Effects of inter-helix spacing and short-term soil setup on the behaviour of axially loaded helical piles in cohesive soil

Axial compressive load tests performed on piles instrumented with strain gauges were completed in order to investigate the effects of inter-helix spacing on the behaviour of helical piles. The test piles had two helices with varied values of helix spacing. The helix-bearing soil layer consisted of a homogeneous clay with an average undrained shear strength of 65 kPa. The test pile failure mechanisms were determined by comparing the measured load distributions to the distributions predicted by the individual bearing and the cylindrical shear models. The results suggest that the individual bearing model dominates the pile behaviour for piles with an inter-helix spacing to helix diameter ratio greater than or equal to 1.5. The helix bearing capacity factor and the shaft adhesion factor were evaluated by comparing the measured pile component resistances to the theoretical estimations. The back-calculated bearing capacity and the adhesion factors were below the values traditionally used in helical pile design. The effects of the soil setup on the pile behaviour were evaluated by comparing the load-settlement response of a pile tested immediately after the pile installation to equivalent piles tested many days after the installation. A piezometer installed near the upper helix edge was used to measure the excess pore pressure generation and dissipation induced by the installation. The results suggest that the pore pressure generation induced by the pile installation was minimal and had little influence on the short-term ultimate capacity.     

Optimization of inter-helix spacing for helical piles in sand

The optimization of the inter-helix spacing is a key issue of the axial bearing capacity of helical piles. In this paper, based on the cavity expansion, an analytical approach considering the small-strain stiffness, strength, compressibility and stress level of sand around the helical pile was proposed to analyze the influence zone of the helices to determine the optimal inter-helix spacing in sand. The calculation results of the proposed method were verified using the centrifuge test data and finite element analysis for helical pile in Congleton HST95 sand. They were also compared with those using the Meyerhof pile foundation theory. The results show that the optimal inter-helix spacing based on Meyerhof pile foundation theory differs significantly from the measurement. The range of the influence zone for the helices in sand calculated by the cavity expansion theory matches with the data from literature. The calculation results with the proposed method are consistent with the range of the optimal spacing ratio inferred in the centrifuge tests. The results based on the two-dimensional (2D) finite element model (FEM) are also basically consistent with the calculated analytical solution.     

Pile under torque in nonlinear soils and soil-pile interfaces

This paper presents a new method for analyzing the nonlinear response of a single, vertical pile with the circular cross-section under torque in layered soils. The nonlinear stress-strain relationships of both soil-pile interface and soil are approximated by the hyperbolic model, whereas the pile material is elastic. The torsional spring stiffness of the soil-pile interface and the soil are determined by traditionally available methods. A four-node finite element model for the soil-pile interface is proposed to represent nonlinear behaviors of the soil-pile interface and the soil, separately. A new iterative scheme for nonlinear analysis of a single pile under torque is also developed that avoids having to solve a large number of simultaneous equations found in traditional solution schemes. The new solution method is based on the tangential stiffness of the soil-pile interface and soil springs, which are determined at each load step. From this solution scheme, an equivalent stiffness of the soil-pile-interface system of each pile element is calculated from the bottom element to the top element while torque and angle of twist are calculated from the top to the bottom elements. The solution gives the distribution of the angle of twist and torque along the pile, and the equivalent stiffness of the soil-pile system and torque-angle of twist curves at any depth. The solution method can be easily applied to the practice field in nonlinear analysis and in designing a single pile under torque in layered soils. The analysis results using the new solution scheme compared well with the results from other analytical methods studied by previous researchers. The proposed method is also used to predict the behavior of two full-scale piles under torques. The predictions are in good to excellent agreement with measurements.     

