不同厚度饱和砂土中群桩结构动力响应试验研究

液化土中桩基础动力响应规律一直是工程抗震领域关注的热点问题。本文基于非液化砂土和不同厚度饱和砂土中的2×2群桩结构模型振动台试验,通过输入一定峰值加速度和频率的正弦波,对群桩在非液化土层和两种不同厚度饱和砂土层中的横向动力响应特性进行振动台试验研究。研究结果表明:在正弦波输入情况下,非液化砂土中群桩承台加速度和位移时程与台面输入时程相比,波形变化规律与峰值大小均相差不大;而对两种不同厚度饱和砂土中承台加速度和位移峰值放大较多,在相对较薄的饱和砂土中群桩承台加速度峰值较台面输入放大了1.83倍,较台面输出位移峰值放大了1.58倍;在相对较厚的饱和砂土中承台加速度和位移峰值则分别放大了2.18倍和1.91倍,说明在相同输入条件下,较厚的饱和砂土在发生液化后群桩承台的动力响应更加显著。     

爆破地震作用下桩-土-结构相互作用的数值模拟

土-结构动力相互作用是地震工程和结构抗震的重要研究内容,但目前对爆破地震作用下土-结构动力相互作用的研究较少。运用大型有限元软件ANSYS/LS-DYNA,建立了桩-土-结构相互作用体系的三维有限元模型,由桩尖输入实测爆破地震波,取得了良好的计算效果。计算结果表明：考虑桩-土-结构相互作用后,群桩基础中每个桩的位移、加速度和剪应力幅值均呈桩顶大、桩尖小的倒三角分布,桩与承台的接合部比较容易受到损坏;桩-土-结构相互作用体系在爆破地震波冲击后,还会发生几次振动,但是这些振动产生的影响要小于爆破地震产生的影响,这与实测结果相符合;爆破地震波冲击下,群桩基础中,角桩顶部表面的桩土接触压力较大,但在爆破地震波冲击后,中心桩顶部表面的桩土接触压力较大,且具有一定的周期性,直至衰减为零。     

近断层脉冲型地震下建筑地桩基础沉降分析

传统基于碰撞回弹系数Stereo-mechanical方法在分析建筑地桩基础的沉降位移过程中未考虑地桩基础碰撞的能量损耗,造成分析结果存在准确性和可信度较低的问题。因此提出改进的Kelvin建筑地桩基础碰撞沉降分析方法,以解决基于碰撞回弹系数Stereo-mechanical法存在的问题。采用线性阻尼构建Kelvin模型,分析地桩基础碰撞靠近阶段和回弹阶段的阻尼做功,构建地桩基础碰撞转换单自由度体系振动模型,实现对地桩基础的相对沉降速度和相对沉降位移的有效运算,基于建筑地桩基础碰撞沉降发生初始和结束时刻的能量和动量守恒定理,得到建筑地桩基础的最大沉降位移,实现对建筑地桩基础沉降的有效分析。实验结果说明,所提方法能提高地桩基础沉降分析的准确性和可信度。     

Mitigation measures for pile groups behind quay walls subjected to lateral flow of liquefied soil: Shake table model tests

This paper presents experimental results of a series of 1g shake table tests on mitigation measures for a model consisting of a 3×3 pile group and a sheet-pile quay wall in which the pile group was subjected to liquefaction-induced lateral spreading. First, general observations associated with the mechanism of lateral spreading and pile response are presented based on tests without remedial measures, followed by in depth discussions. Second, three remedial techniques were deployed to provide an adequate seismic performance of the pile group and the quay wall: (i) mitigating sheet pile of floating type, (ii) mitigating sheet pile of fixed end type, and (iii) anchoring the quay wall to a new pile row. The main objective of these mitigation methods was to restrict ground distortion behind the quay wall, enhancing seismic response of pile group and quay wall. This mitigation philosophy was decided based on the outcome of the first part, which consisted of a series of tests without mitigation measures. In addition, it should be noted that the proposed countermeasures were selected to be applicable for existing vulnerable pile groups, which are at risk of liquefaction and lateral spreading. Results of different mitigation tests are comparatively examined using a parameter called reduction factor, and the effectiveness of each countermeasure is discussed in detail. The results demonstrate that by applying the proposed mitigation measures the seismic performance of both pile group and quay wall can be improved, as a result of reduction in soil displacement and velocity of soil flow.     

