Finite element analysis of effect of soil displacement on bearing capacity of single friction pile

Effect of soil displacement on friction single pile in the cases of tunneling,surcharge load and uniform soil movement was discussed in details with finite element method.Lateral displacement of the pile caused by soil displacement reached about 90% of the total displacement,which means that P-Δ effect of axial load can be neglected.The maximum moment of pile decreased from 159 kN·m to 133 kN·m in the case of surcharge load when the axial load increased from 0 to the ultimate load.When deformation of pile caused by soil displacement is large,axial load applied on pile-head plays the role of reducing the maximum bending moment in concrete pile to some extent.When pile is on one side of the tunnel,soil displacements around the pile are all alike,which means that the soil pressures around the pile do not decrease during tunneling.Therefore,Q-s curve of the pile affected by tunneling is very close to that of pile in static loading test.Bearing capacities of piles influenced by surcharge load and uniform soil movement are 2480 kN and 2630 kN,respectively,which are a little greater than that of the pile in static loading test(2400 kN).Soil pressures along pile increase due to surcharge load and uniform soil movement,and so do the shaft resistances along pile,as a result,when rebars in concrete piles are enough,bearing capacity of pile affected by soil displacement increases compared with that of pile in static loading test.

Evaluation of the Ultimate Capacity of Friction Piles

The precise prediction of maximum load carrying capacity of bored piles is a complex problem because the load is a function of a large number of factors. These factors include method of boring, method of concreting, quality of concrete, expertise of the construction staff, the ground conditions and the pile geometry. To ascertain the field performance and estimate load carrying capacities of piles, in-situ pile load tests are conducted. Due to practical and time constraints, it is not possible to load the pile up-to failure. In this study, field pile load test data is analyzed to estimate the ultimate load for friction piles. The analysis is based on three pile load test results. The tests are conducted at the site of The Cultural and Recreational Complex project in Port Said, Egypt. Three pile load tests are performed on bored piles of 900 mm diameter and 50 m length. Geotechnical investigations at the site are carried out to a maximum depth of 60 m. Ultimate capacities of piles are determined according to different methods including Egyptian Code of practice (2005), Tan-gent-tangent, Hansen (1963), Chin (1970), Ahmed and Pise (1997) and Decourt (1999). It was concluded that approxi- mately 8% of the ultimate load is resisted by bearing at the base of the pile, and that up to 92% of the load is resisted by friction along the shaft. Based on a comparison of pile capacity predictions using different method, recommendations are made. A new method is proposed to calculate the ultimate capacity of the pile from pile load test data. The ultimate capacity of the bored piles predicted using the proposed method appears to be reliable and compares well to different available methods.

Study of LDBPs Shaft Skin Friction for Piles in Cohesiove Soils

The methodology of predicting pile shaft skin ultimate friction has been studied in a systematic way. In the light of that, the analysis of the pile shaft resistance for bored and cast in situ piles in cohesive soils was carried out thoroughly in the basis of field performance data of 10 fully instrumented large diameter bored piles (LDBPs) used as the bridge foundation. The undrained strength index μ in term of cohesive soils was brought forward in allusion to the cohesive soils in the consistence plastic state, and can effectively combine the friction angle and the cohesion of cohesive soils in undrained condition. And that the classical ' α method' was modified much in effect to predict the pile shaft skin friction of LDBPs in cohesive soils. Furthermore, the approach of standard penetration test (SPT) N value used to estimate the pile shaft skin ultimate friction was analyzed, and the calculating formulae were established for LDBPs in clay and silt clay respectively.

Theoretical Formula for Calculating Negative Skin Friction of Pile in Layered Soil and Its Application

Based on Zeevaert＇s method, a theoretical formula was developed to calculate the negative skin friction of pile in layered soil. For practical purpose,a cut-and-try method was proposed to determine neutral point. Case studies indicate that the total calculated negative skin friction was in agreement with the measured one, which verifies the feasibility and practicability of theoretical formula. Furthermore, the methods for calculating efficiency factor of drag load and settlement were also given.

EFFECT OF SKIN FRICTION ON THE DIRECTIONAL STABILITY OF DRIVEN PILES DURING DRIVING

In this paper, the driving forces at a pile top are considered as a periodic load during driving and the Mathieu equation is derived. From the stability charts of this equation, we can obtain the critical length of the pile, and the effect of skin friction upon the critical length is discussed.

