抗浮爆扩桩抗拔机理有限元分析

高地下水位地区构筑地下建(构)筑物必须解决结构上浮问题,抵抗上拔荷载作用已成为地下工程建设中经常需要面临的问题。试验表明爆扩桩具有很好的抗浮能力,其成桩施工工艺方面的研究也比较完善,但是对爆扩桩的承载机理的研究很少,通过运用NCAP-1有限元计算程序,分析总结爆扩桩的抗拔承载机理。     

离岸深水全直桩码头承载性能有限元分析

全直桩码头是适于软土地基上离岸深水海域的新型高桩码头结构型式,其承载机理与传统高桩码头存在较大差异,且软土地基循环软化效应显著。建立全直桩码头结构与地基相互作用三维弹塑性有限元模型,基于二次开发采用拟静力法对土体循环软化效应进行模拟。通过有限元模型研究全直桩码头的承载特性与破坏模式,并探讨水平极限承载力的影响因素。研究表明水平荷载作用下,基桩的塑性破坏是结构失稳的控制因素,地基土体的承载力对结构水平极限承载力不起决定性作用;竖向荷载作用下,结构竖向极限承载力由地基土体强度决定。研究范围内入土深度对结构水平极限承载力影响不大,但桩壁厚度减小或考虑土体软化后,结构水平极限承载力明显降低。设计中,增加入土深度可有效减小土体软化引起的水平极限承载力降低程度,且应考虑结构腐蚀和土体软化对水平极限承载力的双重降低效应,为钢管桩预留足够的腐蚀富裕量。     

黏土中鱼雷锚抗拔承载力数值分析

基于Abaqus软件,针对预埋在一定深度鱼雷锚的抗拔承载力进行3D数值分析,探讨了锚型、土体类型、拉拔荷载倾角、拉拔荷载水平分量与锚翼夹角等多种因素对拉拔承载力的影响。计算结果表明:在均质土、正常固结土和两层土体中,带4个锚翼鱼雷锚增加56.22 m2的锚翼侧面积,其拉拔承载力比无锚翼的提高1.9倍以上;当锚翼侧面积相同时,增加锚翼的宽度可以有效提高锚的承载力;拉拔荷载倾角α在30°～45°范围内,可获得较大拉拔承载力;锚翼宽度越大,拉拔荷载水平分量与锚翼间的夹角β对鱼雷锚水平承载力影响越明显;经归一化的V-H包络线公式对工程应用中预测鱼雷锚的承载力有一定参考价值。     

扩底抗拔桩动态变形全过程承载特性模型试验研究

利用砂土中扩底抗拔桩的模型试验,研究从开始加载到破坏时扩底抗拔桩地基动态变形全过程的承载特性.试验结果表明:半模试验得到的极限荷载和破坏面均略小于全模试验结果,采用半模试验测量地基变形过程与破坏模式有明显优势,用半模试验代替全模试验是可行的;随着桩顶荷载的增加,扩大头上方的土体由压缩变形逐渐发展为局部的压缩—剪切破坏;...     

海上风电大直径加翼桩水平承载性能研究

为改善海上风电大直径钢管桩的水平承载性能,基于ABAQUS有限元软件对单桩改进形式的加翼桩结构进行了系统研究,计算分析了软黏土地基中加翼桩在水平荷载作用下桩身弯矩、应力、位移、桩身泥面处倾斜率和极限承载力,研究了加翼桩面积、形状、埋深和刚度等翼板参数对加翼桩水平承载性能的影响规律,根据加翼桩的桩-土作用机理,参考现行规范模式提出适用于软黏土地基大直径钢管桩的P-Y曲线。研究结果表明,加翼桩通过在泥面处设置翼板可降低桩基泥面处倾斜率50%、提高桩基极限承载力60%以上,加翼桩水平承载性能明显优于单桩。     

