Field Study on the Behavior of Destructive and Non-Destructive Piles Under Compression

This paper presents a series of full-scale load tests on long bored piles instrumented with strain gauges along the shafts, including eight field tests of piles loaded to failure and one non-destructive pile load test. The load-displacement response, skin friction, end resistance, and the threshold of the pile-soil relative displacement for fully mobilizing skin resistance were discussed. A simple softening model was proposed to describe the degradation behavior of the skin friction along the pile-soil interface and the load-displacement relationship developed at the pile base. It is found that the shaft resistance degradation investigated in the non-destructive load test only occurs at a shallow depth, and the skin friction of deeper soil is not fully developed. However, unlike the results of the non-destructive load tests, the softening is accompanied by a reduction in skin friction and observed to be along the whole pile depth. The thresholds of pile-soil relative displacement for fully mobilizing skin resistances in different soils have been found to be in the range 0.6% to 2.4% of the pile diameter. Moreover, in practical applications, a bilinear model is assumed to be feasible in analyzing the load-settlement relationship developed at the end of non-destructive pile, whereas the load transmission curve of the soils below the pile base corresponds to a softening model in the field tests of piles loaded to failure.     

Field and Theoretical Analysis on the Response of Destructive Pile Subjected to Tension Load

Most field tests are carried out using working piles for verification purposes in China, and loading tests are terminated before achieving true pile capacity. In this work, two full-scale destructive loading tests on tension piles instrumented with strain gauges were conducted to capture true pile capacity. The load-displacement response, load transfer, and threshold of the pile-soil relative displacement for fully mobilizing skin resistance in the uplift case were discussed. It was found that the shaft resistance degradation is observed to be along the pile depth with a reduction factor of 0.905 to 0.931, and the thresholds of pile-soil relative displacement for fully mobilizing skin resistance of the tension pile in different soils are found to be in the range 0.67% to 1.34% of the pile diameter. Based on the field test results, a simple softening model was proposed to describe the degradation behavior of skin friction along the pile-soil interface. Further study was conducted to assess the influence of the threshold of pile-soil relative displacement for fully mobilizing skin friction and the reduction factor on the skin friction. As to the analysis of the response of single pile subjected to tension load, a highly effective iterative computer program was developed using the proposed skin friction softening model. Comparisons of the load-settlement response for the well-instrumented tests were given to demonstrate the effectiveness and accuracy of the proposed simple method.     

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.     

A Model Test on the Behavior of a Static Drill Rooted Nodular Pile Under Compression

A static drill rooted nodular pile is a new type of composite pile foundation with high bearing capacity, and mud emissions can be largely reduced using the static drill rooted method. This report presents a model test on the behavior of this composite pile in a test box. The load-displacement response, axial force, skin friction, and mobilized base load are discussed in the report; in particular, the force in the cemented soil was investigated based on the measured data. Moreover, the finite element software ABAQUS was used to help investigate this behavior more thoroughly. It was determined that the function of the cemented soil around the pile shaft was different from that at the enlarged pile base; the stress in the cemented soil around the shaft increased suddenly when nearing the pile base; the ultimate skin friction obtained in the model test was larger than that estimated in the field test; and the relative displacement between the precast nodular pile and the cemented soil could be ignored during the loading process, which corresponded to the result of the field test and demonstrated that the nodular pile and cemented soil act as one entity during the loading process.     

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.     

Model experimental research on uplift single pile in calcareous sand of South China Sea

Vertical uplift static loading tests of single model pile were conducted in the in-lab calcareous sand and quartz sand by emulating practical condition of full-size piles in site. The settlement, lateral deflection, axial force, and friction distribution of the pile are analyzed for each physical test. The pile behaviors in calcareous sand and quartz sand are compared. From the test results, it can be found that the pile top displacement of uplift pile in calcareous sand can be divided into two stages: the pile–sand synchronous stage and pile–sand asynchronous (relative displacement) stage. Data from uplift tests show that the heave of calcareous sand around pile top is very small, which is resulted from the mutually restraint of surface particle. The mutual restriction of surface particle leads to “bottleneck effect” and strengthens ultimate side friction of upper pile segment. In addition, the shear dilatancy and particle breakage of calcareous sand lead to the upper harden and the lower soften of side friction, respectively. Cases of calcareous sand and quartz sand show different responses to pile forming methods, which due to the sands’ different characteristics of particle breakage when compressed as well as plastic deformation under loading–unloading conditions.     

