桩间距对桩筏基础结构性能影响的模型试验研究

 为研究桩间距对桩筏基础结构性能的影响,设计并进行不同桩间距的桩筏基础系列模型试验。结果表明,设置不同的桩间距可调配群桩基础中桩与筏板的分担作用,调整桩筏基础的受力性能。常规桩距桩筏基础以桩为承载主体,地基土的荷载分担次之;当桩间距大于6倍桩径后,桩较早进入塑性支承状态,地基土分担荷载能力增强,桩与筏板荷载分担的主次作用发生转换,筏板逐渐转换为基础的承载主体。随着桩间距增大,筏板内力分布由整体弯曲过渡到以局部弯曲为主。桩筏基础桩顶荷载分布规律受桩间距的影响,随着桩间距增大,桩顶荷载由角桩最大逐渐转化为中间桩最大。建议桩筏基础设计方法应考虑桩间距设置对筏板与桩土体系相对刚度、筏板内力和桩顶荷载不均匀特征的影响。     

不同荷载下的桩筏基础设计

在上部竖向荷载对地基产生的应力分布很不规则导致筏板基础形心与结构重心距离很大，通过不均匀布置摩擦桩，采用桩土共同作用的复合基础调整基础荷载重心，从而使筏板形心与上部结构重心基本重合，本文介绍了一种设计经验，旨在为类似工程提供参考和借鉴。     

桩顶与筏板多种连接构造方式工作性状对比试验研究

针对桩顶与筏板之间不同构造形式下的相互作用,对可压密土中刚性桩复合地基、常规桩筏基础和桩顶预留净空桩筏基础进行了现场模型试验,测量了筏板沉降、筏板内外不同深度地基土沉降、桩身轴力和桩间土反力,分析了不同荷载级别下筏板沉降、筏板内外不同深度地基土沉降、桩土荷载传递特性和桩土荷载分担比分布规律。与刚性桩复合地基和常规桩筏基础相比,对于桩顶预留净空桩筏基础工作性状,研究发现在桩顶与筏板接触前,桩顶预留净空桩筏基础工作性状与刚性桩复合地基相似;桩顶与筏板接触后,其工作性状与常规桩筏基础相似。试验条件下,桩顶与筏板之间接触、设置褥垫层、预留净空(或可压缩垫块)不同连接方式可显著影响桩间土的压缩及桩土相对滑移,对上述三者来说,桩身下部桩、土相对滑移量(桩端刺入量)依次减小。     

桩筏基础中筏板分担荷载作用及影响因素分析

针对高层建筑桩筏基础设计中,一般不考虑筏板分担荷载作用的设计方法,从桩筏基础支承体系的受力和变形问题入手,在参考国内外有关资料以及工程实例分析计算的基础上,通过对桩筏基础中筏板的分担荷载作用分析,由筏板分担作用的时效性引出随着上部结构的建造而逐渐变化的筏底土反力分布规律;通过对筏板分担荷载作用的影响因素分析,提出桩筏基础中筏板分担作用涉及上部结构与地基基础的复杂共同作用全过程.筏板的分担作用不可忽视,在建筑物建造的全过程发生变化.并提出合理的结构设计计算应定量考虑桩筏共同作用的计算方法及建议.     

耦合荷载作用下桩筏基础性状数值分析

采用大型通用有限元软件,模拟分析了耦合荷载作用下桩筏基础力学特性。文章结合国华通辽风电场工程实例,通过对八种分析模式的模拟,分析了上覆土层、筏板刚度、土体承载作用以及土体模量对耦合荷载作用下桩筏基础的受力特性的影响,并研究了上覆土层、筏板刚度和土体模量对桩筏基础桩土分担比的影响。最后,根据有限元分析结果提出耦合荷载作用下桩筏基础设计的优化的一些理论参考依据。     

极限荷载下桩筏基础共同作用性状的室内模型试验研究

利用自制模型槽,通过设计系列单桩带台与群桩的桩筏基础模型试验,研究了极限荷载下桩–筏板–地基土的应力与变形性状。试验结果表明,常规桩距桩筏基础极限荷载下表现出实体深基础性状;而大桩距桩筏基础,基桩先于板下土体达到承载力极限状态,后续荷载基本由板下土体分担,验证了塑性支承桩理论。加载过程中,桩–土的荷载分担比不断变化,6d及以上桩距时,桩达到极限荷载后即趋于稳定。利用桩的极限承载力的桩筏基础设计,应考虑极限荷载与工作荷载下桩–土荷载分担比的不同性状差别。桩间距越大,桩对土体的侧向位移的"遮帘作用"逐渐弱化,板下土体的位移特征趋于天然地基的特征,桩端平面以下土体应力受板下土体分担荷载的影响越明显,6倍桩距可视为常规桩基与复合桩基的分界点。     

