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.

Study of the Bearing Capacity at the Variable Cross-Section of A Riser- Surface Casing Composite Pile

Reducing the cost of offshore platform construction is an urgent issue for marginal oilfield development.The offshore oil well structure includes a riser and a surface casing.The riser,surface casing and oil well cement can be considered special variable cross-section piles.Replacing or partially replacing the steel pipe pile foundation with a variable cross-section pile to provide the required bearing capacity for an offshore oil platform can reduce the cost of foundation construction and improve the economic efficiency of production.In this paper,the finite element analysis method is used to investigate the variable cross-section bearing mode of composite piles composed of a riser and a surface casing in saturated clay under a vertical load.The calculation formula of the bearing capacity at the variable section is derived based on the theory of spherical cavity expansion,the influencing factors of the bearing capacity coefficient N_(c) are revealed,and the calculation method of N_(c) is proposed.By comparing the calculation results with the results of the centrifuge test,the accuracy and applicability of the calculation method are verified.The results show that the riser composite pile has a rigid core in the soil under the variable cross-section,which increases the bearing capacity at the variable cross-section.

Bearing capacity and mechanical behavior of CFG pile composite foundation

CFG pile (i.e., pile constructed by granular materials of cement, fly-ash and gravel) composite foundation is applied in subsoil treatment widely and successfully. In order to have a further study of this kind of subsoil treatment technology, the influencing factors and calculation methods of the vertical bearing capacity of single CFG pile and the CFG pile composite foundation were discussed respectively. And based on the obtained solutions, effects by the cushion and measurements to reduce negative friction area were analyzed. Moreover, the developing law of settlement and bearing capacity eigenvalue controlled by the material strength with the increase of load were given for the CFG composite foundation. The in-situ static load test was tested for CFG pile. The results of test show that the maximum test load or half of the ultimate load is used from all the points of test, the average bearing capacity eigenvalue of single pile is 390 kN, and slightly greater than the design value of bearing capacity. The bearing capacity eigenvalues of composite foundation for 3 piles are greater than 300 kPa, and the mechanical properties of CFG pile composite foundation are almost identical in the case of the same load and cushion thickness. The pile-soil stress ratio and the load-sharing ratio can be adjusted through setting up cushion thickness.

Application of PCC pile in soil improvement of highway

The construction process and load-bearing behaviors of Cast-in-place concrete thin-wall pipe piles are analyzed based on its application on Yantong Expressway Project. The low strain test, static load test and field excavation were also carried out, and the bearing capacity of the new pile can meet the requirements of design. With the increase of pile diameter, the bearing capacity is increased. The settlement of composite foundation is decreased, when the replacement ratio of pile is increased. The test results also show that the load carried by inner soils is neglectable. According to the tests and application, it can be concluded that the new type of pile is convenient to construction with high bearing capacity and reliable quality, which has great potential in practical engineering.

The Effect of Jet Grouting on Enhancing the Lateral Behavior of Piled Raft Foundation in Soft Clay(Numerical Investigation)

Soft clay soils cannot usually support large lateral loads,so clay soils must be improved to increase lateral resistance.The jet grouting method is one of the methods used to improve weak soils.In this paper,a series of 3D finite element studies were conducted using Plaxis 3D software to investigate the lateral behavior of piled rafts in improved soft clay utilizing the jet grouting method.Parametric models were analyzed to explore the influence of the width,depth,and location of the grouted clay on the lateral resistance.Additionally,the effect of vertical loads on the lateral behavior of piled rafts in grouted clay was also investigated.The numerical results indicate that the lateral resistance increases by increasing the dimensions of the jet grouting beneath and around the piled raft.Typical increases in lateral resistance are 11.2%,65%,177%,and 35%for applying jet grouting beside the raft,below the raft,below and around the raft,and grouted strips parallel to lateral loads,respectively.It was also found that increasing the depth of grouted clay enhances lateral resistance up to a certain depth,about 6 to 10 times the pile diameter(6 to 10D).In contrast,the improvement ratio is limited beyond 10D.Furthermore,the results demonstrate that the presence of vertical loads has a significant impact on sideward resistance.

