嵌岩桩的荷载传递及沉降分析

本文通过现场试验，分析了南京市嵌岩桩的荷载传递性状，用双曲线传递函数对桩基的荷载－沉降曲线进行了较好的拟合，文中的方法可应用于条件相似的嵌岩桩。     

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。     

水平荷载作用下非饱和黄土嵌岩灌注桩变形规律试验研究

由于地质条件的复杂性以及计算模型和参数选取等原因,桩-土在水平荷载作用下受力性状难以准确分析计算。通过对某电厂桩基工程在非饱和黄土地层中水平荷载试验,结合桩身应力测试,得到了桩-土共同作用时的工作性状及相互影响。在安全级别较高的工程中,采用原位测试方法确定桩基水平承载力,能有效提高工程质量,降低工程造价。     

超大群桩基桩竖向承载力试验研究

根据超大群桩3根基桩的现场竖向静载试验结果，分析了在复杂地质条件下群桩基桩的荷载传递特点，荷载沿桩身分布规律，竖向承载性状和桩身侧阻发挥特性，得出了一些在山区复杂地质条件下对桩基工程设计和施工具有一定指导意义的结论。     

极限荷载下桩筏基础共同作用性状的室内模型试验研究

利用自制模型槽,通过设计系列单桩带台与群桩的桩筏基础模型试验,研究了极限荷载下桩–筏板–地基土的应力与变形性状。试验结果表明,常规桩距桩筏基础极限荷载下表现出实体深基础性状;而大桩距桩筏基础,基桩先于板下土体达到承载力极限状态,后续荷载基本由板下土体分担,验证了塑性支承桩理论。加载过程中,桩–土的荷载分担比不断变化,6d及以上桩距时,桩达到极限荷载后即趋于稳定。利用桩的极限承载力的桩筏基础设计,应考虑极限荷载与工作荷载下桩–土荷载分担比的不同性状差别。桩间距越大,桩对土体的侧向位移的"遮帘作用"逐渐弱化,板下土体的位移特征趋于天然地基的特征,桩端平面以下土体应力受板下土体分担荷载的影响越明显,6倍桩距可视为常规桩基与复合桩基的分界点。     

摩擦型能源桩荷载–温度现场联合测试与承载性状分析

能源桩集地源热泵技术与建筑桩基于一体,桩基承载性状受荷载–温度耦合作用而不同于常规工程桩。开展了昆山某摩擦型能源桩工程的荷载–温度现场联合测试,测试了多级荷载水平与不同换热工况下桩身的温度、应力分布及桩顶位移变化,整理得到桩身的附加温度荷载、桩身轴力及桩侧摩阻力分布曲线,分析了摩擦型能源桩荷载–温度作用下的承载性状与荷载传递特征。测试结果表明:能源桩的温度变化将引起附加温度荷载,桩身轴力和附加温度荷载分布特征和桩顶荷载作用、桩端约束有关,承载性状不同于单一荷载作用下的工程桩。加热工况引起桩身上、中部多处出现负摩阻力,但荷载的增加有利于减小升温引起的负摩阻力效应;制冷工况下,桩端附近产生负摩阻力,能源桩荷载传递特征受荷载–温度耦合作用而改变。设计荷载作用下,能源桩顶加热时隆起而制冷时下沉,加热工况引起的桩顶位移在停止加热回温后可基本恢复,但制冷工况引起的桩顶位移在回温后会导致桩顶产生附加沉降,荷载–温度耦合作用也引起了能源桩沉降性状的变化。     

上海地铁沿线高层建筑的桩基设计探讨

结合地铁沿线的中金广场工程的桩基设计实例,根据现场单桩竖向极限静荷试验和桩侧摩阻力分布测试,探讨了进入上海⑨层土长钻孔灌注桩的工作性状及其承载力影响因素;介绍了竖向荷载下群桩受力性状的改进弹性理论法,并考虑桩端沉渣对群桩受力性状的影响;最后采用分层总和法分析了高层桩基基础沉降及其对地铁设施的沉降影响;结果可供同类工程设计参考.     

