foundations, based on a load transfer treatment of individual piles, together with elastic interaction between different piles and with the raft. The numerical analysis is used to evaluate a simple approximate method of estimating the overall response of the foundation from the response of the component parts. The method leads to estimates of the overall foundation stiffness, the proportion of load carried by the pile group and the raft, and an initial assessment of differential settlements. Parametric studies are presented showing the effect of factors such as raft stiffness and pile spacing, length and stiffness, and a worked example is included demonstrating the accuracy of the approximate design approach.     

考虑加筋与遮帘效应的层状地基群桩沉降计算

桩群在土中的加筋与遮帘效应是客观存在的,但目前的理论与实践均未能或有效地考虑该效应.基于剪切变形法原理,在计算某一根桩沉降时,考虑了其他各相邻基桩的存在对该桩沉降的折减,即加筋与遮帘效应,得到了桩侧桩-土接触等效剪切弹簧刚度,建立了桩身位移微分方程,分别求得桩顶沉降-桩端沉降、桩顶荷载-桩端压力的递推关系,从而得到了各桩在自身荷载作用下引起自身沉降的柔度系数; 同理,也求得了各邻桩在其桩顶荷载下引起它桩沉降的柔度系数,最终建立了群桩沉降计算的柔度矩阵方程.推导过程中,考虑了地基土的成层性及桩端沉降的相互影响,并提出了基于一定深度内的Mindlin位移解且考虑桩径影响的桩端压力-桩端位移关系新模式.算例结果表明,本文方法与实测值较为接近,且按本文方法求得的群桩中基桩相互作用系数明显小于弹性理论计算结果,且与实测值吻合较好.