基于API规范的海上大直径钢管桩静压载试验分析

大直径钢管桩在海洋工程建设中的使用越来越广泛。海上大直径钢管桩试桩周期长、难度大,相关测试数据及研究成果较为少见。本文基于美国石油行业协会API规范,分析了某海上风电大直径钢管桩静压载试验数据,研究了桩侧阻力与桩端阻力随压荷载变化关系。结果表明,对于本文分析的海上大直径钢管桩,其土塞状态一般为不完全闭塞,计算抗压极限承载力时应计入桩管内侧摩阻力;桩侧摩阻力随压荷载增大而逐渐发挥,相同荷载作用下,无黏性土地层侧摩阻力发挥程度较黏性土高;桩端土体破坏分为弹性变形与塑性变形两个阶段,两阶段端阻发挥程度分别可达约50%和80%,当桩端产生0.05d位移时,桩体达到极限平衡状态。     

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。     

端承型桩承载力离心模型试验中的粒径效应研究

利用modeling of models的方法研究端承型桩承载力离心模型试验中的粒径效应。在模拟同一原型时,不同桩径的模型桩,桩身压缩性及桩长均不同,导致侧摩阻力发挥机理及程度不同,本文分别探讨了桩端阻力,侧摩阻力及承载力(桩顶荷载)的粒径效应对承载机理和承载特性的影响。结果表明,桩端阻力的粒径效应作用规律与浅基础一致,可以借用浅基础的粒径效应定量评价方法评价端承桩承载力离心模型试验中的粒径效应。侧摩阻力的粒径效应比桩端阻力的粒径效应显著。由于侧摩阻力的影响,相同条件下承载力的粒径效应比桩端阻力有所增强。对于极限桩端阻力和极限承载力,粒径效应均随长径比的增加而减弱。     

海上风电单桩基础侧摩阻力研究

单桩基础是目前应用最广泛的海上风电基础形式。现有单桩竖向承载力设计时仅考虑外侧摩阻力与端阻力,并未考虑桩基内侧摩阻力对竖向承载力的贡献。文中基于江苏某沿海海域两个风电场的现场试桩数据,分别采用API规范、美国陆军工程兵团方法以及港口工程桩基规范中的公式计算桩身外侧摩阻力,研究了土层类型、土深度对于钢管桩内外侧摩阻力比值的影响规律,并与实测数据进行了对比分析。研究结果表明:接近泥面处内外侧摩阻力比值较小,内外侧摩阻力比值随深度增加呈增大趋势;在海上风电单桩基础竖向承载力设计中,桩基内侧摩阻力不容忽视。     

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。     

海上风电钢管桩自平衡法现场试验研究

一种新型钢管桩预装荷载箱法被研发出用于自平衡法海上风电钢管桩基检测试验,并通过现场自平衡试验研究探究了海上打入桩桩基础特性。该方法首次成功应用于海外某海上风电场直径1.4 m的超长大直径钢管桩承载力检测,用于探究其承载特性和桩侧桩端阻力发挥规律。现场试验显示,该新型检测方法达到了预期的测试效果和经济效益,与现有钢管桩自平衡法相比,对土的影响更小,可靠度更高,为类似土层和直径的超长钢管桩承载力试验提供了新的途径。     

地基承载力特征值和桩端承载力特征值的比较研究

工程中常涉及到地基承载力特征值fa及桩端阻力qpa,作为同量纲的地基指标,对同一土层后者往往要比前者高得多。运用梅耶霍夫地基极限承载力理论,分析了随基础埋深的加大地基土破坏形式的变化,对浅基础及桩基础破坏形式、地基承载力特征值fa与桩端承载力特征值qpa作了对比。研究结果将有利于建立及区别地基承载力特征值及桩端端阻力特征值的物理概念,对实际工程具有指导意义。     

