Vertical bearing capacity of pile based on load transfer model

The load transfer analytical method is applied to study the bearing mechanism of piles with vertical load in this paper. According to the different hardening rules of soil or rock around the pile shaft, such as work-softening, ideal elasto-plastic and work-hardening, a universal tri-linear load transfer model is suggested for the development of side and tip resistance by various types of soil (rock) with the consideration of sediment at the bottom of the pile. Based on the model, a formula is derived for the relationship between the settlement and load on the pile top to determine the vertical bearing capacity, taking into account such factors as the characteristics of the stratum, the side resistance along the shaft, and tip resistance under the pile tip. A close agreement of the calculated results with the measured data from a field test pile lends confidence to the future application of the present approach in engineering practice.

Finite element analysis of effect of soil displacement on bearing capacity of single friction pile

Effect of soil displacement on friction single pile in the cases of tunneling,surcharge load and uniform soil movement was discussed in details with finite element method.Lateral displacement of the pile caused by soil displacement reached about 90% of the total displacement,which means that P-Δ effect of axial load can be neglected.The maximum moment of pile decreased from 159 kN·m to 133 kN·m in the case of surcharge load when the axial load increased from 0 to the ultimate load.When deformation of pile caused by soil displacement is large,axial load applied on pile-head plays the role of reducing the maximum bending moment in concrete pile to some extent.When pile is on one side of the tunnel,soil displacements around the pile are all alike,which means that the soil pressures around the pile do not decrease during tunneling.Therefore,Q-s curve of the pile affected by tunneling is very close to that of pile in static loading test.Bearing capacities of piles influenced by surcharge load and uniform soil movement are 2480 kN and 2630 kN,respectively,which are a little greater than that of the pile in static loading test(2400 kN).Soil pressures along pile increase due to surcharge load and uniform soil movement,and so do the shaft resistances along pile,as a result,when rebars in concrete piles are enough,bearing capacity of pile affected by soil displacement increases compared with that of pile in static loading test.

Evaluation of ultimate bearing capacity of Y-shaped vibro-pile

Based on Mindlin stress solution, a numerical computational method was proposed to calculate the stresses in the ground induced by side friction and the resistance of Y-shaped vibro-pile. The improved Terzaghi's and ЪерезанцевВГ's methods for ultimate bearing capacity evaluation were proposed by considering the stress strength induced by friction resistance at pile head level of Y-pile. A new method to calculate the ultimate bearing capacity of Y-pile was also proposed based on the assumptions of soil failure mode at the tip of Y-pile and the use of Mohr-Coulomb soil yield criterion and Vesic compressive correction coefficient with the induced stresses in the ground. Based on the comparisons with the field static load test results, it is found that the improved Terzaghi's method gives higher ultimate capacity, while the other two methods shows good agreement with the field results.

Bearing capacity and mechanical behavior of CFG pile composite foundation

CFG pile (i.e., pile constructed by granular materials of cement, fly-ash and gravel) composite foundation is applied in subsoil treatment widely and successfully. In order to have a further study of this kind of subsoil treatment technology, the influencing factors and calculation methods of the vertical bearing capacity of single CFG pile and the CFG pile composite foundation were discussed respectively. And based on the obtained solutions, effects by the cushion and measurements to reduce negative friction area were analyzed. Moreover, the developing law of settlement and bearing capacity eigenvalue controlled by the material strength with the increase of load were given for the CFG composite foundation. The in-situ static load test was tested for CFG pile. The results of test show that the maximum test load or half of the ultimate load is used from all the points of test, the average bearing capacity eigenvalue of single pile is 390 kN, and slightly greater than the design value of bearing capacity. The bearing capacity eigenvalues of composite foundation for 3 piles are greater than 300 kPa, and the mechanical properties of CFG pile composite foundation are almost identical in the case of the same load and cushion thickness. The pile-soil stress ratio and the load-sharing ratio can be adjusted through setting up cushion thickness.

Bearing Behaviors of Stiffened Deep Cement Mixed Pile

A series of investigations were conducted to study the bearing capacity and load transfer mechanism of stiffened deep cement mixed （SDCM） pile. Laboratory tests including six specimens were conducted to investigate the frictional resistance between the concrete core and the cementsoil. Two model piles and twenty-four full-scale piles were tested to examine the bearing behavior of single pile. Laboratory and model tests results indicate that the cohesive strength is large enough to ensure the interaction between core pile and the outer cement-soil. The full-scale test results show that the SDCM piles exhibit similar bearing behavior to bored and cast-in-place concrete piles. In general, with the rational composite structure the SDCM piles can transmit the applied load effectively, and due to the addition of the stiffer core, the SDCM piles possess high bearing capacity. Based on the findings of these experimental investigations and theoretical analysi , a practical design method is developed to predict the vertical bearing capacity of SDCM pile.

