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.

Dynamic soil arching in piled embankment under train load of high-speed railways

Piled embankments have many advantages that have been applied in high-speed railway construction engineering.However,the load transfer mechanism of piled embankments,such as soil arching and tension membranes,is still unclear,especially under dynamic loads.To investigate the soil arching and tension membrane under dynamic train loads on high-speed railways,a large-scale piled embankment model test with X-shaped piles as vertical reinforcement was performed,in which twenty-eight earth pressure cells were installed in the piled embankment and an M-shaped wave was adopted to simulate the high-speed railway train load.The results show that dynamic soil arching only occurs when two bogies of a carriage pass by and disappears at other times.The dynamic soil arching and membrane effect are the most significant under the concrete base.The arching height,stress concentration ratio and pile-soil load sharing ratio have a minimal value at 25 Hz.The dynamic soil arching degrades severely at 25 Hz,whose height at 25 Hz is only 0.35 times that at 5 Hz.The arching height fluctuates over a narrow range with increasing loading amplitude.The stress concentration ratio and the pile-soil load sharing ratio increase monotonically as the loading amplitude increases.

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.

Effects of topography on dynamic responses of single piles under vertical cyclic loading

This paper describes model tests of single piles subjected to vertical cyclic compressive loading for three kinds of topography: sloping ground, level ground, and inclined bedrock. Comprehensive dynamic responses involving cyclic effects and vibration behaviours are studied under various load combinations of dynamic amplitude, mean load,frequency and number of cycles. Test results show that permanent settlement can generally be predicted with a quadratic function or power function of cycles.Sloping ground topography produces more pronounced settlement than level ground under the same load condition. For vibration behaviour,displacement amplitude is weakly affected by the number of cycles, while load amplitude significantly influences dynamic responses. Test results also reveal that increasing load amplitude intensifies nonlinearity and topography effects. The strain distribution in a pile and soil stress at the pile tip are displayed to investigate the vibration mechanism accounting for sloping ground effects. Furthermore, the dynamic characteristics among three kinds of topography in the elastic stage are studied using a three-dimensional finite method. Numerical results are validated by comparing with experimental results for base inclination topography. An inclined soil profile boundary causes non-axisymmetric resultant deformation, though a small difference in vertical displacement is observed.

Shaking Table Test Study on Dynamic Characteristics of Bridge Foundation Reinforcement on Slopes

With the fast development of bridge construction in mountainous and seismic areas,it is necessary to conduct related research. Based on the design of a shaking table model test,here are the following test results: the filtering effect exists in soil and is affected by the dynamic constraint conditions,the amplitude is strengthened around the natural frequency and weakened in other frequency bands in the Fourier spectrum. Since the acceleration scaling effect occurred on a sloped surface,the acceleration response decreases from the outside to the inside in soil. The dynamic response is relatively strong near the slip surface in bedrock due to the reflection of seismic waves. The failure mode of landslide is decided by the slope angle and slipping mass distribution, and the test shows the front row stabilizing piles should keep a proper distance from bridge foundation so that seismic resistance can be guaranteed for the bridge foundation.

The Dynamic Characteristic Experimental Method on the Composite Foundation with Rigid-Flexible Compound Piles

Based on the idea of optimization design of pile type, the composite foundations, which include cememt-flyash-gaavel (for short CFG) long piles and cement-soil (for short CS) short piles, and CS piles with CFG core as well, are formed. The method of the site dynamic characteristic tests of the composite foundations is discussed. The test results show that fireworks bomb may replace demolitions as the vibration resource. Vibration time is about 0.1 sec. Horizontal vibration major frequency is at 22.476 - 56.436 Hz, and vertical vibration major frequency is at 15.538 - 55.884 Hz. The pile arrangements of the composite foundation in the same site have more effect on the acceleration peak value. From the point of vibration, the anti-seismic effect of the CS piles with CFG core is better than others.

