Performance of composite foundations with different load transfer platforms and substratum stiffness over silty clay

The semi-rigid pile-supported composite foundation is widely used in highway projects due to its effectiveness in increasing the bearing capacity and stability of foundations.It is crucial to understand the stress distribution across the embankment width and the behaviour of unreinforced foundations.Thus,five centrifuge tests were conducted to examine the bearing and deformation behaviours of NPRS(Non-Connected Piled Raft Systems)and GRPS(GeosyntheticReinforced Pile-Supported systems)with varying substratum stiffness,then a comparative analysis was conducted on embankment settlement,pressures underneath the embankments,and axial forces along the piles.The results indicated that greater substratum stiffness correlates with reduced settlement and deformation at various depths.Deformation occurring 5 meters from the embankment toe includes settlement in NPRS and upward movement in GRPS.The potential sliding surface is primarily located within the embankment in NPRS,whereas it may extend through both the embankment and foundation in GRPS.The pile-soil stress ratio and efficiency in NPRS are higher than in GRPS across the embankment.The axial force borne by end-bearing piles is significantly greater than that by floating piles.As the buried depth increases,the axial force in GRPS initially rises then declines,whereas in NPRS,it remains relatively constant within a certain range before decreasing.This study aids in assessing the applicability of composite foundations in complex railway environments and provides a reference for procedural measures under similar conditions.

Vision-based Dynamics Monitoring(VDM)for Diagnosing the Variations of Wind Turbine Tower Foundation Conditions

A slight uneven settlement of the foundation may cause the wind turbine to shake,tilt,or even collapse,so it is increasingly necessary to realize remote condition monitoring of the foundations.At present,the wind turbine foundation monitoring system is incomplete.The current monitoring research of the tower foundation is mainly of contact measurements,using acceleration sensors and static-level sensors for monitoring multiple reference points.Such monitoring methods will face some disadvantages,such as the complexity of monitoring deployment,the cost of manpower,and the load effect on the tower structure.To solve above issues,this paper aims to investigate wind turbine tower foundation variation dynamic monitoring based on machine vision.Machine vision monitoring is a kind of noncontact measurement,which helps to realize comprehensive diagnosis of early foundation uneven settlement and loose faults.The FEA model is firstly investigated as the theoretical foundation to investigate the dynamics of the tower foundation.Second,the Gaussian-based vibration detection is adopted by tracking the tower edge points.Finally,a tower structure with distributed foundation support is tested.The modal parameters obtained from the visual measurement are compared with those from the accelerometer,proving the vision method can effectively monitor the issues with tower foundation changes.

SOUND RADIATION FROM PLATES ON ELASTIC FOUNDATION UNDER ACTION OF MOVING LINE FORCES

An analysis is made of the problem of sound radiation from infinite one-dimensional plateson elastic foundation, when the plates are subjected to the action of harmonic line forces movingat subsonic speeds (M 【 1). The expressions of nondimensional sound power are formulated andthe asymptotic forms of sound power in the low frequency regions are derived. The radiatedsound power is shown as a function of the stiffness of elastic foundation, in terms of stiffness fac-torψ, the moving speed of line force, in terms of Math number M, and the frequency, in termsof wavenumber ratio γ . The effects of the parameter ψ in conjunction with the parameters Mand γ on the radiated sound power level and the phenomenon of coincidence radiation are alsoinvestigated in detail.

Numeric simulation analysis of load-settlement relation of pile in layer foundation

Considering the level distribution of soil layers, the soils surrounding pile are simulated with level finite layer elements. Supposing that the vertical deformation of the soil elements surrounding pile varies in the form of exponent function with radial distance, and considering the nonlinear constitutive relation of stress and strain, the stiffness matrix is established. The mechanics behavior of the pile—soil interface is simulated with a nonlinear interface element. This method can truly express the behavior of the pile-soil system. The load-settlement relation Q-S curves of two big diameter prototype piles on bearing test are analyzed, and satisfying results are obtained. This method is reasonable in theory and feasible in engineering.

