End bearing capacity comparison of screw pile with straight pipe pile under similar ground conditions

In the present study, the end bearing capacity of screw and straight pipe pile under similar pile tip area and ground conditions were investigated. The effect of increasing overburden pressure was also considered in this research. Pile load tests on close-ended screw and straight pipe piles were conducted in the small scale. Dry Toyoura sand was used to develop the model ground. The sand was compacted at relative density of 70, 80 and 92 %. It was observed that in case of straight pipe pile, load settlement curve plunges downward without increase in load around settlement equals to 10 % of pile tip diameter, whereas in case of screw pile, the load settlement curve plunges around settlement equals to 15 % of pile tip diameter. Moreover, the screw piles having helix-to-shaft diameter ratio 2–4.1 showed 2–12 times higher end bearing capacity than straight pipe piles with similar pile shaft diameter. It was also observed from the test results that the end bearing capacity of single-helix screw pile was in average 16.25 % less than straight pipe pile with similar pile tip area and ground conditions irrespective of the effect of increasing overburden pressure.     

Static Equilibrium of Screw Anchor Pile Under Lateral Load in Sands

The screw anchor piles are installed in ground by screwing which is done with the help of torque motors. In this paper, the lateral load capacity of screw anchor piles is examined through an experimental investigation carried on model piles embedded in dry sand. The tests were carried on screw anchor piles with different number of helices provided in continuation. Lateral loads were applied at different height above the soil surface. The embedment length of screw anchor piles was also varied to study the behaviour of screw anchor piles under lateral loads. Some tests were conducted on plain shaft pile to compare the lateral load capacity of screw anchor piles with that of plain shaft piles. An empirical equation for computation of lateral loads has been developed considering lateral resistance, bearing resistance, uplift resistance and lateral resistance offered by soil in pile on the basis of experimental results. A theoretical model for predicting lateral load capacity of screw anchor piles in dry sand, consistent with the experimental findings has been developed in this study.     

不同成桩工艺条件下冻结粉土中基桩承载性状试验研究

桩基础作为冻土工程中最适宜的基础形式,主要采用插入桩、静压桩和灌注桩三种桩型,因其成桩工艺不同对冻土地基及桩基自身造成的差异不一。为研究不同成桩工艺对冻土地基及基桩承载性状的影响,通过开展室内-1.5 ℃条件下单桩静载模型试验,分析在冻结粉土中不同成桩工艺对地温场、桩基极限承载力、桩身轴力以及桩侧摩阻（冻结力）的影响规律。试验结果表明：灌注桩对桩周地温场扰动剧烈,桩侧温度较高,地温变化幅度大。随着桩周土体的回冻,地温逐步降低,其中灌注桩桩侧降温速率最大;在承载力方面,2根灌注桩的极限承载力约为12.8 kN,静压桩为11.6 kN,插入桩极限承载力最小,仅为钻孔灌注桩的2/3。并对比4种不同成桩工艺的基桩在不同荷载条件下对应的沉降量,体现出成桩工艺对基桩沉降造成的差异性;随着桩顶荷载的逐级增加,桩侧摩阻（冻结力）和桩端阻力逐渐发挥作用,桩身上部1/3由于温度较低,致使桩侧摩阻（冻结力）较大,在10 cm深度附近温度最低,使桩侧摩阻（冻结力）达到最大值;并对比4种不同成桩工艺的基桩在荷载5.6 kN下轴力沿桩身的传递情况,发现静压桩更易把荷载传递到冻土区深层地基。     

海洋工程中大直径管桩垂直水平荷载试验研究

通过海洋环境条件下大直径管桩的垂直和水平荷载试验,分析了管桩在垂直和水平荷载作用下的受力特点,得到了管桩的垂直极限承载力、侧摩阻力及端承力、轴向反力系数等结果,以及水平荷载作用下桩顶位移和转角关系、弯矩分布、土抗力、水平地基反力系数的比例系数和最大弯矩点等参数。试验结果表明：垂直荷载作用下,极限承载力可达12000kN,在沉桩过程中部分桩有一定程度的闭塞;大直径管桩能够抵抗水平荷载的作用,弹性长桩的受力性质主要受上部土层的影响。根据试验结果计算的水平地基抗力比例系数m值,对本工程及同类地质条件的桩基设计具有参考价值。     