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。

Experimental study of the bearing capacity of a drainage pipe pile under vacuum consolidation

In this study,we propose a drainage pipe pile under vacuum consolidation to integrate foundation treatment and pile foundation engineering in soft soil areas.To study its bearing capacity characteristics and foundation treatment performance,single pile static load tests,vane shear tests,and water content tests were carried out for ordinary piles,perforated piles,and drainage pipe piles under conditions of static and vacuum consolidation.Based on the results,the concept of strong and weak reinforcement areas was proposed and used for bearing capacity prediction.The results showed that the drainage pipe pile did not become silted under vacuum consolidation.The single pile bearing capacity was much higher than that of an ordinary pile,and the pile side friction was exerted mainly in the middle and lower parts.Good results were achieved using the shear strength at the junction of the strong and weak reinforcement areas to estimate the ultimate bearing capacity of a single pile.This study provided important insights into the design and construction of drainage pipe piles in a soft soil foundation.

New Method for Prediction Pile Capacity Executed by Continuous Flight Auger (CFA)

A study of piles is quit complex and the estimation of carrying capacity is calculated from theoretical formula and load test results. The design resistance may be calculated using conventional static pile design theory. The pile founding depths should be predetermined before installation from a site geotechnical investigation. To ascertain the field performance and estimate load carrying capacities of piles, in-situ pile load tests should be conducted. In this study, field pile load test data is analyzed to estimate the ultimate load for end bearing piles. The investigated site is about 100 × 110 m located in Alexandria, Egypt. Geotechnical investigations at the site are carried out to a maximum depth of 45 m. Four borings have been done in field. The tests are conducted at the site for two skelton structure buildings to be constructed on raft foundation rested on piles executed by continuous flight auger. Four pile load tests are performed on 600 mmdiameters and 27 mlengths. Ultimate capacities of piles are determined according to different methods. It is concluded that the percentage of friction load carried by the shaft along the pile length is about 46% of total load while the percentage of load carried by the end bearing is 54% of total load. A new proposed method by the author is presented to calculate the ultimate capacity of pile from pile load test. The proposed method depends on the settlement of pile without taken into consideration the elastic deformation. An empirical formula is presented from the relationship between stress and settlement of pile due to friction and end bearing only after deducting the elastic deformation. However, the obtained results for the ultimate capacity of end bearing piles are considered to be more accurate than other methods. The proposed method appears to give bitter results that agrees well with the theoretical predictions. The proposed method is easier, quicker and more reliable.

上砂下黏地层中桩-筒复合基础V-H承载特性

海上风机基础不仅受自重等竖向力V作用,也因水流、波浪和风等影响而承受水平荷载H。为探讨上砂下黏地层中一种由单桩和吸力筒组成的新型海上风机桩-筒复合基础受V-H组合作用时的承载特性,自主设计完成了一系列室内桩-筒复合基础V-H组合加载模型试验,获得不同组合参数下桩-筒复合基础的荷载-位移曲线和桩身弯矩分布曲线,并绘制出V-H承载力包络线。在此基础上,采用ABAQUS建立了上砂下黏地层中桩-筒复合基础三维数值分析模型,经模型验证与参数分析,进一步讨论了砂土厚度、筒径、筒高以及加载高度等参数对桩-筒复合基础承载特性的影响曲线,并拟合出桩-筒复合基础承载力简化计算公式,分析结果表明:桩-筒复合基础能显著提高桩身水平承载力,增幅达30%~90%,且增加筒径比增加筒高更有利于提高基础水平承载力;上部砂土层较厚时,桩-筒复合基础存在一个使复合基础水平承载力达到最大的预加竖向荷载最佳值,其值随不同载荷工况在(0.4~0.7)Vult范围内变化。

嵌岩桩水平承载力地基反力Matlock法分析

嵌岩桩在海上风电、跨海大桥等结构中的应用十分广泛,但在嵌岩桩水平承载特性方面的研究仍有较多不足。弹性地基反力法中的Matlock法在中国是用于计算水平承载桩承载力时应用最多的方法,但地基反力Matlock法针对嵌岩桩缺少合理的取值方法。基于现场试验数据建立并验证有限差分数值模型,通过数值模拟与参数分析,研究了岩体力学参数和桩基参数等因素对桩岩相互作用的影响,提出了Matlock法中地基反力系数比例系数的拟合计算公式。通过现场试验数据对提出的方法进行了验证,结果表明计算结果与实际情况较为相符,与其他方法比较,所提出的修正Matlock法相对非线性的p-y曲线法具有取值计算较为简便的优势。