U型钢板桩横向承载性能模型试验研究

采用1∶5的比尺模型试验,研究了横向加载过程中U型钢板桩的位移和土压力响应以及破坏模式,并对比分析了不同土质干湿状况、加载速率、埋置深度以及加载高度等影响因素下U型钢板桩位移和土压力的变化规律。试验结果表明,当U型钢板桩凸面加载时,位移随埋置深度、加载等级、加载高度的增加而增加,干砂中的位移大于湿砂中的位移;当凹面加载时,位移随着埋置深度的增加而减小,随着加载等级的增大而增大,在不同加载高度与不同土质干湿情况中差别不大。随着加载力增大,U型钢板桩在受力侧土压力分布呈现“R型”分布,且土压力均随着埋置深度与土体含水率的增大而增大。在加载力作用下桩体产生转动,并随着加载力的增大在距钢板桩底部约1/3埋置深度处发生弯曲。     

江苏海岸辐射沙洲地层中大直径钢管桩基础承载性能试验研究

江苏海岸辐射沙洲是江苏重要的海上风电规划区域,该海域的地层资料和试桩资料极为缺乏。通过钢管桩的现场静荷载试验,对江苏海岸辐射沙洲地层中大直径钢管桩基础承载性能进行研究,以揭示各地层的主要承载性能参数。试桩结果表明,江苏海岸辐射沙洲地层中钢管桩实际的轴向极限承载力明显小于高应变动测结果,只有高应变动测结果的79.09%。总侧摩擦阻力占总承载力的95.61%,而桩端阻力只占总承载力的4.39%。桩内土对管壁的侧摩阻力作用很小,主要是桩外土对管壁的侧摩阻力在发挥作用。辐射沙洲地层中粉土夹粉质黏土的承载性能一般,不适应作为钢管桩的持力层。轴向抗压静载试验得到的极限侧摩阻力高于静力触探的测试结果和API法的计算值。浅部砂层实际的极限水平土体抗力高于API法计算值,在水平荷载作用下上部砂层的p-y曲线具有明显软化效应,土体的软化效应在设计时需进行考虑。     

大直径钢管桩水平载荷试验研究

文章首次对直径为2800mm 的超大直径钢管桩开展了单桩水平静载试验,研究分析了超大直径钢管桩的桩侧水平抗力、桩身截面弯矩、桩身水平位移与桩基水平荷载之间的变化曲线,得到了不同水平位移时桩周地基反力模量的变化规律,为超大直径桩基水平承载力设计提供了现场试验数据。     

变截面劲性水泥土桩承载特性室内模型试验研究

研究变截面劲性水泥土桩的几何特征对承载特性的影响,结果表明:具有1个扩大盘或2个扩大盘间距较大的变截面桩,盘下部的土体发生压缩和局部剪切破坏现象,上部的土体则发生梨形滑落;盘间距较小时,上下两盘之间的土体与两盘成为一体;变截面桩的桩侧荷载分担值均远大于桩端荷载分担值,盘的数量及间距对桩侧及桩端荷载分担值影响不大;1个盘时,其位置对承载力有一定的影响;2个等间距盘的变截面桩,盘位置越高承载力越高;盘间距对承载力影响不显著;3个盘的承载力大于2个盘的承载力,但结果相差不大;变截面桩的承载力得到显著提高,其承载力不小于与扩大盘直径相等的等截面桩;随着桩顶荷载的增大,盘承担的荷载增加显著,盘以下桩身的轴力因盘承担大部分而骤减,其降低幅度与盘的数量、位置及间距有关.     

