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.     

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。     

p-y曲线对成层土体中大直径单桩的适用性研究

API规范推荐的p-y曲线是由均质土体得到的,并未考虑土层间相互作用,Georgiadis基于柔性桩提出了等效深度法修正p-y曲线,把均质土p-y曲线延伸到了成层土体中。为了研究p-y曲线和等效深度法对于大直径单桩在成层土体中的适用性,取4种典型地质条件:成层砂土、砂土-黏土-砂土、成层黏土和黏土-砂土-黏土,通过L-PILE软件计算了6 m直径单桩基础的p-y曲线、桩顶水平荷载-位移曲线、桩身位移和弯矩。并与ABAQUS建立的单桩基础三维有限元模型计算结果进行比较。结果表明等效深度法对于成层砂土影响不大;对于成层黏土影响较大;对于中间为软弱土层的成层土体,在荷载较大时影响显著,等效深度法计算结果更加接近FEM结果。在成层土体中,p-y曲线应用于大直径单桩对于砂土高估了初始刚度,低估了极限抗力;对于黏土则低估了初始刚度和极限抗力。     

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.     

淤泥质海岸螺旋桩的轴向承载特性研究

海上风电高压海缆登陆后往往需要跨越平原淤泥质海岸,高压电缆排管易发生不均匀沉降而裂损,螺旋桩因施工方便、速度快,可用于不均匀沉降的应急控制。螺旋桩的承载力是其工程应用的重要设计内容,其桩型参数的改变,对极限抗压承载力有着较大影响。本文结合工程实践,通过开展抗压承载力数值模拟,研究分析了螺旋桩叶片宽度、叶片个数、叶片间距...     

弱超固结黏土中桩靴贯入形成孔洞对承载力影响

自升式平台作业前需对桩靴基础进行预压安装,使桩靴具备抵抗竖向-水平-弯矩复合荷载的能力。安装过程中,桩靴上部将形成一定深度的孔洞。弱超固结黏土地基中,土体强度较高,桩靴最终贯入深度较浅,而形成的上部孔洞较深,因此孔洞将对桩靴就位后的承载力产生影响。通过有限元分析,研究弱超固结黏土中桩靴上部孔洞对承载力的影响,结果表明:1)与无孔洞的情况相比,孔洞的存在对桩靴的单向和复合承载力有削弱作用; 2)当桩靴与孔洞底部距离大于桩靴直径时,承载力不再受上部孔洞的影响; 3)当桩靴埋深小于等于0.75倍桩靴直径时,无论桩靴上部有无孔洞,现有预测公式都不能较为合理地预测弱超固结黏土地基的复合承载力,为此提出了考虑孔洞影响的桩靴复合承载力包络面预测公式。     

多层砂土地基扩底桩单桩抗压模型试验及颗粒流模拟研究

利用室内半模试验和颗粒流数值模拟,揭示多层砂土地基扩底桩单桩抗压承载特性及变形特征。结果表明,通过对比分析极限承载力与H_h/D(持力层厚度与扩大头直径之比)的关系可以看出,单桩的抗压极限承载力随H_h/D逐渐增加,当H_h/D超过2.0时,极限承载力基本不再增加,此时的单桩抗压极限承载力稳定在300.01~303.25 N,是H_h/D=0.5时极限承载力(183.83 N)的1.65倍。扩大头下部土体发生局部压缩-剪切破坏,破坏面从扩大头底面边缘向斜下方扩展,在水平方向影响范围达到最大后逐渐向桩内侧收缩;荷载作用越大,地基破坏区域越大,相应的极限抗压承载力也越大;持力层厚度增加,扩大头分担的荷载比例增大,分担的荷载达到稳定需要的桩顶位移也越大,H_h=0.5 D试验扩大头分担的荷载比例稳定时为60%,对应的桩顶位移约为29 mm;桩顶位移达到33 mm后,H_h=1.0~3.0 D试验稳定在63%~65%之间;通过细观颗粒流理论对砂土移动特性的研究发现,持力层厚度从0.5 D增大至2.0 D,破坏面的起始扩展角度从31°增大至42°。数值模拟研究结果与模型试验数据吻合效果良好,证明该方法分析多层砂土地基扩底桩单桩抗压荷载传递机理是可行的。     