A Reliable Method of Pile Load Test on Cast-in-Situ Pile Group of Existing Building for Retrofitting

Following the foundation failure of a building, with an aim of economical solution to strengthen other existing buildings of the same project, a new arrangement was implemented experimentally to test the adequacy of load bearing capacity of a few selected cast-in-situ RCC （reinforced cement concrete） pile groups without demolishing the existing buildings. In this test, the column bottom of an existing building was removed by the help of scaffolding and after that a frame system consisting tension piles and hollow beam was constructed over the pile cap of the to be tested pile group. The load was tested by the help of hydraulic jack system and the constructed frame system. This paper contains the detailed plan, arrangement and method of the test with illustrations. The deflection and loading data analysis is also included which was performed to determine the outcome of the test. Through this test method the appropriate assessment of capacity of pile group of existing building could be done successfully and in result the structure could be saved by only super structure retrofitting.

New Method for Prediction Pile Capacity Executed by Continuous Flight Auger (CFA)

A study of piles is quit complex and the estimation of carrying capacity is calculated from theoretical formula and load test results. The design resistance may be calculated using conventional static pile design theory. The pile founding depths should be predetermined before installation from a site geotechnical investigation. To ascertain the field performance and estimate load carrying capacities of piles, in-situ pile load tests should be conducted. In this study, field pile load test data is analyzed to estimate the ultimate load for end bearing piles. The investigated site is about 100 × 110 m located in Alexandria, Egypt. Geotechnical investigations at the site are carried out to a maximum depth of 45 m. Four borings have been done in field. The tests are conducted at the site for two skelton structure buildings to be constructed on raft foundation rested on piles executed by continuous flight auger. Four pile load tests are performed on 600 mmdiameters and 27 mlengths. Ultimate capacities of piles are determined according to different methods. It is concluded that the percentage of friction load carried by the shaft along the pile length is about 46% of total load while the percentage of load carried by the end bearing is 54% of total load. A new proposed method by the author is presented to calculate the ultimate capacity of pile from pile load test. The proposed method depends on the settlement of pile without taken into consideration the elastic deformation. An empirical formula is presented from the relationship between stress and settlement of pile due to friction and end bearing only after deducting the elastic deformation. However, the obtained results for the ultimate capacity of end bearing piles are considered to be more accurate than other methods. The proposed method appears to give bitter results that agrees well with the theoretical predictions. The proposed method is easier, quicker and more reliable.

上砂下黏地层中桩-筒复合基础V-H承载特性

海上风机基础不仅受自重等竖向力V作用,也因水流、波浪和风等影响而承受水平荷载H。为探讨上砂下黏地层中一种由单桩和吸力筒组成的新型海上风机桩-筒复合基础受V-H组合作用时的承载特性,自主设计完成了一系列室内桩-筒复合基础V-H组合加载模型试验,获得不同组合参数下桩-筒复合基础的荷载-位移曲线和桩身弯矩分布曲线,并绘制出V-H承载力包络线。在此基础上,采用ABAQUS建立了上砂下黏地层中桩-筒复合基础三维数值分析模型,经模型验证与参数分析,进一步讨论了砂土厚度、筒径、筒高以及加载高度等参数对桩-筒复合基础承载特性的影响曲线,并拟合出桩-筒复合基础承载力简化计算公式,分析结果表明:桩-筒复合基础能显著提高桩身水平承载力,增幅达30%~90%,且增加筒径比增加筒高更有利于提高基础水平承载力;上部砂土层较厚时,桩-筒复合基础存在一个使复合基础水平承载力达到最大的预加竖向荷载最佳值,其值随不同载荷工况在(0.4~0.7)Vult范围内变化。

