Centrifuge model test on dynamic behavior of group-pile foundation With inclined piles and its numerical simulation

In this paper, dynamic behavior of a grouppile foundation with inclined piles in loose sand has been investigated with centrifuge model tests. The test results are also simulated with elastoplastic dynamic finite element method, in which, not only sectional force of piles, stress of ground, but also deformation of piles are calculated using a three-dimensional elastoplastic dynamic finite element analysis (Code name: DGPILE-3D). The numerical analyses are conducted with a full system in which a superstructure, a pile foundation and surrounding ground are considered together so that interaction between pile foundation and soils can be properly simulated because the nonlinearities of both the pile and the ground are described with suitable constitutive models. Different types of piles, vertical pile or inclined pile, are considered in order to verify the different characteristics of a group pile foundation with inclined piles. The validity of the calculation is verified by the model tests.     

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.     

横向荷载下群桩相互作用的积分方程解法及参数分析

针对横向受荷桩的相互作用问题,将桩土体系分解为弹性半空间地基土和虚拟桩,基于水平位移协调条件建立两桩相互作用的第二类 Fredholm 积分方程,采用 MATLAB 软件编制程序进行求解。基于 Chen 等提出的横向受荷桩相互作用系数解法,得到了桩顶转角固定条件下水平受荷桩的积分方程解答,与已有算例的对比验证了本文算法的正确性。参数分析表明,固定转角约束条件下两桩的水平位移相互作用系数随着桩间距和荷载方向夹角的增加而减小,随着桩土模量比的增加而增加。基于相互作用系数解法,该方法可应用于转角固定的水平受荷群桩分析。     

基于p-y曲线法分析隧道开挖条件下邻近桩基的水平反应

在分析隧道开挖条件下邻近桩基的水平反应时,基于考虑桩土界面非线性的p-y曲线,引入统一极限桩土相互作用力的概念,利用有限差分方法通过迭代求解给出隧道开挖与邻近单桩相互作用的弹塑性解答.在群桩计算中引入考虑群桩效应的p-因子(f_m),从而得到群桩中各单桩的p-y曲线,进而通过有限差分方法求得考虑轴力情况下群桩中各单桩的变形,转角,弯矩和剪力.同时通过已有试验和实测数据对本文方法进行验证分析,结果表明本文方法与试验与实测结果得到了较好的一致性;分析隧道开挖条件下邻近桩基的水平反应时,群桩中的第一排桩与同位置处的单桩性质相似,后排桩由于群桩效应的存在受到的影响将明显小于前排桩.     

新型板桩码头群桩基础被动段桩侧压力试验研究

在新型板桩码头结构中,带卸荷承台板的直立灌注桩群桩基础被用来与前墙一起共同承担土体侧向位移所产生的侧向荷载,这些直立桩的工作机制类似抗滑被动桩,但其桩土相互作用程度远没有达到极限状态。结合20万吨级卸荷式板桩码头结构设计方案的验证,开展了多组土工离心模型试验,测量了各排灌注桩两侧土压力,得到了均质细砂地基中两种承台群桩基础中桩身侧压力分布,其特征是桩身上部侧压力为正,作用方向与土体位移一致,而桩身下部侧压力为负。各排桩的侧压力零值点约位于港池泥面线下4倍桩径位置处,可根据侧压力零值点分界线将桩身划分为上部被动段和下部主动段进行设计处理。为了确定被动段侧压力大小,引入被动段桩宽比,将桩侧压力与朗肯主动土压力相关联,结果发现,海侧桩被动段桩宽比接近3.0,明显大于中间桩和陆侧桩,因此,承台群桩基础设计中,被动段可以近似按3倍朗肯主动土压力作为设计值考虑。     

浅层土体加固对倾斜桩竖向承载力影响研究

桩身倾斜可能导致在小于对应于相同条件下竖直桩的极限承载力的荷载下提前发生弯曲破坏,并可在弯曲破坏前产生较大的桩顶水平位移和桩身挠曲。根据某工程现场载荷试验结果,对倾斜桩桩周土体进行浅层加固数值模拟,以研究其改善倾斜桩竖向承载性状的机理。分析结果表明：加固体深度、加固体面积及加固体尺寸均可对加固效果产生影响。加固体对竖向荷载作用下倾斜桩的承载性状的改善体现在对被动区土体强度和压缩性的改善以及约束桩身上部桩体转动和挠曲的作用。进一步研究了加固体压缩性的影响,结果表明：加固体弹性模量越大,加固效果越明显;但随着弹性模量的增大至一定值后,加固效果基本不再变化。     

