Embedded beam element with interaction surface for lateral loading of piles

This paper presents a numerical formulation of a three dimensional embedded beam element for the modeling of piles, which incorporates an explicit interaction surface between soil and pile. The formulation is herein implemented for lateral loading of piles but is able to represent soil–pile interaction phenomena in a general manner for different types of loading conditions or ground movements. The model assumes perfect adherence between beam and soil along the interaction surface. The paper presents a comparison of the results obtained by means of the present formulation and by means of a previously formulated embedded pile element without interaction surface, as well as reference semi‐analytical solutions and a fully 3D finite element (FE) model. It is seen that the proposed embedded element provides a better convergence behavior than a previously formulated embedded element and is able to reproduce key features of a full 3D FE model. Copyright © 2015 John Wiley & Sons, Ltd.     

软土地基中PHC管桩水平受荷性状的试验研究

桩在水平荷载作用下的受力性状是一个复杂的桩土相互作用过程。针对软土地集中预应力管桩受水平荷载的问题,结合工程实例,通过水平静载试验,实测得到了水平荷载作用下PHC（超长预应力）桩与土共同作用时的工作性状,分析了PHC桩受水平力作用时的内力、变形和临界承载力以及地基土的水平抗力比例系数。试验结果可为今后同类地区水平受荷桩的设计与研究提供参考。     

横向载荷作用下刚性桩变位规律研究

目前对横向受载刚性桩的研究主要集中在其承载力方面,对变化规律研究很少。为此提出了利用刚性桩上两点位移求桩上任意一点位移、桩回转中心位置及转角的方法。通过对粉质黏土中的刚性桩进行模型试验与数值计算发现,回转中心位置随位移和载荷的增大,先是急剧下降,然后变缓,最后基本趋于稳定,而且桩埋置参数与土力学参数对回转中心位置的变化规律影响很小;而桩的转角随位移增大近似线性变化。比较发现,由试验和数值计算得到的回转中心极限位置与由极限地基反力法得到的结果相差不大     

考虑桩土侧移的被动桩中土拱效应数值分析

被动桩对侧向位移的土层起到遮拦作用的机制主要是土拱效应。采用土工有限元软件Plaxis Tunnel 3D 1.2,对堆载荷载作用下邻近桩基中的土拱效应产生机制和性状进行三维数值分析,指出目前被动桩中土拱效应二维有限元分析存在的问题。考虑桩土侧移与相对位移,再利用土工有限元软件Plaxis2D 8.2详细地研究了侧向土体位移大小、桩身水平位移大小、土体性质以及桩土接触面性质等影响因素对土拱效应性态和桩土荷载分担比的影响。     

桩基水平位移的有限元模拟

有限元数值计算方法在岩土工程中已有较多的应用,但在水平受荷桩分析计算中的应用尚不普遍。有关水平受荷桩土体系水平承载力及变形的数值模拟方法少有文献报道。采用有限元数值模拟方法来研究水平受荷桩土体系的水平位移。首先简要分析了水平受荷桩的主要计算方法及原理。然后,基于ABAQUS软件建立了桩土系统二维及三维的有限单元模型,分别通过按现行规范推荐的m法和按Poulos的弹性理论法计算的两个算例,将数值计算方法与解析方法进行了对比计算分析。有限元数值计算结果与解析解基本吻合,说明了所采用有限元数值模拟方法的可靠性及有效性。     

Selection of p–y curves for the design of single laterally loaded piles

Two-dimensional finite element analysis has been used to find load–transfer relationships for translation of an infinitely long pile through undrained soil for a variety of soil-constitutive models. It has been shown that these load–transfer curves can be used as p–y curves in the analysis of single piles undergoing lateral pile head loading in undrained soils with non-linear stress–strain laws. Lateral pile response deduced from 2-D analysis input to the subgrade reaction method has been compared to the behaviour of a single pile analysed using three-dimensional finite element analysis. Good agreement between the two methods for non-linear soils suggests that the 2-D analysis may form a useful design method for calculation of p–y curves. Copyright © 1999 John Wiley & Sons, Ltd.     