波浪荷载下海上风力机桩基础滞回效应及水平承载力变化特性研究

该研究以海上风电桩基础为研究对象,采用自主设计的水平加载装置模拟波浪荷载,结合粒子图像测速(PIV)技术开展缩尺模型试验。着重探究不同幅值的波浪荷载加载过程中,桩周土体的循环变形特征及加载前后,海上风力机桩基础水平承载能力变化。试验结果表明:桩顶累积转角随加载次数呈线性增长、缓慢增长和稳定发展的三阶段变化特性;桩周土体呈现典型的刚性桩两区域破坏形式,桩右被动区土体位移矢量场和位移云图影响范围受加载幅值变化影响显著;桩顶循环加载荷载位移曲线表现出明显的滞回性,滞回圈随加载次数右移并产生累积位移。伴随着滞回圈面积减小和割线刚度增大,桩周土体由以塑性变形向弹性变形为主导变化;循环加载后的滞回圈有明显捏缩,耗能能力减弱。桩基础水平承载能力及弹性变形能力较加载前均明显提高。

卷积神经网络与随机场分析桩梁基础承载力

岩土体的参数在空间上随机分布,为能更好地反应实际工程地质条件,在桩基础承载力研究中考虑土体的不确定性,并建立具有重要工程价值的承载力预测模型,将基于随机场理论的岩土参数空间变异性引入桩梁基础的研究中,采用数值方法建立桩梁基础与群桩基础的二维随机有限元模型分析承载能力,并与模型试验结果验证。随后通过卷积神经网络建立土体参数随机场图像与基础极限承载力之间的模型进行承载力预测,并基于预测模型研究不同参数的影响。结果表明:考虑土体空间变异性的基础承载力与试验结果基本吻合,随机结果均高于确定性分析;随机场下桩梁基础与群桩基础的承载力均为正态分布;采用卷积神经网络建立的基础承载力预测模型精度较高,且可以用于参数分析,基础承载力随着土体参数的增加而增加,随变异系数的增加而下降。随机条件下,桩梁基础的承载力高于群桩基础,可以充分发挥土体强度并抵御参数不确定性带来的承载力损失。

中风化花岗岩中嵌岩桩承载性能原位试验与极限承载力预测

为深入探究中风化花岗岩中嵌岩桩的竖向抗压承载特性,对12根嵌岩桩进行了单桩竖向抗压静载原位试验与ABAQUS有限元数值模拟,通过多种方法对嵌岩单桩极限承载力进行评价,明确中风化花岗岩中嵌岩桩竖向抗压承载性状。研究表明:12根中风化花岗岩中嵌岩桩并非表现出完全端承桩,而是呈摩擦型桩或摩擦端承桩的性状;中风化花岗岩地基中的嵌岩桩竖向抗压极限承载力较高,桩顶沉降小,满足工程对基础的承载要求;有限元模拟荷载-沉降曲线与实测荷载-沉降曲线走势吻合度较高,桩顶沉降误差较小;本试验条件下,桩端阻力占桩顶荷载的56.9%,桩侧摩阻力占比为43.1%,桩侧摩阻力在荷载传递过程中发挥较充分;有限元模拟得到的单桩极限承载力与指数函数模型的预测结果较为吻合,可用于嵌岩桩单桩竖向抗压极限承载力的预测,以及嵌岩桩承载性状和荷载传递规律的分析。

Bearing capacity and load transfer mechanism of a static drill rooted nodular pile in soft soil areas

The static drill rooted nodular pile is a new type of pile foundation consisting of precast nodular pile and the surrounding cemented soil.This composite pile has a relatively high bearing capacity and the mud pollution will be largely reduced during the construction process by using this type of pile.In order to investigate the bearing capacity and load transfer mechanism of this pile,a group of experiments were conducted to provide a comparison between this new pile and the bored pile.The axial force of a precast nodular pile was also measured by the strain gauges installed on the pile to analyze the distribution of the axial force of the nodular pile and the skin friction supported by the surrounding soil,then 3D models were built by using the ABAQUS finite element program to investigate the load transfer mechanism of this composite pile in detail.By combining the results of field tests and the finite element method,the outcome showed that the bearing capacity of a static drill rooted nodular pile is higher than the bored pile,and that this composite pile will form a double stress dispersion system which will not only confirm the strength of the pile,but also make the skin friction to be fully mobilized.The settlement of this composite pile is mainly controlled by the precast nodular pile;meanwhile,the nodular pile and the surrounding cemented soil can be considered as deformation compatibility during the loading process.The nodes on the nodular pile play an important role during the load transfer process,the shear strength of the interface between the cemented soil and the soil of the static drill rooted pile is larger than that of the bored pile.