Numerical Analysis of the Interaction between Shallow （Square, Circular and Strip） Foundations and Subsoil

In this paper, the effects of the stiffness of circular, square and strip foundation structures and bonding effects were analyzed. Presented analysis was oriented on the influence of stiffness system ＂foundation--subsoil＂ and bonds （bi-directional bond and one-directional bond with and without friction）. The results of numerical calculations have proved that the relative stiffness of system ＂foundation--subsoil＂ affect considerably the value and the distribution of contact stresses （vertical normal and shear stresses） in the foundation gap and value of the displacements （settlement, deflection and relative deformations） of foundation. From the numerical point of view, this problem was solved by deformation variant of the FEM （finite element method）. The numerically obtained results were presented in the graphical and tabular forms. Obtained results were qualitative and quantitative compared with one another. From the calculation results it is obvious that relative stiffness of the system ＂foundation structure--subsoil＂ substantially affects distribution of contact stresses in the foundation subsoil and displacements （settlement, deflection and relative deformations, flexibility） of foundation. In the case of flexible foundations, the bond on the contact surfaces must be considered during the calculation. On the other hand, the effects of friction on the contact surface between the foundation and subsoil affect the distribution of contact stresses and deformations only to smaller extent.

开挖顺序及内支撑早期刚度对软土基坑稳定性影响规律

深基坑开挖变形控制是软土基坑工程的难题,工程中常采用密集的混凝土内支撑来提高支撑刚度,然而实际工程中未能合理考虑软土深基坑的开挖顺序和混凝土内支撑早期刚度影响,往往导致基坑实际变形远大于设计值。结合珠三角水资源配置工程的软土深基坑,利用验证后的数值模拟方法研究了不同开挖顺序及混凝土内支撑早期刚度对基坑受力和变形特性的影响规律,得出了分层分区非对称开挖和对称开挖对地连墙水平位移和地表沉降的影响规律。结果表明:尽管非对称开挖会造成基坑两侧的地连墙位移和地表沉降差异,但是密集内支撑条件下非对称开挖比对称开挖对于基坑周围地表沉降和地连墙水平位移影响更小并能够大量节约工期;受混凝土内支撑早期刚度影响,施作内支撑后越早进入下一步开挖,地连墙水平位移和周围地表沉降越大;考虑内支撑早期刚度影响时基坑上部的内支撑轴力会增大,而下部的内支撑轴力会减小,早期刚度越小,这种趋势越明显;现场施工时难以实时动态监测混凝土的弹性模量,极易造成内支撑轴力的早期监测数据失真,甚至给工程带来错误决策。

变刚度地基上隔震结构群桩基础动力响应试验研究

通过控制输入地震动持时压缩比和强度的方法,提出了变刚度地基上桩基基础隔震结构振动台模型试验方法,并结合已完成的不同地基上桩基基础隔震结构系列振动台模型试验,分析了地基刚度变化对隔震结构群桩基础动力学特性的影响规律。结果表明,结构-土体相对刚度比和输入地震动强度显著影响隔震结构群桩基础弯矩反应,强震下结构-土体相对刚度比越大,隔震结构群桩基础中间桩体上部弯矩反应幅值增大越显著,而中间桩体下部弯矩幅值较小。当输入加速度峰值和结构-土体相对刚度比R大于一定限值时,群桩基础震陷显著增加,群桩基础承台转动反应强烈,同时群桩基础桩顶水平位移在主震后出现明显的单边累积水平位移,说明强震作用下地基刚度变化过程中隔震结构群桩基础上部更易出现地震破坏,而群桩基础震陷与基础承台强烈转动反应的共同作用可能是导致隔震结构群桩基础上部弯矩骤增的主因。

模块化建筑刚性桩复合地基共同作用试验分析

本文依据某模块化示范楼,制作了简化后的1∶10的模块化建筑刚性桩复合地基的室外缩尺模型,考虑了上部模块结构与地基基础共同作用。通过模型试验分析了上部模块刚度变化对上部模块钢柱、筏板基础和刚性桩复合地基各受力体的影响。结果表明:模块化建筑中部柱轴力>边部柱轴力>角部柱轴力;随上部模块刚度的增加,中部柱轴力逐渐减小,边部柱和角部柱轴力逐渐增大;筏板基础中部沉降最大,边部沉降最小,整体沉降随上部模块楼层的增加逐渐增大;角部桩身轴力>边部桩身轴力>中部桩身轴力,桩身轴力沿着桩深度方向先增大后减小;桩身上部一定范围内产生负摩阻力。分析结果能够为模块化建筑单元构件设计及刚性桩复合地基基础优化设计提供参考。