水平循环荷载下海上大直径单桩累积变形特性

受风、波浪等荷载作用海上大直径单桩的水平循环受荷特性在其设计中更为关键。基于有限差分程序,通过开发用户子程序实现土体刚度衰减模型的嵌入,分析循环次数和荷载特征对桩基累积变形的影响规律,并讨论累积变形预测模型的适用性与可靠性。研究结果表明,密砂地基中荷载值的增加将导致累积变形发展加快,设计最大循环荷载值应控制在0.58倍的桩基静承载力内;对数模型能较好地预测小循环荷载值作用下桩基累积变形,而低估较大荷载值引起的累积变形;幂函数模型较好地反映不同荷载作用下的桩基累积变形发展规律,且模型参数α的取值随着荷载幅值的增加呈线性增大,并给出了不同荷载下幂函数模型的设计参数。     

水平荷载作用下根式基础受力特性分析

根式基础是一种新型的结构式基础,利用自身的结构性,可以在增加少量成本的情况下大幅度提高基础的承载力。但是,目前还没有关于根式基础的理论研究成果,有必要对根式基础的受力特性进行分析。采用弹塑性有限元法,对淮河特大桥1#桥的根式基础原位试验进行数值模拟,验证了数值模拟方法和参数选取的合理性。在此基础上,通过无根键沉井基础和有根键沉井基础的受力特性对比,分析了根式基础的受力特性,对根键上的弯矩分布型式进行了研究,并探讨了根键沿深度位置分布对根式基础承载力的影响。研究证明,根键分布在合理的位置可以有效地提高沉井基础的水平承载力。     

Nonlinear response of laterally loaded piles and pile groups

In spite of extensive studies on laterally loaded piles carried out over years, none of them offers an expedite approach as to gaining the nonlinear response and its associated depth of mobilization of limiting force along each pile in a group. To serve such a need, elastic–plastic solutions for free‐head, laterally loaded piles were developed recently by the author. They allow the response to be readily computed from elastic state right up to failure, by assigning a series of slip depths, and a limiting force profile. In this paper, equivalent solutions for fixed‐head (FixH) single piles were developed. They are subsequently extended to cater for response of pile groups by incorporating p‐multipliers. The newly established solutions were substantiated by existing numerical solutions for piles and pile groups. They offer satisfactory prediction of the nonlinear response of all the 6 single piles and 24 pile groups investigated so far after properly considering the impact of semi‐FixH restraints. They also offer the extent to ultimate state of pile groups via the evaluated slip depths. The study allows ad hoc guidelines to be established for determining input parameters for the solutions. The solutions are tailored for routine prediction of the nonlinear interaction of laterally loaded FixH piles and capped pile groups. Copyright © 2008 John Wiley & Sons, Ltd.     

Study on stability of thin-walled circular pipe piles embedded shallowly under lateral loads

The interaction between soil and rigid piles loaded laterally is analyzed with finite difference method. There is a little difference between computational and experimental value of bearing capacity of rigid piles. Effects of embedding dimensions of piles and mechanical parameters of soil on lateral bearing capacity of rigid piles are studied. It is found that the lateral bearing capacities of rigid piles have important relation with the elastic modulus of soil, have approximate linear relations with the cohesion of soil and the friction coefficient of pile-soil interface and increase obviously with the internal friction angle of soil. But the effects of dilation angle on the lateral bearing capacities of rigid piles are negligible. The relations of bearing capacities of rough piles with smooth piles and the friction coefficient of pile-soil interface are also obtained.     

软基上沉入式大圆筒结构的水平承载力分析

大圆筒结构是一种适宜于软土地基上的新型港口水工建筑物,但目前对于软基上大圆筒结构的水平极限承载力尚缺乏合理的计算模式和分析方法.结合长江口深水航道治理二期工程,针对大圆筒结构设计方案提出了一种简便而实用的上限极限分析方法,采用有限元数值分析解验证了所提出计算方法的合理性,进而,据此进行了一系列变动参数比较计算,以此对大量的计算结果进行了系统的分析,所得到的数值计算结果与分析结论为工程设计提供参考依据.     