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。     

桩-筒组合基础在单层黏土中水平承载性能分析

桩-筒组合基础是将单桩基础与筒型基础组合的一种新型海上风电基础形式,其受力模式不同于传统桩基基础,优点是可通过合理减小桩长、桩壁厚等途径提高承载性能。基于极限地基反力法,提出单层黏土中桩-筒组合基础受力模式及水平承载力的计算方法。利用力加载和位移加载两种控制方法进行数值分析,研究水平承载性能的影响因素。结果表明,在一定范围内,桩-筒组合基础水平承载性能随桩入土深度、桩壁厚、筒外径的增大而提高。基于实际工程,对等用钢量的单桩基础与组合基础进行比较计算,结果表明,桩-筒组合基础可有效降低基础倾斜度和位移,提高承载性能。     

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。     

Postgrouted helical piles behavior through physical modeling by FCV

Piling procedure may disturb the surrounding soil, due to the installation particularly for cast-in-place piles. It causes a reduction in the soil strength parameters and, consequently, pile capacity. To overcome shortcomings and also for improving piles’ capacity, postgrouting as a compensation method is recognized and more developed in recent years. Helical piles, those are used widely in marine and land projects, although, are driven by torque implementation, but soil disturbance is noticed, where number of the helices become up to 3 and more. In this paper, an experimental study program is performed by frustum-confined vessel (FCV) to investigate bearing capacity of model helical piles and also postgrouted cases’ performance. FCV has been used because of its linear distribution of vertical and horizontal stresses from zero at top to maximum at bottom which simulates real field stress conditions. Through experimental study, small-scale helical model piles were made of 4-mm-thick steel plate and have been used with a length of 750?mm. The shaft and helix diameters of model piles have been 32 and 89?mm, respectively. So, the helix-to-shaft ratio (wing ratio) was about 2.8. The helical model piles installed in fine-grained sand as a surrounding soil and then axial loading tests before and after grouting were performed to achieve ultimate pile capacity. Results indicated postgrouting can improve both ratios of toe and frictional soil–pile interactions including upgrading β and N_t factors. In addition, the post grouting phenomena can change the pile geometry due to treated soil bond, resulting better functioning. Therefore, it is a proper method to improve helical piles performance and compensate installation effects in capacity mobilization.     

Field Evaluation of the Vertical Bearing Capacity of a Screw Pretensioned Spun High-Strength Concrete Pile

This study has evaluated the vertical bearing capacity by conducting static load tests for noise-free and vibration-free screw pretensioned spun high-strength concrete (PHC) piles installed using two different methods (end-squirting shoe and pre-boring methods). Vertical bearing capacity differences seem to occur due to the displacement of soils near the external circumference of a pile, depending on the installation method. A method by which to evaluate the bearing capacity of screw concrete piles is suggested by considering the equations that already have been used to calculate the bearing capacity of piles. Based on static load tests and analysis, the pile installed using the end-squirting shoe method was assumed to be a bored pile and it was reasonable to use the equation proposed by the Japanese Geotechnical Society. At the same time, the pile installed using the pre-boring method was deemed a low soil displacement pile and so it was reasonable to apply the equations proposed for calculating the bearing capacity of the driven pile suggested by the Architectural Institute of Japan.     