两阶段变刚度桩筏基础工作机理研究

本文利用ABAQUS有限元软件,对天然筏板基础、常规端承型桩筏基础以及两阶段变刚度桩筏基础三种基础形式进行模拟。通过对比分析桩土荷载分担比、桩侧摩阻力以及三种基础筏板下地基土的沉降,研究三种筏板基础的工作特性,探讨并总结两阶段变刚度桩筏基础的工作机理。分析结果显示,两阶段变刚度桩筏基础工作机理介于天然筏板基础与复合桩筏基础之间,具有较明显的两阶段受力特性,能够充分发挥地基土的承载力,有效提高桩与土的共同承载能力。     

大型格池桩筏基础共同作用的试验研究

结合南京市城北污水处理厂曝气沉淀池地基及基础工程设计,对格池闭水载荷试验过程中桩筏基础的变形、筏板底土压力分布、桩顶荷载受力分布及桩与筏板的荷载分担测试方法与结果进行分析和研究,探讨了曝气沉淀池在实际运行工况时,桩筏基础与地基共同作用的机理和变化规律。试验结果表明:大面积筏板后浇带部分筏板在受力转变过程中,对桩筏基础的沉降值影响甚微;在桩径相同时,可通过调整不同区域内的桩长来优化桩基承载力的设计;两相邻格池,当一池为卸载(排水),另一池为加载(注水)时,卸载一侧的桩顶反力会达到最大值;扣除水浮力的影响,当桩距径比s/d=3.8~4.5时,筏板分担荷载平均比值:满池工况为9.0%,空池工况为11.3%;桩筏基础最大沉降量及最大差异沉降量均远小于设计值和允许值,对类似工程设计取s/d=5.0~6.0,筏基可承担15%~20%的荷载,经济效益会非常显著。     

桩筏基础的筏板分块探讨

采用三维有限元方法,就工程中常见的桩筏基础在竖向荷载作用下桩—地基土—筏板共同作用时的特性进行探讨,重点分析筏板分块面积大小对桩土分担比、筏板内力(特指弯矩和扭矩)、最大沉降量和最大沉降差的影响,从而得出有益的结论并提出了设计建议。     

基于减小筏板差异沉降的刚性桩复合地基试验研究

针对高层建筑筏板荷载分布特点,采用内密外疏布桩和仅在筏板中心布桩方式,通过现场缩尺(1:10)模型试验,完成了带上部结构无桩筏板和刚性桩复合地基筏板静载荷试验。分析了筏板沉降、桩端平面以下地基沉降和筏板外侧地面沉降、筏板下桩土反力分布、桩土荷载传递及桩土荷载分担比。研究了工作荷载下不同布桩方式降低筏板差异沉降的效果。     

桩筏基础设计方案优化若干问题

从优化设计的角度出发,探讨了桩筏基础的设计思路、布桩方式、桩土共同作用等一系列问题,得到了一些有益的结论.还提出了一些对设计进行优化的思路和具体方法供读者参考.     

Physical modeling of behaviors of cast-in-place concrete piled raft compared to free-standing pile group in sand

Similar to free-standing pile groups, piled raft foundations are conventionally designed in which the piles carry the total load of structure and the raft bearing capacity is not taken into account. Numerous studies indicated that this method is too conservative. Only when the pile cap is elevated from the ground level, the raft bearing contribution can be neglected. In a piled raft foundation, pile–soil–raft interaction is complicated. Although several numerical studies have been carried out to analyze the behaviors of piled raft foundations, very few experimental studies are reported in the literature. The available laboratory studies mainly focused on steel piles. The present study aims to compare the behaviors of piled raft foundations with free-standing pile groups in sand, using laboratory physical models. Cast-in-place concrete piles and concrete raft are used for the tests. The tests are conducted on single pile, single pile in pile group, unpiled raft, free-standing pile group and piled raft foundation. We examine the effects of the number of piles, the pile installation method and the interaction between different components of foundation. The results indicate that the ultimate bearing capacity of the piled raft foundation is considerably higher than that of the free-standing pile group with the same number of piles. With installation of the single pile in the group, the pile bearing capacity and stiffness increase. Installation of the piles beneath the raft decreases the bearing capacity of the raft. When the raft bearing capacity is not included in the design process, the allowable bearing capacity of the piled raft is underestimated by more than 200%. This deviation intensifies with increasing spacing of the piles.     

上部结构与桩筏基础共同作用的非线性分析

通过对上部结构与桩筏基础的共同作用进行非线性分析,可以来优化基础设计。本文建立了一种上部结构与桩筏基础共同作用的非线性分析方法,编制了分析程序。通过实例分析,研究其工作机理,探讨其合理的布桩方式和优化设计方法。     

桩基础设计中若干问题探讨

根据建筑物桩基础设计经验,就桩基础设计中几个值得注意的问题作了探讨,具体对高层带裙房桩筏基础、桩筏基础变刚度调平设计、桩的承载力水平等作了论述,从而为桩基础形式的选择奠定了基础。     

分层土中桩筏基础荷载分担问题的有限元分析

采用三维有限元方法建立桩筏基础模型来考虑各种影响因素对桩土分担比的影响。地表硬壳层的存在能够有效地提高筏板的分担作用。通过对不同桩距和布桩方式桩筏基础的计算,发现当桩距径比增大时,桩间土的承载作用有显著提高,但当大于7d之后则无显著变化。此外,还研究了不同桩端土的相对刚度以及应力水平下桩筏基础共同作用性状,可供工程设计人员参考,以优化设计并满足较高的经济性。     