粉质粘土中的刚性桩在横向载荷作用下失稳的模型试验研究

为解释横向载荷作用下刚性桩的失稳机理,针对桩头自由的刚性桩做了一系列横向加载试验。基础土为粉质粘土,含水量介于9.85 %～13.85 %之间。由载荷-位移全过程曲线发现,刚性桩在横向载荷达到一定值时会失稳;由试验录像及土体剖面发现,由于土体的软化破坏,在桩后土体内会出现贯穿的局部破坏并形成一楔体,同时在土面伴随一不完全的椭圆形鼓包及一条平行于加载方向的拉伸裂缝,而在桩前土中,由于桩的挤压会形成一条侵彻沟。分析认为,对大位移刚性桩桩后土体的破坏是桩失稳的根本原因。     

Numerical modelling of the driving response of thin‐walled open‐ended piles

The driving response of thin‐walled open‐ended piles is studied using numerical simulation of the wave propagation inside the soil plug and the pile. An elastic finite element analysis is carried out to identify the stress wave propagation in the vicinity of the pile toe. It is found that the shear stress wave has the highest magnitude above the bottom of the soil plug. Below the bottom of the soil plug, the vertical stress wave has the highest magnitude. Although the shear stress wave propagating in the radial direction is similar in magnitude to the vertical stress wave at the bottom of the soil plug, it decays rapidly while travelling downwards. The highest vertical stress at the bottom of the soil plug appears after the vertical stress wave interacts with the shear stress wave travelling in the radial direction. Initially, the vertical stress wave propagates with the dilation wave velocity in both the radial and vertical directions. After it interacts with the shear stress wave, the vertical stress wave starts to propagate with the shear wave velocity in the radial direction and with the axial wave velocity downwards. It is concluded that at the bottom of the soil plug, the interaction between the waves travelling in radial and vertical directions is important. The capabilities of several one‐dimensional pile‐in‐pile models to reproduce the driving response given by a two‐dimensional axisymmetric finite element model is studied. It is seen that when the base of the soil plug fails, a one‐dimensional pile‐in‐pile model can be used to achieve results in agreement with the finite element model. However, when the pile is unplugged, where the base of the soil plug does not fail, a reduced finite element mesh that permits the radial wave propagation inside the soil plug must be used. Copyright © 2001 John Wiley & Sons, Ltd.     

A numerical interpretation of load tests on bored piles

The finite element interpretation is discussed of two load tests carried out on bentonite slurry piles bored in granular soils. The first case concerns a pile belonging to a 12 pile group. An axisymmetric finite element model that reproduces, with reasonable accuracy, the experimental results is developed. The model is then extended to three-dimensional conditions and applied to the analysis of the entire group. The results suggest some comments on the different assumptions that can be adopted in the calculations and on their effects on the global load–settlement curve of the pile group. The second case concerns a load test in which, in addition to the load–settlement data, also the axial strains along the pile were measured through electrical extensometers. The numerical back analyses highlight an apparent contradiction between the two sets of experimental data. On their bases some conclusions are drawn on the possible causes of the observed inconsistency and on the influence of the construction technology on the interaction between the pile tip and the soil underneath it.     

Numerical analysis of soil plug behaviour inside open-ended piles during driving

The plugging mechanism of infinitely-long open-ended piles is examined using numerical simulation of the wave propagation inside the soil plug and pile. It is shown that the key parameters for the plugging mechanism are the pile radius, the shape of the impact load, the shear wave velocity of the soil inside the pile, and the friction at the pile–soil interface. Consequently, the tendency of the pile to plug during driving can be assessed prior to the driving process by consideration of these key parameters. Existing one-dimensional models for the shaft response of open-ended piles are discussed and an improved model is presented. The differences between using one-dimensional models and finite element models to simulate the plugging process are examined. The differences are found to vary with the key parameters. Pile-in-pile and lumped-mass one-dimensional models are found to give satisfactory performance for some parameter combinations, while for others an axisymmetric finite element model must be used. © 1998 John Wiley & Sons, Ltd.     

