刚性桩复合地基中性面深度与桩土应力比计算

刚性桩复合地基中性面深度及桩土应力比的简化计算主要基于桩侧摩阻力线性分布假设,当桩身较长时,桩端侧摩阻力的计算值会远大于实际值,致使中性面深度及桩土应力比的计算结果与实际差别较大,故有必要对线性分布模式予以修正。据此将桩侧摩阻力分布简化为分段线性模式,考虑负摩阻力作用及桩上、下刺入变形,根据褥垫层-桩-土变形协调关系推导了刚性桩复合地基中性面深度、桩顶面桩土应力比、中性面桩土应力比计算公式。最后通过模型试验与工程实例验证,计算值与实测值吻合较好。     

考虑桩土非等应变的路堤荷载下搅拌桩复合地基沉降计算方法

为全面考虑路堤荷载下搅拌桩复合地基的桩土非等应变特性,将路堤、桩体、桩间土及下卧土层整体考虑,在总结前人试验结果基础上提出简化的桩土应力比和桩土差异沉降两阶段模型,改进了现有的桩顶平面处的桩土应力比计算方法。根据实际的应力状态计算桩侧摩阻力,计算桩土荷载分担,进而得到桩体和桩间土沉降。通过各子系统界面处的位移和应力边界条件考虑其相互作用,得到了路堤荷载下搅拌桩复合地基总沉降计算模型。对比淮盐高速公路试验段搅拌桩复合地基的实测沉降、文中模型以及传统复合模量法计算结果,验证了该模型的正确性,结果表明模型计算沉降与实测数据吻合,较常规的复合模量法计算结果更接近实测值。     

路堤荷载作用下柔性桩复合地基的沉降分析

为了寻求路堤荷载下柔性桩复合地基沉降计算的简便方法,基于已有的研究成果,将桩侧摩阻力分布简化为分段线性模式,根据桩长与临界桩长的大小关系,结合桩?土?垫层三者在交界面上的应力与压缩变形协调条件,运用单位元法推导了柔性桩复合地基加固区的沉降计算公式,并采用分层总和法计算复合地基下卧层的沉降量。结合工程实例对计算结果进行验证,结果表明：采用理论方法计算的柔性桩复合地基沉降量与现场实测沉降结果吻合较好,证明理论计算方法的合理性,且能较好地反映路堤荷载作用下柔性桩复合地基的工作性状。进一步分析表明：在临界桩长范围内,桩与桩间土相互作用,最大限度发挥了复合地基桩间土的承载能力;此外,由于桩侧负摩阻力对桩体有拖拽作用,桩身轴力在桩体中性面位置处达到最大值。因此,在工程设计中要高度重视和运用临界桩长和中性面的概念。     

群桩沉降简化计算方法

将均质土和成层土中的单桩桩顶沉降分成桩端力引起的沉降、桩身压缩和桩侧阻力引起的沉降3部分分别计算,获得单桩沉降后,运用等代墩法可获得群桩的平均沉降。单桩沉降计算方法可考虑桩端力与桩端位移的非线性关系和桩侧阻力引起沉降的非线性特性,同时计算方法可考虑桩身压缩对桩顶沉降的贡献。算例分析表明,计算值与实测值和其他方法的计算值有较好的一致性,验证了该方法的合理性。     

考虑负摩阻力的刚性桩复合地基工作性状分析

以带褥垫层的刚性桩复合地基为研究对象,在考虑桩体负摩阻力作用的条件下,讨论了桩身侧摩阻力分布状况,建议了侧摩阻力线性简化计算模型。分析了带褥垫层刚性桩复合地基桩体与褥垫层的相互作用,计算了桩顶对褥垫层的刺入量。推导了桩体位移微分方程,并根据方程,同时结合中性面的定义,给出了求解中性面深度比的计算方法,讨论了其影响因素及取值范围。算例显示,按该方法计算的结果符合工程经验。     