Automatic optimization of piled raft design based on branch and bound method

Piled rafts are foundation systems that combine a shallow raft with the minimum number of piles beneath the raft, in order to achieve the necessary bearing capacity and the acceptable settlement restrictions. Its use is increasing in many countries, especially in skyscrapers in Europe and Asia, for which the foundation design can result in hundreds of piles due to the high load magnitude. However, controlling the most influencing variables on the foundation behaviour, while achieving an acceptable cost, is a challenge for designers. This paper presents an optimization procedure to determine the best raft thickness, number of piles and their location. The branch and bound optimization method was combined with a specific computational programme for piled raft analysis (GARP) providing an automatic technique to obtain the lower cost design. A building in Brazil and theoretical examples of a piled raft with different load sets are presented and analysed using this numerical approach. The results show that the presented automatic procedure provides interesting solutions, economically enhancing the design of piled raft foundation systems. The optimization process provides arrangements using lower number of piles while satisfying the constraints in both analysed cases. Copyright © 2014 John Wiley & Sons, Ltd.     

Innovative piled raft foundations design using artificial neural network

Studying the piled raft behavior has been the subject of many types of research in the field of geotechnical engineering. Several studies have been conducted to understand the behavior of these types of foundations, which are often used for uniform loading on the raft and piles with the same length, while generally the transition load from the upper structure to the foundation is non-uniform and the choice of uniform length for piles in the above model will not be optimally economic and practical. The most common method in identifying the behavior of piled rafts is the use of theoretical relationships and software analyses. More precise identification of this type of foundation behavior can be very difficult due to several influential parameters and interaction of set behavior, and it will be done by doing time-consuming computer analyses or costly full-scale physical modeling. In the meantime, the technique of artificial neural networks can be used to achieve this goal with minimum time consumption, in which data from physical and numerical modeling can be used for network learning. One of the advantages of this method is the speed and simplicity of using it. In this paper, a model is presented based on multi-layer perceptron artificial neural network. In this model pile diameter, pile length, and pile spacing is considered as an input parameter that can be used to estimate maximum settlement, maximum differential settlement, and maximum raft moment. By this model, we can create an extensive domain of results for optimum system selection in the desired piled raft foundation. Results of neural network indicate its proper ability in identifying the piled raft behavior. The presented procedure provides an interesting solution and economically enhancing the design of the piled raft foundation system. This innovative design method reduces the time spent on software analyses.     

Centrifuge model tests on piled raft foundation in sand subjected to lateral and moment loads

Piled raft foundation has been widely recognized as a rational and economical foundation system with the combined effects of raft and piles. However, the behavior of laterally loaded piled raft foundation has not been well understood due to the complicated interaction of raft–ground–piles. A series of static horizontal loading tests were carried out on three types of foundation models, i.e., piled raft, pile group and raft alone models, on sand using a geotechnical centrifuge. In this paper, the influences of relatively large moment load and rotation on the overall performance of laterally loaded piled raft foundation were examined. From the centrifuge model tests, it is found that the vertical displacement due to horizontal loads is different between piled raft and pile group foundation, and this vertical displacement has significant influences on the performance of laterally loaded piled raft foundation. The horizontal resistance of the pile part in the piled raft foundation is higher than those observed in the pile group foundation due to raft base contact pressure. The vertical displacement of the foundation due to the horizontal loads affects the vertical resistances of piles, which results in the different mobilization of moment resistances between the piled raft and pile group foundations.     

可控刚度桩筏基础桩土共同作用的工程实践

端承型桩等支承刚度较大的桩基础,在正常工作荷载作用下,桩顶较小的竖向变形无法保证桩土共同作用的实现,这可能导致良好的天然地基承载力得不到充分利用,造成极大的浪费。介绍的典型工程成功解决了支承刚度较大的桩基础难以实现桩土共同作用的问题,同时还实现了桩筏基础的变刚度调平,通过对工程设计及数值分析过程的详细论述与描写,并将现场测试数据与分析结果进行对比分析,验证可控刚度桩筏基础应用于该项目的合理性与可靠性。数值分析及现场实测结果显示,本工程采用可控刚度桩筏基础后,建筑物总沉降和差异沉降均满足设计要求,地基土分担了上部结构63%的的荷载,桩基数量大幅度降低,减少了高能耗建材的使用和桩基施工对周边环境的影响,取得了显著的经济效益和社会效益。本工程可控刚度桩筏基础实施所取得的相关经验与成果可供类似工程借鉴与参考。     

高层建筑与裙房基础整体设计的若干问题

通过带裙房高层建筑与地基基础共同作用的理论研究以及实测调查，就高层建筑与裙房基础整体设计的一些问题提出看法与建议。其中包括整体设计裙房基础悬臂长度的建议值和相对弯曲参考值。推荐在软土地基合适的桩箱（筏）基础沉降计算方法以及上海各类桩持力层基床系数的建议值，以供设计者参考。