碎石土斜坡水平受荷桩承载特性研究

斜坡上的桩基础的承载性能是复杂多变的。对于四川西部山地地形较广泛,且地基覆盖层多为特有的碎石土地层来说,水平受荷桩的相关研究还较少。为了研究碎石土地基斜坡上单桩基础的水平承载特性及桩土间的相互作用,通过现场水平静载荷试验在坡度为0°、15°、30°、45°的条件下,探讨桩身变形、桩身弯矩、土压力的变化。运用FLAC3D有限元分析软件得出水平荷载作用下,碎石土斜坡不同坡度的桩基础与桩周土之间的应力云图、位移云图的变化特点。将数值模拟结果与现场试验结果进行了对比,提出了单桩水平临界荷载和极限荷载在不同坡度区间内取值时的折减系数,为实际工程提供一定的参考。     

斜坡地基刚性桩水平承载力上限分析

为探讨斜坡地基刚性桩水平极限承载力的计算方法，介绍柔性桩的等效刚性桩有效嵌入深度并引入极限水平地基反力分布形式。根据荷载指向坡外及坡内两种情况，提出适用于斜坡地基桩前土体的两种极限破坏模式。然后，基于极限分析上限定理，推导出两种荷载方向下的刚性桩极限承载力，并引入多组现场试验，验证了理论方法的合理性。探讨了边坡坡角、内摩擦角、黏聚力及荷载方向对极限承载力的影响，得出了一些规律性结论，并基于以上分析结果，提出斜坡地基刚性桩水平极限承载力随坡角变化的拟合公式。这些分析为斜坡地基上基桩设计提供了一定的参考，具有理论及工程应用价值。     

Influence of seismic motions on behavior of piles in liquefied soils

In the present study an analytical procedure based on finite element technique is proposed to investigate the influence of vertical load on deflection and bending moment of a laterally loaded pile embedded in liquefiable soil, subjected to permanent ground displacement. The degradation of subgrade modulus due to soil liquefaction and effect of nonlinearity are also considered. A free headed vertical concrete elastic nonyielding pile with a floating tip subjected to vertical compressive loading, lateral load, and permanent ground displacement due to earthquake motions, in liquefiable soil underlain by nonliquefiable stratum, is considered. The input seismic motions, having varying range of ground motion parameters, considered here include 1989 Loma Gilroy, 1995 Kobe, 2001 Bhuj, and 2011 Sikkim motions. It is calculated that maximum bending moment occurred at the interface of liquefiable and nonliquefiable soil layers and when thickness of liquefiable soil layer is around 60% of total pile length. Maximum bending moment of 1210 kNm and pile head deflection of 110 cm is observed because of 1995 Kobe motion, while 2001 Bhuj and 2011 Sikkim motions amplify the pile head deflection by 14.2 and 14.4 times and bending moment approximately by 4 times, when compared to nonliquefiable soil. Further, the presence of inertial load at the pile head increases bending moment and deflection by approximately 52% when subjected to 1995 Kobe motion. Thus, it is necessary to have a proper assessment of both kinematic and inertial interactions due to free field seismic motions and vertical loads for evaluating pile response in liquefiable soil.     

水平荷载作用下长桩的合理截面形式优化研究

利用计算推力桩的综合刚度原理和双参数法,选择合理参数,计算出钢筋混凝土空心桩、实心桩和矩形桩以及钢管混凝土桩这4种不同截面形式的推力桩在砂土、黏土和风化岩3种不同地质条件中相同水平推力作用下地面处的转角、位移及其最大弯矩,并验算混凝土桩的裂缝宽度以及钢管混凝土桩的强度,分析比较不同土质中的不同截面类型长桩在相同水平荷载作用下的承载力。为大型建筑结构的桩基础的合理截面形式的优化设计提供参考。研究认为,砂土和黏土中使用空心桩较好,风化岩使用钢管混凝土桩较好。     

考虑坡度效应的水平受荷桩应变楔计算方法

由于坡度效应的存在,常规方法并不适用于斜坡上水平受荷桩的计算,首先开展了斜坡上基桩的横向加载破坏试验,以确定斜坡上基桩的破坏模式。在此基础上,沿坡体方向对破坏土楔体进行斜向单元划分,提出了考虑坡度效应的土体应变楔模型,对于其中的关键参数应变楔深度与应变楔土体应变采用迭代求解。迭代过程中,建立基桩横向受荷的桩-土相互作用方程并用有限杆单元法求解,当求解得到的桩身地面处位移与应变楔模型中地面处土体位移之差小于某一允许值时,得到的基桩的水平位移及内力即为最终解答。通过与试验测试数据的对比,验证了该方法的合理性。最后,将土体破坏深度与边坡斜率的比值定义为陡坡效应影响范围,并对其影响因素进行了对比分析。结果表明,陡坡效应的影响范围受土体强度参数及基桩尺寸等多因素影响,其随着桩径的增加而减小,且随土体强度的增强而减小。     