Self-expanded piles: A new approach to unconventional piles development

Abstract

In onshore and offshore fields of ocean engineering, piles are used as foundation systems for various structures. Piles are classified into different types depending on their materials, geometries, and particularly, installation methods, which have advantages or limitations. Companies and engineers have developed a new group of piles, because of necessity to improve their performance in terms of increasing the bearing capacity, reducing impacts of traditional installation procedures, implementing by low- torque power equipment, and utilizing them in widely different ground conditions, including in a marine environment. In the present study, three different models of a new pile with an expander body are introduced to increase the shaft and pile-toe diameters and its self-expansion in the embedment depth under the titles of the Bubble pile (BP), Self-Expanded pile (SEP) and Wing pile (WP). The main subject of this research is to achieve increased bearing capacity, reduced installation effects, and decreased required installation torque. The frustum-confining vessel of Amirkabir University of Technology (FCV-AUT) was employed for this purpose. Up to 14 axial compressive and tensile load tests were carried out on different model piles on sand collected from Anzali shore located on the northern coast of Caspian Sea in Iran, with relative densities of 45% to 50% within FCV-AUT. Comparing the performance of introduced pile with traditional pile corresponding to the same characteristics, the results indicated a significant increase in the axial bearing capacity and reduced disturbance effect of the pile. Also, a lower installation torque of the SE pile was required compared to the helical pile. The test results also demonstrated that the new pile could resist considerable compressive and uplift loads, and could be a possible alternative to traditional piles in the onshore sector.     

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.     

海上风电嵌岩桩水平抗力影响因素研究

桩基础是我国海上风电工程中应用最为广泛的基础形式,其中嵌岩桩因其施工难度大,承载力高备受关注。与其他类型的桩基础不同,嵌岩桩的水平承载力不仅受到围岩强度的影响,更与其成桩质量与灌浆材料的强度相关。采用有限元方法分析了嵌岩深度、桩基直径与壁厚、桩身倾斜度等多种因素对嵌岩桩水平承载力的影响,提出了确定嵌岩桩水平极限抗力的标准。研究表明：桩与围岩间的灌浆环会先于桩身发生破坏,因此可将灌浆环受拉破坏作为判断嵌岩桩达到水平极限承载力的标准;桩身倾斜度对嵌岩桩的水平极限承载力影响较大,直径和壁厚的增加,均能提高桩基的水平承载力。     

Performance of small group of geosynthetic-reinforced granular piles

Granular piles are frequently used as a method of improving soft grounds as they provide increased bearing capacity and reduce foundation settlements. However, in very soft clayey soils, they may not derive their load-carrying capacity by low confining pressure provided by the surrounding soil. In such circumstances, granular piles may be reinforced with suitable geosynthetic to increase its load-carrying capacity and to reduce excessive bulging. In this study, the performance of small group of geosynthetic-reinforced granular piles (GRGPs) is examined in terms of load-carrying capacity, settlement, and modulus by laboratory model tests. The parameters investigated include modulus of reinforcement material, area replacement ratio (ARR) based on the column diameter and reinforcement length. The results indicated that increasing the modulus of the reinforcement and the ARR based on the column diameter enhances the overall performance of the GRGP group. It was also observed that reinforcement on top portion of the granular pile is sufficient to substantially increase the load-carrying capacity of granular pile group.     

Numerical analysis of bearing capacity of drilled displacement piles with a screw-shaped shaft in sand

Drilled displacement (DD) piles with a screw-shaped shaft (referred to as DD piles) are installed using a continuous full thread hollow rod (without a displacement body) inserted and advanced in the soil by both a vertical force and a torque. As a type of newly developed pile, current understanding of the bearing mechanism of DD piles is unsatisfactory, which restricts their further applications in engineering. The primary aim of this paper is to study the bearing mechanism of this type of pile using a numerical method. First, a numerical model for calculating the bearing capacity of the DD piles was created and validated by a laboratory test. Then, the effects of the parameters of pile–soil interface, soil strength, and pile geometrical parameters on the bearing mechanism of the DD piles were investigated in parametric studies. The results of parametric studies show that the limit shear stress on the pile–soil interface, the friction angle of surrounding sand, screw pitch, and thread width significantly influence the bearing capacity of the DD piles, whereas the friction coefficient at the pile–soil interface and the thread thickness have little effect. Based on the results of the parametric studies, the failure mechanism of the DD piles under vertical load is analyzed. Finally, an equation for predicting the ultimate bearing capacities of helical piles based on cylindrical shear failure was used for estimating the bearing capacity of the DD piles, and the calculated results were verified with the numerical results.     

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.     

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.