Shaking table test for reinforcement of soil slope with multiple sliding surfaces by reinforced double-row anti-slide piles

Despite the continuous advancements of engineering construction in high-intensity areas,many engineering landslides are still manufactured with huge thrust force,and double-row piles are effective to control such large landslides.In this study,large shaking table test were performed to test and obtain multi-attribute seismic data such as feature image,acceleration,and dynamic soil pressure.Through the feature image processing analysis,the deformation characteristics for the slope reinforced by double-row piles were revealed.By analyzing the acceleration and the dynamic soil pressure time domain,the spatial dynamic response characteristics were revealed.Using Fast Fourier Transform and half-power bandwidth,the damping ratio of acceleration and dynamic soil pressure was obtained.Following that,the Seism Signal was used to calculate the spectral displacement of the accelerations to obtain the regional differences of spectral displacement.The results showed that the overall deformation mechanism of the slope originates from tension failure in the soil mass.The platform at the back of the slope was caused by seismic subsidence,and the peak acceleration ratio was positively correlated with the relative pile heights.The dynamic soil pressure of the front row piles showed an inverted"K"-shaped distribution,but that of the back row piles showed an"S"-shaped distribution.The predominant frequency of acceleration was 2.16 Hz,and the main frequency band was 0.7-6.87 Hz;for dynamic soil pressure,the two parameters became 1.15 Hz and 0.5-6.59 Hz,respectively.In conclusion,dynamic soil pressure was more sensitive to dampening effects than acceleration.Besides,compared to acceleration,dynamic soil pressure exhibited larger loss factors and lower resonance peaks.Finally,back row pile heads were highly sensitive to spectral displacement compared to front row pile heads.These findings may be of reference value for future seismic designs of double-row piles.

Shaking Table Tests on Bridge Foundation Reinforced by Antislide Piles on Slope

Based on the requirement of seismic reinforcement of bridge foundation on slope in the Chengdu-Lanzhou railway project,a shaking table model test of anti-slide pile protecting bridge foundation in landslide section is designed and completed. By applying Wenchuan seismic waves with different acceleration peaks,the stress and deformation characteristics of bridge pile foundation and anti-slide pile are analyzed,and the failure mode is discussed. Results show that the dynamic response of bridge pile and anti-slide pile are affected by the peak value of seismic acceleration of earthquake,with which the stress and deformation of the structure increase. The maximum dynamic earth pressure and the moment of anti-slide piles are located near the sliding surface,while that of bridge piles are located at the top of the pile. Based on the dynamic response of structure,local reinforcement needs to be carried out to meet the requirement of the seismic design. The PGA amplification factor of the surface is greater than the inside,and it decreases with the increase of the input seismic acceleration peak. When the slope failure occurs,the tension cracks are mainly produced in the shallow sliding zone and the coarse particles at the foot of the slope are accumulated.

Fictitious soil pile model for dynamic analysis of pipe piles under high-strain conditions

A fictitious soil pile(FSP)model is developed to simulate the behavior of pipe piles with soil plugs undergoing high-strain dynamic impact loading.The developed model simulates the base soil with a fictitious hollow pile fully filled with a soil plug extending at a cone angle from the pile toe to the bedrock.The friction on the outside and inside of the pile walls is distinguished using different shaft models,and the propagation of stress waves in the base soil and soil plug is considered.The motions of the pile—soil system are solved by discretizing them into spring-mass model based on the finite difference method.Comparisons of the predictions of the proposed model and conventional numerical models,as well as measurements for pipe piles in field tests subjected to impact loading,validate the accuracy of the proposed model.A parametric analysis is conducted to illustrate the influence of the model parameters on the pile dynamic response.Finally,the effective length of the FSP is proposed to approximate the affected soil zone below the pipe pile toe,and some guidance is provided for the selection of the model parameters.

软黏土中含桩地下结构地震响应离心试验模拟

针对大空间含桩地下结构(如地下车行系统)在地震作用下的动力响应特性以及场地震陷、结构上浮等危害展开离心振动台试验。试验采用饱和软黏土作为地基土,设计了自由场、整体浅埋地下结构以及等效单元浅埋地下结构3种试验情况,利用层状剪切箱消除边界反射效应,在50g的离心加速度下开展水平地震试验。试验结果表明,不同结构在地震过程中会产生自振在特定的频率区段放大响应,在整体质量、静态浮力及转动惯量等效的前提下,整体结构相对单元结构形式能够降低地震作用下结构的自振效应,且整体结构具有更好地震后抗浮性能。