大位移条件下水平受荷单桩的简明弹塑性计算方法

软土地层中当桩顶水平荷载较大时,采用传统m法计算容易低估桩身弯矩与挠曲变形,有必要针对该问题提出相关计算方法。将地基土体简化为理想弹塑性体,假定桩身某一深度处存在土体的弹塑性变形临界点,临界点以上的土体进入塑性变形状态,而临界点以下的土体仍处于弹性变形状态,分段建立桩身挠曲微分控制方程,得到水平受荷单桩简明弹塑性计算方法。现场单桩实测和参数敏感性分析结果表明：采用简明弹塑性计算方法得到的桩身最大弯矩较传统m法计算精度提高38.1%;桩身最大水平位移计算精度提高22.3%;桩顶边界条件对桩身水平位移与弯矩沿桩身的分布规律影响显著;桩身最大弯矩和水平位移对土体的极限抗力系数及其形状参数较敏感,设计中宜按下限值选取。     

坡顶面复合荷载刚性短桩工程设计要素分析

位于斜坡顶面的复合受荷桩,除受水平及竖向荷载共同作用产生的交叉影响外,还需考虑边坡效应,其承载力特性存在多种影响因素相互交叉作用,复杂程度远高于水平地面受荷桩。本文采用三维有限元软件对坡顶复合受荷桩进行多方面的数值模拟分析,研究发现同级荷载下随复合荷载加载角度增加其极限承载力呈先减小后增大的规律;竖向荷载对桩身水平位移的影响大于其对内力的影响;桩周均布土压力增大将使得桩身位移及内力随之增大;随临坡距增加边坡的影响作用将减小。     

复合桩基重复加卸载过程中侧向约束桩变位规律试验研究

复合桩基上堆载可能引起侧向约束桩侧移,导致侧向约束桩和复合桩基失稳破坏。采用室内模型试验研究复合桩基重复加、卸载过程中侧向约束桩水平变位规律。结果表明：（1）复合桩基上加载时,侧向约束桩的桩身侧移沿深度先增大、后减小、存在峰值,峰值随加载增大而增大,出现在距离地面0.4倍地面以下桩长处,峰值位移与桩顶（或地面）处位移的比例系数和发生侧移的桩身长度随荷载增大而增大;（2）复合桩基上重复加、卸载时,侧向约束桩的桩顶侧移随桩间距增大而增大,弹性变形则变小。中桩侧移比边桩大。桩顶侧移增长率随重复加载次数增加而减小。若荷载超过首次加载的最大荷载,侧向复合地基上加载-桩水平位移曲线将回到首次加载曲线的延长线,具有记忆效应。第 次卸载曲线与第i次卸载曲线线型相似且相互平行。每次卸载的初、中期,荷载的减小不影响桩顶侧移,只有卸载到最后1～2级时,侧移才开始减小,直到永久塑性变形。侧向约束桩的顶部水平位移回弹曲线特征与岩石或土在垂直加、卸载作用下的回弹曲线相似。     

Method of initial parameters for laterally loaded piles embedded in layered soils

The method of initial parameters (MIP) was originally developed to solve the problem of a beam on an elastic foundation with applied concentrated forces along the span, which introduce discontinuities in the mathematical formulation of the problem. MIP is modified in this paper so that it can be used for solving the problem of a laterally loaded pile with discontinuities due to soil layering along the length of the pile. In this paper, the basis of MIP is outlined, and its use to find the deflection, slope, bending moment and shear force of laterally loaded piles is illustrated. Example problems of laterally loaded piles embedded in multi-layered soil media are provided.     

Elastic analysis of laterally loaded pile in multi-layered soil

An analysis is developed to determine the response of laterally loaded piles in layered elastic media. The differential equations governing pile deflections in different layers due to a concentrated static force and/or moment acting at the pile head are obtained using the principle of minimum potential energy and calculus of variations. The differential equations are solved analytically using the method of initial parameters. Pile deflection, slope of the deformed axis of the pile, bending moment and shear force can be reliably obtained by this method for the entire pile length. The input parameters needed for the analysis are the pile geometry and the elastic constants of the soil and pile. It is observed that soil layering has a definite impact on pile response and must be taken into account for proper analysis and design. The analysis forms the basis for future formulations that can consider stress–strain nonlinearity.     

