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。     

竖向循环荷载作用下砂土中单桩承载特性模型试验研究

采用自主设计的竖向循环加载装置,通过室内模型试验研究竖向循环荷载作用下砂土中单桩承载特性和桩周土体变形机制。根据试验结果,桩体累积位移可以划分为不发展区域、渐变发展区域、破坏区域3种区域;滞回曲线的滞回圈包络面积随着循环次数的增加,呈现逐渐减小的趋势,滞回曲线由不闭合发展为闭合曲线,桩周土体由弹塑性变形逐渐转变为弹性变形。采用粒子图像测速（particle image velocimetry,简称PIV）技术对循环荷载作用下桩周土体变形进行实时量测,得到桩周土体完整的位移场和剪切应变场。结果表明：循环周期、幅值和密实度为桩周土体变形的主要影响因素,随着循环周期的增加,剪切破坏带在接近土体表面处呈现内敛趋势,剪切破坏面最终近乎平行于桩-土界面。循环荷载幅值越大,表层土体在循环荷载作用下越趋于密实,侧向土压力增大,位移影响区域减小,对应剪切应变场呈现“耳”状分布,幅值比循环周期更容易导致桩周土体出现沉陷。不同砂土密实度中的桩体累积位移随着循环周期呈现出不同的特征,松砂状态下桩周土体位移场呈现倒截锥形,密砂则呈现圆柱形。     

被动桩侧土体位移场的透明土模型试验

常规室内模型试验中,土体侧向位移作用下桩身变形特性能够获得,而桩周土体内部的位移很难量测。因此,结合透明土材料、粒子图像测速(PIV)技术和光学测量系统,设计了侧向位移作用下被动桩桩周土体变形测量试验系统。通过非插入式测量,得到桩不同埋置深度下桩周土体的位移场。试验结果表明:相同侧向位移模式下,桩埋置深度越大,桩对桩前土体的遮拦效应越明显,中间与两侧土体的相对位移越大。基于模型试验的结果建立了数值模型,参数敏感分析表明土体侧向位移形状对桩前土体位移遮拦效应的影响程度依次为矩形、抛物线形和三角形,且遮拦效果随着桩径的增加而更加明显。     

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。     

带桩筏基系统的弹塑性分析与设计方法研究

采用ABAQUS有限元程序中的线性扩展D-P模型对带桩筏基下的地基土进行弹塑性模拟,建立了带桩筏基系统的ABAQUS程序三维有限元模型。经比较各种桩、筏基础的内力与沉降关系计算结果,得到了带桩筏基的一些工作特性。在此基础上结合《建筑地基基础设计规范》(GB5007-2002),改进了传统的复合桩基设计方法,讨论了“三阶段”设计理念的可行性,并将其应用于工程实例。分析结果表明,改进的复合桩基设计方法和“三阶段”设计方法较常规设计方法更加经济合理。     

超高层建筑桩筏基础的桩顶反力计算研究

根据上海高88层、筏板厚度为4 m的金茂大厦和高101层、筏板厚度为4.5 m的上海环球金融中心桩筏基础的实测沉降资料,论证超高层建筑的桩筏基础为弹性体。对以上两幢超高层建筑和正在建造中的高121层、筏板厚度为6 m的上海中心大厦的桩筏基础,采用偏心受压公式和高层建筑与地基基础共同作用理论方法(混合法)进行详细对比计算,论证按弹性体计算桩顶反力的合理性,阐明《建筑桩基技术规范》(JGJ 94–2008)的3.1.8条的正确性和合理性。期望能够改变过去按偏心受压公式计算桩顶反力的传统观念,提高设计水平。     

桩筏基础变形分析简化程序DRAB的扩充

桩筏基础变形分析简化程序PRAB采用了以下混合模型。柔性筏、桩和土体分别被模型化为薄板、弹性梁和弹簧。将其扩充到能够考虑土体有限深度的影响。对设置在有限深度的均质土体中的桩筏基础和群桩基础的变形,采用PRAB和三维有限元法分别进行了分析,两种计算方法的结果非常吻合。结果表明,与群桩相比,桩筏基础中桩体所受的力和变形都相对较小。因而仅讨论了采用线弹性模型的情况,非线性模型也可以扩充到PRAB程序中去。     

管桩挤土效应的现场试验和数值模拟

以PHC管桩加固某物流加工二期标准仓库、厂房项目为依托,进行了吹填土地区PHC管桩的挤土效应研究。利用预先埋设的孔压计和测斜计,借助沉桩过程的现场测试手段,研究了PHC管桩在沉桩过程中桩周孔压变化和土体位移分布特征,发现沉桩过程中超孔隙水压力随距桩心距离的增加而近似呈线性规律衰减,影响范围约为10倍桩径,土体水平位移不仅与地基土性质有关,而且其分布特征也与深度有很大关系,表现为0.2～0.4倍桩长处土体水平位移较大。采用数值模拟有效地模拟了现场测试结果,进而将结果推广,得到了沉桩引起的桩周土体位移随深度和距桩心距离的变化规律。数值模拟结果表明：桩-土界面摩擦特性和桩径对土体位移场的影响较大。     

