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。     

Field Investigations of Two Super-long Steel Pipe Piles in Offshore Areas

A research on super-long piles has been primarily based on cast-in-place bored piles. In this article, field tests associated with selected measuring technologies were conducted on two super-long steel pipe piles in offshore areas to investigate the behaviors and performance of super-long steel pipe piles. The strain along the pile shaft was monitored by adopting the Brillouin optical time domain reflection and fiber Bragg grating techniques. Static load tests were also conducted on two test piles to determine the bearing capacities. In addition, the axial forces, relative displacements between piles and soils and pile shaft resistances were calculated based on the measured strain. According to the results of the static load tests, the ultimate bearing capacities of the two test piles are greater than 15,000 and 15,500 kN. Both of these values meet the design requirements. In addition, the two test piles can be treated as pure friction piles, and the load transfer mechanism and relationships between the pile shafts and relative displacements are also discussed. Finally, recommendations for practical engineering and significant conclusions are presented.     

大型钢管桩的动力测试和分析研究

本文介绍了用高应变法对某专用码头大直径钢管桩进行动力量测及ＣＡＰＷＡＰＣ法计算分析结果，提出了单桩极限承载力及该地区停锤标准，供设计部门参考。     

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。     

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.     

码头钢管桩潮差段聚乙烯防腐层

本文介绍了码头钢管桩潮差段底漆、底胶、聚乙烯包覆三层防腐结构的施工工艺及防腐材料的性能指标并简要介绍了防腐钢管桩的使用情况。     

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。     

Influence of large-diameter pipe pile driving on surrounding marine soils and adjacent submarine pipelines

Abstract

With the large-scale development and utilization of ocean resources and space, it is inevitable to encounter existing submarine facilities in pile driving areas, which necessitates a safety assessment. In this article, by referring to a wharf renovation project as a reference, the surrounding soil response and buried pipe deformation during pile driving in a near-shore submarine environment are investigated by three-dimensional (3D) numerical models that consider the pore water effect. Numerical studies are carried out in two different series: one is a case of a single pile focusing on the effect of the minimum plane distance of the pile–pipe, and the other is a case of double piles focusing on the effect of the pile spacing.     

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.     

某外海工程超长大直径钢管桩变形原因简析与加强方案研究

某外海工程在沉设超长大直径钢管桩施工过程中,发生钢管桩在海床泥面附近严重变形的情况。通过对本工程超长大直径钢管桩产生变形的可能性原因进行分析,同时对其变形处局部加强处理方案进行一系列的研究论证,并对加强处理方案进行比选优化后,最后采用一可靠的加强处理方案。为类似工程避免发生此类质量事故和钢管桩局部变形水下加强处理提供参考。     

Premature refusal of large-diameter,deep-penetration piles on an offshore platform

Open-ended pipe piles are commonly driven into the seabed to support offshore platforms. This paper presents a case of practical offshore driven pile installation experiences associating with premature refusal. Pile drivability and capacity are analyzed using sufficient driving records. Dynamic loading tests were performed three months after the driving in order to determine the pile capacity after refusal. The test results are detailed in this paper compared with back analysis of measured pile driving records. Empirical equations are provided to predict soil resistance during driving and after setup according to the driving records and dynamic loading tests. Analyzing this practical engineering case is hoped to lead to a better understanding of pile driving, especially when premature refusal occurs. The sufficient details of the engineering data in this paper are also expected to enrich the engineering experience and literature of offshore piles in offshore engineering.     

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。     

Influence of vacuum preloading on vertical bearing capacities of piles installed on coastal soft soil

Abstract

Land reclamation has increased significantly in the eastern coastal areas of China. The increased exploitation of offshore resources has made cast-in-situ piles more preferable in these regions. However, precise prediction of axial forces and shaft resistances of piles is particularly difficult because geological conditions are complex after the foundation is treated by vacuum preloading. In this study, two groups of cast-in-situ piles, each of which consisted of two piles installed in soft soil in Oufei Project, Wenzhou, China, were compared by conducting tests using the slow static loading method to evaluate the influence of applying vacuum preloading to deal with soft soil foundation on the vertical bearing capacities of the piles. Two piles were located in an untreated area, while the other two were located in a vacuum preloading treating area. All the piles had the same length and diameter. In addition, the axial forces and shaft resistances of piles were calculated based on the measured strains. The field tests revealed that the ultimate bearing capacities and shaft resistances of test piles were significantly improved compared to those of the piles in untreated area. The experimental results presented in this study are expected to be highly beneficial for practical engineering.     

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.     

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.     

Study of the Bearing Capacity at the Variable Cross-Section of A Riser-Surface Casing Composite Pile

Reducing the cost of offshore platform construction is an urgent issue for marginal oilfield development.The offshore oil well structure includes a riser and a surface casing.The riser,surface casing and oil well cement can be considered special variable cross-section piles.Replacing or partially replacing the steel pipe pile foundation with a variable cross-section pile to provide the required bearing capacity for an offshore oil platform can reduce the cost of foundation construction and improve the economic efficiency of production.In this paper,the finite element analysis method is used to investigate the variable cross-section bearing mode of composite piles composed of a riser and a surface casing in saturated clay under a vertical load.The calculation formula of the bearing capacity at the variable section is derived based on the theory of spherical cavity expansion,the influencing factors of the bearing capacity coefficient N_c are revealed,and the calculation method of N_c is proposed.By comparing the calculation results with the results of the centrifuge test,the accuracy and applicability of the calculation method are verified.The results show that the riser composite pile has a rigid core in the soil under the variable cross-section,which increases the bearing capacity at the variable cross-section.     

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.     

Flexural performances of prestressed high strength concrete piles reinforced with hybrid GFRP and steel bars

Abstract

This study developed prestressed high-strength concrete (PHC) piles reinforced with high-strength materials (glass fiber-reinforced polymer (GFRP) bars) for flexural performance enhancement. Flexural strengths and behaviors of PHC piles reinforced with hybrid GFRP and steel bars were experimentally investigated, respectively. Large-scale specimens with total lengths of 12,000?mm and diameters of 600?mm were constructed and tested under bending, accompanied by evaluation of effects of non-prestressed reinforcement type and longitudinal reinforcement ratio. J-factors were calculated to evaluate deformability of all the specimens. PHC piles reinforced with GFRP bars were demonstrated to have much higher flexural capacity than those reinforced with steel bars. Moreover, strains at the midspans of cross sections of all the specimens basically conformed to the assumption of plane section. Failure of PHC piles reinforced with GFRP bars was attributable to gradual concrete crushing, while that of PHC piles reinforced with steel bars resulted from steel yielding. Results of this study were expected to provide theoretical basis for wide engineering applications of PHC piles reinforced with hybrid GFRP bars and steel bars in marine structures.     

辽河三角洲潮道软土及其工程灾害研究

辽河三角洲潮道沉积物为软弱淤泥质土，管桩在波流荷载作用下的弹性变形或振动，可以导致潮道软土强度衰减并发生变形位移，随着悬臂段加长和波流荷载振动作用，下部扮土会自上而下发生液化。简单管桩的抗倾验算可以按照波流力与被动土压力的作用力矩分析，并考虑到海底的冲刷淘空深度。管桩打入下卧高强度粉土时，在保证打入深度的同时，应适当增大管桩的强度，加大管桩自振周期与波浪周期的差值。