斜坡基桩水平极限承载力及影响因素模型试验和数值模拟

斜坡上的基桩具有承重和阻滑的双重功能,其受力变形性状远比平地上的情形复杂。采用模型试验和数值模拟相结合的方法研究了其水平承载特性及影响因素。模型试验结果表明：临坡距对基桩的水平承载变形性能有较大影响。同一级荷载下,临坡距较大的桩身水平位移小于临坡距较小的基桩;临坡距较大基桩的临界荷载和极限承载力也大于临坡距小的基桩。数值模拟研究结果表明：基桩水平极限承载力随着斜坡坡比的增大而减小,随着临坡距的增大而增大,与模型试验的结果基本一致。对比分析了斜坡和平地基桩水平承载变形性能的差别,得出了可考虑坡比和临坡距的斜坡基桩水平极限承载力简便计算方法,可为有关规范的修订以及工程设计提供参考。     

Analysis of laterally loaded piles with rectangular cross sections embedded in layered soil

An analysis is developed to determine the response of laterally loaded rectangular piles in layered elastic media. The differential equations governing the displacements of the pile–soil system are derived using variational principles. Closed‐form solutions of pile deflection, the slope of the deflected curve, the bending moment and the shear force profiles can be obtained by this method for the entire pile length. The input parameters needed for the analysis are the pile geometry and the elastic constants of the soil and pile. The new analysis allows insights into the lateral load response of square, rectangular and circular piles and how they compare. Copyright © 2007 John Wiley & Sons, Ltd.     

盾构隧道施工对邻近承载桩基影响研究

针对苏州轻轨1号线成层非均质土地基,选用Mohr-Coulomb弹塑性本构模型,建立三维有限元数值模型,研究非均质土中盾构隧道施工对邻近承载桩基工作性状的影响规律。数值计算结果表明,随着成层非均质土中各土层软硬程度差异的增大,隧道开挖会在邻近承载单桩引起明显反弯点,且桩体沉降亦随之增大;位于上软下硬成层土中的承载单桩桩身正弯矩更大,且该正弯矩出现在桩身中上部的反弯点部位,而上硬下软成层土中的承载单桩下部出现更大的负弯矩;与均质土中同位置承载单桩相比,位于上软下硬成层土中承载单桩桩顶及桩端轴力均更大,而位于上硬下软成层土中承载单桩桩顶轴力则更小。不同竖向集中荷载作用下,非均质土中盾构隧道开挖引起的承载群桩中前桩水平位移沿桩身分布与同位置承载单桩重合,后桩挠曲程度小于承载单桩;盾构隧道施工对承载群桩内力的影响明显高于对变形的影响。     

悬臂抗滑桩加固边坡地震动力响应模型试验研究

为研究悬臂抗滑桩加固边坡的地震响应和抗滑桩桩身弯矩分布规律,利用北京工业大学结构实验室的大型振动台进行悬臂抗滑桩加固边坡模型的振动试验。在试验过程中,输入汶川地震重华镇波,记录边坡不同位置加速度的时程变化,并作对比分析,采集抗滑桩桩身的应变,用于分析桩身弯矩分布。结果表明,地震过程中边坡内部加速度自下而上逐渐放大,边坡顶部放大效果达到最大;悬臂抗滑桩的加固效应和桩间土体成拱作用使附近土体的动力响应受到限制;抗滑桩的嵌固端与悬臂部分分界面随着地震波的输入应变急剧增大,而悬臂部分随着高度增加应变减小,反映了悬臂抗滑桩弯矩的“凸”形分布规律。     

General Analytical Solution for Laterally-Loaded Pile-Based Miche Model

Piles subjected to lateral loading can create problems in soil-structure interaction. Several differing methods of analysis have been proposed to solve the problem of laterally loaded piles, resulting in the determination of pile bending and the bending moment as a function of depth below soil surface. These piles are widely used to support laterally loaded piles, such as bridge pillars, offshore platforms, communication towers and others. This study presents an analytical solution to Miche’s problem as a continuous function of depth: deflection and moment, as well as a dimensional plots to be used in projects involving piles subjected to laterally loading only including data concerning laterally loading test and pile geometry. A new formula is presented to calculate the pile head displacement as well as an equation to determine maximum moment for a generalized Miche model and further analysis. In addition, this paper proposes an equation for the determination of constant horizontal subgrade reaction \((n_{h})\) based on the CPT in-situ test and the geometric characteristics of the pile. Calibration of the analytical model showed good fit and conservative results regarding inclinometer data from an bored pile and good agreement with the literature results.

