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。     

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。     

海洋平台大直径钢管桩打桩过程有限元分析研究

近年来海上工程的规模越来越大,为了满足工程需要,桩基设计常常采用大直径,大长度的钢管桩。打桩过程是个相当复杂的过程,不仅涉及到几何非线性、材料非线性、边界非线性,而且是个动力过程。有限元法在处理打桩分析方面具有很强的优势,采用PLAXIS对不同条件下的打桩问题进行了动力模拟分析。分析显示在打桩过程中,桩端土体会产生较大的水平位移和竖向位移,桩端土体和靠近桩端的部分土塞内会产生较大的超孔隙水压力。在砂土中,停锤较短时间也会使孔压迅速消散,这也是打桩中间的停锤会造成后续打桩困难的主要原因。     

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。     

海上复杂地质条件下大直径钢管桩时效性试验研究

通过对3根海上复杂地质条件下的大直径钢管桩采取高应变初打与不同休止时间复打相结合的试验方法,得到不同休止时间钢管桩承载力、侧阻力及端阻力大小,以此对不同桩侧土及持力层对钢管桩时效性的影响进行了研究。研究结果表明:1)钢管桩承载力时效性现象明显,且随时间增长迅速; 2)钢管桩侧阻力的恢复系数远大于端阻力; 3)桩侧黏性土强度的恢复是钢管桩侧阻力增加的主要原因; 4)砂土层虽提供的侧阻力较大,但其对侧阻力增长的贡献不如黏性土; 5)持力层越硬,端阻力与承载力的恢复性越差。     

海上大直径钢管桩打桩过程中桩周土强度弱化研究

海上大直径钢管桩打桩过程中,桩周土体受到强烈扰动而发生强度弱化,掌握桩周土体强度弱化规律对于准确预测打桩过程、保证工程安全具有重要意义。为研究土体强度弱化规律,开展了环剪试验模拟打桩对桩周土体的扰动,测试土体强度随剪切速率的变化规律,建立了描述土体强度弱化规律的拟合公式,引入到打桩分析软件中。研究结果表明：土体的强度折减程度不仅与土体本身的性质有关还受到土体的埋深和剪切速率的影响,埋深越深土体强度折减程度越低,剪切速率越高土体强度折减越高,在打桩分析中可采用这里推荐的线性折减方法来模拟不同深度处土体强度的折减规律。     

基于能量法的大直径超长钢管桩溜桩问题分析

海洋桩平台采用大直径超长桩,由于桩、锤的重量很大,沉桩过程中经常发生溜桩现象。因此为了便于打桩控制,判断溜桩的范围是非常必要的。结合实际工程对溜桩的过程和发生机理进行了探讨;利用PCPT原位测量资料,基于能量法建立了判断溜桩范围的分析计算方法。针对南海油田典型的平台桩沉桩过程中的溜桩问题进行了分析计算,验证了所提出的方法的合理性,可供桩设计以及沉桩施工参考。     

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。     

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。     

桩基尺寸对砂土中水平受荷桩-土作用力的影响

p-y 曲线法在水平受荷桩的设计中得到了广泛应用,然而近年来在海上风机超大直径单桩基础设计中受到了学术界和工程界的质疑,当前研究表明,桩基尺寸是产生这一问题的主要原因。本文通过三维数值仿真技术,研究了砂土地基中水平受荷桩-土作用特征的演变规律,探讨了当前超大直径单桩设计方法存在的主要问题及产生的原因。计算结果显示：超大直径单桩基础因其桩径大、长径比（入土长度与直径的比） 小的尺寸特征,在水平荷载作用下桩土作用具有明显的三维空间效应,即除受水平向土反力外,还受到桩端水平向的摩擦力、沿桩身不均匀摩阻力产生的抵抗力矩及桩底不均匀土反力提供的抵抗力矩;进一步研究表明,后三类抗力对桩基水平承载力的贡献占比与桩基尺寸有关,即桩基直径越大、长径比越小,这些抗力的贡献越大。因此,在水平受荷超大直径单桩承载特性计算与分析的过程中,除当前水平向p-y 弹簧提供的抗力外,还应考虑桩底的水平剪力、桩身摩擦力和桩底反力不平衡产生的抵抗力矩。     

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.     

Pit bearing capacity effect on status of soil plug during pile driving in ocean engineering

Foundation piles of the offshore oil platforms in the Bohai Bay are usually longer than 100 m with a diameter larger than 2 m.Driving such long and large-sized piles into the ground is a difficult task.It needs a comprehensive consider ation of the pile dimensions,soil properties and the hammer energy.Thoughtful drivability analysis has to be performed in the design stage.It has been shown that judging whether the soil column inside the pile is fully plugged,which makes the pile behave as close-ended,strongly influences the accuracy of drivability analysis.Engineering practice repeatedly indicates that the current methods widely used for soil plug judgment often give incorrect results,leading the designers to make a wrong decision.It has been found that this problem is caused by the ignorance of the bearing capacity provided by the soil surrounding the pile.Based on the Terzaghi’s bearing capacity calculation method for deep foundation,a new approach for judging soil plug status is put forward,in which the surcharge effect has been considered and the dynamic effect coefficient is included.This approach has been applied to some practical engineering projects successfully,which may give more reasonable results than the currently used method does.     