基于透明土技术的开口管桩施工模拟

开口管桩因为土塞效应的存在,其施工力学机理比实心桩要复杂。传统的测量方法无法测得整个桩周土体的位移场以及土塞的具体情况。利用透明土技术,模拟不同桩径,壁厚的开口管桩的沉桩过程,得到管桩外侧土体的全部位移场,并观测到土塞高度的变化、土塞中土颗粒的位移向量场和土塞土颗粒位移中性面。通过对实验结果的分析,可以得到以下结论:不同截面尺寸的管桩施工引起的土体变形区域的横向范围与管桩内径与壁厚的比值（r/w）成反比,且每施工2倍管桩外径R深度,横向影响范围扩大67%;当外径相等时,r/w值相差150%时,土塞高度相差186%;土塞效应和桩径无关,和r/w有关;当壁厚保持一定的同时,土塞高度变化量约为管桩内外径比（R/r）的改变量的2倍;土塞达到完全闭塞状态时还会发生桩端土体的剪切破坏,从而管桩内腔涌入新土形成新的土塞。     

深基坑工程逆作法的理论与计算

把高层建筑与地基基础共同作用理论引伸到深基坑工程的逆作法,考虑地下墙的共同作用,形成比较完整的深基坑工程的逆作法理论和方法。利用编制的程序,可以计算逆作法和半逆作法施工过程中各个工况的地下墙、楼板、立柱桩以及基础(筏或箱)板浇注后的受力和变形,此外,还可计算地下墙和桩的荷载分担等。结合上海一幢高238 m,60层的超高层建筑带10层的裙房,桩筏基础,深为18.95～24.00 m和面积22 000 m2的基坑工程进行计算,以图与表阐述逆作法的特点,并与实测结果对比分析,说明该理论的合理性和可行性,有助于提高深基坑工程的设计与施工水平。     

Centrifuge study on behavior of rigid pile composite foundation under embankment in soft soil

The rigid pile composite foundation is widely used in highway projects in soft soil area as it can effectively increase the bearing capacity and stability of the foundation. While the research on the behavior and failure mode of rigid pile composite foundation under embankment is not enough, instability failure of rigid pile composite foundation often occurs in practical projects. This paper presents a centrifuge model test to investigate the load transfer mechanism, settlement characteristic and failure mode of rigid pile composite foundation under embankment. The test results show that: the soil displacement of different region in rigid pile composite foundation was different, obvious vertical displacement occurred in the soil under the center of embankment and the horizontal displacement was very small in this region; both vertical and horizontal displacement occurred in the soil under the shoulder of embankment; and obvious horizontal displacement occurred in the soil under the slope toe of embankment; moreover, ground heave also occurred near the slope toe of embankment. The soil displacement in rigid pile composite foundation had a large influence on the stress characteristic and failure mode of rigid piles, the compressive failure and bending failure would probably occur for the piles under the center and shoulder of the embankment, respectively, and the tension-bending failure would probably occur for the piles under the slope toe of embankment. The different failure modes of piles at different regions should be considered in the design of rigid pile composite foundation under embankment. The test results can be used to improve the design method for rigid pile composite foundation under embankment in practical projects.

     

Design of raft–pile foundation using combined optimization and finite element approach

This paper describes the application of a structural optimization approach combined with the finite element method for the optimal design of a raft–pile foundation system. The analysis takes into account the non-linear behaviour of the soil medium and the piles. For the optimization process, the sensitivity analysis is carried out using the approximate semi-analytical method while the constraint approximation is obtained from the combination of extended Bi-point and Lagrangian polynomial approximation methods. The objective function of the problem is the cost of the foundation. The design variables are the raft thickness, cross-section, length and number of piles. The maximum displacement and differential displacement are selected as the constraints. The proposed method is shown to achieve an optimum design of raft–pile foundation efficiently and accurately. Copyright © 1999 John Wiley & Sons, Ltd.     

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.     

极限荷载下纵向截面异形桩破坏形式对比模型试验研究

由于基桩纵向截面形式的差异,竖向荷载作用下桩侧摩阻力和桩端阻力发挥存在明显的差异,尽管纵向截面异形桩在工程中得到了一定的应用,然而针对极限荷载下桩端和桩侧土体破坏形式的研究却相对较少。基于透明土材料和粒子图像测速（particle image velocimetry）技术,针对等体积的扩底楔形桩、楔形桩和等截面桩的承载特性及破坏形式进行对比模型试验,测得桩顶荷载-沉降曲线,研究了各级荷载下桩端和桩侧土体位移场的变化规律以及极限荷载下桩端和桩侧土体的破坏形式;同时分析了不同桩长情况下各类型桩的承载力特性。研究结果表明,在此试验条件下,扩底楔形桩的极限承载力约为常规楔形桩的3.5倍和等截面桩的2.5倍;极限荷载作用下各类型纵向截面异形桩桩端的破坏形式规律基本一致。     

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.     