柔性基础下柔性桩复合地基沉降计算研究

研究表明,柔性桩复合地基存在有效桩长。首先,根据柔性基础下柔性桩复合地基桩侧摩阻力分布形式,假定桩土界面处桩土变形协调,基于剪切位移法推导出有效桩长的计算公式;其次,提出将复合地基压缩层分为三层来计算,利用弹性理论建立了加固区压缩量计算方法,同时采用Mindlin应力解建立了复合地基下卧层沉降的计算方法,在探讨相关计算参数取值的基础上建立了柔性基础下柔性桩复合地基沉降计算方法。最后,通过工程实例分析并与其它方法计算结果比较,表明本文方法的合理性与可行性。     

嵌岩桩负摩阻力现场试验与计算方法研究

桩-土剪切位移对桩基负摩阻力发展有重要的影响。在既有研究的基础上,通过桩周土体沉降与桩身应变的监测结果建立了计算桩-土剪切位移量的方法。利用该方法对浅中层桩-土监测数据进行分析,确定了在桩顶自由条件下该试验场地桩-土剪切位移量在4~5 mm;对于深层桩-土没有达到该剪切位移量,负摩阻力则无法发挥至极值。根据负摩阻力的发挥特征,改进了规范中嵌岩桩负摩阻力的计算方法,该方法同时考虑了桩周土竖向有效应力与桩土剪切位移的影响。使用该方法计算的负摩阻力与实测结果较为吻合,获得的下拉荷载能够达到安全与经济的效果,可供工程设计参考、借鉴。     

带帽刚性桩复合地基荷载传递机理研究

为了研究带帽刚性桩复合地基荷载传递机理,基于弹性理论和合理假定,采用荷载传递函数法,建立了带帽刚性桩复合地基中桩体沉降及其轴向应力、桩帽下土体竖向位移及其竖向应力、桩帽间土体竖向位移及其竖向应力、桩身侧摩阻力、桩帽边缘土体之间的侧摩阻力与荷载水平、深度之间的控制微分方程。采用微分方程的近似解法,推导出相应地解析表达式。利用桩体荷载沉降关系作为已知条件进行求解,计算结果能够反映带帽刚性桩复合地基荷载传递的一般力学性状规律。     

浆固碎石桩单桩荷载传递特性研究

浆固碎石桩是加固软土地基的一种新型桩基技术,已成功应用于高速公路和高速铁路等工程软土地基处理。为了研究浆固碎石桩单桩荷载传递机制,假定浆固区压缩模量呈指数分布,应用圆形承载板作用下的半空间体位移的解,对竖向荷载作用下单桩复合地基各部分沉降进行分析,推导出桩轴向压应力、桩侧摩阻力和桩顶沉降计算公式。现场试验实测结果与理论计算结果对比表明,理论计算值与实测值相吻合,验证了理论计算公式的合理性     

桩基负摩阻力时间效应试验研究

由于黏土固结缓慢,桩基负摩阻力存在明显的时间效应,然而目前相关研究仍显不足。设计实施了能实现桩顶加载及较大超载值的单桩及双桩负摩阻力模型试验,桩周土采用砂土和软黏土夹层,测定了模型桩身应力、桩顶位移以及土体分层沉降随固结时间的变化。试验结果显示,沉降、负摩阻力具有明显的时间效应。土表超载作用下土体沉降带动桩沉降,桩与土体的沉降均表现出早期快、后期慢的趋势。试验加载初期,黏土夹层处的负摩阻力略小于砂层,但随土体固结而增长,其基本变化规律与沉降相同。因桩端砂土层沉降稳定迅速,中性点随桩身沉降增长略呈上移趋势。此外,相同荷载作用下桩间距较小的双桩,由于下拉力较小,其沉降较小。试验条件下,3D桩间距的负摩阻力群桩效应系数在0.71～0.77之间,6D桩间距时不存在负摩阻力群桩效应。     