基坑开挖对管桩竖向承载性状影响研究

针对基坑开挖引起管桩挤土效应的弱化,以及挤土效应弱化对管桩承载性质影响问题,开展了室内模型试验,并通过对模型试验进行数值建模,验证了数值模型的可靠性。在此基础上,从桩长、桩间距、桩侧摩阻力系数对开挖卸荷影响下管桩基础承载性状的影响展开研究。结果表明:开挖卸荷对管桩承载特性造成较大影响,极限承载力对应桩顶沉降量提高54.5%;数值计算发现,卸载前后极限荷载所对应最大沉降差率为9.52%,满足工程精度要求,验证了所提数值模型的可靠性;数值计算结果表明,随桩长提高,最大桩身轴力位置不断上升,而中性点位置却不断下降;桩间距越小,开挖卸荷对桩身轴力影响越大,而对负摩阻力影响越小;摩阻力系数为0.3时,摩阻力系数对桩身轴力及桩侧摩阻力发挥敏感度最高。

Three-dimensional elasto-plastic finite element analysis of a soil-pilebridge interaction

The soil-pile-bridge interaction of super-large pile groups is a very complex issue for the design of deep pile group foundations. In this paper, the load distribution on the pile top of a super large bridge foundation and its influential factors are analyzed comprehensively using a three-dimensional elasto-plastic finite element method. The adopted model and its input parameters are firstly verified by comparing the numerical results with the measured data of static loading tests of a single pile. Numerical analysis is then performed to investigate the load distribution and the load-settlement characteristics of super-large pile groups, and the models are verified using centrifuge laboratory model testing data. The mechanism of the interaction between pile groups and soil under different conditions is explored.

THE APPLICATION OF THE FINITE PRISM ELEMENT METHOD TO THE ELASTO PLASTIC ANALYSIS OF SINGLE PILE SOIL INTERACTION SYSTEM

In this paper, a finite prism element procedure is presented, and a new infinite prism element for modelling the semi infinite domain boundary condition is developed to expedite the (3D) calculations in a more accurate way for the far stress field to the pile. It has been taken into account that the soil is restrained by the pile cap, and the relative displacement happens between the pile and the soil. Revised Cambridge model is incoporated in the procedure for modelling elasto plastic behavior of the soil. In this paper, the single long friction pile with a pile cap and soil system are analyzed. The proper load displacement curve and frictional resistance distribution curve are obtained. The results can provide the basis for design and calculation of the long piled foundation.

杂填土地基中螺杆桩竖向承载特性研究

为了研究杂填土地基中螺杆桩竖向承载特性,基于极限平衡理论分析了桩-土咬合机理,结合桩-土之间的相互作用得到螺纹段等效侧阻力增大系数,在此基础上建立了单桩竖向承载力计算公式,并对杂填土地基中螺杆桩单桩进行缩尺模型试验。结果表明,螺杆桩在竖向荷载作用下,荷载-沉降曲线经历了弹性、弹塑性、塑性破坏3个阶段。根据模型试验所得桩身轴力和桩侧等效侧阻力沿深度的分布规律,总体上螺纹段等效侧阻力高于直杆段。当桩体达到极限承载力时,试验所得等效侧阻力扩大系数与理论分析结果基本一致。

Analysis of bearing capacity of pile foundation in discontinued permafrost regions

Piles are the main building foundation in permafrost regions. Thawing the permafrost foundation would have a negative effect on a pile, and may cause damage to the building. This paper focuses on the effects of negative friction force due to the melt of permafrost, and presents four calculated methods for bearing capacity of a pile. An engineering station was taken as an example, where the lengths of a pile were compared based on four methods. Finally, quick field load tests were carried out, and some meaningful conclusions are presented. Thus, these analytical results can be used to design a pile for permafrost regions.

抗拔桩承载能力影响因素与群桩变形规律试验研究

利用自主研发的桩基室内抗拔测试装置,结合数值模拟技术对抗拔桩的承载破坏过程及影响因素、群桩的协同工作特征展开了深入研究。结果表明:抗拔桩的承载破坏经历4个阶段,承载初期,桩顶侧摩阻力最先发挥作用,桩顶土体发生塑性破坏;随上拔荷载不断增大,桩体产生相对位移,桩周土体由于桩身侧摩阻力产生塑性破坏;当桩身轴力自桩顶传递至桩底时,桩身底端产生抗拔“吸附力”,并伴随局部土体塑性破坏;随着桩周土体塑性区的拓展、连通,抗拔桩承载能力达到极限;桩身长径比、桩-土界面摩擦因数、桩侧土体压力与其承载极限呈正相关关系,其中桩身长径比对桩端“吸附力”具有重要影响;群桩抗拔过程中,角桩侧摩阻力发挥最充分,桩身位移量最小,极限承载力最大,中心桩桩身位移最大,极限承载力最低;距径比影响抗拔桩的群桩效应,当距径比从2增大至8时,桩身侧摩阻力提高30%,将距径比8作为群桩工程的推荐值,6~10作为群桩距径比的推荐范围。

The Effect of Structural Parameters on The Compression Resilience of Uncut Woven Pile Fabrics

If an uncut woven pile fabric is to be subjected to compressive load in use its compression resilience performance becomes very crucial.In order to optimize the extent of recovery in fabric thickness when the applied force is removed,thirteen uncut woven pile fabric samples were woven and tested on KES-G5 Hand Type Compression Tester (small compress Hammer).The results show that the above parameters influence compression resilience property of the uncut woven pile fabrics,and the structural factors have a more pronounced effect as compared to effect of fibre and yarn type.

