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.

Model tests on XCC-piled embankment under dynamic train load of high-speed railways

Piled embankments,which offer many advantages,are increasingly popular in construction of high-speed railways in China.Although the performance of piled embankment under static loading is well-known,the behavior under the dynamic train load of a high-speed railway is not yet understood.In light of this,a heavily instrumented piled embankment model was set up,and a model test was carried out,in which a servo-hydraulic actuator outputting M-shaped waves was adopted to simulate the process of a running train.Earth pressure,settlement,strain in the geogrid and pile and excess pore water pressure were measured.The results show that the soil arching height under the dynamic train load of a high-speed railway is shorter than under static loading.The growth trend for accumulated settlement slowed down after long-term vibration although there was still a tendency for it to increase.Accumulated geogrid strain has an increasing tendency after long-term vibration.The closer the embankment edge,the greater the geogrid strain over the subsoil.Strains in the pile were smaller under dynamic train loads,and their distribution was different from that under static loading.At the same elevation,excess pore water pressure under the track slab was greater than that under the embankment shoulder.

Energy Absorption of Pultruded Glass-Graphite/Epoxy Hybrid Composites under High Strain-Rate Induced Transverse Tension

This paper focuses on the dynamic tensile response of glass-graphite/epoxy composites illustrating improvement in energy absorption through hybridization. The dynamic response and energy absorption characteristics of pultruded hybrid combinations of glass and graphite fibers in an epoxy matrix subjected to induced transverse tension at high strain-rate in a modified Split Hopkinson Pressure Bar (SHPB) apparatus, are presented. Transverse tensile strength was determined by diametral compression of disc samples (Brazilian indirect tensile test method). Diametral crack initiation and strain to failure were monitored with a Shimadzu HPV-2 high-speed video camera at a recording speed of 500,000 fps and Digital Image Correlation (DIC). Adequate measures were taken to ensure that initiation of specimen failure occurred at the exact center of the disc specimen, and propagated through the diameter along the compressive loading axis, for the induced transverse tension tests to be valid. A study of the strength and specific energy absorption demonstrates the benefits of hybridization. Under induced transverse tensile loading condition, the pure glass/epoxy (GL60) exhibited higher strength than pure graphite/epoxy (GR60). Pure graphite/epoxy (GR60) has higher specific energy absorption capacity than pure glass/epoxy (GL60) in transverse tension. Among all hybrids, GR30 has the highest specific energy absorption under transverse tension. Overall, hybrid GL48, with 48% low-cost glass fibers in the inner core and 12% high-cost graphite fibers in outer shell, was found to exhibit better performance under induced transverse tension at high strain-rates, showing the benefits of hybridization.

南京某高桩码头靠泊能力仿真分析与在线监测预警

高桩码头服役环境复杂,码头结构在数十年的运行期内会发生不同程度的损伤,影响码头的安全运营。在码头运行过程中,船舶荷载是高桩码头结构的主要荷载,对码头结构的安全性、耐久性影响较大。本文对南京港某码头靠泊能力进行了有限元仿真分析,提出船舶靠泊过程中的三级安全预警阈值;结合长期在线监测,为码头结构安全服役提供技术支撑。研究结果表明:在船舶撞击1#排架的情况下,船舶撞击力达到238 kN时,码头结构响应达到三级预警指标,码头位移为3.8 mm,桩基顶部应变为69×10-6;船舶撞击力达到323 kN时,码头结构响应达到二级预警指标,码头位移为5.1 mm,桩基顶部应变为100×10-6;船舶撞击力达到471 kN,码头结构响应达到一级预警指标码头位移为7.5 mm,桩基顶部应变为156×10-6。在线监测预警结果表明,码头结构具备5 000吨级船舶靠泊能力,在规范靠泊的前提下,能够满足8 000吨级船舶靠泊的结构安全要求。

高桩码头结构安全监测预警模型及工程应用

高桩码头结构在服役期内会出现不同程度的损伤,导致码头结构安全性降低。为保证码头长期安全运行,可通过实时监测确定码头运行状态并及时预警。提出一种高桩码头结构安全监测预警模型,采用不同结构状态的设计控制值作为不同级别的预警阈值,每级预警阈值对应不同的风险程度,针对不同级别的预警提出相应处置措施。将该预警模型应用于南京港某高桩码头的安全监测,对可变荷载作用下码头结构位移和桩基应变进行实时监测,并计算设置码头结构位移和桩基应变三级安全预警阈值。结果表明,码头安全预警阈值的设置及安全告警的应对措施科学有效,实现了码头结构全工况整体技术状态评估和安全预警。

