海上风电桩–筒复合基础承载性能研究

基于有限元软件ABAQUS平台,建立了非匀质饱和黏土场地的海上风电桩–筒复合基础数值计算模型,对比研究竖向荷载V、水平荷载H和弯矩荷载M作用下不同筒结构尺寸的桩–筒复合基础的承载力系数,并采用正交试验法开展桩–筒复合基础的各向承载性能的影响因素研究.结果表明,饱和黏土的非匀质特性系数K对竖向承载力系数N_cV影响较小;K对水平承载力系数N_cH和抗弯承载力系数N_cM的影响呈指数型递减.筒结构直径D和入土深度L对各向承载力系数的影响存在交互作用.D对桩–筒复合基础承载力系数的影响最大,可以通过增加筒结构直径从而有效地提高桩–筒复合基础的承载性能.研究结果为海上风电桩–筒复合基础的设计提供了依据.     

水平荷载作用下桩基础与桩筏基础承载性状数值模拟对比

运用有限元软件ABAQUS建立桩基础与桩筏基础模型,通过控制桩长、桩数、竖向荷载及桩间距,对其施加水平荷载进行全过程模拟,分析受荷过程中桩体水平位移变化、桩身弯矩和剪力变化及土体变形规律。模拟结果表明：相同水平荷载作用下,桩基础的桩体水平位移随荷载增大而增大,以桩底为旋转中心发生破坏;桩筏基础由于筏板的作用,倾斜非常小,群桩较单桩有更好的稳定性。桩基础的桩周土塑性区域明显大于桩筏基础,二者在荷载作用下土体均会出现土拱效应,但桩基础的土拱效应更明显;桩基础的桩底会出现塑性区域,桩筏基础几乎不会出现,桩数的增加会减小塑性区域范围。桩基础的最大弯矩均出现于0.8倍桩身处,桩筏基础的最大弯矩出现在0.2倍桩身处。桩基础和桩筏基础最大正剪力出现在桩顶和桩底附近,最大负剪力出现在桩身中部。桩筏基础弯矩值和剪力值都小于桩基础,因此桩筏基础具有更好的抗弯、抗剪能力。     

大直径钻孔嵌岩灌注桩竖向承载力的估算

通过桩土体系荷载传递机理分析,认为按端承桩公式估算桩竖向承载力而忽略桩侧阻力的作用与工程实际情况误差较大,为此提出设计中估算单桩竖向承载力的几点建议。     

土塞效应对预应力高强混凝土管桩承载力的影响

为了分析土塞效应对预应力高强混凝土(PHC)管桩单桩竖向承载力的影响程度,利用FLAC3D软件对不同地质条件下的开口桩和闭口桩进行数值模拟,得出预应力高强混凝土管桩的荷载-沉降曲线(Q-S曲线),研究表明:对于端承桩,土塞效应导致单桩竖向承载力降低,而对于摩擦型桩,可不考虑土塞效应的影响,按闭口桩计算PHC管桩的单桩竖向承载力。     

非均质土中海上风电单桩基础动力响应特性

采用有限元分析软件ABAQUS建立了非均质土中海上风电单桩基础数值计算模型,将桩基础受到的波浪、洋流及风荷载等效成双向对称循环荷载,对水平循环荷载作用下桩身水平位移、桩身剪力、桩身弯矩和桩侧土抗力进行了研究,并对不同循环次数下桩身水平位移进行了对比分析。研究表明,桩身水平位移随时间变化逐渐累积,随着循环次数的增加,泥面处桩身最大位移发生的时间点滞后;桩身剪力出现负值;桩身弯矩最大值发生在浅层土体;桩身外壁土抗力曲线随时间的变化在埋深约2/3处出现分界点,分界点上下范围内土抗力变化规律正好相反,在淤泥土和粉砂土分界面处增加显著;不同时间点桩身内壁沿埋深承担的荷载基本不变。      

岩土工程勘察中桩基的选型分析

岩土勘察是一项专业性极强且复杂的工程,岩土工程的桩基选型结果关系到岩土工程的质量,为此提出岩土工程勘察中桩基的选型分析。分析桩基选型需要确定施工技术条件与环境,计算得出桩基荷载,得知桩基需要承载的荷载,静载试验确定桩基,需要先根据桩基需要承载的荷载,计算桩基的长度和直径,只有通过了单桩竖向静载荷试验,才能选择计算所得出的桩基的规格。     

不同桩体材料复合地基承载及变形性状对比试验研究

通过室内模型试验,实测得到碎石桩、夯实水泥土桩和CFG桩复合地基桩土荷载分担比、桩土应力比和桩间土深层变形,并对三类不同桩体材料复合地基的承载及变形性状进行了对比分析.认为碎石桩复合地基和夯实水泥土桩复合地基均存在有效桩长或有效复合土层厚度;碎石桩桩长超过有效桩长,对提高复合地基承载力和压缩模量、减小变形效果不明显,除一些特别情况如为处理可液化地基外,设计桩长可适当超过有效桩长,但不宜过长;夯实水泥土桩复合地基的有效桩长与桩身强度相关性显著,应以桩身强度控制进行夯实水泥土桩桩体设计,使按桩身强度确定的单桩承载力大于或等于由桩周土及桩端土的抗力所提供的单桩承载力;CFG桩复合地基桩身强度高,桩体自身压缩性小,可全桩长发挥侧阻作用,当桩端落在好的持力层时,能很好地发挥端阻,提高承载力,减小变形,设计时应优先选择好的桩端持力层进行设计.      