能量桩群桩工作特性简化分析方法研究

采用双曲线模型模拟桩土界面上的力学行为,利用剪切位移法反映剪应力在土层中的传递,考虑群桩之间的相互作用,建立了热?力耦合作用下能量桩群桩基础工作特性的简化分析方法。该方法能反映桩土界面上的非线性、桩顶的约束条件和能量桩位置的影响,可直接计算所有桩的位移和轴力。与现有方法相比,计算得到的双桩相互作用因子更加合理。通过与文献中试验数据的对比表明,若只有局部桩经历温度变化,能量桩运行过程中各桩之间存在差异变形,基础出现倾斜,桩顶荷载发生重分布。所建立方法计算方便,能合理模拟能量桩群桩基础的主要工作特性,可用于大规模能量桩群桩基础的设计计算。     

桩（墙）单位侧摩阻力与轴向应变关系的解析解及其应用

根据弹性力学原理,提出了模型筒桩（闭合墙）的简化力学模型。采用半逆解法先求出桩左右侧受均布摩阻力、桩顶受均布荷载两种情况下的应力分量,根据叠加原理得出桩两侧均受相同摩阻力、桩顶受均布荷载情况下的应力分量。据此推导了普通桩基的轴力和侧摩阻力计算式,分析结果表明,该计算式与传统计算方法是一致的。当桩壁两侧受不同大小的侧摩阻力、桩顶受均布荷载时,仍采用叠加原理求得各应力分量,进而求得桩表面处的轴向应变与单位侧摩阻力关系的解析解。通过矩形闭合墙基础模型试验实例,对该解析解的应用方法加以说明。该解析解的求出,为竖向载荷试验中模型筒桩（闭合墙）所量测的内外侧应变进行数据处理提供了理论依据,进而为基础-土相互作用研究提供了真实、准确的试验结果。     

Analytical solution for distributed torsional low strain integrity test for pipe pile

Low strain integrity tests (LSITs) are the most popular non-destructive methods for pile testing. However, traditional LSITs have encountered unprecedented challenges as the need for long pile and existing pile testing keeps multiplying. Compared to traditional longitudinal excitations, the torsional wave is less influenced by the velocity attenuation effect and can be subjected at the pile shaft for existing piles. Distributed torsional LSIT is proposed in this article with the presentation of the corresponding analytical solutions that exhibiting the velocity responses along the pile shaft. The solution is verified with previous simplified theoretical and rigorous finite element method (FEM) answers. At the end, the application of this method is exhibited through the identification of necking and concrete segregation defects on pipe piles, which shows the advantage of this method on long pile testing.     

A simple critical state interface model and its application in prediction of shaft resistance of non-displacement piles in sand

A simple semi-hyperbolic state-dependent constitutive model for sand-structure interfaces is proposed. The model formulation is consistent with critical state soil mechanics since void ratio evolves continuously with shear strain from initial state towards asymptotic critical state at extremely large shear strains. The model takes into account influence of normal stiffness on volume change and stress path. The proposed interface model is implemented in a pile segment analysis scheme for simulation of shaft resistance mobilization in non-displacement piles. Results reveal that the proposed pile segment analysis can well predict shaft resistance of model piles embedded in different sands.     

Prediction of the shaft resistance of nondisplacement piles in sand

Using pile segment analysis, the mobilized shaft resistance of axially loaded nondisplacement piles in sand is investigated here. It is accepted that the shaft capacity of piles constructed in granular soils is highly influenced by the mechanical behavior of soil–structure interfaces forming adjacent the piles skin. Adopting the thin interface layer as a load transfer mechanism, a simple but accurate critical state compatible interface constitutive model is introduced. After evaluation, the interface model in conjunction with the pile segment analysis is applied for the prediction of the shaft resistance mobilized in nondisplacement piles. The proposed approach takes into account the influences of pile diameter and surface roughness together with the effects of the surrounding soil density and stiffness on the mobilized shaft resistance. The performance of the proposed method is verified by comparing its predictions with the experimental data of various model piles covering wide ranges of length, diameter, roughness, and surrounding soil properties. Copyright © 2012 John Wiley & Sons, Ltd.     