珊瑚礁地质大直径钢管打入桩承载特性研究

马尔代夫中马友谊大桥采用钻孔灌注桩基础,主墩基础施工过程中,将35根大直径钢护筒施沉至中等—强胶结礁灰岩地层,作为钻孔平台的临时桩基础。以主墩大直径钢护筒沉桩记录为依据,并结合高应变动力检测方法,对珊瑚礁地质大直径打入桩的承载性能进行研究。研究结果表明:1)以钙质砂为主的覆盖层侧阻力较小; 2)礁灰岩侧阻力随胶结程度的增加而增大; 3)中等—强胶结礁灰岩可以作为打入桩的持力层,端阻力约占总承载力的70%; 4)打入桩的承载力恢复系数较小,仅为1.1。     

Direct CPT and CPTu methods for determining bearing capacity of helical piles

Helical piles are structural deep foundation elements, which can be categorized as torque-driven piles without any limitations to implement in marine situations. Different methods are used to predict the axial capacity of helical piles, such as static analysis, but have some limitation for this type of piles on marine conditions. In situ testing methods as supplement of static analysis have been rarely used for helical piles. In geotechnical engineering practice, the most common in situ tests particularly applicable for coastal or offshore site investigation are cone penetration test (CPT) and piezocone penetration test (CPTu). The CPT is simple, repeatable, and prepares the continuous records of soil layers. In this paper, a data bank has been compiled by collecting the results of static pile load tests on thirty-seven helical piles in ten different sites including CPT or CPTu data. Axial capacities of thirty-seven helical piles in different sites were predicted by direct CPT methods and static analysis. Accuracy estimation of ten direct CPT methods to predict the axial capacity of helical piles was investigated in this study. Comparisons have been made among predicted values and measured capacity from the pile load tests. Results indicated that the recently developed methods such as NGI-05 (2005), ICP-05 (2005), and UWA-05 (2005) predicted axial capacity of helical piles more accurately than the other methods such as Meyerhof (1983), Schmertmann (1978), Dutch (1979), LCPC (1982), or Unicone (1997). However, more investigations are required to establish better correlation between CPT data and axial capacity of helical piles.     

A review on the behavior of helical piles as a potential offshore foundation system

Abstract

Helical piles are used mainly to resist tension forces generated by uplift and overturning moments of various structures, therefore they have been suggested as a potential alternative to driven piles as offshore pile because they provide a large uplift capacity due to the anchor effect of the helix. To date no standards are available for the assessment of the use of helical piles in the offshore environment. State-of-the art installation and uplift capacity assessment is based on field onshore tests on small helical piles. The purpose of this review is to critically evaluate the current knowledge on helical piles considering uplift capacity, cyclic load, installation torque models and the parameters affecting the installation torque, to understand whether they can be considered for potential offshore applications. The paper could be of valuable interest for engineers and contractors involved in the offshore installation of piles.     

Shaft capacity of pre-bored grouted planted nodular pile under various overburden pressures in dense sand

Abstract

This paper presents the results of a series of model tests performed to study the shaft capacity of pre-bored grouted planted nodular (PGPN) pile in dense sand. The influence of the vertical overburden pressure on the shaft capacity of the PGPN pile is also investigated based on the test results. The test piles were equipped with strain gauges to measure the axial loads during the loading process, moreover, a foam plate was buried beneath pile tip to eliminate the influence of tip resistance on the shaft capacity. Some conclusions can be drawn based on the test results: the peak skin friction of PGPN pile increases with the increase of vertical overburden pressure applied on the foundation soil, while the rate of increase decreases with the increasing overburden pressure; the surface of the pile–soil interface of PGPN pile is relatively rough, and significant dilatant increase in lateral stress occurs during the loading process.     

Discussion: A review on the behaviour of helical piles as a potential offshore foundation system

Abstract

Helical piles have emerged as an attractive foundation system for offshore applications with renewed interest from the offshore community. Significant research gap currently exists in transferring this technology offshore and this paper discusses how existing and emerging knowledge can be successfully used to bridge some of the gaps. We focus on the Coupled Eulerian Lagrangian (CEL) large deformation finite element (LDFE) modelling technique that is commercially available and can be used to model the three-dimensional installation process with consideration of strain rate and softening effects in soft offshore clays. A helical pile of L?=?7.5?m long is modelled with one or two large-diameter helices (D?=?2?m) attached to a central shaft of d?=?0.5?m in diameter.The net effect of strain rate and softening is to increase the installation torque. The measured torque is within the range of 200–400?kN.m for the offshore clay and the pile geometry studied. Additional helices increase the uplift force but to a lesser degree than that of the measured torque. Remoulding induced strength reduction is found to be within the range of 25–33% of the intact clay strength. Issues of extracting and reusing offshore helical pile foundations are discussed.     