大直径钢管桩土塞效应的判断和沉桩过程分析

港口工程和海洋工程中出现了越来越多的大直径超长钢管桩。由于这种桩直径较大,土塞的形成对桩的可打入性和承载力有较大的影响。鉴于此,根据大直径和超大直径钢管桩土塞性状的特殊性,考虑了桩直径对侧壁摩阻力、端阻力的影响,引入了尺寸效应系数,重新建立了土塞微分体的静力平衡方程,提出了采用改进的静力平衡法进行土塞效应判断,同时采用波动方程法近似模拟土塞与桩管内壁的相互作用,建立了简化的土塞与桩壁相互作用模型,并用该方法进行实际工程的打桩分析,分析结果表明该方法对土塞效应的判断、打桩过程的预测等与工程实测数据吻合较好。     

海上风电楔形单桩基础竖向承载力特性分析

为提高基础利用率增加海上风电设施的可行性，对楔形单桩基础竖向承载力特性进行研究分析。采用PLAXIS 3D 有限元软件建立楔形单桩基础模型，从桩侧摩阻力、桩侧法应力及土体位移对比分析楔形单桩基础与等截面单桩竖向承载特性差异，并探讨内摩擦角、楔角及楔高对承载力的影响。研究表明：楔形单桩基础竖向承载力高于等截面单桩基础，且承载力随着楔角、楔高的增大而增大，提高率最大达24.786%。倾斜侧壁的引入改变了桩侧摩阻力的传递规律；倾斜侧壁挤密桩周土体，桩侧摩阻力与法向应力增大，从而有效提高单桩基础的竖向承载力。研究成果可为今后海上风电单桩基础截面型式的设计提供参考。     

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.     

Field study of set-up effect in open-ended PHC pipe piles

Large-scale field tests were conducted to study set-up effect in open-ended prestressed high-strength concrete pipe piles jacked into stratified soil. Four open-ended prestressed high-strength concrete pipe piles with 13 and 18 m in embedment depth were fully instrumented with fiber Bragg grating sensors and installed. Several restrike dynamic tests were performed on each test pile, with the time interval from 21.5 to 284 hours after installation. Static loading tests (SLTs) were later performed on each test pile at 408 hours after installation to substantiate the dynamic tests. Changes with time in pile bearing capacity and in the shaft and toe resistances were studied based on the results of the pile tests. The development of shaft resistance set-up in different layers was studied in particular. It was found that set-up effect in the shaft resistance is significant and the toe resistance increment was minor. The overall set-up factor of total bearing capacity was found to range from 0.09 to 0.53, and the set-up effect of friction pile is much larger than the end bearing pile. More significant set-up in shaft resistance was observed in fill and alluvium layer. The dimensionless set-up factor A for shaft resistance in marine deposits ranges from 0.5 to 1.43, and it contributes the most to the shaft resistance as the shaft resistance in marine deposits is higher.     

压力注浆螺旋钢管桩抗拔承载性能试验研究

针对海相软土地区螺旋钢管桩承载力低与腐蚀问题,提出一种新型压力注浆螺旋钢管桩,并设计5根足尺试验桩,进行现场抗拔承载性能试验,研究螺旋叶片直径与排布方式对成桩直径与桩基抗拔承载性能的影响.结果表明,成桩直径与螺旋叶片直径呈正相关,在每节延长段钢管末端设置螺旋叶片利于提高水泥土柱完整性,使成桩直径更为饱满,提高桩基的抗拔承载性能.将试验结果和现行规范抗拔极限承载力计算结果进行对比,计算结果约为实测平均值的94％,在此基础上提出压力注浆螺旋钢管桩抗拔承载力计算参数修正建议,为后续的设计提供参考.     

Soil Plug Effect Prediction and Pile Driveability Analysis for Large-Diameter Steel Piles in Ocean Engineering

Long steel piles with large diameters have been more widely used in the field of ocean engineering.Owing to the pile with a large diameter,soil plug development during pile driving has great influences on pile driveability and beating capacity.The response of soil plug developed inside the open-ended pipe pile during the dynamic condition of pile-driving is different from the response under the static condition of loading during service.This paper addresses the former aspect.A numerical procedure for soil plng effect prediction and pile driveability analysis is proposed and described.By taking into consideration of the pile dimension effect on side and tip resistance,this approach introduces a dimensional coefficient to the conventional static equilibrium equations for the plug differential unit and proposes an improved static equity method for the plug effect prediction.At the same time,this approach introduces a simplified model by use of one-dimensional stress wave equation to simulate the interaction between soft ping and pile inner wall.The proposed approach has been applied in practical engineering analyses.Results show that the calculated plug effect and pile driveability based on the proposed approach agree well with the observed data.     