Small- and large-scale model tests on the buckling property of slender pile

Slender piles embedded in soft ground or liquefied soil may buckle under vertical load. In this paper, both small- and large-scale model tests are conducted to investigate the buckling mechanisms of a slender pile and the lateral earth pressure acting on the pile. To observe the buckling of a slender pile, the strain-controlled loading method is adopted to apply a vertical load. When the two ends of a slender pile are hinged, the buckling mechanisms of small- and large-scale model tests are same. It should be noted that this applies only to a system with a small ratio of pile bending stiffness to soil bending stiffness. An applied vertical load increases with an increasing pile head settlement until it reaches the critical buckling load. By further increasing the pile head settlement, the measured load approaches the critical buckling load. In the large-scale model test, the measured lateral earth pressure (i.e., active and passive) acting on the slender pile varies linearly with the lateral pile displacement when the measured range is 3–5?m beneath the ground. A critical buckling calculation method has been adopted to compare with the conventional “m” method. The two-sided earth pressure calculation method can achieve more approximate results with the model test.     

海上风电单桩基础土相互作用特性影响因素分析

采用有限元软件ABAQUS建立了海上风电单桩基础与土相互作用数值计算模型,将波浪、洋流及风荷载等效成双向对称循环荷载,研究了水平循环荷载作用下不同因素对桩身水平位移、剪力和弯矩的影响规律。研究表明,随着循环荷载比的增加,桩身位移零点和桩身剪力反弯点沿埋深逐渐下移,桩身弯矩最大值点位于浅层土体;不同荷载频率时桩身位移在零点以上变化较大,桩身弯矩随着频率的增加逐渐增大;单向循环荷载作用下桩身位移最大,双向对称循环荷载作用下桩身位移最小;壁厚较小时对桩身水平位移影响较大;在位移零点之上范围内可以考虑设计"上厚下薄"的钢管桩,以减小桩身水平位移;不同桩壁厚时桩身剪力曲线在埋深约6D处出现交点,且泥面处桩身弯矩变化不明显。     

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;扩大头对其上部的桩侧摩阻力有增强作用,对其下部的桩侧摩阻力有削弱作用;扩大头在工作荷载、极限荷载和破坏荷载作用时分担的荷载比例为15%~20%。     

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.     

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.     

Development of Prebored Screw Pile Method and Evaluation of Its Bearing Characteristics

The pile-driving method produces considerable noise and vibrations. Hence, an auger-drilled pile method was developed as a low-noise and -vibration substitute. However, this method does not guarantee the bearing capacity of the pile unless some amount of pile-driving is performed. Therefore, the noise and vibration problems cannot be completely solved. In this study, a prebored screw pile method is proposed to solve these problems. In this method, piles are constructed by the rotary penetration of a screw pile into a prebored hole filled with some cement milk and whose diameter is smaller than that of the screw pile. To determine the shape of the screw pile, laboratory tests with model screw piles were conducted. Also, field load tests were conducted on an actual screw pile fabricated based on the laboratory test result and on a smooth-surfaced pile. In addition, the behavior of the screw pile was estimated by using three-dimensional finite element analysis. The results of the field load test and the numerical simulation showed that the ultimate bearing capacity and the unit skin friction of the screw pile are very superior to those of the smooth-surfaced pile and the cement milk is an important factor in the prebored screw pile method.     

Research on shaft resistance of rock-socketed piles based on the cavity expansion theory

The shaft resistance of rock-socketed piles depends not only on the properties of its surrounding rock, but also on the radial force induced by the load imposed on the top of the pile. This paper deduced a plastic zone of rock around a single pile and obtained the shaft resistance of a rock-socketed pile based on the theories of cavity expansion and shear strength. The research results showed that the magnitude of the radial force in the socketed portion of a rock-socketed pile was related to the pile diameter, Poisson’s ratio, and properties of the surrounding rock. The influence area of rock decreased with increasing pile diameter. The radial force and lateral friction decreased with the increasing relative stiffnesses of the pile and rock. The radial stress on the elastic–plastic interface can be analytically determined based on the rock properties and depth. A field test was used to validate the proposed method, and a good agreement was obtained between the field data and predicted results of the proposed method. The research results in this paper are beneficial to guide actual practice.     