桩基础的探讨

桩基础是高层建筑常采用的基础形式,其设计的合理性、经济性关系到整个建筑的安全和造价,文章通过工程实例分析用抗压静载试验确定单桩承载力及合理选择桩型、桩长在桩基础设计中的重要性并针对施工中的一些问题提出相应的解决方案.     

饱和均质土层中单桩沉降特性研究

单桩沉降计算理论是桩基设计的理论基础。迄今为此,静荷载作用下的单桩沉降特性研究在国内外都取得了众多硕果,但就基岩埋置深度对单桩沉降的影响研究不多。在这一背景下,本文将桩基纵向振动中桩土相互作用的分析理论运用到分析静荷载下的单桩沉降特性研究,并提出虚土桩法来进行单桩沉降分析,着重分析了基岩埋置深度对单桩沉降变化的影响。     

成孔工艺对灌注桩承载力影响的探讨

文章通过分析桩土体系的荷载传递机理,以及基桩侧阻力、端阻力的成孔工艺影响因素,对目前常用成孔方法对基桩承载力的影响进行了探讨,提出了增加单桩承载力的有效措施.     

Experimental Investigations of the Response of Suction Caissons to Transient Combined Loading

Combined loading of foundations is a fundamental problem in civil engineering, particularly in the offshore industry where harsh environmental conditions occur. Large moment and horizontal loads may be applied to the foundation as well as vertical loads. Also, as the waves pass a structure, there can be rapid changes in the loads, so that transient effects need to be considered. When designing shallow foundations, such as suction caissons, there is uncertainty in the current understanding of how the foundation responds to these loads. This paper presents experiments, performed on model suction caisson foundations, where typical cyclic loading conditions are applied. The footing is embedded in oil-saturated sand so that dimensionless drainage times are comparable with the typical offshore conditions. Most of the testing was carried out with the vertical load held constant, to mimic the structural dead weight, while realistic “pseudorandom” moment and horizontal cyclic loads were applied. Experiments were carried out at different vertical loads, showing that the response depends on the vertical load level. Nondimensional relationships were established which accounted for this dependency. Surprisingly, the rate of loading had little impact on the load–displacement behavior for the experiments undertaken.     

Lateral Resistance of Short Single Piles and Pile Groups Located Near Slopes

Many transmission towers, high-rise buildings, and bridges are constructed near steep slopes and are supported by large-diameter piles. These structures may be subjected to large lateral loads, such as violent winds and earthquakes. Widely used types of foundations for these structures are pier foundations, which have large diameter with high stiffness. The behavior of a pier foundation subjected to lateral loads is similar to that of a short rigid pile, because both elements seem to fail by rotation developing passive resistance on opposite faces above and below the rotation point, unlike the behavior of a long flexible pile. This paper describes the results of several numerical studies performed with a three-dimensional finite-element method (FEM) of model tests and a prototype test of a laterally loaded short pile and pier foundation located near slopes, respectively. Initially, in this paper, the results of model tests of single piles and pile groups subjected to lateral loading, in homogeneous sand with 30° slopes and horizontal ground were analyzed by the three- dimensional (3D) finite-element (FE) analyses. Furthermore, field tests of a prototype pier foundation subjected to lateral loading on a 30° slope was reported. The FE analyses were conducted to simulate these results. The main purpose of this paper is the validation of the 3D elasto–plastic FEM by comparisons with the experimental data.     

Contact Interface Model for Shallow Foundations Subjected to Combined Cyclic Loading

It has been recognized that the ductility demands on a superstructure might be reduced by allowing rocking behavior and mobilization of the ultimate capacity of shallow foundations during seismic loading. However, the absence of practical reliable foundation modeling techniques to accurately design foundations with the desired capacity and energy dissipation characteristics and concerns about permanent deformations have hindered the use of nonlinear soil–foundation–structure interaction as a designed mechanism for improving performance of structural systems. This paper presents a new “contact interface model” that has been developed to provide nonlinear relations between cyclic loads and displacements of the footing–soil system during combined cyclic loading (vertical, shear, and moment). The rigid footing and the soil beneath the footing in the zone of influence, considered as a macroelement, are modeled by keeping track of the geometry of the soil surface beneath the footing, along with the kinematics of the footing–soil system, interaction diagrams in vertical, shear, and moment space, and the introduction of a parameter, critical contact area ratio (A/Ac); the ratio of footing area (A) to the footing contact area required to support vertical and shear loads (Ac). Several contact interface model simulations were carried out and the model simulations are compared with centrifuge model test results. Using only six user-defined model input parameters, the contact interface model is capable of capturing the essential features (load capacities, stiffness degradation, energy dissipation, and deformations) of shallow foundations subjected to combined cyclic loading.     