湿陷性黄土地基中桩基负摩阻力计算新方法

负摩阻力对湿陷性黄土地基中桩基的承载变形性状具有重要影响,现行桩基规范及黄土规范推荐的负摩阻力计算方法不能反映黄土桩基负摩阻力的实际性状。对近30年来在黄土地区开展的钢筋混凝土灌注桩现场浸水试验实测数据进行统计,分析黄土浸水完成后桩身中性点深度、桩身最大负摩阻力深度及负摩阻力系数与桩长径比的关系,通过线性拟合得到了中性点深度比、最大负摩阻力深度比及负摩阻力系数与桩长径比的经验表达式,提出湿陷性黄土地基中桩基负摩阻力计算的新方法。新方法用三角形表示负摩阻力沿深度的分布,能反映负摩阻力从桩顶向下先逐渐增大、达到最大值后随深度增加逐渐减小、最终在中性点处减小为零的分布特征。将提出的方法、桩基规范及黄土规范方法的计算结果与实测结果进行对比,结果表明,桩基规范预测的中性点深度比优于黄土规范,黄土规范预测的最大下拉荷载优于桩基规范,而用新方法计算的负摩阻力最接近实测结果。

Numerical Study of Piles Group under Seismic Loading in Frictinal Soil—Inclination Effect

Recent devastating earthquakes in some countries, such as Pakistan, Turkey, Algeria and China, call to the mind the high risk exposure of Lebanon which is located over an active seismic zone. Many experts shared the view that major seismic event may occur in Lebanon in the future. Moreover, many earthquakes, of low magnitudes between three and four, have been registered in Lebanon during 2008. These events have increased the anxiety of Lebanese people because of the poor quality of the constructions and their behavior under moderate or severe earthquake events. The efficient way to minimize seismic effects, material and human losses, is the prevention. The system piles-foundation is an appropriate way and widely used to ensure the stability of constructions when subjected to seismic excitation. It seems necessary to study the interaction of pile-foundation-pile-cap-structure in the case of non linear soil behavior and the interface pile-soil. The study will be also conducted by using measures recorded during real earthquakes for example in Turkey (Kocaeli, 1999). In this paper, we present a numerical modeling of the interaction of using FLAC3D software. According to soil behavior and pile inclination, parametric studies are also performed. The analysis of the results could give the better piles group configuration in order to minimize the seismic effect on the structures.

软土中单侧边载作用下桩基负摩阻力研究

软土中因土体压缩性较高,在边载状况下易产生桩基负摩阻力,从而对工程产生安全隐患。本文通过有限元法对软土中受边载作用的桩基础所受到的桩基负摩阻力进行数值模拟,分析了不同边载等级、不同边载位距对软土中桩基负摩阻力、中性点位置和桩身轴力的影响。结果表明,在边载等级增加时,软土中桩身轴力有明显上升趋势,并随桩体埋深增加呈非线性趋势增长,而桩基负摩阻力随深度变化呈现非线性减小的趋势,桩体的下拉力增加,边载效应增强;同等级边载效应时,当边载与桩基之间的距离增加时,软土中桩基负摩阻力逐渐减小,中性点位置位于桩体靠近底部位置处,并随边载位置改变而受到一定影响。

大直径嵌岩灌注桩竖向承载特性试验研究

为了探讨泥岩地基大直径灌注桩竖向承载性能,依托青岛某机场桩基工程,对嵌入泥岩的9根大直径灌注桩进行了竖向抗压静载试验和桩身应力监测,分析大直径嵌岩灌注桩的竖向抗压性能,明确大直径灌注桩的荷载传递规律。结果表明,试桩桩身完整性较好,桩身质量可靠;试桩荷载沉降(Q-s)曲线均为缓变型,加载初期存在明显的压密段,桩顶沉降量为10.31 mm,最低回弹率大于50%;最大荷载作用下,各试桩的竖向抗压极限承载力特征值均不小于6480 kN,试桩竖向抗压承载能力仍有较大潜力;试桩均表现出端承摩擦桩的特性,轴力自桩顶到桩端逐渐衰减,桩侧摩阻力峰值发生在中风化泥岩段,桩端阻力占桩顶荷载的10.55%~34.48%。