刚性桩、柔性桩复合地基力学性状三维有限元分析

采用ANSYS软件对刚、柔性桩复合地基的力学性状做了计算分析,比较了刚、柔性桩复合地基的荷载传递形式,同时与桩基的荷载传递形式做了比较,通过计算分析了桩身模量以及下卧层模量的改变对桩身荷载分布形式的影响。分析结果表明,刚性桩复合地基与柔性桩复合地基荷载传递形式是完全不同的。     

耦合荷载作用下桩基础的响应性状分析研究

采用PLAXIS有限元分析软件建立数值模型,分析在竖向荷载与水平动力荷载耦合的情况下,桩基础的响应性状分布特征。结果表明,竖向荷载较小时,桩基的水平位移随着竖向荷载的增加呈减小的趋势;当竖向荷载较大时,横向位移响应随着竖向荷载的增大而增加;对于强度较低的桩基,竖向荷载成为影响水平承载力及变形性状的主要因素;循环水平荷载若长期作用于承受较大竖向荷载下的桩基,会使桩身出现反向弯曲变形。     

地面水对黄土地区桥梁桩基承载力影响试验研究

黄土因其特有的工程性质,使位于黄土地区的桥梁桩基础在桩周土受地面水浸湿后产生湿陷变形,该变形引起桩的负摩阻力,降低了桩的承载力。通过陕西芝川河特大桥桩周浸水前、后的荷载试验,对黄土区域桩基浸水前、后的承载性状进行了分析研究,揭示了地面水对黄土区域桥梁桩基承载力性状具有较大的影响;分析了桩及桩周土浸水期间的沉降变化规律。研究成果对黄土区域公路桥梁桩基础的设计与施工具有重要理论价值与指导意义。     

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.     

预制节桩的荷载试验及荷载传递性能分析

节桩是一种带节的新型预制管桩,其桩侧摩阻力要大于相同直径的普通桩,同时具有良好的抗液化性能。本文通过现场试验,对节桩的荷载传递性能进行了分析和计算,为节桩的应用提供了依据。     

模型边界对大型哑铃型承台群桩基础承载变形特性的影响

利用三维弹塑性有限元,考虑群桩基础周围外侧土体竖向变形影响范围和桩端土体压缩层厚度的因素,研究分析其对大型哑铃型承台群桩基础承载变形特性的影响,从而研究大型超深群桩基础边界问题以及离心模型试验边界对其承载变形特性的影响。研究表明:群桩基础边界大小由群桩基础变形特性控制,成桥阶段时大型哑铃型承台群桩基础周围外侧土体竖向变形影响范围和桩端土体压缩层影响深度,分别为一倍承台宽度和一倍桩长;设计的离心模型试验边界对群桩基础桩身受力大小与分布影响不大,而对承台沉降和桩端土体整体压缩变形比等变形特性影响较大。     

水下干作业复合灌注桩试验研究

简要介绍一种新桩型---水下干作业复合灌注桩的成桩原理与工艺 ,通过试验阐明了其承载力、荷载传递特性、水泥土环桩侧阻力、破坏特性 ,以及水泥土环桩钻取土芯后发生的空气增强效应     

Influence of deep excavation induced ground movements on adjacent piles

In urban areas, excavations for cut-and-cover tunnels and basement construction cause detrimental effects on adjacent existing piles. Hence quantifying the excavation induced lateral deformations and bending moments on piles are important to ensure the stability of structures. In this paper, behaviour of a single pile subjected to excavation induced ground movements is analysed using the finite element method, which has the ability to simulate the construction sequence comprising soil excavation, deformations due to dewatering within the excavation and installation of struts. A fully coupled analysis is carried out based on the effective stress principle. The numerical model was verified using the centrifuge test data found in the literature. A parametric study was carried out to establish the excavation induced pile behaviour varying the depth of the excavation, soil properties, wall support system, pile fixity conditions and pile location with respect to the excavation. Increasing axial load does not have a significant influence on the pile behaviour. However, pile head fixity condition, and stiffness and spacing of the wall support system have a significant influence on the pile behaviour adjacent to the excavation. Finally, based on the parametric study, a set of design charts are developed to predict the pile behaviour by taking into account the depth of excavation, undrained shear strength, width of the pile, spring stiffness, spacing of vertical supports, and unsupported depth of the excavation. The capability of the proposed design charts are demonstrated using a three-dimensional finite element analysis, a case study from the literature and a previously published simplified analysis procedure.     