自平衡法和高应变检测在码头灌注桩中的应用

海南某高桩码头工程地质条件特殊,表面覆盖硬岩层,下部为黏土层,码头基础采用灌注桩结构,以硬黏土层为持力层。施工后分别采用自平衡法静载试验和高应变法检测桩基承载力,将检测结果与规范理论计算结果进行比较。结果表明,通过自平衡检测的荷载-位移关系曲线判断,桩端依旧处于弹性阶段,并根据在加载过程中桩身轴力沿桩身轴向的分布情况,可分别判断各桩侧摩阻力、端承载力的发挥值,从而判断桩阻力并没有全部发挥,桩基的实际极限承载力大于试验值。高应变检测法检测桩侧摩阻力结果大于理论计算结果,桩端承载力小于理论计算结果,总桩基承载力二者比较接近,因此可以作为静载试桩的辅助验证。在港口码头工程中采用自平衡静载测试法,并用高应变检测法进行验证,可以解决水域条件下大质量桩基承载力的检测问题。

高聚物胶凝戈壁土的动残余变形特性试验研究

高聚物胶凝戈壁土是将高聚物与戈壁土按一定比例拌合而成的,可有效提高戈壁土的力学性能。为研究高聚物胶凝戈壁土的动残余变形特性,采用中型动三轴试验研究了高聚物质量比、围压、固结比和动应力比等对高聚物胶凝戈壁土残余剪应变和残余体应变的影响。结果表明:高聚物胶凝材料能有效降低戈壁土的动残余变形,高聚物胶凝戈壁土在经受30次循环荷载后产生的残余剪应变为天然戈壁土的15.0%~18.8%,残余体应变为天然戈壁土的12.1%~22.2%;掺入高聚物质量比为3%和12%时,残余剪应变的减小幅度为85.0%、95.2%,残余体应变的减小幅度为87.9%、95.5%,且高聚物质量比越大,减小幅度越大。采用投影寻踪回归(projection pursuit regression,PPR)分析了各影响因数对残余变形的影响权重,得出高聚物胶凝戈壁土动残余剪应变和残余体应变的影响权重依次为高聚物质量比、围压、动应力比和固结比。采用指数函数对高聚物胶凝戈壁土的残余变形与振次的关系进行拟合,建立了能反映高聚物质量比影响的高聚物胶凝戈壁土的修正残余变形模型。

高应变动测技术在如东海域大直径钢管桩基工程中的应用研究

基于江苏如东海域三根大直径敞口钢管桩的初打和复打高应变动测试验数据,提出了适用于该海域的桩端折减系数η的推荐值和承载力恢复计算公式,为该海域大直径钢管桩基础的承载力评估提供了技术依据。敞口钢管桩高应变试验数据显示,试验桩侧阻力的恢复系数都远大于端阻力的,且桩侧阻力随时间增长显著,但端阻力的增长却很小。基于此,推荐如东海域大直径敞开钢管桩的桩端阻力折减系数η取0.05进行估算。此外,通过桩基土体恢复和增长机理探讨,提出桩基承载力恢复计算公式,且验证误差在10%以内。

高强度马氏体不锈钢的小能量冲击疲劳行为研究

随着钻具失效的不断发生,越来越多的研究者进行服役工况下钻具失效-断裂行为的机理研究。在服役过程中钻具受到复杂载荷的作用,既有静载荷也有大能量单次冲击载荷和小能量多次冲击载荷,小能量多次冲击载荷会造成材料冲击疲劳。研究了新型高强度马氏体不锈钢在不同冲击能量下的材料的冲击疲劳失效行为。结果表明,材料的冲击疲劳寿命的影响因素主要是加载的冲击能量,即冲击能量越高材料的冲击疲劳寿命越低,但冲击能量的变化不会改变材料的断裂形式。在疲劳裂纹萌生阶段,冲击能量的提高可使裂纹快速萌生。在疲劳裂纹扩展阶段,冲击载荷作用于裂纹尖端并使裂纹逐渐扩展。最后,建立了裂纹长度a与循环周次N的函数关系,计算了裂纹的扩展速率。

复杂场地条件下PHC桩沉桩可行性分析评价研究

在PHC桩的施工过程中,会遇到复杂的地质条件,如强风化岩层和深厚的硬黏土,从而导致沉桩过程中可能出现无法顺利沉桩到位等质量事故。本文研究利用GRLWEAP分析程序对软土地区的PHC管桩在穿越强风化岩层和深厚硬黏土等复杂场地下进行沉桩的可行性分析,同时,还将分析结果与现场高应变监测试验的结果进行了综合比较。研究发现:高应变监测结合GRLWEAP工具评估沉桩可行性是关键有效的方法;只要合理选择打桩系统和沉桩工艺,PHC桩在穿越强风化岩层场地方面是完全可行的;PHC桩穿越硬黏性土地基时,桩身损伤主要是由土塞效应引起的环向张力造成的。

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。

宝鸡峡漆水河渡槽钻孔灌注桩承载力检测试验

宝鸡峡灌区漆水河渡槽工程对基桩承载力检测试验综合采用了静载法和高应变法,通过两种检测法验证了单桩极限承载力满足设计要求,并对两种检测方法的优缺点、准确性和可靠性进行了对比分析。在进行工程基桩检测时合理进行二者选择、结合,才能满足工程需求,也为类似工程基桩承载力检测提供了参考。