Thermo-hydro-mechanical response of a large piled raft equipped with energy piles: a parametric study

This paper presents the results of a parametric study in which a series of fully coupled, 3-dimensional thermo-hydro-mechanical Finite Element (FE) analyses has been conducted to investigate the effects of the thermal changes imposed by the regular performance of a GSHP system driven by energy piles on a very large piled raft. The FE simulation program has been focused mainly on the evaluation of the following crucial aspects of the energy system design: the assessment of the soil–pile–raft interaction effects during thermal loading conditions; the quantification of the influence of the thermal properties of the soil and of the geometrical layout of the energy piles on the soil–foundation system response, and the evaluation of the influence of the active pile spacing on the thermal performance of the GSHP–energy pile system. The results of the numerical simulations show that the soil–pile–raft interaction effects can be very important. In particular, the presence of a relatively rigid raft in direct contact with the soil is responsible for axial load variations in inactive piles of the same order of those experienced by the thermo-active piles, even when the latter are relatively far and temperature changes in inactive piles are small. As far as the effect of pile spacing is concerned, the numerical simulations show that placing a high number of energy piles in a large piled raft with relatively small pile spacings can lead to a significant reduction of the overall heat exchange from the piles to the soil, thus reducing the thermal efficiency of the system.     

复杂环境中大规模桩筏基础的优化设计方法研究

变刚度优化设计对于复杂环境中的大规模桩筏基础始终是一个重要难题。基于有限元分析提出一种两阶段优化设计方法,该方法首先根据传统均匀布桩方案的桩顶应力分布对群桩进行分区,然后依据每个子区域桩顶应力之间的关系确定基桩数量调整系数,最后通过调整桩间距来改变各子区域的基桩数量,从而实现变刚度优化设计。运用该方法对多层土体中承担非均匀上部结构荷载的大规模桩筏基础进行变刚度优化设计,计算结果表明优化设计后筏板的差异沉降、平均整体弯矩和群桩顶部的差异应力均显著降低。该方法计算简单,应用范围广,且不受复杂土层条件、非均匀上部结构荷载以及桩基础规模大小形状的限制。     

超大面积深厚软土桩-网复合地基现场试验研究

结合潮汕车站软基处理工程设置监测断面,埋设相关监测仪器,对桩-网复合地基上部路堤填土施工过程中地表沉降、深部分层沉降、深部水平位移、桩顶应力、桩间土应力、土工格栅伸长量等的变化进行观测分析,结果表明,在加载初期,桩间土和桩顶土的应力都存在一个迅速增大的过程,但桩顶土应力增大的速率要大于桩间土应力增大的速率;当填土达到一定高度时,桩间土应力出现极值,产生的土拱效应会使4桩中心处的应力小于2桩中心处的应力值;管桩的轴力、摩阻力和地层情况密切相关,且均随时间、荷载的增加而增大;地基分层沉降的速率与路堤填土的速率呈正相关,沉降量的大小与地层深度和地层特征有关;土工格栅的拉伸位移量随着填土高度的增加而增加,且其增长速率经历了由慢到快再到缓的过程;地基水平位移随荷载的增加而增大,随深度增加而减小,管桩有效地限制了地基的水平位移。     

上部结构－桩基—地基共同作用数值分析及桩基变刚度调平优化设计

本文采用有限元法对高层建筑上部结构—桩筏基础—地基共同作用及相互影响进行了研究。研究表明:高层建筑上部结构—桩筏基础—地基共同作用及相互影响时,基础总体沉降和差异沉降随楼层的增加呈非线性变化趋势,上部结构中存在次应力,弯矩和轴力比常规法设计偏大;随楼层的增加,桩体对荷载的分担比在减少,土体分担比在增加;随着上部结构刚度的增加,荷载向角桩、边桩集中;增加筏板厚度,能减少一定的差异沉降和基础平均沉降,从而减少上部结构的次应力,提高地基土的荷载分担比,同时筏板下桩顶反力分布更不均匀,因此需要从筏板受力,以及考虑筏下桩、土的受力来综合确定一个合理的筏板厚度,使设计安全经济;随着地基土变形模量的提高,地基土分担的上部荷载增加,桩顶反力趋向平均,筏板最大弯矩逐渐减小。桩筏基础在均匀布桩条件下呈中间大边缘小的“碟型”分布。差异沉降是由于上部结构次生应力和筏板内力产生的。通过对地基土刚度以及桩长、桩径、桩距等五种桩基刚度的调整,并分析不同刚度对基础差异沉降影响可知:改变桩长的布桩形式并结合地基土刚度调整的中心布桩形式是高层建筑桩筏基础最佳设计方案。