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。     

静压敞口预应力高强混凝土管桩界面剪切性状试验研究

桩土界面剪切行为对静压敞口预应力高强混凝土(PHC)管桩沉贯性状及长期承载力特性具有至关重要的作用。通过成层土地基中桩身预埋光纤光栅(FBG)传感器的静压桩足尺试验,分别对敞口PHC管桩贯入及静载荷试验中的桩土界面剪切行为进行研究。结果表明：在贯入阶段,桩身轴力及侧摩阻力沿桩的深度方向逐渐传递,传力幅值与桩周土体性状密切相关,土层界面处轴力传递效率依次为98.2%、92.2%、96.3%、83.8%、80.5%。随着压桩循环次数的增加,同一深度土层摩阻力呈逐渐减小趋势。经历5个压桩循环后,深度6 m处的砂质粉土层摩阻力减小幅度约为46.25%,深度10m处的粉质黏土层经历3个压桩循环后摩阻力减小幅度约为12.1%;载荷试验过程中,桩侧摩阻力随着桩顶荷载施加自上而下逐步发挥。摩阻力完全发挥所需的桩土相对位移,粉质黏土层的最大,约为6.96~7.46mm,淤泥质黏土层的次之,约为6.05mm,砂质粉土层的最小,约为4.23mm;与原状土相比,重塑区土体含水量、孔隙比参数指标降低,重度、黏聚力及内摩擦角增大。桩周重塑区土体物理力学指标变化是贯入及载荷试验阶段桩土界面剪切行为不同的重要原因。     

黏土热–力学特性对能量桩力学特性的影响

能量桩运行会导致土体温度场的改变,从而影响桩周土的热–力学特性,进而影响能量桩的变形、桩–土界面应力及承载性能。将ACMEG-T土体热本构模型在ABAQUS软件中进行二次开发,通过编写UMAT子程序对能够反映黏土热–力耦合特性的三轴试验结果进行模拟与分析,验证了模型的可靠性。建立数值模型,研究了土体热–力学特性对能量桩位移、桩–土界面应力及桩身轴力的影响规律。研究结果表明,温度变化会导致土体产生累计沉降,并进一步导致桩侧产生负摩阻力;在负摩阻力的影响下,能量桩会产生额外的沉降以及不可恢复的轴力;土体热–力学特性对能量桩力学特性的影响效应随着土体超固结比的增加逐渐减弱。     

Pile-clayey soil interaction analysis by boundary element method

This paper is an attempt to solve the soil-pile interaction problems using the boundary element method(BEM).A computer package called PGroupN,which deals mainly with the analysis of the pile group problem,is employed in this study.Parametric studies are carried out to assess the impacts of the pile diameter,pile length,ratio of spacing to diameter and the thickness of soil stratum.The external load is applied incrementally and,at each increment,a check is made that the stress state at the pile-soil interfaces does not violate the yield criteria.This is achieved by specifying the limited stresses of the soil for the axial pile shaft capacity and end-bearing resistance.The elements of the pile-soil interface yielded can take no additional load,and any increase in load is therefore redistributed between the remaining elements until all elements have failed.Thus,by successive application of loading increments,the entire load-displacement relationship for the pile group is determined.It is found that as the applied load reaches the ultimate bearing capacity of the pile group,all the piles will share the same amount of load.An exception to this case is for the center pile in a group of 9 piles embedded in clay,which is not consistent with the behaviors of the other piles in the group even if the load reaches the ultimate state.For the 4 piles group embedded in clay,the maximum load carried by the base does not exceed 8% of the load carried by each pile with different diameters.This low percentage ascertains that the piles embedded in cohesive soils carry most of the load throughout their shafts.     

长短桩组合路堤桩荷载分担规律离心模型试验与数值模拟

采用离心模型试验和三维有限元法,探讨路堤荷载下长短桩组合路堤桩的荷载分担规律。共进行4组离心模型试验,得到不同桩长比时长桩和短桩的桩土应力比随分级加载过程的变化曲线,以及相应的长桩和短桩桩身轴力的传递规律。基于离心模型试验的基本参数建立三维有限元模型,采用Gibson地基模拟离心场中软土地基强度沿深度方向逐渐增大的特性。计算结果与离心模型测试结果的对比验证模型建立的合理性;并且进一步研究长短桩组合路堤桩应用于存在持力层时的层状地基中的桩土荷载分担规律。研究结果表明:随着荷载的增加,长桩分担的荷载逐渐增大,而短桩和桩间土分担的荷载逐渐减小,且都趋于稳定值;桩长比越大,长桩分担的荷载越大;存在持力层时的层状地基更有利于长短桩组合路堤桩加固效果的发挥。     