长摩擦桩高应变法测桩分析探讨

高应变法测桩在国内应用已有几十年的历程,广东区域在高应变应用领域更加成熟。高应变相对于单桩竖向抗压静载试验而言,它能够兼具检测桩的承载力和桩身完整性,能够拟合分析桩侧和桩端摩阻力情况。本文通过工程实例介绍高应变法在长摩擦桩中的应用,探讨分析桩身缺陷位置LTD值和桩身完整性系数β值的合理性,以期能够有一定的借鉴价值。

某热电厂工程PHC桩基原体试验研究

文章基于工程勘察报告和桩基原体试验方案,选取代表性场地进行了PHC桩试验施工,并对其分别做了低应变法桩身完整性检测,高应变法抗压承载力试验,以及单桩竖向抗压承载力和单桩竖向抗拔承载力静载试验。桩基原体试验成果表明,PHC桩在本工程中是适应的,其承载力和桩身完整性等满足桩基原体试验任务书要求,为后续的工程桩设计和施工等提供了依据性资料。

静载试验和高应变法在桩基承载力检测中的对比试验

以巴基斯坦某电厂桩基承载力检测为目的,分别采用静载试验法和高应变动力检测法对旋挖法和反循环法两种成桩工艺的四根试桩进行承载力检测,并对结果进行了对比分析。结果表明,静载试验与高应变法实测四根试桩的承载力特征值均大于设计承载力特征值,均满足设计要求;两种方法对不同成桩工艺桩基承载力的检测结果均具有较好的一致性,采用高应变法代替静载试验在该地区进行桩基承载力检测具有较强的可行性和适用性;为该地区桩基承载力检测提供参考和借鉴。

海上大直径超长钢管桩打桩贯入特性及竖向静载试验分析

针对海上大直径超长钢管桩承载力恢复时间长以及现场试验匮乏等问题,依托浙江省宁波舟山港六横公路大桥项目二期工程,对2.0m直径、113m长钢管桩进行初打和复打高应变测试以及静载测试,研究了大直径钢管桩沉桩贯入特性以及不同休止时间的承载力恢复特性。钢管桩沉桩后承载力存在显著的时间效应,沉桩6天后根据复打检测承载力恢复系数达到1.41,沉桩21天后根据轴向抗压试验承载力恢复系数达到2.22,比单桩设计极限值大22.15%。同时实测发现大直径钢管桩内外泥面高差较小,没有明显的土塞效应。该工程现场试验成果可为类似海上大直径钢管桩设计和施工提供技术参考。

Propagation characteristics of transient waves in low-strain integrity testing on cast-in-situ concrete thin-wall pipe piles

The three-dimensional effects of pile head and the applicability of plane-section assumption are main problems in low-strain dynamic tests on cast-in-situ concrete thin-wall pipe piles.The velocity and displacement responses were calculated by a theoretical formula deduced by the authors.The frequency and influencing factor of high-frequency interference were analyzed.A numerical method was established to calculate the peak value and arrival time of incoming waves on top of the piles.The regularity along circumferential and the influence of radius or impulse width were studied.The applicability of plane-section assumption was investigated by comparison of velocity responses at different points in the sections at different depths.The waveform of velocity response at different points forked after the first peak,indicating that the propagation of stress waves did not well meet the plane-section assumption.

灌注桩承载特性测试技术研究进展

灌注桩凭借承载力高、施工工艺成熟、施工机械化程度高等优点被广泛应用于城市建设中。有效确定灌注桩的承载特性和充分掌握承载特性的测试方法,是确保建(构)筑物安全使用的前提和保证。为此,围绕灌注桩承载特性和承载特性关键测试方法进行了梳理,总结和评述中外相关的最新研究现状,系统归纳灌注桩承载特性的影响因素和荷载传递规律,重点介绍低应变法、声波透射法、高应变法、静载荷试验、自平衡试验等灌注桩完整性和承载特性测试方法的原理与特点,简述分布式光纤测试技术在灌注桩承载性能测试方面的最新研究进展,综合现有研究成果对灌注桩桩端阻力、桩侧摩阻力以及桩身荷载传递规律进行概述与分析,最后基于目前灌注桩承载特性及测试方法的研究不足与缺陷,提出了具有针对性的建议措施以促进灌注桩承载特性测试方法的进步与发展。

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。

静动荷载下桩网结构路基土工格栅承载特性试验研究

以京沪高速铁路徐沪段路基为研究对象,通过室内大比例模型试验,研究了静动荷载作用下桩网结构路基土拱效应与土工格栅承载性能的变化规律,分析10万次动荷载作用后土工格栅的承载特性。结果表明:在路基填筑初期未形成稳态土拱前土工格栅应变增长较快,当形成稳态土拱后土工格栅应变增长趋于稳定,且桩帽边缘处土工格栅应变增长最快,土工格栅使得桩顶上方多承担了7.6%的荷载;10万次动荷载作用下,土拱效应依然有效,但是会产生一定程度的弱化,土工格栅减小了动荷载对土拱效应的削弱作用;相比静载动荷载作用下土工格栅应变增长迅速,最大增长了约19%;动荷载作用后土工格栅会产生拉力退化与软化现象。