土体疲劳对打桩分析的影响

在动力沉桩过程中,桩长时间连续运动导致桩侧土体强度的降低,使土体产生疲劳效应。结合一维应力波动理论,桩土相互作用模型和土体疲劳的不同计算方法,编制计算软件,对渤海某油田采油平台的桩基工程进行打桩分析,预测沉桩过程所需的锤击数、桩周土静阻力以及桩的极限承载力。比较不同的桩侧土体疲劳计算模式的分析结果,并用工程实测数据验证计算值。为工程设计和施工提供参考。     

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.     

An Approach to Stability Analysis of Embedded Large-Diameter Cylinder Quay

The large-diameter cylinder structure, which is made of large successive bottomless cylinders placed on foundation bed or partly driven into soil, is a recently developed retaining structure in China. It can be used in port, coastal and off-shore works. The method for stability analysis of the large-diameter cylinder structure, especially for stability analysis of the embedded large-diameter cylinder structure, is an important issue. In this paper, an idea is presented that is, em-bedded large-diameter cylinder quays can be divided into two types, i.e. the gravity wall type and the cylinder pile wall type. A method for stability analysis of the large-diameter cylinder quay of the cylinder pile wall type is developed and a method for stability analysis of the large-diameter cylinder quay of the gravity wall type is also proposed. The effect of sig-nificant parameters on the stability of the large-diameter cylinder quay of the cylinder pile wall type is investigated through numerical calculation.     

Behavior of soil plugs in open‐ended model piles driven into sands

Calibration chamber tests were conducted on open‐ended model piles driven into dried siliceous sands with different soil conditions in order to clarify the effect of soil conditions on load transfer mechanism in the soil plug. The model pile used in the test series was devised so that the bearing capacity of an open‐ended pile could be measured as three components: outside shaft resistance, plug resistance, and tip resistance. Under the assumption that the unit shaft resistance due to pile‐soil plug interaction varies linearly near the pile tip, the plug resistance was estimated. The plug capacity, which was defined as the plug resistance at ultimate condition, is mainly dependent on the ambient lateral pressure and relative density. The length of wedged plug that transfers the load decreases with the decrease of relative density, but it is independent of the ambient pressure and penetration depth. Under several assumptions, the value of earth pressure coefficient in the soil plug can be calculated. It gradually reduces with increase in the longitudinal distance from the pile tip. At the bottom of the soil plug, it tends to decrease with increase in the penetration depth and relative density, and to increase with the increase of ambient pressure. This may be attributed to (1) the decrease of friction angle as a result of increase in the effective vertical stress, (2) the difference in the dilation degree of the soil plug during driving with ambient pressures, and (3) the difference in compaction degree of soil plug during driving with relative densities. Based on the test results, an empirical equation was suggested to compute the earth pressure coefficient to be used in the calculation of plug capacity using one‐dimensional analysis, and it produces proper plug capacities for all soil conditions.     

水射流辅助动力沉桩技术研究

水射流破坏桩内土塞辅助动力沉桩是一种有效处理土塞问题使桩达到标准贯入深度的辅助沉桩方法,而该项技术的关键是水射流对桩内土塞的破坏。借助理论分析、数值模拟和模型实验进行水射流破坏桩内土塞辅助动力沉桩研究。首先进行射流破坏土塞理论分析,定性地说明水射流破坏土塞能力与水射流压力、流量等参数有关;然后对不同形状喷嘴形成的射流的破坏土塞能力进行数值模拟,得到六种喷嘴射流作用下土塞内的最大、最小应力;再通过模型实验对有无射流辅助沉桩的沉桩锤击数进行对比,结果表明水射流辅助沉桩的锤击数少于无射流辅助沉桩的锤击数,说明水射流可用于辅助动力沉桩。最后设计出水射流破坏桩内土塞辅助动力沉桩工艺步骤。     

Vertical dynamic impedance of large-diameter pile considering its transverse inertia effect and construction disturbance effect

The vertical dynamic impedance of the large-diameter pile is theoretically investigated considering the construction disturbance effect. First, the Rayleigh–Love rode model is introduced to simulate the large-diameter pile with the consideration of its transverse inertia effect. The shear complex stiffness transfer model is proposed to simulate the radial inhomogeneity of the pile surrounding soil caused by the construction disturbance effect. Then, the pile–soil system is divided into finite segments, and the governing equation of the pile–soil system subjected to vertical dynamic loading is established. Following this, the analytical solution of vertical dynamic impedance at the pile head is obtained by means of the shear complex stiffness transfer method and the impedance function transfer method. Based on the present solution, a parametric analysis is conducted to investigate the influence of the transverse inertia effect on the vertical dynamic impedance at the pile head and its relationship with the pile–soil parameters. Finally, comparisons with published solutions are carried out to verify the reliability of the present solution.     

Case study on pile running during the driving process of large-diameter pipe piles

The super-long and large-diameter steel pipe piles are often adopted for the construction of offshore oil platforms in deep sea. One constructability issue related to driving heavy pipe piles is the pile running. The term pile running refers to the quick penetration of a pile into the seabed as a result of its high self-weight and low resistance from the seabed. The unexpected pile running can cause the steel wire of the hammer to break or even the loss of the hammer. A case study of pile running at an oil platform is introduced in this paper. A simplified theoretical method is proposed to explain the mechanisms of the pile running in this case. A factor of friction degradation is proposed to calculate the dynamic skin friction from the static ultimate skin friction of surrounding soil. The comparisons between the predictions to the case history show that the proposed simplified method can be used to predict the pile running condition.