大直径嵌岩灌注桩承载特性试验与有限元模拟

基于青岛某工程的4根大直径嵌岩灌注桩,在现场开展了单桩竖向抗压静载试验和桩身完整性测试,以明确大直径嵌岩灌注桩在竖向荷载下的承载特征,拟揭示不同荷载水平下嵌岩灌注桩的承载力发挥机制。结合不同嵌岩深度的单桩承载性能有限元模拟,重点分析了单桩在不同嵌岩深度下的竖向抗压承载特性和桩土位移分布特征。研究表明:在笔者试验条件下,大直径嵌岩灌注桩单桩竖向抗压承载力具有一定离散性,桩头部位的损坏会极大影响单桩承载力的有效发挥;加载完成后,数值模拟得到的桩顶沉降量为31.56 mm,约为现场实测值的1.1倍,模拟结果与实测结果较为吻合;通过不同嵌岩比的模拟结果对比发现,嵌岩深度越大桩端阻力占桩顶荷载的比例越小,且桩端阻力占比的变化速率随着嵌岩比的增加逐渐减缓;嵌岩深度取2.5倍桩径为宜。

局部冲刷下砂土中桩-盘复合基础H-T联合承载特性

海上风机基础工作时既承受复杂组合荷载作用,也受波流冲刷影响。单桩-摩擦盘复合基础同时结合了单桩与重力式基础的特点,摩擦盘还可起防冲板作用。为探讨砂土地基中H-T联合作用时局部冲刷对这种复合基础承载特性的影响,首先基于室内水槽试验装置,完成了一系列模型载荷试验;获得了冲刷前后桩-盘复合基础的荷载-位移曲线,经无量纲化处理与曲线拟合,获得H-T联合受荷桩-盘复合基础的承载包络线及其简化计算公式。然后,采用有限元细化分析了主要冲刷参数(冲刷深度、冲刷宽度、冲刷角度)及加载点高度对复合基础承载特性的影响,结果表明:冲刷坑的存在使复合基础的横向及扭转抗力明显降低,削弱程度分别可达25%及45%;进一步分析表明,桩身弯矩值随加载点高度的增加而显著增大,随预加扭矩的增加呈下降趋势;局部冲刷会明显削弱复合基础的承载力,同时H-T联合作用时相应的承载力包络线向内发生不同程度的收缩;其中,冲刷深度的影响最大,对横向抗力与扭转抗力的削弱程度分别可达约35%及60%,冲刷角度次之,冲刷宽度的影响最小;此外,不同加载路径下桩-盘复合基础承载力包络线呈现一定程度的差异。

多元荷载作用下软基深水高桩码头结构承载特性研究

软基深水高桩码头结构受到恶劣外海环境荷载及复杂工作荷载等多元荷载的共同作用。研究表明,船舶撞击荷载是该结构水平向控制荷载,但波浪长期循环荷载作用会引起桩周土体软化,进而导致码头结构在其它荷载作用下承载特性的劣化。鉴于此,首先基于ABAQUS有限元软件建立了深水高桩码头结构-地基土体相互作用的三维弹塑性有限元模型;然后,借助USDFLD子程序实现了同时考虑土体强度弱化和刚度衰减的模拟,进而开展了未考虑土体软化、仅考虑土体强度弱化、仅考虑土体刚度衰减以及同时考虑土体强度弱化和刚度衰减对码头结构承载特性影响的对比分析。研究结果表明,与未考虑土体软化时撞击荷载作用下码头结构安全系数相比,仅考虑土体强度弱化时其值降低14.72%,仅考虑土体刚度衰减时其值降低15.28%,同时考虑土体强度弱化和刚度衰减时其值降低19.44%,且考虑土体软化后桩周土体的塑性应变范围明显增大,极限状态时桩身应力值减小,结构稳定性显著降低。

刚性桩复合地基的载荷试验问题

《建筑与市政地基基础通用规范》(GB 55003—2021)对刚性桩复合地基承载力验收的条文是:复合地基承载力的验收检验应采用复合地基静载荷试验,对有粘结强度的复合地基增强体尚应进行单桩静载荷试验。在实际工程中,可能会遇到这样的情况:(1)复合地基静载荷试验合格,单桩静载荷试验不合格,复合地基承载力真的不合格吗?(2)复合地基和单桩静载荷试验都合格,复合地基承载力就一定合格吗?本文对此进行了分析讨论,认为与地质条件有关,规范方法主要是针对单一均匀地基的情况,对于多层土地基,规范方法有局限性,由于试验压板尺寸小,压板试验还不能反映深层土的影响。对于大尺寸的基础,还要进行科学的分析,应对实际尺寸的基础沉降进行计算分析,才能判断和保证工程的安全,仅靠小尺寸的压板载荷试验是不够完善的。

