High-frequency interference waves in low strain dynamic testing of X-section concrete piles

Stress waves propagate along vertical,radial and circumferential directions when a non-uniformly distributed load is applied at one end of a three-dimensional shaft.As a result,the receiving signals are usually mixed with undesired interference components,often featuring as high-frequency fluctuations.Previous studies have revealed that sectional geometry(shape and size)greatly affects the high-frequency interference.In this study,low strain dynamic testing on full-scale X-section concrete is conducted in order to investigate the influences of high-frequency interference on velocity responses at the pile head.Emphasis is placed on the frequency and peak value of interference waves at various receiving points.Additionally,the effects of the geometrical,and mechanical properties of the pile shaft on high-frequency interference are elaborated on through the three-dimensional finite element method.The results show that the measured wave is obscured by interference waves superposed by two types of high-frequency components.The modulus and cross-sectional area are contributing factors to the frequency and peak value of the interference waves.On the other hand,the position with the least interference is determined,to some extent,by the accurate shape of the X-section.

Conversion factor analysis of self-balanced loading test of cast-in-situ piles based on analogue test method

Based on the characteristics of pile-soil interaction and the Mohr-Coulomb strength theory,a new method of determining the side friction at a pile-soil interaction is proposed.Combined with the actual engineering cases,the effectiveness of the analogue test method is verified by comparing it with the traditional anchor pile method and self-balanced method.Taking the self-balanced test of the bridge pile foundation in the Songhua River as an example,the conversion factor of sandy soil and weathered mudstone are confirmed by the analogue test method.The results show that the conversion factor of sandy soil and weathered mudstone in the Songhua River area should consider the geological conditions and the construction technology,etc.The standard values are relatively conservative.It is suggested that the engineering application should be properly revised.The recommended range of the conversion factor of sandy soil in this area is 0.65 to 0.85,and that of weathered mudstone is 1.0.

Deformation and failure modes of composite foundation with sub-embankment plain concrete piles

With the development of high-speed railway in China, composite foundation with rigid piles has become a stamdard solution of meeting the high requirements of stability and post-construction settlement of embankment on soft subgrade. Among several im- provement pattems, plain concrete piles have been extensively used to treat soft ground supported embankment. To investigate the deformation and failure modes of unimproved soft ground and soft ground reinforced by sub-embankment plain concrete piles, and to learn the influences of track and vehicle load, the effect of pile spacing, as well as the compression moduli of soil layers and upper load condition on the failure modes, a series of centrifuge model tests were performed. Test results indicate that the dis- placement of unimproved soft ground under the embankment increases continuously as embankment, track and train loading, and slip circle failure takes place. The deformation law of soft ground reinforced by sub-embankment plain concrete piles depends on pile spacing, compression modulus of the soft ground, and loading conditions. It was also found that plain concrete piles show displacement and failure patterns depending on its location, compression modulus of soft soil around the pile, and loading condi- tions. Furthermore, the evaluation of improved ground stability as well as the model test procedure is also presented.

Uplift behavior and load transfer mechanism of prestressed high-strength concrete piles

Prestressed high-strength-concrete (PHC) tube-shaped pile is one of the recently used foundations for soft soil. The research on uplift resistance of PHC pile is helpful to the design of pile foundations. A field-scale test program was conducted to study the uplift behavior and load transfer mechanism of PHC piles in soft soil. The pullout load tests were divided into two groups with different diameters, and there were three piles in each group. A detailed discussion of the axial load transfer and pile skin resistance distribution was also included. It is found from the tests that the uplift capacity increases with increasing the diameter of pile. When the diameter of piles increases from 500 to 600 mm, the uplift load is increased by 51.2%. According to the load-displacement (Q-S) curves, all the piles do not reach the ultimate state at the maximum load. The experimental results show that the piles still have uplift bearing capacity.

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.