短螺栓型膨胀式自应力灌浆卡箍滑动承载力试验研究

对短螺栓型膨胀式自应力灌浆卡箍的滑动承载力进行了试验研究,首先从自应力产生机理介绍了短螺栓型灌浆卡箍结构,分析了这种结构的特点和优越性,然后在不同膨胀剂掺量和长细比下进行了这种卡箍试件的滑动承载力测试。试验结果表明:滑动承载力随着膨胀剂掺量的增加线性增加;在长细比为1.02~1.67的范围内,滑动应力差异不大,但当长细比增加到3.33时,滑动应力明显降低;而且短螺栓型膨胀式自应力灌浆卡箍比近似结构尺寸的传统长螺栓型卡箍能提供更大的滑动应力。试验结果为短螺栓型膨胀式灌浆卡箍的工程应用提供了一定的设计依据。     

Closure to discussion “A review on the behavior of helical piles as a potential offshore foundation system”

Abstract

A closure on the recent discussion regarding the review on the behavior of helical piles as a potential offshore foundation system by the Authors is presented. Installation torque, uplift capacity, installation effects, model pile, installation speed and results proposed by Discussers are commented.     

海上风机基础灌浆连接段压弯性能试验研究

对四根不同轴压比的灌浆连接段试件进行压弯试验,研究了不同轴压比灌浆连接段试件的压弯承载力、延性和破坏模式,并且分析了压弯荷载作用下的灌浆连接段钢管应变分布规律。试验结果表明:灌浆连接段试件具有良好的延性和较高的承载力,且随着轴压比的增加,灌浆连接段试件水平承载力与延性不断减小。     

实际尺寸膨胀式自应力灌浆卡箍承载性能测试

以中试研究的试验结果为基础,介绍了实际尺寸膨胀式自应力灌浆卡箍承载性能的测试情况。通过2个鞍板壁厚不同模型的膨胀压力和滑动应力的试验测试,表明实际尺寸样机承载性能的测试结果与小尺寸模型试验基本一致,15%膨胀剂掺量下的卡箍膨胀压力均超过1.8 MPa、滑动应力均超过2.5 MPa,实际尺寸卡箍样机仍然具有较高的承载性能,并且海试卡箍的滑动应力为2.56 MPa,达到了模型试验的水平。     

透空式水平板波浪上托力冲击压强试验研究

首先对以往波浪作用下透空式平板上托力研究进行了回顾,在此基础上,通过系列模型试验,对波浪上托力产生的机理进行了详细的分析和论述,认为透空式平板下波浪上托力主要由一个迅速上升的冲击压强和一个缓慢变化的压强构成,其中冲击压强通常远大于缓变压强,起主导作用,影响其大小的最直接因素为几何因素(即波浪冲击角)、动力因素(包括波高、波速等)和空气垫层因素,由此通过对各影响因素的分析,得到最大冲击压强计算公式,大量试验资料表明,该公式与实验值有着较好的一致性。     

A CPT-based model to predict the installation torque of helical piles in sand

Helical piles present a possible alternative to driven displacement piles in the offshore sector. While this type of pile is used widely in the onshore environment, design methods tend to be highly empirical and there is considerable uncertainty around the bearing resistance and the installation resistance required to install large diameter piles necessary to resist uplift. The paper combines a theoretical model for estimating torque resistance from the literature with a cone penetration test (CPT)-based model originally developed to estimate the axial resistance of helical piles and to predict the installation torque required to install piles in sand. The model appears to be able to capture the general installation behavior of piles across a range of scales and in various sand states.