考虑轴力与底板约束的埋入式钢柱脚刚度模型

为准确预测埋入式(特别是浅埋)钢柱脚节点的初始转动刚度,将柱脚的埋入段简化为地基梁,基于初参数法完善考虑轴力影响的Winkler地基上的Timoshenko梁理论,提出受锚栓和混凝土约束的柱脚底板转动刚度的计算方法,进而建立可以综合反映柱脚埋深、埋入段剪切变形、钢柱轴力、底板约束影响的埋入式钢柱脚转动刚度模型。基于已验证的有限元模型进行参数分析,研究埋深比、轴力和锚栓布置对柱脚转动刚度的影响。与有限元结果的对比表明,提出的刚度模型较过往文献中理论模型具有更高的精度和更好的适用性,尤其是对于埋深比为0.5~1.5的浅埋柱脚节点。理论与有限元结果表明:埋深比较小时,柱脚转动刚度随埋深比的增加快速增大,而埋深比增加到一定程度后,其对转动刚度几乎无影响;柱脚转动刚度随钢柱轴压力增加呈增大的趋势;对于浅埋柱脚节点,底板约束对其转动刚度的影响较大,而随着埋深的增大,其影响逐渐减小;底板转动约束对埋深比小于1.5的节点刚度具有显著影响,当埋深比增大至2.5,其影响可以忽略。

基坑工程中冠梁刚度的计算方法研究

针对目前基坑设计中存在冠梁刚度计算偏大的问题,开展了冠梁刚度计算方法的研究。首先对现行商业软件中冠梁刚度计算模型进行分析,然后讨论了简支梁受均布荷载和两端固定梁受均布荷载的冠梁刚度计算模型,并给出了相应的刚度计算公式。考虑了圆形基坑和多边形基坑,推导了圆形基坑冠梁刚度的理论计算公式,并将多边形基坑的冠梁刚度与有限元计算的精确解进行了对比。结果表明:简化模型计算的冠梁刚度误差与基坑形状密切相关,因此不宜直接使用简化模型进行计算。相反,将完整的基坑冠梁作为研究对象,在冠梁上施加荷载,求解对应位移,然后根据刚度计算原理得出冠梁的等效刚度是一种可行和可靠的方法。

基于HSS模型海上风电桶形基础水平循环承载特性对比研究

采用土体本构模型对黏土中的桶形基础进行了有限元模拟,以研究桶形基础的水平循环承载特性。利用有限元方法模拟了在不同单调组合荷载作用下黏土中的桶形基础离心机试验,与试验结果对比验证了使用小应变硬化土(HSS)本构模型的适用性。在此基础上,基于HSS模型和硬化土(HS)模型对比研究了不同循环荷载和小应变模量对桶形基础累积转角、旋转中心和桶-土相对刚度的影响。结果表明,黏土中桶形基础的累积转角随着循环次数和水平荷载的增加而增加,然而每个循环中累积转角的增加逐渐减小。另外,提出了Tb和ζb之间的拟合关系式预测黏土中桶形基础的累积旋转。无论处于疲劳极限状态(ζb=0.3)还是正常使用极限状态(ζb=0.5),旋转中心类型都是从深中心变为浅中心。桶-土相对刚度随循环次数的增加而减小,并且循环荷载越大刚度衰减越大。另外,小应变模量对基础累积转角和桶-土相对刚度有较大影响而对旋转中心无显著影响。与已有的研究结果对比发现,HSS模型更加适用于精细化设计。

锚碇式支护在市政深基坑工程中的应用研究

结合福州市三环路某箱型通道基坑采用锚碇式支护体系的工程实例,推导锚碇结构的弹性支点刚度系数,并将其应用于计算。建立有限差分数值模型,分析锚碇式支护结构的变形特征,验证其可靠性,分析讨论锚碇结构发生整体侧移对支点刚度产生的不利影响,提出计算软土基坑时对弹性支点刚度系数进行折减使用的方案。施工及监测结果表明,锚碇式支护结构在工程中获得良好的应用效果。