Horizontal Bearing Capacity of Piles in a Lateritic Soil

The behavior of pile foundations subjected to horizontal loading is typically evaluated using horizontal load tests. Although load tests are valuable to understand site-specific soil-structure interaction phenomena, validated predictive methods are also useful during the design phase. In this study, the results from horizontal load tests are compared with methods which predict the horizontal bearing capacity of piles using in situ measurements of soil behavior. Specifically, several horizontal load tests were performed in order to evaluate the behavior of two 12-m long Strauss piles and four bored piles with similar length, all installed in a lateritic soil profile. Two prediction methods were evaluated using p-y curves computed from the results of Marchetti’s dilatometer test (DMT) results. The predictive methods using the p-y curves from the DMT showed good agreement with the behavior observed in the pile loading test.     

Centrifuge Modeling of Large-Diameter Bored Pile Groups with Defects

In this research, centrifuge model pile-load tests were carried out to failure to investigate the behavior of large-diameter bored pile groups with defects. The model piles represented cast-in-place concrete piles 2.0?m in diameter and 15?m in length. Two series of static loading tests were performed. The first series of tests simulated the performance of a pile founded on rock and a pile with a soft toe. The second series of tests simulated the performance of three 2×2 pile groups: One reference group without defects, one group containing soft toes, and one group with two shorter piles not founded on rock. The presence of soft toes and shorter piles in the defective pile groups considerably reduced the pile group stiffness and capacity. As the defective piles were less stiff than the piles without defects, the settlements of the individual piles in the two defective pile groups were different. As a result, the applied load was largely shared by the piles without defects, and the defective pile groups tilted significantly. The rotation of the defective pile groups caused large bending moments to develop in the group piles and the pile caps. When the applied load was large, bending failure mechanisms were induced even though the applied load was vertical and concentric. The test results confirm findings from numerical analyses in the literature.     

Pullout Behavior of Granular Pile-Anchors in Expansive Clay Beds In Situ

Granular pile anchors (GPA) are one of the recent innovative foundation techniques devised for mitigating the problems posed by swelling clay beds. In a granular pile anchor, the footing is anchored to an anchor plate at the bottom of the granular pile. This makes the granular pile tension resistant and enables it to absorb the tensile force caused on the foundation by the swelling clay. An understanding of the amount of uplift resistance offered by the GPA is important in the design of granular pile-anchor foundations in field situations causing tensile forces on foundations, such as in expansive clay beds. This paper presents the results of a field-scale test program conducted to study the pullout response of GPAs embedded in expansive clay beds. Pullout load tests were conducted on GPAs of varying lengths and diameters. It was found from the field pullout load tests that granular pile anchors of larger surface area resulted in higher pullout capacity. Of the various single granular pile anchors with l/d values between 2.5 and 10, the GPA of length 1000?mm and diameter 200?mm (l/d = 5) showed the best pullout load response when tested alone, resulting in a failure uplift capacity of 14.71?kN. Increase in diameter and length of granular pile anchor increased the uplift capacity. When the length of the GPA was increased from 500 to 750 and 1000?mm, the percentage increase in the uplift load required for an upward movement of 25?mm was 33.3 and 55.5% respectively. The pullout load of the GPA when tested under group was 18?kN as against a 12?kN for the GPA when tested single.     

Response of Laterally Loaded Large-Diameter Bored Pile Groups

This paper presents results of full-scale lateral load tests of one single pile and three pile groups in Hong Kong. The test piles, which are embedded in superficial deposits and decomposed rocks, are 1.5 m in diameter and approximately 30 m long. The large-diameter bored pile groups consist of one two-pile group at 6 D (D = pile diameter) spacing and one two-pile and one three-pile group at 3 D spacing. This paper aims to investigate the nonlinear response of laterally loaded large-diameter bored pile groups and to study design parameters for large-diameter bored piles associated with the p-y method using a 3 D finite-element program, FLPIER. Predictions using soil parameters based on published correlations and back-analysis of the single-pile load test are compared. It is found that a simple hyperbolic representation of load-deflection curves provides an objective means to determine ultimate lateral load capacity, which is comparable with the calculated values based on Broms' theory. Lateral deflections of bored pile groups predicted using the values of the constant of horizontal subgrade reaction, suggested by Elson and obtained from back-analysis of the single pile load test, are generally in good agreement with the measurements, especially at low loads.