Influence of the penetration of adjacent X-section cast-in-place concrete(XCC)pile on the existing XCC pile in sand

A series of small-scale 1g X-section cast-in-place concrete(XCC)pile-penetration model tests were conducted to study the effects of soil density and pile geometry on the lateral responses of an existing pile and the variations in surrounding soil stress.The results showed that the bending patterns of existing XCC piles varied with penetration depth.The lateral response of the existing pile was sensitive to the change in relative density and pile geometry.For example,the bending moment of the existing pile increased along with these parameters.The development of the radial stressσ′r/σ′v0 of the soil around an existing pile showed different trends at various depths during the penetration of the adjacent pile.Moreover,the change in radial stress during the penetration of the XCC pile did not exhibit the“h/R effect”that was observed in the free-field soil,due to the shielding effect of the existing piles.The peak value of radial stressσ′r_max/σ′v0 decreased exponentially as the radial distance r/R increased.The attenuation ofσ′r_max/σ′v0 with r/R in the loose sand was faster than in the medium-dense or dense sands.Theσ′r_max/σ′v0 at the same soil location increased with the cross-section geometry parameter.

钙质砂地基中群桩基础荷载传递特性研究

为研究钙质砂地基中群桩基础承载过程中的性能特征,开展不同桩距条件下群桩基础静载试验.得到不同桩距条件下群桩的Q-S曲线和S-lgt曲线,测得3d、4.5d、6d(d为桩径)群桩在静载条件下的极限承载力F_(u),研究各个桩段的桩身轴力、桩侧摩阻力以及桩端阻力的变化趋势.研究结果表明:群桩的Q-S曲线为缓变型,S-lgt曲线为渐变型,桩基承载变形大致分为稳定、渐进以及破坏3个阶段,相较于3d群桩,4.5d、6d群桩沉降发展更缓慢,F_(u)更高;同等桩距下,钙质砂地基表现出比陆源砂地基更强的群桩效应,钙质砂地基中群桩最佳桩距应在4.5d~6d之间.该研究为群桩在钙质砂地基中的应用提供数据支撑和理论参考.

桩—土—钢、砼结构动力相互作用试验对比研究

为了分析不同上部结构-桩-土相互作用规律,分别进行了钢框架结构-桩-土模型和混凝土结构-桩-土模型的振动台试验,并对试验模型进行了相应的有限元数值模拟分析。试验采用三维叠层剪切模型箱,土体为均匀粉质黏土,钢结构和混凝土结构模型为简化的3层框架结构,桩基为3×3根群桩,桩径为10 cm,桩长为200 cm,输入为人工地震动时程,按时间相似比压缩1/5。振动台对比试验结果表明,相同几何尺寸的结构试验模型,混凝土结构的整体刚度大于钢结构,因此振动频率大于钢结构;相同地震作用下,钢框架结构模型加速度反应明显大于混凝土结构,桩身加速度放大系数前者为后者1.15~1.2倍,上部结构可达2倍,钢框架结构模型反应谱的卓越周期更长。有限元数值模拟的结果定性地验证了试验结果的合理性。

预制混凝土桩引扩孔复合成桩技术及其工程特性分析

针对预制桩传统沉桩工法穿越较硬土层困难且易引发挤土效应的问题,结合合肥绕城高速路口至陇西互通立交改扩建应急工程,分别提出了引扩孔灌浆后植入预制混凝土桩技术和引扩孔浆石埋入预制混凝土桩技术,丰富了预制混凝土桩的复合成桩技术。通过现场载荷试验,验证了两工法成桩技术的适用性和可靠性,并基于数值分析对引扩孔灌浆后植入桩的承载性能进行了分析。结果表明:外围包裹层的设置可有效提升预制桩的承载性能,工程设计荷载作用下的桩顶沉降仅约1.1~3.0mm;考虑复合桩身结构特点,外围水泥砂浆包裹层对后植预制桩具有扩径和增强作用;其合理设置与选用有利于改善后植入桩的承载性能。

砂岩-混凝土结构面剪切特性试验研究及离散元模拟

为从宏、细观角度揭示常法向刚度(Constant Normal Stiffness,CNS)条件下锯齿状砂岩-混凝土结构面剪切机制,首先采用湖南大学自主改造的CNS岩石直剪仪开展3组砂岩-混凝土结构面室内剪切试验.在此基础上,采用刚性墙替代法建立与室内试验相应的离散元数值模型,并引入3个显式运动学方程控制砂岩试样运动轨迹从而实现CNS条件加载,将数值模拟与试验结果对比验证了模型的合理性.随后,开展16个工况的数值剪切试验,通过观察剪切裂纹扩展和力链演化形态,从细观角度揭示结构面破坏模式和荷载传递机制,并从宏观角度分析锯齿几何尺寸(半波长λ、起伏角θ)和加载边界条件(初始法向应力σn0、法向刚度K)对剪切强度和法向膨胀的影响.结果表明:剪切裂纹按照“平稳增加—急剧增加—增幅减小”的发展趋势由结构面区域逐渐向岩石内部扩展,结构面破坏模式随起伏角的增大由滑移磨损向剪断破坏过渡,剪切强度随λ、θ、σn0和K的增加呈指数函数型增加.