深基坑剪力键支护模型优化研究

为寻求深基坑剪力键支护模型的优化形式,在剪力键支护体系构想与模型试验的基础上,设计了3组剪力键与直立桩之间不同组合形式的试验方案,在模拟基坑开挖过程中,对各方案中支护模型的桩顶水平位移、桩身内力及基坑外侧填土表面的沉降进行监测,同时设计了4个系列12组剪力键模型方案进行有限元数值模拟。结果表明:斜向桩与腰梁连接的节点位于直立桩桩身处的剪力键组支护效果优于斜向桩与腰梁连接的节点位于相邻直立桩中间的剪力键组; 腰梁位于直立桩上部的剪力键组支护效果优于腰梁位于直立桩中部和下部的剪力键组; 腰梁高度对支护效果的影响大于斜向桩与直立桩连接节点位置的影响; 剪力键模型的支护效果与斜向桩和竖向的夹角非正相关,并且在实际工程中夹角越大所占用的地下空间越大,基于数值模拟可认为斜向桩与竖向的夹角30°为剪力键支护结构的适宜角度; 斜向桩与腰梁连接的节点位于直立桩桩身处、斜向桩与竖向夹角30°且腰梁位于直立桩上部的剪力键组是较优的支护形式,这些成果可为剪力键支护技术的开发与应用中提供借鉴。     

A case study on the soil–pile–structure interaction of a long span arched structure

Different concepts for modelling of soil-foundation in complete dynamic interaction analysis for a 110-m height 70-m span arched structure on 180 piles were investigated in this paper. The modelling approaches consisted of a sophisticated procedure to account for soil compliance and foundation flexibility by defining frequency-dependent springs and dashpots; namely, flexible-impedance base model. The results of this model were compared with those of the conventional modelling procedures; namely, fixed base model and flexible base model by defining frequency-independent springs. In the flexible-impedance base model, the substructure approach was employed through finite element modelling. To account for the kinematic interaction, the numerical model of the soil, foundation and piles were developed using a verified finite element model in ABAQUS. The free field time history and design spectrum were modified to obtain the foundation input motion. The impedance of pile groups with different length was obtained by the finite element model to assess the inertial interaction. The comparison of the results of the employed models showed that rocking and torsional responses were greatly affected by soil–structure interaction, indicating redistribution of seismic demands. It was also proven that the internal demands of the conventional model considering frequency-independent Winkler springs might be higher than those of the model including pile–soil–structure interaction effects.     

土体侧移作用下既有轴向受荷桩性状的室内模型试验研究

堆载或开挖等引起的土体侧移对邻近轴向受荷桩的承载和变形性状可能会产生负面影响,对于这一问题目前尚缺乏充分的研究。采用室内模型试验研究轴向受荷桩在土体侧移作用下的承载和变形特性,重点分析了土体侧移、桩顶轴向荷载以及群桩效应等对桩基性状的影响。试验结果表明,桩顶轴向荷载和土体侧移的耦合作用使得桩身弯矩和位移均相应的增大;而两桩中的前桩对后桩具有遮拦效应,前桩的存在使得后桩的弯矩和变形明显变小。在工程实践中应充分重视土体侧移与轴向荷载的耦合效应对桩基性状的影响。     

塑料套管现浇混凝土桩倾斜对承载性能影响的模型试验研究

 通过模型试验研究竖直桩与倾斜角度为5°,8°,10°和15°倾斜桩的承载性能,分析倾斜对塑料套管现浇混凝土桩(以下简称TC桩)单桩承载力、桩顶沉降、桩身水平位移、桩身弯矩、桩身轴力及侧摩阻力和端阻比的影响。模型试验结果表明：(1) 当TC桩倾斜度不大于8°时,对承载力和桩顶沉降影响不明显,对于倾斜10°,15°的桩,承载力明显降低;(2) 倾斜桩桩身水平位移和弯矩主要发生于1/3桩长范围内,且均随着荷载和倾角的增加而增大;(3) 竖直桩及各倾斜桩的侧摩阻力随深度的增加呈先增大后减小的趋势,随着倾角的增大桩极限侧摩阻力的平均值略有增大,侧摩阻分担比较大;(4) 端阻力和端阻比基本随荷载的增加而增加,随倾角的增加而减小。采用Origin软件应用Sigmoidal函数拟合,得出模型桩极限承载力随桩体倾角的计算公式,并根据现场实测数据,给出现场应用时修正系数的取值范围。     