Axial response and material efficiency of tapered helical piles

Different techniques have been proposed to increase the bearing capacity of open-ended piles.Welding helices to the shaft and tapering the pile shaft could be used simultaneously to enhance the static and dynamic behaviors of these piles.This paper subjects the bearing capacity,stiffness,frictional behavior,and material efficiency of the tapered helical piles to scrutiny.Tapered helical piles are introduced herein as an alternative option to improve the material efficiency of hollow piles.Based on the Taguchi method,a series of experiments was designed and conducted.The axial responses of tapered helical piles are also investigated using finite element analyses.The results derived from loadedisplacement curves and strain gages are used to characterize the axial compression responses of tapered helical piles.The effects of tapered angle,helices diameter and helices distance are examined using dimensionless parameters,and the degree of contribution of these factors is calculated on each of the enumerated variables individually.Experimental results show that the shaft friction resistance of tapered helical piles increases continuously with the pile head settlement.Furthermore,the effect of tapered wall on the shaft friction resistance is more tangible at low stress levels.The results showed that the relative material efficiency factor of the optimum pile could be 2.5 times that of unoptimized pile with a similar quantity of material.     

基于CPTU测试的K_0固结黏土中静压桩时变承载力研究

基于空间滑动面(SMP)准则改进的K_0固结各向异性修正剑桥模型,考虑K_0固结饱和黏土初始应力各向异性、应力历史及应力诱发各向异性对土体三维力学特性的影响,推导了静压沉桩柱孔扩张问题的弹塑性解析解。在此基础上,根据桩侧土体应力状态与单剪试验中试样应力状态的相似性,结合桩周土轴对称固结理论提出采用孔压静力触探仪(CPTU)锥尖阻力、锥肩孔隙水压力及相应孔压消散数据预测静压桩时变承载力的理论方法。通过离心机模型试验实测结果和理论预测值的对比,验证了理论方法的有效性,研究了静压桩承载力随时间的变化规律。研究结果表明,本文理论预测方法避免了土体基本参数测定等繁琐过程,且可以较为合理地预测静压桩时变承载力;静压桩沉桩结束后其承载力在短时间内迅速增加,之后承载力增加幅度变缓且逐渐趋于稳定值;静压桩桩径越大,沉桩结束后承载力增加的速度越慢,承载力达到稳定值的时间越长。     

桩-土-承台共同作用的模型试验研究

通过带承台单桩及双桩基础的模型试验,对低承台桩基桩间土变形发展及其与承台板板底应力、桩侧摩阻力及桩端阻力间的相互影响进行较为细致的研究。试验表明:在相同基础荷载作用下,桩数的增加使桩端刺入变形量占基础沉降的比例降低。双桩基础桩体的存在对板底应力体现出增强作用,在相同桩间土变形量下,双桩基础板底应力大于带承台单桩基础。桩土相对位移的发展从桩端部位开始,逐步向承台板扩展,同一部位基础外侧的桩土相对位移要大于基础内侧。靠基础内外,桩的不同侧面表现出不同的侧阻发挥过程及极限值。同样桩间土变形量下,带承台双桩基础在桩端平面上土体的竖向应力要大于带承台单桩基础,从而发挥出较大的桩端阻力。     