大直径超长管桩打桩过程中土塞性状研究

大直径超长桩的可打入分析是海洋平台打桩施工顺利进行的重要保障,土塞是否闭合的判断对于桩基可打入性分析具有较大的影响,因此,合理准确的土塞判断结果对提高桩的可打入分析的准确性具有重要的意义。以现场静力触探(CPTU)试验数据为依据,采用孔扩张理论推导了基于CPTU测试结果的桩端土的极限承载力计算公式;在求解桩端土体承载力时考虑了管桩与土体的刚度差异,同时考虑到打桩过程中的土体扰动。采用Randolph推荐的方法得到了土塞阻力,将两者进行比较,进而判断土塞的状态。通过实际工程的实测数据,对各个土层的土塞状况进行了判别,并根据判别情况采用波动方程的方法对桩基的可打入性进行了分析,将预测结果和现场的打桩记录进行了比较。计算结果显示,提出的方法与实测结果更为接近,有效地提高了桩的可打入性的预测精度。     

Model tests on open-ended concrete pipe piles jacked in sand

Abstract

An experimental study of the performance of concrete pipe piles during installation under different penetration speeds and static load tests on the piles in sand is presented. The applied jacking force, the amount of pile penetration, length of soil plug formed and ultimate bearing capacity were measured during the model tests. The results showed that the concrete pipe piles were partially plugged and the behavior of the soil plug was significantly affected by the penetration speed. The lower the penetration speed, the larger the soil plug formed which in turn leads to a greater ultimate bearing capacity. The size of soil plug can be evaluated by the m value defined as the ratio of the volume of the soil plug to that of the penetrated pile wall. The relationship between the m value and the penetration speeds can be used to estimate the amount of soil plug and the depth of penetration for an open-ended concrete pipe pile jacked into sand.     

A new look at the phenomenon of offshore pile plugging

Abstract

Open‐pipe piles are widely used for offshore structures. During the initial stage of installation, soil enters the pile at a rate equal to the pile penetration. As penetration continues, the inner soil cylinder may develop sufficient frictional resistance to prevent further soil intrusion, causing the pile to become plugged. The open‐ended pile then assumes the penetration characteristics of a closed‐ended pile. The mode of pile penetration significantly alters the soil‐pile interaction during and after installation. This affects the ultimate static bearing capacity (mainly in granular materials), the time‐dependent pile capacity (in clays), and the dynamic behavior and analysis of the piles.

Following a summary demonstrating the effects of pile plugging, a review of the common view of offshore pile plugging is undertaken. The interpretation of plugging by referring to the average plug length has led to the erroneous conclusion that in most piles significant plugging action does not occur.

Establishment of an analogy between soil samplers and open‐ended piles enabled correct identification of plugging by referring to the incremental changes in plug length. Examination of case histories of plugging of offshore piles revealed that beyond a certain penetration depth‐to‐diameter ratio, most piles are plugged.     

Bearing Behavior of Cast-in-Place Expansive Concrete Pile in Coral Sand Under Vertical Loading

The low side friction of piles in coral sand results in the low bearing capacity of foundations. In this paper, expansive concrete pile is utilized to improve the bearing capacity of pile foundations in coral sand. Both model tests and numerical simulation are performed to reveal the bearing mechanism of expansive concrete pile in coral sand.Results showed that the lateral earth pressure near pile increases obviously and the side friction of piles is improved,after adding expansion agent to the concrete. The horizontal linear expansion is 1.11% and the bearing capacity increased 41% for the pile, when 25% expansion agent is added. Results in finite element numerical simulation also show that ultimate bearing capacity increases with the increase of the linear expansion ratio. Besides, the area for obvious increase in side friction is below the surface of soil about three times the pile diameter, and the expansion leads to a high side friction sharing of the pile. Therefore, the cast-in-place expansive concrete pile is effective in improving the bearing capacity of piles in coral sand.