某工程长短桩组合桩基础设计方案分析

针对土层中存在上下两层或多层可供利用的桩端持力层,且建筑物对沉降要求较高的情况,基于减沉桩原理,根据长桩控制变形、短桩提供承载力的基本思路,提出长短桩组合桩基础设计思想。结合某一实际工程桩基设计,采用长短桩组合桩基础方案进行了三维有限元分析。结果表明,长短桩组合桩基础不仅可以大量减少长桩用量,而且可有效地控制基础整体沉降和差异沉降,显示了其良好的应用前景。     

Numerical analysis of the geotechnical behaviour of energy piles

Energy geostructures are rapidly gaining acceptance around the world; they represent a renewable and clean source of energy that can be used for the heating and cooling of buildings and for de‐icing of infrastructures. This technology couples the structural role of geostructures with the energy supply, using the principle of shallow geothermal energy. The geothermal energy exploitation represents an additional thermal loading, seasonally cyclic, which is imposed on the soil and the structure itself. Because the primary role of the piles is the stability of the superstructure, this aspect needs to be ensured even in the presence of the additional thermal load. The goal of this paper is to numerically investigate the behaviour of energy pile foundations during heating–cooling cycles. For this purpose, the finite element method is used to simulate both a single and a group of energy piles. The piles are subjected to a constant mechanical load and a seasonally cyclic thermal load over several years, imposed in terms of injected–extracted thermal power. The soil and the pile–soil interface behaviours are reproduced using a thermoelastic‐thermoplastic constitutive model. The thermal‐induced stresses inside the piles and the additional displacements of the foundations are discussed. The group model is used to investigate the interactions between the piles during thermo‐mechanical loading. The presented results are specific to the studied cases but lead to the conclusion that both the thermal‐induced displacements and stresses, despite being acceptable under normal working conditions, deserve to be taken into account in the geotechnical design of energy piles. Copyright © 2014 John Wiley & Sons, Ltd.     

砂土中边坡附近单桩水平抗力的模型试验研究

针对平地p-y曲线对临近边坡桩基的不适用性,开展了砂土中边坡存在对单桩水平抗力影响的室内物理模型试验。依据边坡坡角及边坡到单桩距离的不同,共进行了11次3组试验。采用分段3次多项式对实测桩身应变分布进行拟合,以推算桩身变形和桩侧抗力,有效克服了数据拟合点的奇异。对比无边坡时的试验p-y曲线、美国API规范曲线和双曲线模型,确认了平地p-y曲线的计算表达式。其次获得了边坡条件下的单桩荷载-位移曲线,揭示了边坡坡角和边坡到桩的距离对水平受荷单桩桩身弯矩分布的影响规律。根据试验结果采用折减系数修正浅层桩侧极限抗力,并引入边坡对桩侧初始刚度的削弱表达式,得到边坡条件下水平受荷单桩浅层区域双曲线p-y曲线表达式。最后通过与离心试验结果对比,验证了给出的p-y曲线考虑边坡影响时的合理性。     

软土地基邻近堆载对桥梁桩基偏位的影响研究

在软土地基中,邻近堆载不仅将引发桥梁桩基发生侧向偏位,还将导致桩身产生附加弯矩,这对于桥梁的安全使用将产生极其不利的影响。通过对具体工程实例的介绍,利用有限元分析手段,并结合现场桩基偏位的实测结果,对邻近单侧堆载及双侧堆载所引发桩基偏位情况进行深入剖析。通过分析结果可知,在单侧堆载的作用下,桩基将产生侧向偏移及附加弯矩,且反弯点位于软土层与硬土层交界处附近,严重时将导致桩顶区域发生开裂破坏;在双侧堆载的作用下,桩基的偏位情况取决于两侧的堆载作用,而双侧卸载对桩基偏位影响较小,但对缓解桩身附加弯矩具有显著的作用。     

岩溶地区桩基的有限元分析

岩溶地区长桩存在着遇洞率的问题。为了研究岩溶地区桩的受力特点,采用有限元法模拟岩溶地区桩基的工作方式,分析了岩层厚度对桩的影响以及桩的受力状态,并研究了去桩的可能性。计算结果表明:岩层厚度对桩的影响较大,岩层的位置对桩的应力不产生影响,仅对桩的位移产生影响。