A simplified analysis method for the influence of tunneling on grouped piles

One of key issues of tunneling in urban areas is to assess the likely impact on adjacent piled buildings of tunnel construction. Simple and reliable predictions of tunneling-induced bending and axial stresses in pile foundations are important to the safety of tunneling. In this paper, a simple two-stage analysis method for determining the response of pile groups caused by tunneling was presented. At the first stage, an analytical solution proposed by Loganathan and Poulos [Loganathan, N., Poulos, H.G., 1998. Analytical prediction for tunneling-induced ground movement in clays. J. Geotech. Geoenviron. Eng., ASCE 124 (9), 846–856] is used to estimate the free-field vertical and lateral soil movements induced by tunneling. At the second stage, assuming no slippage at the soil-pile interface, the Winkler model is first adopted for simulating the pile-soil interaction, combined with finite difference method in the case of multi-layered soils. Then, shielding effect is considered for the interaction between two passive piles using a logarithmic attenuation function suggested by Randolph and Wroth [Randolph, M.F., Wroth, C.P., 1979. Analysis of the vertical deformation of pile groups. Géotechnique 29 (4), 423–439] for vertical response and Mindlin’s solution for lateral response. Finally, the response of a passive pile group due to tunneling is obtained by the superposition principle. Solutions obtained by the proposed approach for the analysis of single piles and piled groups subjected to ground movements induced by tunneling are compared with those using the boundary element program GEPAN. Comparisons are also made between the observed behavior of centrifuge model tests as well as field measurements and those computed by the proposed method. It is demonstrated that the present method can in general give a satisfactory prediction of the response of passive piles subjected to tunneling.     

Investigation on in-situ test of penetration characteristics of open and closed PHC pipe piles

To investigate the load transfer mechanism of open and closed Pre-stressed High-strength Concrete (PHC) piles jacked into layered soil, a full-scale in-situ test was conducted. In this test, the axial stress experienced by one open and two closed PHC pipe piles during jacking into layered soil was monitored using Fiber Bragg Grating (FBG) sensors mounted on pile shaft. The test results showed that (1) The jacking force on pile depended on the soil mechanical properties at the end of pile; (2) The variation of axial force on pile reflected that the compaction effect of soil for closed pile was larger than that for open pile; (3) At the beginning of penetration depth, the variation of shaft and end resistances on the open pile were different from those on the closed piles; (4) The variation of average shaft resistance relied on the mechanical properties of soil at the side of pile.     

Centrifuge studies on the effects of existing piles on the end resistance and shaft friction of a newpile

The effects of existing piles on the vertical bearing capacity of piles of a new building were examined using vertical static loading centrifuge tests on a new pile located among existing piles. The results suggest the following conclusions: (1) Existing piles increased the total shaft resistance of the new pile with a rough surface because the existing piles restrained the soil around the new pile and the positive dilatancy of the sand increased the confining pressure of the soil. (2) The shaft resistance of the lower part of the new pile with a rough surface fell rapidly during the loading tests, regardless of whether there were existing piles or not. The diminution of shaft resistance, known as ‘friction fatigue’, was probably caused by sand particle crushing in the vicinity of the pile end. (3) For a new pile with a rough surface, existing piles did not affect the end resistance when the new pile head settlement normalized by the pile diameter, s/D_p, was less than 0.2. (4) Existing piles did not affect the shaft resistance or the end resistance of the new pile with a smooth surface. Dominance of the sliding displacement along the pile–sand contact surface engenders the extremely small variation of the confining pressure of the soil around the new pileshaft.     

弱化饱和砂土中桩的p–y曲线与极限抗力研究

利用给土层施加反压的方法控制饱和砂土中的超孔隙水压力,以此模拟饱和土层液化过程中具有一定残余孔压时的弱化状态。进而对不同弱化状态饱和砂土与桩的相互作用进行模型试验,研究p–y关系与水平极限抗力随饱和砂土中残余孔压增加时的变化规律。结果表明:对于相对密度为30%的饱和砂土,随土层中残余孔压增加,同一水平位移下桩受到的土层抗力逐渐降低;如果土层中无残余孔压,桩的水平极限抗力实测结果与理论计算结果接近;当土层中的残余孔压分别达到土层上覆有效压力的0.25,0.5,0.75倍时,桩的水平极限抗力分别降低30%,55%与80%;土层液化后,桩的水平极限抗力大约降低90%,这点与已有的振动台及离心模型试验结果基本一致。     