不同场地条件下长桩的承载性状研究

根据软土地基、非软土地基中长钻孔灌注桩静载荷试验和桩身轴力的测试结果,分析探讨了竖向荷载下长桩的受力性能及沉降特征的一些规律。桩侧土模量较高的非软土地区的长桩静力试桩所测得的结果表明,荷载传递和桩身压缩与软土地区超长桩性状相似,长桩的桩身压缩量相当可观,计算中应予以考虑。极限侧阻力与端阻力不同步发挥,不同深度处的不同土层处基桩的侧阻也不能同步发挥;同时指出相关规范中极限承载力计算公式的不严密之处,并且探讨说明了竖向荷载下的长群桩基础变形性状及沉降计算有待进一步的研究。本项研究对今后超长桩的理论研究和工程设计应用具有一定的借鉴作用。     

轴向荷载对斜桩水平承载特性影响试验及理论研究

斜群桩受水平荷载作用时,群桩中的基桩受到径向荷载、轴向荷载和弯矩的共同作用。为研究轴向荷载对斜桩水平承载特性的影响,完成了3根单桩以及1组1×2斜桩的大尺寸模型试验。试验结果表明：轴向拉力作用会降低斜桩的水平刚度和极限承载力;而轴向压力作用则会使其水平刚度和极限承载力提高。基于桩侧浅层土体楔形破坏假定,推导了考虑轴向荷载影响的斜桩水平极限土抗力计算公式,提出了桩侧土抗力的p-y曲线方法,并通过模型试验及现场试验验证其合理性。     

素混凝土桩复合地基支承路堤变形破坏模式

为了分析素混凝土桩复合地基支承路堤沉降变形特征和失稳破坏机制,建立了3组不同桩间距的素混凝土桩复合地基支承路堤离心试验模型及其数值模型。结果表明：在路堤填土自重、轨道和车辆荷载作用下,改变桩间距对素混凝土桩复合地基支承路堤沉降变形、桩体应变、加筋垫层和桩体破坏模式具有显著的影响;当桩间距不大于4倍桩径时,加筋垫层整体基本保持完好,路堤下素混凝土桩复合地基沉降能逐渐趋于稳定,而桩间距达到6倍桩径后,桩顶刺穿加筋垫层,加筋垫层对桩土变形协调和传递荷载作用失效,素混凝土桩复合地基支承路堤沉降持续增大;当桩间距达到4倍桩径时,素混凝土桩最大应变值发生随上部荷载的增大反而减小的突变现象,最靠近坡脚的素混凝土桩最先产生弯曲破坏而不是剪切破坏,当桩间距增大至6倍桩径时,桩体弯曲破坏逐渐往路堤中心方向发展。     

竖向荷载下群桩-土-承台相互作用数值分析

在大桩径、小桩距的群桩条件下,不仅有来自桩侧、桩端和承台传递的多重应力叠加,还有群桩对桩间土的夹持作用影响,桩-土-承台之间作用更加复杂。用有限差分软件模拟固定桩距、桩径,变化竖向荷载下桩-土-承台的相互作用。从各层土的侧摩阻力、不同位置桩的桩顶荷载、荷载-沉降关系、桩间土体位移等方面的计算结果分析桩-土-承台之间的相互影响。结果表明,荷载超出117.8 MN（略大于Pu/2,Pu为群桩极限承载力）后,群桩对上部桩间土的夹持作用开始减小,桩侧上部侧摩阻力增大;桩侧下部侧摩阻力在多重应力叠加作用下呈减小趋势,不同位置的桩侧摩阻力影响范围有差异;用群桩沉降达到5%倍桩径时的荷载作为群桩的竖向极限承载力是可取的;当沉降与桩径的比值超出1%后,承台分担荷载的比例逐渐增大,群桩分担荷载的比例减小。     

加载次序对湿陷性黄土中基桩承载力检测的影响

湿陷性黄土地区基桩承载力检测中桩顶荷载是在土体沉降后施加的,桩基使用条件下是桩顶荷载施加之后再出现土体沉降。简要分析了这两种加载次序之间桩土相互作用差异的机制。采用有限元方法分析了这种差异。计算结果表明,在这两种加载次序中,桩顶荷载的增加都会引起沉降增加,中性点下降,下拉力减小。结果还表明,与桩顶荷载先施加的相比,桩顶荷载后施加得到的中性点的位置比较低,桩基沉降比较小,下拽力也较小,这些差异随着桩顶荷载的增大而更加明显。在两种加载次序的各个加载阶段,桩周摩阻力的分布与发挥也会出现细微的变化。这会使湿陷性黄土地区桩的承载力检测结果略微偏于不安全     