Flexible support of a pile embedded in unsaturated soil under Rayleigh waves

Under the action of Rayleigh waves, pile head is easy to rotate with a concrete pile cap, and pure fixed-head condition is rarely achieved, which is a common phenomenon for it usually occurs on the precast piles with insufficient anchorage. In addition, the propagation characteristics of Rayleigh wave have been changed significantly due to the existence of capillary pressure and the coupling between phases in unsaturated soil, which significantly affects the pile-soil interaction. In order to study the above problems, a coupled vibration model of unsaturated soil–pile system subjected to Rayleigh waves is established on the basis that the pile cap is equivalent to a rigid mass block. Meanwhile, the soil constitution is simplified to linear-elastic and small deformations are assumed to occur during the vibration phase of soil–pile system. Then, the horizontal dynamic response of a homogeneous free-field unsaturated soil caused by propagating Rayleigh waves is obtained by using operator decomposition theory and variable separation method. The dynamic equilibrium equation of a pile is established by using the dynamic Winkler model and the Timoshenko beam theory, and the analytical solutions of the horizontal displacement, rotation angle, bending moment and shear force of pile body are derived according to the boundary conditions of flexible constraint of pile top. Based on the present solutions, the rationality of the proposed model is verified by comparing with the previous research results. Through parametric study, the influence of rotational stiffness and yield bending moment of pile top on the horizontal dynamic characteristics of Rayleigh waves induced pile is investigated in detailed. The analysis results can be utilized for the seismic design of pile foundation under Rayleigh waves.     

液化侧扩流场地桥梁群桩效应分析

基于u-p有限元公式模拟饱和砂土中水和土颗粒完全耦合效应,建立液化侧向流场地群桩动力反应分析的三维数值模型。模型中,砂土采用多屈服面弹塑性本构模型模拟、黏土采用多屈服面运动塑性模型模拟,群桩在计算过程中保持线弹性状态;采用20节点的六面体单元和考虑孔压效应的20-8节点分别划分黏土层和饱和砂层;选用剪切梁边界处理计算域的人工边界,模拟地震过程中土层的剪切效应;应用瑞利阻尼考虑体系的阻尼效应。随后对比分析2×2群桩中各单桩的地震反应规律,结果表明,各单桩的弯矩、位移时程规律基本一致,峰值弯矩及峰值位移出现时刻滞后于输入加速度峰值时刻,上坡向桩的弯矩和位移峰值大于下坡向的桩的反应值。接着通过改变桩间距研究群桩效应,随着桩间距增加,群桩中各单桩的弯矩最大值均出现在土层分界处,且各单桩的弯矩、桩顶位移逐渐增大。最后给出液化侧向流场地群桩效应的基本原因,得出该类场地群桩抗震设计的基本认识。     

PHC管桩单桩振动台试验与数值模拟对比分析

利用振动台对PHC管桩进行单桩承台模型试验;同时将土体PHC管桩-上部结构视为共同工作整体,建立三维有限元模型,再以ABAQUS软件为计算平台,对单桩试验工况下的地震反应进行数值模拟计算,并将试验结果与数值模拟计算结果进行对比分析。结果表明：数值模拟和振动台模型试验基本吻合,两者得到的地震反应影响体现出相似的规律性,印证了模型试验的可靠性和数值模型的合理性。     

Prediction of high-speed train induced ground vibration based on train-track-ground system model

The development of analysis on train-induced ground vibration is briefly summarized. A train-track-ground integrated dynamic model is introduced in the paper to predict the ground vibration induced by high-speed trains. Representative dynamic responses of the train-track-ground system predicted by the model are presented. Some major results measured from two field tests on the ground vibration induced by two high-speed trains are reported. Numerical prediction with the proposed train-track-ground model is validated by the high-speed train running experiments. Research results show that the wheel/rail dynamic interaction caused by track irregularities has a significant influence on the ground acceleration and little influence on the ground displacement. The main frequencies of the ground vibration induced by high-speed trains are usually below 80 Hz. Compared with the ballasted track, the ballastless track structure can produce much larger train-induced ground vibration at frequencies above 40 Hz. The vertical ground vibration is much larger than the lateral and longitudinal components.     