Influence of installation method on performance of screwed pile and evaluation of pulling resistance

In the present study, we performed installation and pull-out loading tests on screwed piles in sand deposits using a calibration chamber. These tests focused on how the installation method influenced the performance of the piles. The results revealed that the load-displacement relationship strongly depended on the installation method, but that the second-limit uplift resistance was almost unaffected. Next, we observed the movement of the soil near the pile after both the installation and pull-out loading tests. Shear failure of the soil, which occurs in a cylindrical region in the periphery through which the wing plate of the pile passes, regardless of how the screwed pile is installed, was found to be one of the determinants of the pulling resistance. Finally, we evaluated the pulling resistance of the screwed pile based on these soil observations and an analysis of the loading test results. We found that the pulling resistance of the wing plate could be determined based on the change in earth pressure near the pile due to installation and pull-out loading of the pile.     

Load–settlement modeling of axially loaded steel driven piles using CPT-based recurrent neural networks

The design of pile foundations requires good estimations of the pile load-carrying capacity and the settlement. Designs for bearing capacity and settlement have been traditionally carried out separately. However, soil resistance and settlement are influenced by each other, and thus, the design of pile foundations should consider the bearing capacity and the settlement together. This requires that the full load–settlement response of the piles be accurately predicted. However, it is well known that the actual load–settlement response of pile foundations can only be obtained through load tests carried out in-situ, which are expensive and time-consuming. In this technical note, recurrent neural networks (RNNs) were used to develop a prediction model that can resemble the load–settlement response of steel driven piles subjected to axial loading. The developed RNN model was calibrated and validated using several in-situ full-scale pile load tests, as well as cone penetration test (CPT) data. The results indicate that the developed RNN model has the ability to reliably predict the load–settlement response of axially loaded steel driven piles, and thus, can be used by geotechnical engineers for routine design practice.     

斜向抗拔螺旋群桩基础承载性状试验研究

通过3桩等边三角形承台原型群桩基础的4次斜向抗拔、2次垂直抗拔和5次单桩垂直抗拔试验,绘制了相应的荷载位移关系曲线。依据地基变形特点和曲线变化特征,建议采用具有明确物理特征的H-(△x/△H)曲线判定螺旋群桩基础水平方向极限荷载,垂直方向抗拔极限荷载依据单位荷载的桩顶位移变化率、桩顶位移增量以及地基变形特征判定,并讨论了群桩基础在斜向荷载和垂直荷载上拔过程中的群桩效率。试验表明:抗拔群桩基础的群桩效率与外荷载的作用角度有关,斜向抗拔螺旋群桩基础的群桩效率小于垂直抗拔群桩基础的群桩效率,荷载水平分量加快了该种群桩基础的上拔破坏,斜向抗拔群桩的极限破坏荷载和极限位移要小于垂直抗拔螺旋群桩基础。     

Horizontal impedance of pile groups considering shear behavior of multilayered soils

The impedances and internal force distributions of a pile and pile groups subjected to horizontal harmonic pile-head loads have been studied. A layered Pasternak model, which overcomes the limitation of the Winkler model, that ignores the shear stiffness of the soil, is developed to describe the reaction of the soil on piles. The differential equation of the horizontal damped vibrations of a pile is solved by the initial parameter method, combined with transfer-matrix formulations, to deal with the layered property of the soil. The superposition approach, based on the interaction factors, is taken to estimate the effects of pile groups. The precision and the applicability of the method are demonstrated through this study. Comparing the results of the Pasternak model and the Winkler model, it is revealed that the role of soil shear has an influence on the impedances of a single pile and pile groups, especially in the case of a low pile–soil modulus ratio.     