液化场地大直径变截面单桩动力响应研究

为研究不同地震动强度下液化场地大直径变截面单桩的动力响应规律,基于振动台试验,选取5010波,在地震动强度0.10g~0.45g作用下,研究液化场地砂土孔压比和大直径变截面单桩桩顶水平位移、桩身弯矩、桩身加速度时程响应及桩基损伤等变化规律.试验结果表明:饱和砂土孔压比随着地震动强度的增大上升明显,地震动强度≥0.30g时,饱和砂土孔压比稳定值在0.9附近,此时砂土完全液化;在0.45g地震动强度作用下,桩身加速度、桩顶水平位移及桩身弯矩均达到最大;桩身不同位置处加速度峰值出现时刻均滞后于输入地震波加速度峰值出现时刻,且桩顶及变截面的加速度响应比桩端的响应更弱;不同地震动强度作用下,桩身弯矩最大值均出现在液化土层和非液化土层分界处,且变截面处弯矩小于土层分界面处;地震动强度达到0.30g时,大直径变截面单桩桩身发生损伤.因此,液化场地下大直径变截面桥梁单桩基础抗震设计时,应该重点考虑饱和砂土层分界处、变截面处的抗弯能力,以确保单桩桩身强度满足抗震要求.

震陷土层变化下变截面单、群桩动力响应差异

为研究地震动作用下震陷土层厚度不同时大直径变截面单桩与群桩基础的动力响应差异,依托厦门第二东通道翔安大桥,通过振动台模型试验,开展震陷土层厚度30、40、50 cm时,单桩与群桩基础桩周土震陷量、桩顶水平位移、桩身加速度及弯矩动力响应差异研究.结果表明:随着震陷层厚度增大,单桩与群桩基础桩周土震陷量、桩顶水平位移、桩身加速度及弯矩均逐渐增大,且加速度及弯矩在变截面处突变;同一震陷层厚度下,群桩基础桩周土震陷量较单桩大,但群桩基础加速度、桩顶水平位移及桩身弯矩均小于单桩.震陷场地桩基础设计时,应着重考虑变截面单桩与群桩动力响应差异,保证桩基础抗震性能.

爆破振动作用下桥梁桩孔稳定性振动台试验研究

为了解爆破振动对邻近桥梁桩基成孔后桩孔稳定性的影响,以与山体爆破工程计划同时施工的某主跨280 m的斜拉桥为背景,设计、制作土箱和缩尺比1∶40桩孔模型开展振动台试验,研究桩孔孔壁的破坏特征、土体振动速度和动土压力的变化规律。结果表明:土体的振动速度峰值和动土压力峰值均随着爆破强度的增大而增大,在土体产生破坏后,土体的振动速度峰值和动土压力峰值均呈现出非线性增大趋势;爆破强度较小时土体的动土压力峰值曲线为U形,爆破强度较大时桩孔区域土体的动土压力峰值曲线为W形,桩孔中上部区域的动土压力峰值较大;山体爆破振动会使钢护筒底部与土体交界面以下桩孔的中上部区域出现破坏,应对山体爆破施工加强现场监测,桩孔附近的振动速度峰值不宜超过2 cm/s,以保障桩孔的稳定性。

单、双排桩支护路堑边坡动力响应特性的振动台试验研究

结合西部艰险山区铁路实际工程背景,开展了El-Centro波作用下单、双排桩支护路堑边坡大型振动台模型试验,基于位移、加速度等参数深入研究了两种模型边坡动力响应特性的差异,并结合快速傅里叶变化(fast Fourier transform,简称FFT)谱探讨了两种边坡出现差异的原因。结果表明:抗滑桩的支护效果和差异均随输入波幅值的增大而逐渐体现;当输入波幅值为0.1g~0.3g时,单、双排桩边坡均处于稳定状态,总体动力响应特性差异较小;当输入波幅值为0.4g时,两边坡动力响应开始出现较大差异,单排桩边坡宏观破坏更明显,坡表位移增大较多,土体损伤累积,非线性特征显现;当输入波幅值为0.5g~0.6g时,两种模型边坡的地震波峰值加速度(peak ground acceleration,简称PGA)放大系数高程效应明显,且均在路堑面和桩后坡面处出现塑性区,抗滑桩的存在有效地抑制了PGA放大系数沿坡面向上的增大趋势和坡表双塑性区的贯通,而单排桩边坡PGA放大效应更明显,塑性区范围更广,深度更大,局部失稳更严重,抗震支护作用相对较弱;引入FFT谱比可知,单、双排桩边坡在5~10 Hz频段振幅放大效应的不同是造成单、双排桩边坡动力响应差异的主要原因;在地震作用下,单排桩边坡破坏历经边坡自稳定→坡表土体塑性变形→边坡局部塌陷溃散3个阶段,双排桩边坡破坏历经前两阶段。