软土地区桩端后注浆桩承载性状对比试验研究

桩端后注浆技术是在通过预埋的压浆管对桩底压注水泥浆液,从而减少桩底沉渣和桩周泥皮的负面影响,在工程上逐渐得到广泛应用。结合宁波软土地基中某工程桩基静载荷试验,分析2根相邻后注浆和未注浆试桩的承载性状;通过对桩身预埋钢筋应力计的测试,对后注浆和未注浆桩的桩身轴力传递特性和桩侧阻力发挥特性进行比较分析研究。表明桩端后注浆不仅能提高端阻,还能提高桩端以上一定范围内土层的侧摩阻力,同时,后注浆桩的单桩极限承载力约为未注浆桩的2.5倍。根据以上研究结果表明,桩端后注浆技术能有效改善桩土界面条件,提高单桩承载力和减小桩基沉降。     

桩端后注浆对单桩承载性状的作用效应研究

以桩的长径比l/d、桩间距与桩径之比s/d、注浆加固体厚度h为因素,以单桩极限承载力Qu与某一荷载下的桩顶沉降Sp为评价指标,设计L9(34)正交试验方案,采用有限元法进行了桩端后注浆对单桩承载性状的作用效应研究,得出各因素作用效应的重要性次序和作用规律。研究表明,无论对桩基承载力还是对桩基沉降,桩间距与桩径之比都是最重要的影响因素,s/d较大时采用桩端后注浆来提高桩基承载力,减小桩基沉降,效果更显著;桩端后注浆使桩的端阻增大,承载性状改变,由摩擦桩逐渐过度为端承摩擦桩、摩擦端承桩或端承桩。     

刚性桩复合地基水平承载力特性

利用三维非线性弹性有限元方法,研究了刚性桩复合地基在不同垂直荷载作用下的水平承载力特性,分析了不同褥垫层厚度及其地基土的性质对复合地基水平受力特性的影响,得出了水平荷载作用下桩身弯矩和挠度的分布规律。研究表 明,垂直荷载的大小改变了复合地基水平荷载作用下的破坏模式,直接影响着其水平承载力;适当的垫层厚度可降低桩所分担的水平荷载,有利于桩身的安全。     

Bearing capacity and settlement of concrete-cored DCM pile in soft ground

A new kind of composite pile named Concrete-cored DCM (Deep-Cement-Mixing) pile is introduced for supporting medium loads in soft clay ground. This composite pile is composed of an inner precast concrete pile and an external DCM pile socket, where the high strength concrete pile is designed to bear the load of building, and DCM pile socket acts to transfer axial force into the surrounding soils by skin frictions. In this paper we introduce its application in soft ground, and tentatively investigated its bearing capacity and settlement using static loading tests and Finite Element Method. Our study indicates that concrete-cored DCM pile acts as a friction pile and is an economical and effective method for settlement control and capacity improvement in soft clay ground. This study is also valuable for research on the mechanism of other composite piles. This revised version was published online in July 2006 with corrections to the Cover Date.     

An experimental study on static and dynamic bending behaviour of piles in soft clay

Static and dynamic lateral load tests were carried out on model aluminium single piles embedded in soft clay to study its bending behaviour. Model aluminium piles with length to diameter ratios of 10, 20, 30 and 40 were used. Static lateral load tests were conducted on piles by rope and pulley arrangement upto failure and load–deflection curves were obtained. Dynamic lateral load tests were carried out for different magnitudes of load ranging from 7 to 30 N at wide range of frequencies from 2 to 50 Hz. The load transferred to the pile, pile head displacement and the strain variation along the pile length were measured using a Data Acquisition System. Safe static lateral load capacity for all piles is interpreted from load–deflection curves. Dynamic characteristics of the soil–pile system were arrived from the acquired experimental data. The soil–pile system behaves predominantly in nonlinear fashion even at low frequency under dynamic load. The displacement amplitude under dynamic load is magnified by 4.5–6.5 times the static deflection for all piles embedded in soft clay. But, the peak magnification factor reduces with an increase in the magnitude of lateral load mainly because of increase of hysteretic damping at very soft consistency. The maximum BM occurs at the fundamental frequency of the soil–pile system. Even the lower part of the pile affects the pile head response to the inertial load applied at the pile head. The maximum dynamic BM is magnified by about 1.5 times the maximum static BM for model piles in tested consistency of clay. The maximum dynamic BM occurs at a depth of about 1.5 times the depth of maximum static BM for model piles, which indicates an increase of active pile length under dynamic load.     