     

水平荷载短桩桩土共同作用全过程分析

利用三维有限元、无限元和界面单元相耦合的方法对短桩桩土共同作用全过程进行数值模拟,计算了短桩桩土体系中土体对桩身的抗力分布及桩前土体隆起位移和桩后土体与桩分离大小等,较详细地探讨了短桩桩土共同作用机理.     

强震区桩−土−断层耦合作用下桩基动力响应

为研究桩-土-断层耦合作用下桩基动力响应特性,利用振动台试验选取0.35g地震动峰值加速度时4种类型地震波,研究断层上下盘桩基加速度响应、桩顶相对位移、弯矩及桩基损伤情况。试验表明：断层上盘桩基各项参数明显大于断层下盘,呈现出上盘效应;桩顶峰值加速度大于桩底峰值加速度,上部土层对输入地震波具有滤波作用;桩顶加速度响应相较于桩底具有滞后性;桩顶峰值加速度与桩顶加速度放大系数α在输入El-Centro波时最大;上、下盘α 差值在输入Kobe波时最大;弯矩和桩顶相对位移峰值在输入Kobe波时最大;弯矩在土层分界面处较大,输入不同地震波时弯矩峰值均未超过桩身抗弯能力;提出了强震区近断层桩基可根据验算内容选取合理地震波进行验算的抗震设计建议。     

Development of p–y curves for undrained response of piles near slopes

The response of laterally loaded piles placed near the crest of clay slopes is analysed. Three-dimensional finite element analyses are presented for piles of different geometries, installed at several distances from slopes of various inclinations. The results of these analyses are used to establish the pattern of lateral load distribution along the pile length in relation to slope inclination and pile to slope distance. Subsequently, p–y curves are developed for the case of undrained lateral loading of piles near the crest of clay slopes, a case for which no such curves exist so far. The proposed p–y curves are implemented into a commercial subgrade reaction computer code and used to perform a series of parametric numerical analyses. The results of these analyses show that the predicted response of piles near slopes with the proposed p–y curves is in good agreement with the response observed in some pile tests reported in the literature.     

单桩与群桩基础动力时程响应差异振动台试验

为探明强震作用下大直径深长单桩与群桩基础的动力时程响应差异,依托海文大桥实体工程,通过大型振动台试验,开展了4种不同类型地震波作用下单桩及群桩基础的桩顶加速度、桩顶相对位移、桩身弯矩时程响应变化规律及其差异性研究。研究结果表明：由于群桩效应的存在,桩顶加速度时程响应呈现双面性,群桩基础桩顶加速度峰值大于单桩基础,但峰值出现时刻滞后于单桩0.29～1.06 s;群桩基础的桩顶相对位移最大值显著小于单桩基础,且出现时刻明显滞后,Kobe波作用时滞后高达3.82 s;群桩基础的桩身弯矩最大值小于单桩基础7.54%～9.22%,且单桩基础受地震波影响较大,弯矩时程响应振幅明显大于群桩基础。桩基础抗震设计时,可充分发挥群桩基础动力时程响应滞后性特点,合理设计并选择最优桩型。     

Centrifuge shaking table tests on 4 × 4 pile groups in liquefiable ground

A series of centrifuge shaking table model tests are conducted on 4?×?4 pile groups in liquefiable ground in this study, achieving horizontal–vertical bidirectional shaking in centrifuge tests on piles for the first time. The dynamic distribution of forces on piles within the pile groups is analysed, showing the internal piles to be subjected to greater bending moment compared with external piles, the mechanism of which is discussed. The roles of superstructure–pile inertial interaction and soil–pile kinematic interaction in the seismic response of the piles within the pile groups are investigated through cross-correlation analysis between pile bending moment, soil displacement, and structure acceleration time histories and by comparing the test results on pile groups with and without superstructures. Soil–pile kinematic interaction is shown to have a dominant effect on the seismic response of pile groups in liquefiable ground. Comparison of the pile response in two tests with and without vertical input ground motion shows that the vertical ground motion does not significantly influence the pile bending moment in liquefiable ground, as the dynamic vertical total stress increment is mainly carried by the excess pore water pressure. The influence of previous liquefaction history during a sequence of seismic events is also analysed, suggesting that liquefaction history could in certain cases lead to an increase in liquefaction susceptibility of sand and also an increase in dynamic forces on the piles.