单根嵌岩桩在水平荷载作用下原型测试分析

通过对单根嵌岩灌注桩在竖向荷载下的水平静荷试验 ,分析该桩在水平荷载作用下的工作性状 ,为确定单桩水平承载力及钢筋笼的设置长度提出依据。     

Full-scale model tests of load transfer in geogrid-reinforced and floating pile-supported embankments

Well-designed field full-scale model tests were carried out to enhance the understanding of geogrid-reinforced and floating pile-supported (GRFPS) embankments constructed on medium compressibility soil (MCS). Two comparative test sections of GRFPS embankments with and without pile caps were built over silty clay with medium compressibility for field monitoring, over 25 months. The heavily instrumented embankments produced comprehensive high-quality data. Settlement, earth pressure, and geogrid strain measurements during embankment filling stages and the postconstruction placement stage were conducted. The influence of pile cap installation on the differential deformation and load transfer behaviour of the GRFPS embankment was evaluated. The results demonstrate the installation of pile caps can significantly improve the evolution characteristics of the stress increment ratio on the pile, facilitating a change in load sharing of the pile top from a “softening” feature to a “hardening” feature. The state of the “arching structure” heavily depends on the relative displacement. After the maximum arching is formed, although the subgrade load continuously increases, the arching enters the damage and recovery state, and the transfer of the overburden load increment is largely conducted by the tensioned membrane effect.     

Zeitabhängige Traglaststeigerung von Pfählen am Beispiel der Elbphilharmonie

Time‐dependent set‐up of piles considering as example the Elb‐philharmonica. The extension of the Kaispeicher in the Hafencity Hamburg to the Elb‐philharmonica leads to a local load increase of the existing pile foundation. To determine the load bearing behaviour of the existing piles and the need for any extension of the foundations pile load tests on three piles have been performed. A comparison of the results with the data of three old pile load tests from the construction of the building in 1963 reveals that the bearing capacity as well as the stiffness of the piles have increased significance. A finite‐element‐analysis of the pile load tests based on additional measurements shows that the capacity increase can be partly explained by different boundary conditions of the load tests like e.g. pre‐loading, but that a big part of the increase must be due to other time‐dependent mechanisms. Most plausible explanations for this set‐up effect is the relaxation of hoop stresses around the pile after installation as well as a densification of the sand at the pile shaft due to cyclic ground water fluctuation.     

改进的桩土界面荷载传递双曲线模型及其在单桩负摩阻力时间效应研究中的应用

桩土界面荷载传递模型对预测桩的承载变形性状有重要影响。本文在总结前人研究成果的基础上,改进了反应桩土界面荷载传递性状的双曲线模型。改进后的模型可以描述随着地基土的固结,桩侧土初始剪切刚度随时间增长及桩土界面的加载、卸载循环剪切特性。利用该模型分析了大面积堆载下,在桩顶作用大小不同的竖向荷载以及桩侧土达到不同固结度时再施加桩顶荷载情况下,桩身摩阻力的发展变化规律。研究结果表明:随着地基土的固结,桩身中性点位置处于一个变化过程中,桩顶作用的荷载大小不同,桩身中性点位置也不同;地基土固结一段时间后再打桩能减小桩侧负摩阻力。     

Rapid pile load tests in the geotechnical centrifuge

Centrifuge experiments were conducted to learn about the factors that affect mobilised resistance during rapid load testing of piles in sand. We studied the influence of pore water pressure during rapid load tests and its effect on the widely used unloading point method to derive static pile capacity. This paper describes the testing programme and the test set-up. We present typical measurement results from a total of 36 rapid and 12 static load tests, as well as the effects of the loading rate and excess pore pressures on pile resistance. The tests confirm that a rapid load test can overestimate static capacity due to pore water pressure when testing piles in medium to fine sands. The results of the pore pressure measurements show a combination of positive and negative excess pore pressure in the zone around the pile base, which can be explained by compression, volumetric behaviour during shearing and pore fluid flow around the pile.