弱胶结砂岩地基承载力深度修正系数k2试验研究

为探究弱胶结砂岩深度修正合理性及修正系数取值,依托南京龙潭长江大桥北锚碇大型块体基础,对25 m埋深的弱胶结砂岩持力层分别开展基岩深层和浅层平板载荷试验,得到持力层深度修正前后的地基承载力特征值,研究深度修正规律,研究结论:修正浅层载荷试验地基承载力的修正系数k2t取3.3、修正规范法地基承载力的修正系数k2c取8.0,保守考虑时可不区分未修正的承载力特征值获得途径,k2统一取3.3;深度修正后地基承载力有较大提升,说明本工程中对弱胶结砂岩承载力进行深度修正是很有必要的;对比现有规范中土质地基k2的取值规律,弱胶结砂岩的k_(2t)和k_(2c)取值较为保守,该值可为类似地质条件下的工程提供参考。

寒区冻土桩基承载特性研究现状与展望

冻土地区季节性的温度变化、大气升温以及人类工程活动加剧,常引起桩基冻胀融沉、倾斜、混凝土开裂等灾害,造成桩基的承载力降低,给寒区冻土桩基带来新的更大挑战。通过回顾国内外相关文献,对冻土桩基的承载性能目前的研究现状进行了总结。从特殊的冻土性质出发,描述了冻土桩基体系主要的受力形式及其作用特征;从桩–冻土界面力学特性试验及荷载传递机制两个方面,阐述了冻土区桩–土界面力学特性变化机制;从试验研究、理论分析以及数值模拟三个方面,阐明了冻土桩基承载性能变化规律及分析、预测方法;归纳了冻土区桩–土体系温度、水分、应力应变等方面的监测技术。对寒区冻土桩基承载特性未来的研究提出了展望。

基于桩-土界面剪切特性的单桩沉降和承载问题研究

近年来,西北地区出现了许多高填方场地,为减小建筑物基础的不均匀沉降,基础类型广泛使用桩基础。与一般场地不同,黄土填方场地中的单桩桩周土受力后仍会产生较大的变形,该类场地单桩沉降机制复杂。桩顶总沉降计算是桩基设计的重要依据,为此,建立了高填方黄土场地单桩桩顶总沉降计算模型。基于传统的荷载传递法和剪切位移法,分别考虑桩-土界面的桩-土相互作用和桩-土界面外剪切带土体的剪切变形。依据桩端边界,将单桩类型分为摩擦桩和端承摩擦桩,分别建立桩周土弹性阶段和塑性阶段的桩身位移控制微分方程,结合边界条件进行求解,得到桩身位移、轴力、侧摩阻力,并通过弹塑性理论求解了桩周土剪切带土体剪切变形,进而通过叠加原理求得桩顶总沉降。用桩长与桩周土塑性发展深度的比值,定义了桩基承载力安全系数K。通过算例分析与现场试验数据对比分析,研究结果表明:使用新的模型计算得到的桩顶总沉降与现场试验结果相近;当桩顶荷载较小、桩周土处于弹性阶段时,桩端边界对桩身轴力、位移和侧摩阻力影响很小,但桩周土进入塑性滑移阶段后,桩端边界的影响开始变大,考虑桩端土的承载能力会极大提高单桩极限承载力;建立了将荷载传递法和剪切位移法综合起来的计算模型,不仅可以考虑桩-土界面的相对滑移,还可以计算桩-土界面外土体剪切带的剪切变形,使得桩顶总沉降计算更加精确,可为类似场地中单桩沉降的分析与控制提供一定的参考。

地基承载力特征值确定土变形模量的探讨

目前我国地基沉降计算主要采用土压缩模量来计算,再用经验系数对计算结果进行修正。这种方法对于一些结构性较强的硬土地基误差较大,主要原因是取样扰动影响。为此,探讨了变形模量与地基承载力特征值的理论联系,在笔者提出的确定变形模量经验方法基础上,总结出依据地基承载力特征值确定变形模量的方法,该方法具有理论依据,可便捷地通过岩土工程勘察报告中常提供的地基承载力特征值确定土的变形模量,能够更为准确计算地基沉降值。采用不同方法(压板试验法、压缩模量推求法、标贯击数法、承载力经验法、依据地基承载力特征值确定变形模量法)计算了若干试验案例,通过对不同方法结果的对比,初步验证了依据地基承载力特征值确定变形模量法是可行的。