地铁车站矩形顶管壳-接头理论模型及纵向变形

针对矩形顶管隧道易受影响产生纵向变形、进而引发接头破坏的问题,提出一种矩形顶管隧道壳-接头理论模型,并以此研究隧道在不均匀沉降作用下的纵向变形。首先,通过室内模型试验,探究矩形顶管隧道接头力学特性;其次,建立壳-接头理论模型,给出接头力学元件的刚度矩阵及其计算方法;然后,建立数值模型,并与室内模型试验结果进行对比;最后,依托20环管节的矩形顶管隧道工程实例,研究不均匀沉降作用下的隧道纵向变形。结果表明:接头破坏表现为闭合间隙、钢套环受力和变形破坏3个阶段,接头的抗剪和抗弯均由钢套环承担;壳-接头理论模型的模拟计算结果与模型试验结果较吻合,验证了模型的实用性和准确性;矩形顶管隧道的纵向变形受地基刚度影响显著,砾砂、粉砂和黏土3种地层对应的隧道最大附加沉降分别为3.72,7.66和12.14 mm,随着地基刚度的减小,隧道接头受不均匀沉降的影响将会增大。

海上风机桩-筒复合基础基频特性分析

桩-筒复合基础作为海上风机的一种新型基础型式,在提高单桩承载力方面相较于单桩基础具有一定优势,然而筒体对风机结构基频特性的影响规律尚不明确。为研究该问题,对风机塔筒进行解析建模,结合有限元,建立了不同几何尺寸的单桩基础和桩-筒复合基础,将数值模型获得的基础刚度与上部塔筒解析模型进行耦合,提出了分析风机整机结构基频特性的有限元-解析耦合法。研究结果表明有限元-解析耦合法能够快速地实现风机整机结构基频预测。对比桩-筒复合基础与单桩基础基频特性发现:筒体的加入能有效提升单桩基础刚度,可以在不改变单桩基础桩径的前提下,有效提升结构第一阶固有频率,筒外径对基础基频的影响比筒壁高的影响更大。

岩体地基抗剪刚度系数试验研究

地基抗剪刚度系数反映了地基对上部结构在水平方向的约束强弱程度,直接影响地基上浇筑的混凝土结构温度约束应力的大小,是控制混凝土底板温度裂缝的关键参数之一。为完善温度约束应力计算中地基抗剪刚度系数的取值,基于岩体地基防水层底板温度应力监测现场试验,计算得到岩体地基防水层构造条件下地基抗剪刚度系数取值;而后进行了岩体地基上的防水层、垫层、滑动层3种构造条件的动力特性测试试验,基于量纲分析计算出了垫层和滑动层构造条件下的地基抗剪刚度系数取值。

邻接已建地下室深大基坑支护结构设计实践

随着城市地下空间开发利用的迅猛发展,临近甚至邻接已建地下结构的项目也越来越多,如何利用已有地下结构或其支护结构成为面临的新课题。本文以某综合体项目基坑支护设计为例,阐述了项目特点及支护结构选型,并根据项目特点,针对与已建地下室及原围护结构间传力转换体系、支锚刚度引起的变形协调和地下水处理等关键问题提出了处理措施。工程实践表明,利用已有地下结构作为支撑点是可行的,但需要考虑支撑计算长度引起的支锚刚度变化,其可通过调整支撑杆件布置、加强截面参数及覆板边桁架等措施实现。

上海某超深基坑分区开挖数值计算及模型参数分析

以上海某挖深超过40m的超深基坑为分析案例,建立了超深基坑精细化流固耦合数值分析方法,通过实测数据和参数反演分析,研究了超深基坑多因素叠加耦合效应的围护结构变形分布规律和变形模式。研究表明:相较于浅层土体开挖,深层土体开挖的变形效应更加明显,分层开挖的变形量和变形速率逐渐增加;模拟基坑开挖变形效应时,考虑围护结构的刚度折减是必要的,坑底承压水层降水有利于控制基坑变形,降水后深层土体开挖变形速率显著减小;围护结构变形的空间效应显著,地下连续墙角部几何形式对围护结构侧移影响较大;计算结果可以较好拟合实测结果,预测最大误差可控制在20%以内。