非均质地基中群桩竖向荷载沉降关系分析

运用剪切位移法计算了桩轴向荷载传递因子。对于桩端采用线性的荷载传递函数,推导了基于弹塑性模型的单桩竖向荷载沉降的解析解。分析过程中考虑了土体强度沿深度线性变化的特性和桩土间的滑移现象,因此更符合大部分土体的实际性状。在此基础上,建立了考虑桩土滑移的桩-桩相互作用系数的计算公式,并将上述方法应用于群桩的分析,获得了群桩的荷载沉降特性。该分析方法克服了目前应用较多的弹性理论方法夸大桩土相互作用的缺点,单桩和群桩的荷载沉降曲线的分析结果和实测数据吻合,证明了该方法的合理性。     

The Shaft Capacity of Displacement Piles in Clay: A State of the Art Review

The rapid expansion of the offshore wind sector, coupled with increasing demand for high rise structures, has placed renewed demand on the driven piling market. In light of this industry growth, this paper reviews the evolution of design approaches for calculating the shaft capacity of displacement piles installed in cohesive soils. The transition from traditional total stress design towards effective stress methods is described. Complex stress–strain changes occur during pile installation, equalisation and load testing and as a consequence, the selection of parameters for use in conventional earth-pressure type effective stress approaches is not straight-forward. These problems have led to the development of empirical correlations between shaft resistance and in situ tests, such as the cone penetration tests. However, many of these approaches are limited because they were developed for specific geological conditions. Significant insight into pile behaviour has been obtained from recent model pile tests, which included reliable measurements of radial effective stresses. These tests have allowed factors such as friction fatigue and interface friction to be included explicitly in design methods. Whilst analytical methods have been developed to investigate pile response, these techniques cannot yet fully describe the complete stress–strain history experienced by driven piles. The use of analytical methods in examining features of pile behaviour, such as the development of pore pressure during installation and the effects of pile end geometry on pile capacity, is discussed.     

考虑沉桩效应的群桩非线性荷载-沉降解析

考虑沉桩效应对桩周土体力学特性的影响,采用指数函数型荷载传递曲线分别建立了静压桩的桩侧和桩端荷载传递模型。在此基础上,根据群桩加载过程中桩周土体的变形模式,基于荷载传递法描述桩-土界面的非线性行为,采用剪切位移法考虑群桩之间的相互作用,提出了考虑沉桩效应的群桩非线性荷载-沉降混合计算方法。通过开展离心模型试验对该计算方法解答进行了验证,研究了沉桩效应和桩-土界面非线性特征对群桩承载特性的影响。研究结果表明,沉桩效应对桩周土体起到挤密作用,使得桩周土体的强度和刚度增大,从而提高了群桩的承载特性。群桩加载过程中桩-土界面刚度随沉降变形而逐渐减小,使得群桩荷载-沉降曲线呈现出明显的非线性特征。     

Shaft and base resistance of non-displacement piles in sand

This paper presents the results of three-dimensional, finite element analyses performed with an advanced, two-surface-plasticity, constitutive sand model to investigate the response of non-displacement piles to axial loading. The analysis domain is carefully meshed such that the formation and evolution of shear bands next to the pile shaft and near the pile base can be properly captured. Analyses considering various soil profiles and pile geometries show that the mobilized lateral earth pressure coefficient K along the pile shaft increases with increasing relative density and decreasing initial confining stress. The ultimate unit base resistance is independent of pile diameter, increasing with increasing relative density and increasing initial confining stress at the pile base. Based on the analysis results, design equations are proposed to estimate the limit shaft resistance and ultimate base resistance of non-displacement piles in sandy soil. In proposing these relationships, the pile slenderness ratio is considered. The effect of layer proximity to the base of the pile or pile base embedment in a layer is also considered.     

复杂地基反力模式下高承台嵌岩灌注桩的屈曲稳定分析

为探讨桩侧地基土反力对高承台嵌岩灌注桩桩身屈曲稳定的影响,假定地基反力系数呈更为复杂的幂分布,基于弹性地基梁理论建立桩土体系总势能方程,采用最小势能原理,导出了桩身屈曲临界荷载与稳定计算长度的解析解,并据此获得了地基反力系数分布模式、桩身自重及桩侧摩阻力等对桩身屈曲稳定的影响规律。工程应用分析结果表明,考虑地基反力系数为一般幂分布时,桩身屈曲分析结果更趋合理。