多元荷载作用下软基深水高桩码头结构承载特性研究

软基深水高桩码头结构受到恶劣外海环境荷载及复杂工作荷载等多元荷载的共同作用。研究表明,船舶撞击荷载是该结构水平向控制荷载,但波浪长期循环荷载作用会引起桩周土体软化,进而导致码头结构在其它荷载作用下承载特性的劣化。鉴于此,首先基于ABAQUS有限元软件建立了深水高桩码头结构-地基土体相互作用的三维弹塑性有限元模型;然后,借助USDFLD子程序实现了同时考虑土体强度弱化和刚度衰减的模拟,进而开展了未考虑土体软化、仅考虑土体强度弱化、仅考虑土体刚度衰减以及同时考虑土体强度弱化和刚度衰减对码头结构承载特性影响的对比分析。研究结果表明,与未考虑土体软化时撞击荷载作用下码头结构安全系数相比,仅考虑土体强度弱化时其值降低14.72%,仅考虑土体刚度衰减时其值降低15.28%,同时考虑土体强度弱化和刚度衰减时其值降低19.44%,且考虑土体软化后桩周土体的塑性应变范围明显增大,极限状态时桩身应力值减小,结构稳定性显著降低。

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。

基于岩溶三维地质模型的桩基承载力特性研究

本文依托中铁十九局集团有限公司岑大公路YTHJ4标桥梁桩基项目,通过Civil 3D软件建立了三维地质模型;并介绍了桩基作用下溶洞两种不同的破坏模式;然后采用基于FLAC 3D数值分析软件的单因子变量法分析了不同溶洞顶板厚度、顶板跨度、洞高、溶洞中心偏离桩轴线距离等对嵌岩桩承载力特性(极限承载力、极限侧摩阻力、桩土相对滑移距离等)的影响。研究结果表明:溶洞顶板是影响桩基极限承载力的主要因素;溶洞极限状态下的破坏模式强烈依赖于溶洞顶板厚度、顶板跨度和岩体力学性质;当溶洞顶板厚度达到三倍的桩径时,桩侧摩阻力可完全发挥;溶洞顶板跨度和偏心距离对桩基承载力特性的影响存在一个稳定点。这些研究结论为岩溶地区的嵌岩桩设计提供了一定的参考依据,进一步丰富了岩溶地区桩基设计理念。

考虑桩土变形协调的复合地基设计方法

目前在复合地基的设计中,由于采用桩体压缩模量计算得到的复合压缩模量偏大,造成复合地基加固区沉降被低估;另外桩和桩间土承载力发挥系数取值主观性较强,导致复合地基承载力计算结果偏大。针对上述两个问题,从复合地基形成机理出发,指出桩及桩间土共同直接承担荷载时必须保证桩土变形协调。桩土变形协调的条件是:(1)桩刺入垫层的深度不能超过垫层厚度;(2)桩宜设置为摩擦桩。在此前提下,分别计算桩及桩间土的沉降,利用桩土变形协调方法得到了桩土应力比和承载力发挥系数理论值,进而得到加固区的沉降计算方法。工程算例证实,从保证复合地基有效设计和工程安全的角度出发,应该根据桩土变形协调来进行复合地基设计计算。

桩网结构路基动静承载性状分析

为研究桩网结构路基在静动荷载作用下的承载性状,采用有限元分析方法对桩网结构路基在静动两种荷载下的承载特性、荷载传递机理、土拱形成及发展规律进行了数值计算。通过分析静动两种工况下桩土应力、桩周土压力分布、桩土应力比及土拱效应,并与试验结果进行了对比分析。结果表明:静载作用下基桩轴向应力约为400 kPa,最大土压力出现在路基中部,且沿路基横向向外逐渐减小;动载作用下沿路基横向布置的桩体的动应力在埋深方向呈先增大后减小趋势,其竖向应力最大值约为静载下的160%;动载下路基内部土拱效应较静载有大幅削弱,其中削弱最大的为四桩中心处的空间土拱效应,其桩土应力比约为静载的29.3%。