考虑系泊点深度的单点系泊系统锚桩水平承载性能理论分析

单点系泊系统因其克服恶劣海况的优越性成为我国海洋石油开发的重要系泊形式。然而,水下单点系泊系统锚桩-土体相互作用机制尚不清楚,尤其缺少考虑系泊点深度的水平承载性能计算方法。基于复合地基反力法,建立水平荷载下单点系泊系统锚桩的挠曲控制方程和边界条件,并编制相关计算程序;通过迭代法求解,获得不同系泊点位置下的锚桩荷载-位移曲线以及桩身位移、弯矩沿桩身的分布规律曲线;通过与数值模拟和既有离心模型试验结果的对比分析,验证了其准确性和可靠性;继而考虑系泊点深度对锚桩水平承载力的影响,进行参数分析,给出了最优系泊点深度取值范围。分析结果表明,增加系泊点深度可以显著减小锚桩的水平位移及弯矩最大值,从而提高锚桩的水平极限承载力。

复杂地质条件下抗滑桩水平承载特性影响因素分析

抗滑桩是治理滑坡时经常采用的一种有效的边坡防护工程措施,其水平承载力是评价边坡加固效果的关键。为研究复杂地质条件下边坡抗滑桩的水平承载特性,以卡拉水电站上田镇滑坡体防护工程为例,采用现场试验和数值模拟相结合的方法研究边坡抗滑桩的水平承载特性。基于抗滑桩现场单桩水平静载试验,验证数值计算模型的准确性,在此基础上进行抗滑桩水平承载力影响因素分析。结果表明:前后排桩受水平承载力呈现不均匀性,在相同水平承载力作用下后排桩桩顶最大弯矩大于前排桩桩顶最大弯矩;不同桩布置对群桩水平承载特性影响较大,其中双排交错布置方式桩顶弯矩最小,单排交错次之,双排水平桩顶弯矩最大;相同水平荷载作用下,与1.2 m和1.3 m桩间距相比,1.4 m桩间距的群桩效应较为明显;桩入岩深度对群桩水平承载能力影响较小,只在土-岩体分层界面有明显差异。

基于Rayleigh分布扰动状态概念的单桩承载特性研究

基于扰动状态概念(Disturbed State Concept,DSC),结合Rayleigh分布从微观角度描述桩-土界面荷载渐进性传递特性,建立了基于DSC理论的桩侧及桩端荷载传递模型,给出了模型参数确定方法,并验证了模型的合理性.同时结合桩侧及桩端荷载传递模型,提出了一种分析单桩承载特性的迭代算法并验证其合理性.通过算法计算得到的单桩承载特性与案例实测值有较好的一致性,且可较好地反映侧阻及端阻硬化、软化等特性.变参数分析结果表明,基于DSC理论的桩侧及桩端荷载传递模型对不同土层、不同桩基施工工艺都有好的适用性,可较准确地描述桩-土界面荷载传递特性.

穿越溶洞桥梁桩基嵌岩深度计算方法研究

为准确计算岩溶区桥梁桩基嵌岩深度,依托莆炎高速公路岩溶区桥梁桩基工程,分别考虑桩基竖向极限承载力、桩身稳定性和桥梁全寿命周期内溶洞溶蚀量,提出3种穿越溶洞桥梁桩基嵌岩深度计算方法,并验证3种方法的可靠性。选取34根代表性桩基对比嵌岩深度设计值与所提方法计算值,并选取1根试桩对比规范值与所提方法计算值。结果表明:34根代表性桩基中,1根桩基的嵌岩深度设计值小于考虑全寿命周期内溶洞溶蚀量的嵌岩深度计算值,占比2.9%;10根桩基的嵌岩深度设计值同时满足竖向极限承载力、桩身稳定性和桥梁全寿命周期内溶洞溶蚀量要求,且安全储备合理,占比29.5%;23根桩基存在安全储备较高问题,占比67.6%,其中14.7%的桩基嵌岩深度设计安全储备高达5倍。各规范关于桩基嵌岩深度的取值差异较大,为确保桥梁桩基承载安全,同时降低施工难度和成本,建议设计穿越溶洞桥梁桩基的合理嵌岩深度时,充分考虑桥梁全寿命周期内溶洞溶蚀量。