非挤土嵌岩预应力高强度混凝土管桩的桩端承载性能研究

施工装备和工艺的不断创新使得非挤土嵌岩预应力高强度混凝土(prestressed high-strength concrete,简称PHC)管桩逐渐得到推广和应用。为揭示嵌入中微风化岩PHC管桩的桩端承载机制,以随钻跟管工法桩为背景,基于泥质粉砂岩地区5组桩径为500 mm的PHC管桩桩端现场静载破坏性试验,分析了不同条件下嵌岩PHC管桩的桩端承载性能及宏观破坏模式,并提出桩端承载力计算方法。试验表明:对于采用敞口桩靴的PHC管桩,未封底时,敞口桩靴削弱了PHC管桩桩端承载性能,呈现刺入岩基的破坏,极限荷载下桩端沉降为11~15 mm;混凝土封底后,显著提高了桩端承载性能,极限承载力较非封底时提升340%,封底混凝土和桩靴的桩端阻力分担比分别为78%和22%,桩端呈整体剪切破坏。基于Hoek-Brown强度准则,提出了嵌岩PHC管桩桩端承力简化计算方法,计算精度得到了试验验证,可供非挤土嵌岩PHC管桩的设计与施工参考。

钢渣混凝土桩土相互作用机理试验研究

通过设计桩土相互作用可视化试验装置,模拟钢渣混凝土桩加固吹填土地基,对桩周土体的水分含量、孔压值以及桩体对土体的扰动情况进行监测分析,并利用宏观、细观和微观相结合的试验手段,深入研究桩土间产生的物理化学反应及其产物。试验结果表明,桩体具有膨胀挤密土体的作用;桩体能够加快土体的排水减压过程;桩土间产生的物理化学反应及其产物对地基稳定性有着正向加强作用。

不同工况下螺锁式预应力混凝土实心方桩的桩身完整性分析

螺锁式预应力混凝土方桩的桩身完整性检测方法主要采用低应变法,实测曲线中接桩位置经常出现明显同向反射,对桩身完整性的合理判定产生影响。在没有桩周土和施工工艺影响的工况下,采用低应变法对 3 组桩的桩身完整性进行对比验证,结果表明同时采用螺锁式和密封材料连接工艺的桩的桩身完整性可满足要求。在实际项目的桩基承载力问题处理过程中,低应变法无法准确判定接桩位置的缺陷程度,而高应变法可以作为基桩施工质量综合判定的有效手段。

钢筋混凝土建筑桩基截面承载力现场测试研究

桩基承载力直接影响建筑的施工质量,因此进行钢筋混凝土建筑桩基截面承载力测试研究。以宁波某商业用地工程项目作为研究对象,根据建筑设计方案制作多个桩基试件,在试件周围布设承载力测试仪器。在钢筋混凝土建筑桩土体系荷载传递理论的支持下,制定桩基截面加载方案,通过荷载施加、数据记录等环节,完成桩基截面承载力测试操作,并得出截面的抗压、抗弯、极限承载力指标测试结果。测试结果显示,试件截面抗压承载力、抗弯承载力和极限承载力测试结果的平均值分别为438.4、518.6和552.8 kN。

拉哇水电站下游河道防护工程防冲桩施工重难点控制

根据钻孔灌注桩的成孔方式,可将其划分为钻孔灌注桩、沉管灌注桩、人工挖孔、挖孔扩底等,在工程实践中,最常见的有:钻孔灌注桩和沉管灌注桩。现浇混凝土桩是在场地位置上直接进行钻孔施工,在钻孔中浇筑混凝土,或者放置钢筋笼,最后浇筑混凝土。笔者以拉哇水电站下游河道防护工程为研究对象,详细描述了混凝土灌注桩施工工艺及施工方法,通过现场施工过程对其施工重难点进行分析。