基于桩顶沉降的桩基竖向承载力和位移分析

基于按桩顶沉降分析桩基承载力和位移的思想 ,提出了一种改进的竖向受荷桩荷载传递计算方法。该方法以桩身压缩量为迭代变量 ,根据桩身的轴向变形与桩侧土变形协调关系 ,通过编制的计算程序 ,可按桩顶沉降直接计算桩顶荷载、桩身轴力分布以及桩身各点的位移 ,特别适用于对桩顶沉降有严格控制要求的桩基设计。结合某工程试桩实测数据进行了分析 ,理论计算与实测结果吻合较好。     

深长桩力学性态计算分析

首先阐述了基于能量守恒定律的单桩沉降计算方法,通过计算总结出桩身轴力和桩身沉降分布的一般规律.分别计算不同桩长、桩径、桩体弹性模量、桩侧极限摩阻力条件下深长桩桩身轴力和桩身沉降的分布特点,探讨各参数变化对深长桩力学性态的影响.结果表明,桩长、桩径、桩体弹性模量、桩侧极限摩阻力的变化对深长桩力学性态均有显著影响,且随荷载水平的变化而变化.     

倾斜液化场地桩基地震响应离心机试验研究

 倾斜液化场地中群桩地震响应受液化土层侧向流动和桩土相互作用影响和控制,故倾斜液化场地中桩基抗震性能问题是一个极其复杂问题。基于动态土工离心机试验来探讨考虑倾斜液化土侧向流动特点的群桩地震响应规律。试验设计不同地震强度下2个50 g典型土工离心模型试验,以研究倾斜液化场地中桩土加速度、位移、桩身弯矩和土体超孔隙水压力响应特性。试验提出倾斜饱和土层的制备方法,再现倾斜液化场地中桩基础在强震作用下的破坏程度、状态和机制,并进一步对比分析试验结果,取得较好的成果,此为倾斜液化场地桩基础的抗震设计提供可靠依据,对确保液化场地桩基础的抗震稳定性和安全性具有重要意义。     

软土地区大吨位超长试桩试验设计与分析

温州350 m超高层中超长桩加载2800 t的试桩静载试验设计与分析表明：在地表土质承载力较低场地进行大吨位堆载试验时,可选择桩梁式堆载支墩–反力架装置来完成试验。对超长桩来说,在最大加载条件下,实测桩端阻力仅为桩顶荷载的25%左右,超长桩表现为端承摩擦桩性状。在使用荷载下,桩顶沉降的90%以上来自桩身压缩,在进行超长桩设计时,要充分考虑桩身质量对试桩沉降的影响。同时,桩底沉渣清除的干净与否,也直接影响超长桩的沉降。超长桩桩侧上部土层摩阻力具有不同程度的软化现象,而中下部土层侧摩阻力具有较弱的强化效应,因此在超长桩承载力计算时,不同深度土层的桩侧阻力和桩端阻力都应乘以相应不同的修正系数。试验结果显示淤泥土、淤泥质黏土、淤泥夹粉砂土中极限侧阻充分发挥所需的桩土相对位移阀值分别约为5～7 mm、6～8 mm和8～10 mm。     

VERTICAL CAPACITY OF PILES USING FUZZY SETS

Because prediction of the load-carrying capacity of piles continues to challenge geo-technical engineers, new solutions are needed. The problem is aggravated by the lack of understanding of the phenomena of soil-pile interaction, and the limited quantity and inexact quality of subsurface soil information that can be provided for analysis. The use of fuzzy set theory improves the engineer's ability to handle the uncertainty in the soil parameters and the prediction methods and thus improves the reliability of the pre dicted capacity. A methodology for predicting pile capacity based on fuzzy set theory is developed and implemented in a computer program. The validity of the program re sults is evaluated using a US Federal Highway Administration (FHWA) pile load test database. The comparison between predicted and measured ultimate capacities of piles in sand, clay and mixed soils shows that the predictive capability of the developed program is very good.