基于沉降控制的竖向下压循环荷载作用下的桩基设计方法

低幅值、大周次竖向循环荷载作用下桩基发生累积沉降及承载力弱化,影响到风机塔架、高速铁路路基等沉降敏感结构物的服役性能。首先总结了竖向循环荷载作用下桩基设计要求及若干设计关键问题。基于模型试验结果,分析了竖向循环荷载作用下的桩基承载力和累积沉降演化规律;定义了3种桩顶累积沉降发展模式及对应的循环荷载比和静荷载比组合,建立了桩基循环累积沉降分析图,提出了组合循环荷载作用下的桩基循环加载性状简化判定公式。提出了针对竖向循环荷载作用下桩基的设计方法及设计流程,并给出了设计实例。     

海洋高桩基础水平单调及循环加载现场试验

开展了海洋软黏土中 2 根大直径高桩基础的现场水平单调和循环加载试验,实测获得了桩顶荷载 – 位移关系、桩身变形和桩身弯矩及桩侧土压力和孔隙水压力,揭示了水平单调和循环荷载作用下桩土相互作用规律及桩基水平位移和桩身弯矩发展规律。利用实测桩身水平位移推算了桩周土反力,在此基础上提出了相应的双曲线型 p – y 曲线,通过引进 Poulos 循环弱化模型建立了水平循环荷载作用下的桩基双曲线型 p – y 曲线分析模型,水平单调及循环荷载作用下桩顶荷载 – 位移关系、桩身变形和桩身弯矩及桩侧土压力等计算结果与实测值均吻合良好。通过现场试验发现规范 p – y 曲线法计算结果偏保守的主要原因是所采用的 p – y 曲线的刚度偏小;不同时段的循环荷载对桩基循环累积变形有叠加效应。 建议设计中应考虑桩基全寿命服役期内所承受的所有循环荷载的影响, 对于重要工程应开展相应的现场水平加载试验,实测桩身水平位移或桩身弯矩,进而利用所推算的桩周土反力来分析桩基受力变形及承载力。     

考虑时间效应的斜桩基负摩阻力室内模型试验研究

斜桩基是桥梁支墩、码头等工程中常用的桩基础形式,但是针对斜桩基础负摩阻力问题的研究却相对较少。进行了黏性土层中斜单桩及斜群桩在桩周土堆载固结条件下负摩阻力性状的室内模型试验研究,测得了桩侧负摩阻力、桩端阻力以及桩周土体分层沉降随固结时间的变化情况;并进行了同等条件下竖直单桩及竖直群桩试验作为比较分析。试验结果表明,斜桩和竖直桩桩侧负摩阻力引起的下拽力都存在明显的时间效应和群桩效应;在本文试验条件下,当桩–土相对位移达到2mm时,桩侧负摩阻力将达到其最大值的80%～90%左右。     

基于三维数值仿真分析竖向荷载对水平承载力的影响

随着我国近年来大跨径桥梁及高层建筑的迅速发展,基桩的承载能力及桩长日益增大,竖向、横向组合荷载(倾斜荷载)作用下基桩的受力分析和设计理论已成为目前土木工程界急待解决的重要问题之一。针对我国现行规范将桩顶倾斜荷载分解为竖向和水平荷载分别计算,进行单方向荷载下的承载力、强度、构造计算、校核存在的不足。采用PLAXIS 3D FOUNDATION对竖向荷载作用下,非扩底桩和扩底桩的水平承载力进行三维数值仿真分析,提出了对规范进行修正的建议。     

一种改进型的管桩桩尖及其在倾斜地层中的应用

针对预应力高强度混凝土管桩在上软下硬的倾斜地层中施工时容易发生斜桩这一问题,笔者根据多项工程实践,通过详细分析产生斜桩的施工影响因素、常用桩尖的工程特点与沉桩时桩尖的工作机理,提出了施工技术防治措施,并设计了一种改进型的凹弧十字型管桩桩尖以解决斜桩问题。经工程实践证明,能够达到降低斜桩率、提高工程质量和降低生产成本等目的。     

斜桩承台桩基沉降影响有限元分析

马来西亚槟城二桥部分基础采用斜桩双承台,由于其采用斜桩基础,且桩基间距小于规范要求,沉降不方便计算。为控制其变形,需预估沉降值,通过有限元软件建立其中两个最具代表性的斜桩承台的空间模型,分为单承台和双承台两种情况,分析两种情况的沉降情况以及两承台间的相互影响,得出单承台和双承台两种情况下的沉降计算值以及变形情况,进而确定出双承台间的影响系数。同时比较桩身压缩量与总沉降的关系,结果表明沉降主要是由桩身压缩量引起,且总沉降满足要求。