Comparison between bearing characteristics of pervious concrete pile composite foundations with different replacement ratios

The replacement ratio is an essential factor in evaluating the bearing capacity characteristics of compositefoundations. This study focuses on the bearing capacity of a pervious concrete pile with different replacementratios. The axial force, skin friction, and settlement were evaluated using a model test to assess the performance ofthe pervious concrete pile composite foundation. When the replacement ratio was reduced from 9.26% to 2.32%,the characteristic bearing capacity value was only 14%. Therefore, it may be unreasonable to use the settlementratio method to evaluate this composite foundation's bearing capacity in a model test. Appropriate loading cansignificantly improve the bearing capacity of a pervious concrete pile composite foundation with a lowreplacement ratio. The pile–soil stress ratio exhibited different decreasing ranges in the later loading stage. As theload increased, the axial force of the pervious concrete piles was small and nonobvious, and the average sidefriction resistance of the piles in the foundation with a lower replacement ratio slowly increased.

大直径预应力高强度混凝土管桩在黄泛区高速公路桥梁基础中的设计与应用研究

预应力混凝土管桩具有单桩承载能力高、抗弯性能好、施工方便快捷、工程造价较低等优点。介绍了大直径高强度预应力混凝土管桩在黄泛区高速公路桥梁基础中的设计与应用实例,通过开展静载试验确定管桩单桩承载能力,并依据试验结果和相关规范标准制定详细的施工流程,为大直径预应力混凝土管桩在黄泛区高速公路桥梁基础中的应用提供指导与参考。

第四系地层预应力混凝土管桩承载性状现场试验研究

基于印尼某工程15根预应力混凝土管桩(PC管桩)的单桩竖向抗压、抗拔及水平静载荷试验,分析PC管桩分别在竖向荷载和水平荷载作用下的承载特征,揭示不同荷载水平下PC管桩的承载力发挥机制。基于单桩竖向抗压极限承载力预测模型,对比分析指数曲线模型、双曲线模型及调整双曲线模型的可行性,并对PC管桩单桩竖向抗压极限承载力进行预测;结合水平静载试验,探讨地基土水平抗力系数的比例系数m的取值问题。研究结果表明:PC管桩单桩竖向抗压承载力主要取决于桩端持力层的支承力,同时也受桩径、桩长的影响较大;PC管桩的竖向抗拔承载力主要取决于桩侧摩阻力,桩径越大、桩长越长,单桩竖向抗拔承载力越高;PC管桩水平承载力主要取决于桩侧土体的力学性质。就本试验而言,指数曲线模型对单桩极限承载力的预测最精确;m在桩顶水平位移超过10 mm时变化平稳并逐渐收敛为常数,通过试验结果反推的m接近甚至超过JGJ 106—2014中推荐m的上限值。

界面状态对PHC管桩填芯抗拔承载性能影响试验研究

对考虑两种界面状态和3种填芯长度的PHC⁃600⁃B⁃130管桩混凝土填芯足尺试件开展轴心抗拔静载试验,获得了极限轴心抗拔荷载值、界面滑移量、分段黏结强度和平均黏结强度等试验数据。分析试验数据表明:相同填芯长度下,管桩内壁的两种界面状态对极限轴心抗拔荷载值影响并不明显;相同界面状态下,极限轴心抗拔荷载值随着填芯长度的增加呈线性增大趋势;混凝土填芯界面平均黏结强度试验值介于0.618~0.692MPa。相对试件加载远端,加载近端处的填芯混凝土界面黏结强度和滑移量均相对较大,表明加载端的填芯混凝土承担了大部分的轴拉荷载;无端板试件的PHC管桩在加载端附近出现竖向开裂,表明加载端填芯混凝土在轴拉作用下存在剪胀现象,应引起重视。

光纤光栅应变检测技术在预制桩中的应用研究

光纤光栅表面式GFRP桩基应变计具有分布式和长距离检测等特点,并且此种桩基应变计易埋入到成型的预制工程构件中,在桩身内力测试中具有独特的优势。介绍了在软土地区预制桩检测中采用布里渊光时域反射技术(BOTDR)的传感原理及GFRP桩基应变计的植入工艺,根据测试结果对比分析桩身内力、桩周摩阻力及桩端阻力,更加全面地了解地层特性及预制桩承载特性,为设计参数的优化提供可靠依据。