局部冲刷下砂土中桩-盘复合基础H-T联合承载特性

海上风机基础工作时既承受复杂组合荷载作用,也受波流冲刷影响。单桩-摩擦盘复合基础同时结合了单桩与重力式基础的特点,摩擦盘还可起防冲板作用。为探讨砂土地基中H-T联合作用时局部冲刷对这种复合基础承载特性的影响,首先基于室内水槽试验装置,完成了一系列模型载荷试验;获得了冲刷前后桩-盘复合基础的荷载-位移曲线,经无量纲化处理与曲线拟合,获得H-T联合受荷桩-盘复合基础的承载包络线及其简化计算公式。然后,采用有限元细化分析了主要冲刷参数(冲刷深度、冲刷宽度、冲刷角度)及加载点高度对复合基础承载特性的影响,结果表明:冲刷坑的存在使复合基础的横向及扭转抗力明显降低,削弱程度分别可达25%及45%;进一步分析表明,桩身弯矩值随加载点高度的增加而显著增大,随预加扭矩的增加呈下降趋势;局部冲刷会明显削弱复合基础的承载力,同时H-T联合作用时相应的承载力包络线向内发生不同程度的收缩;其中,冲刷深度的影响最大,对横向抗力与扭转抗力的削弱程度分别可达约35%及60%,冲刷角度次之,冲刷宽度的影响最小;此外,不同加载路径下桩-盘复合基础承载力包络线呈现一定程度的差异。

海上试桩不同桩型承载力特性研究

天津南港某高桩码头拟建场地有深厚密实砂层,通过试桩测定设计桩型对砂层的穿透及贯入能力,研究不同基桩承载力特性。试验结果表明:混凝土预制桩及钢管桩均有较好的穿透及贯入能力;基桩轴向极限承载力高应变检测结果小于静载试验,误差均在5%以内;混凝土预制桩桩端承载力占总承载力不小于44.6%,钢管桩桩端承载力占总承载力不小于21.9%;相同荷载作用下,混凝土预制方桩静载与动载沉降量之比为1.40,钢管桩为0.90,两者差异明显;基桩承载力达到极值时桩侧摩阻力要明显大于规范极限侧摩阻力,规范取值偏保守。

赛车道软土路基PHC桩负摩阻力中性点的时间效应

武汉赛车场跑道下伏的深厚软土层给桩基的设计与施工带来不小阻碍,桩基的负摩阻力及其中性点的确定对工程的稳定性具有非常重要的影响。为检验武汉赛车场跑道软土路基的预应力高强度混凝土管桩(PHC)设计的合理性以及为后续桩基改进方案的提供技术支持,选择典型地层开展了堆载作用下桩与桩间土的沉降监测,根据桩与土的相对沉降变化规律研究负摩阻力中性点的时间效应。结果表明,该场地的PHC桩的桩周土沉降约是桩沉降的6倍,具有明显的负摩阻力特性,桩周软弱土的下限深度为13.0 m;随着时间增加,中性点向上移动;中性点深度随时间变化具有三阶段特征,采用指数函数的中性点时间效应预测模型能较好反映中性点初始值、中性点变化规律与速率以及最终稳定值,与规范法比较,预测模型计算的中性点深度具有更高的准确性,由此得到了两个试验场地的中性点稳定值分别为4.43 m与5.72 m。研究还表明三向土工格栅能有效降低桩间土的沉降,也能降低桩-土的差异沉降,使中性点上移,减小负摩阻力,提高桩的承载力。本研究对了解负摩阻力对桩基性能发挥有一定的参考价值。

海上风电大直径超长钢管桩竖向承载特性现场试验研究

钢管桩是海上风电工程最主要的基础型式,但针对海上大直径超长钢管桩竖向承载力问题的研究却相对较少。结合广东汕头海上风电工程,进行了海上大直径超长钢管桩竖向承载力特性的现场试验研究,对比了桩底沉降和桩顶沉降随上部荷载的变化特点,分析了桩身侧摩阻力和桩端阻力随上部荷载的变化规律,并依据试验结果对大直径超长钢管桩极限承载力判定方法进行了探讨。试验结果表明:大直径超长钢管桩桩身变形较大,利用桩底沉降随荷载的变化曲线更容易对现场加载量进行控制和调整,也更容易对承载力的极限状态进行判断;大直径超长钢管桩端阻力随桩顶荷载的变化曲线呈“S”形;现行规范中采用Q/Qmax-s/d曲线斜率开始转变为0.2的点所对应的荷载作为极限承载力,将过高的估计大直径超长钢管桩的竖向抗压极限承载力。