Recent Construction Technology Innovations and Practices for Large-Span Arch Bridges in China

Arch bridges provide significant technical and economic benefits under suitable conditions.In particular,concrete-filled steel tubular(CFST)arch bridges and steel-reinforced concrete(SRC)arch bridges are two types of arch bridges that have gained great economic competitiveness and span growth potential due to advancements in construction technology,engineering materials,and construction equipment over the past 30 years.Under the leadership of the author,two record-breaking arch bridges—that is,the Pingnan Third Bridge(a CFST arch bridge),with a span of 560 m,and the Tian’e Longtan Bridge(an SRC arch bridge),with a span of 600 m—have been built in the past five years,embodying great technological breakthroughs in the construction of these two types of arch bridges.This paper takes these two arch bridges as examples to systematically summarize the latest technological innovations and practices in the construction of CFST arch bridges and SRC arch bridges in China.The technological innovations of CFST arch bridges include cable-stayed fastening-hanging cantilevered assembly methods,new in-tube concrete materials,in-tube concrete pouring techniques,a novel thrust abutment foundation for nonrocky terrain,and measures to reduce the quantity of temporary facilities.The technological innovations of SRC arch bridges involve arch skeleton stiffness selection,the development of encasing concrete materials,encasing concrete pouring,arch rib stress mitigation,and longitudinal reinforcement optimization.To conclude,future research focuses and development directions for these two types of arch bridges are proposed.

桩基础变刚度调平设计量化控制指标研究

考虑到当前规范缺乏可以实操的桩基变刚度调平具体设计细则和方法,实际设计环节也尚无明确、可控的量化指标,对模型试验和设计实践成果进行分析,提出桩基变刚度调平设计计算方法和设计原则.通过室内大型模型试验成果对变刚度调平具体控制指标进行量化分析研究,得出强化指数在1<λ_(s)<1.30范围内时,标准沉降比、标准刚度比与强化指数大致成线性关系,进而提出核心筒区域强化指数宜为1.05~1.30,外框区域的弱化指数宜为0.70~0.95.采用上述控制指标进行野外大型变刚度调平桩筏基础模型试验设计研究.结果表明,不均匀沉降大幅减小,实测最大差异沉降均小于规范规定允许值.可见采用上述指标进行变刚度调平设计对于控制差异沉降效果显著.

裹覆隔振层的实心桩纵向振动特性研究

为分析实心桩裹覆隔振材料后的纵向振动特性,基于Rayleigh-love杆件模型和Novak平面应变理论,综合考虑桩周振动传播介质的径向和纵向非均匀性以及桩身横向惯性效应,建立了桩周振动传播介质-实心桩耦合作用纵向动力阻抗解析模型。利用Laplace变换、复刚度传递和阻抗函数传递法,得到黏性阻尼介质中任意竖向荷载作用下实心桩桩顶动力阻抗的解析解答,并与已有解析解答对比验证其合理性。通过现场试验发现,桩顶动刚度幅值的大小对桩顶动力响应有明显影响,两者之间高度相关。在此基础上,研究了橡胶隔振层不同厚度和纵向深度以及隔振层材料种类变化对实心桩桩顶动刚度的影响。结果表明,在1~80 Hz频段内:(1)随着橡胶隔振层厚度的增加(减小),动刚度幅值曲线会向低频(高频)移动,但整体上厚度变化对特征频率和动刚度幅值的影响较小;(2)随着橡胶隔振层纵向深度的增加(减小),动刚度幅值曲线的振荡幅度和振荡频率也会随之明显提高(降低);(3)当采用剪切模量小于桩周土体的隔振材料时,随着剪切模量的减小,动刚度幅值曲线的振荡幅度和振荡频率也会随之提高;(4)当采用剪切模量大于桩周土体的隔振材料时,随着剪切模量的增大,动刚度幅值曲线的振荡属性基本消失,且整体会出现小幅抬升。