Influence of cap size and strength on settlements of TDM-piled embankments over soft ground

Abstract

This study investigates the impact of pile cap size, soft layer thickness and pile strength on load transfer and settlement behaviors of embankments supported by floating and fixed T-shaped deep cement mixing piles and conventional DCM piles under volume control. Preliminary investigation is performed by a series of small-scale physical model tests. The results reveal that the differential settlement can be substantially reduced with an enlarging pile cap as a result of larger embankment load transferred to the piles. The extended numerical analysis results demonstrate that the pile efficacy is related to the individual pile bearing capacity, which, in turn, depends on the pile cap size. The soft layer thickness has an insignificant effect on differential settlement but a significant effect on average settlement, while the pile strength plays an important role in differential settlement only when the cap size is not very large. Shape factor of at least 3.0 is recommended to ensure the reduction in differential settlement and minimize the effect of the change in pile strength.     

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.     

钻井船插桩对导管架平台群桩影响的CEL有限元分析

采用CEL大变形非线性有限元方法并结合非线性地基梁模型对海洋石油941钻井船在番禺10-2平台钻井插桩时对邻近导管架平台群桩的影响进行了分析,并得到以下结论:1)钻井船插桩过程中,桩身最大位移及出现的位置随钻井船插桩深度增加而下移且钻井船插桩位置与群桩距离越近,对桩的影响越大;2)在插桩过程中,桩身最大弯矩出现的位置与桩身最大位移出现的位置一致,而桩身最大剪力出现的位置较桩身最大弯矩出现的位置偏下;3)与没有插桩影响的群桩相比,桩身最大弯矩与桩身最大剪力明显增加。     

Modulus of Subgrade Reaction for Laterally Loaded Piles in Multiple Layered Foundation

-According to the field test data of laterally loaded steel piles at home and abroad,an empiri-cal formula for evaluating the modulus of subgrade reaction under static load in layered soil is proposed.The suggested formula takes account of not only non-linearity of K_s with the depth but also its variationwith the load.It is shown that the computed bending moments along the pile shaft and the deflection atthe pile top are consistent with the measured ones.     

The Effect of Stabilizing Piles on a Self-Supported Earth-Retaining Wall

The behavior of a self-supported earth-retaining wall with stabilizing piles was investigated using a numerical study and field tests in urban excavations. Special attention is given to the reduction of lateral earth pressures acting on a retaining wall with stabilizing piles. Field tests at two sites were performed to verify the performance of the instrumented retaining wall with stabilizing piles. A number of 3D numerical analyses were carried out on the self-supported earth-retaining wall with stabilizing piles to assess the results stemming from wide variations of influencing parameters such as the soil condition, the pile spacing, the distance between the front pile and the rear pile, and the embedded depth. Based on the results of the parametric study, the maximum horizontal displacement and the maximum bending moment are significantly decreased when the retaining wall with stabilizing piles is used. In engineering practice, reducing the pile spacing and increasing the distance between the front pile and the rear pile can effectively improve the stability of the self-supported earth-retaining wall with stabilizing piles.     

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。     

Reinforcement and Arching Effect of Geogrid-Reinforced and Pile-Supported Embankment on Marine Soft Ground

The effectiveness of constructing a geogrid-reinforced and pile supported embankment on soft ground to reduce differential settlement has been studied by pilot scale field tests and numerical analysis. Three-by-three pile groups with varying pile spacing were driven into a layer of soft ground, and a layer of geogrid was used as reinforcement over each pile group. Further, a 2-D numerical analysis has been conducted using the computer program FLAC 2D. The mechanisms of load transfer can be considered as a combination of embankment soil arching, geogrid tension, and stress transfer due to the difference in stiffness between pile and soft ground. Based on the pilot scale field tests and results of numerical analysis, we find that the geosynthetic reinforcement slightly interferes with soil arching, and helps reduce differential settlement of the soft ground. Also, the most effective load transfer and vertical stress reduction at the midspan between piles occurs when the pile cap spacing index D/b (D: pile cap spacing, b: diameter of pile) is 3.0.