几种缺陷单桩竖向承载性状的现场模型试验研究

为评价灌注桩在施工过程中因形成缩径、扩径、断桩、泥皮等缺陷导致单桩竖向极限承载力变化的程度,针对缺陷桩单桩开展了现场模型试验研究。进行正常桩和缺陷桩的竖向静载模型试验,测试单桩竖向极限承载力,对比缺陷桩和正常桩的单桩承载特性,分析了缩径、扩径、断桩、泥皮等缺陷对单桩承载性状的影响。对比正常桩和缺陷桩的荷载-沉降关系曲线,得出了基于文中缺陷桩设计方案的结论,缩径缺陷和泥皮缺陷均使单桩竖向极限承载力降低,降幅在正常桩极限承载力的15%范围内;扩径缺陷桩的荷载-沉降关系曲线无明显陡降点,桩顶沉降较正常桩递增缓慢;断桩缺陷影响荷载-沉降关系曲线中反弯点的出现位置,即反弯点出现时的桩顶位移与断桩缺陷距地表的距离有关。     

砂土中螺纹桩承载特性的模型试验研究

通过室内模型试验,研究砂土中螺纹桩的竖向承载特性,比较分析螺纹桩与普通直桩承载性能的差异以及螺距对螺纹桩承载力的影响。静载荷试验后进行一系列土工试验,研究桩周土的物理力学性质。试验结果表明,在相同条件下螺纹桩的极限承载力约是普通直桩极限承载能力的1～4倍,极限状态下的平均桩侧阻力是普通直桩的3~4倍;在一定范围内,随着螺距的减小,螺纹桩的桩侧阻力可以得到有效的发挥,极限承载能力和控制沉降的能力逐渐增强。     

基于cut-off方法刚性承台下群桩基础优化分析

针对刚性筏板下群桩基础优化设计问题进行分析,通过改变桩长分布来调整各桩的荷载分担,群桩分析采用基于弹性理论的积分方程方法,并通过cut-off方法来反映桩的弹塑性特性,实现群桩基础的弹塑性优化分析,改进了常规弹性分析方法的缺陷。算例分析表明,随着外荷载的不断增加,角桩首先达到极限荷载,对角桩超出极限荷载的部分进行重新分布,继而使得边桩逐渐达到极限承载状态,继续加载直至内桩也达到极限承载状态,从而引起桩筏基础的整体破坏。采用cut-off方法可以改进基础变刚度优化设计,使之与实际情况更为吻合。     

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.

     

高速铁路沉降控制复合桩基的性状试验研究

基于沉降控制设计理念,无砟轨道京沪高速铁路地基处理采用筏板+垫层+疏桩的方法,形成复合桩基以实现有效减少工后沉降和充分利用地基承载力的优化加固方案。为探索该新方法沉降控制机制,选用CFG桩开展了复合桩基现场试验研究,对复合桩基在高速铁路路基填筑、静置、预压卸载过程中的地基沉降变形、桩和桩间土土压力、筏板顶与底部压力进行了长期观测,分析了路基沉降变形、桩-土应力比和荷载分担比以及筏板的受力随填筑高度和固结时间的变化规律。研究表明：筏板＋垫层＋疏桩联合加固地基方案在初期充分发挥了桩间土承载作用,导致桩与桩间土产生差异沉降;随着垫层的调节作用,筏板可集中发挥桩体的承载能力及显著提高桩顶应力集中程度,地基土沉降主要发生在加固区范围内,从而揭示了复合桩基在路基荷载下的承载机制和变形特性。现场试验结果可为指导高速铁路CFG桩复合桩基设计参数的进一步优化提供试验依据。     

成层土中倾斜荷载作用下桩承载力有限元分析

利用有限元方法对现场单桩水平载荷试验进行模拟,在此基础上,分析了成层土中桩在倾斜荷载作用下其竖向分量的有利作用和横向土抗力分布特点。计算结果表明,在地面下一定范围内,倾斜荷载作用下的桩侧摩阻力比水平荷载作用下的桩侧摩阻力大。在土层分界处土抗力分布有明显的跳跃。达到一定深度后,横向土抗力主要是静止土压力,而由荷载引起的横向土抗力很小。承台能有效减小土体及桩的水平位移。模拟的灌注桩和钢管桩桩顶在地面以上的自由长度较小,竖向分量由于桩身挠曲变形而产生的P-Δ效应较小,所以就算例中的灌注桩和钢管桩而言,荷载倾斜度不大时,荷载竖向分量提高了桩的侧阻并由此增大桩侧土竖向应力,对桩水平承载力总体上起到了有利的作用。