框架梁与锚索桩板墙加固隧道洞口边坡的动力响应特性

针对山岭隧道洞口在降雨、地震等因素作用下易产生变形破坏的问题,以某处采用框架梁与锚索桩板墙支护的隧道洞口边坡工程为原型,开展几何比尺为1∶50的振动台模型试验,分析降雨后地震作用下边坡的动力响应特性及失稳变形规律。结果表明:降雨后坡面无局部破坏,地震作用下边坡的破坏过程可归纳为坡顶张拉破坏—坡脚剪切溃裂—坡体整体失稳滑动,边坡表现为张拉—剪切型破坏;边坡峰值加速度放大系数呈层状分布,“高程效应”与“趋表效应”显著,均随输入正弦波频率、幅值的增加而增大;边坡土体峰值应变分布与坡体滑动破坏面较吻合;桩体锚索轴力与输入正弦波频率、幅值呈正相关,强震作用下锚索轴力增幅显著;桩后峰值土压力近似呈三角形分布,桩体嵌固段内桩土压力最大;桩体弯矩呈“b”型分布,嵌固段内桩体峰值弯矩最大,抗震设计中应注意验算其抗弯强度;桩后土压力与桩体加速度FFT谱幅值主要集中于低频段,结构动力响应主要受地震波低频段部分影响。

强夯置换对软土基坑边坡开挖稳定性的影响研究

强夯置换作为一种地基处理方法,主要用于提高地基承载力,减小沉降,而将其作为基坑支护的相关研究却鲜有报道。通过一系列室内模型试验探讨了不同强夯置换率、基坑边坡坡率条件下,强夯置换后软土基坑在开挖过程中的变形位移规律。通过构建墩土合并计算、墩土分开计算、墩土耦合3种有限元数值模型,对室内模型试验结果进行了对比验证。结果表明:水平位移向四周呈辐射式递减,室内模型试验与数值模拟的水平位移变化趋势一致且吻合度较高;坡体稳定系数随置换率的增加而不断上升;在墩与土之间加入界面单元提高了墩土之间的相互作用,可以有效提高数值模拟的准确性。

粉喷桩与强夯处理技术在软土路基加固中的应用

该文以某公路工程为例,探究粉喷桩法和强夯法在软土路基加固处理中的应用方法。粉喷桩法采用高压喷射水泥的方式,让水泥和岩土在搅拌中均匀混合并成为具有一定强度的柱状体,达到加固软土路基的效果,该方法具有施工速度快、加固效果好等特点,但是施工成本较高;强夯法利用夯锤自由下落夯击软土,达到密实、加固的效果,该方法具有操作简单、成本较低、适用范围广等特点,但是效率不高,且存在噪音污染。经过多方面的综合对比后,最终选择粉喷桩法进行该公路工程的软土路基加固,在保证工程建设质量的前提下降低工程造价、提升社会效益。

填方场地水厂结构桩基动力响应三维数值研究

桩基础是填方场地广泛采用的地基基础。文章依托西南地区某水厂建设工程,采用ABAQUS建立三维计算模型,分析了桩体的加速度、弯矩及剪力响应,探讨了地震动作用下地基-桩基-结构的动力响应规律。结果表明,桩体加速度随桩高的增加整体呈增大趋势,在桩顶处放大效应略有减小;群桩基础不同位置的动力响应有所差异,最靠近填土区域角桩的弯矩与剪力最大。

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。

基于动静比对试验的圆砾持力层预应力管桩选型分析

根据某工地4根不同桩径预应力管桩在沉桩后不同间歇时间的高应变和静载试验结果,分析高应变法与静载试验的机理差异,指出由于圆砾层的多成分颗粒结构,桩端阻力的发挥存在明显的“尺寸效应”,应谨慎对待大直径管桩高应变法的检测结果,为提高基桩承载力、降低单位承载力成本,宜优先选用大直径管桩。