打桩引起的地面振动的研究

为了对打桩引起的地面振动进行研究,应用一维应力波理论建立了粘弹性成层土中等截面弹性桩的力学模型,得到了桩中任意一点处位移的半解析解。利用桩与土的相互作用将桩对土的作用力简化到各土层面上。在复阻尼理论和纳维方程的基础上,利用分层法得到了任意荷载作用下土的位移、速度和加速度的解。从而得到了打桩引起的地面振动的衰减特性。通过实测结果和计算结果的比较说明了该方法的可行性。     

单体边坡地形的地震动力响应及其放大效应的数值分析

本文采用隐式动力有限单元法研究了不同的边坡角度和边坡高度对地形放大效应的影响,并以位移峰值放大系数为衡量地震动放大效应的标准,计算了不同边坡角度和边坡高度条件下的地震响应,在此基础上对模型关键监测点的输出波形以及位移峰值放大系数的变化趋势进行了分析,获得了不同监测点处的地震动时程曲线,揭示了坡角和坡高对单体边坡地震动放大效应的定量作用规律。数值结果表明,相同高度处坡面监测点的水平向位移峰值放大系数大于坡内监测点的,地形放大效应在水平方向具有趋表效应。由于坡面存在入射波和反射波的叠加,因此竖直向位移峰值放大系数的最大值出现在坡体内部。      

承台型式对可液化场地桥梁桩-柱墩地震响应影响振动台试验

基于相同土层结构地基条件下,分别采用低承台群桩-独柱墩与高承台群桩-独柱墩结构,完成了两次可液化场地群桩-土-桥梁结构地震反应振动台试验,据此研究了承台型式对桥梁桩-柱墩地震反应的影响。研究表明,与高承台桩相比,可液化场地中低承台桩的抗震性能更优;地震中砂层尚未液化或液化不充分时,低承台更多表现出减弱桩尤其桩上段的加速度反应的作用,相反高承台更多起到放大桩的加速度作用,而高承台桩与低承台桩的峰值应变自下而上更多表现出逐渐增大趋势;即使砂层完全液化时,低承台桩的峰值应变自下而上仍以渐增为主;与低承台桩相比,高承台桩更有助于放大墩顶加速度、位移反应,对结构体系整体稳定性产生了不良影响;虽然低承台桩未出现严重破坏,但砂层中部桩的应变却很大,液化砂土-桩运动相互作用对桩的抗震性能影响不容忽视。     

端承桩在层状地基波动与辐射阻尼影响下的横向地震反应分析

通过对层状地层中端承桩的分析,建立了合理的几何,数学及力学模型。考虑层状地基ＳＨ与ＳＶ波波动及非线性辐射阻尼影响,应用Ｄｉｒａｃ广义函数及Ｗｉｎｋｌｅｒ地基理论计入各种动力,抗力,地震力及内力等,建立地基波动方程,桩振动方程,地基与桩耦合振动方程。给出了柯西型函数的自由振动解。利用非经典理论正交条件和卷积定理给出了强迫振动地震反应解析解,算例结果充分表明了动力特性及振动规律非常正确,充分体现了本文     

强震作用下桩-土-断层非线性动力相互作用特性

为研究强震区跨断层桥梁桩基非线性动力相互作用特性,依托海文大桥实体工程,利用MIDAS/GTS有限元软件,建立了桩-土-断层相互作用模型,分析0.20~0.60g地震动强度下断层上下盘桩基加速度响应、桩顶水平位移、桩身弯矩以及桩身剪力响应情况。结果表明:覆盖层土体对桩身加速度放大作用明显,且随着输入地震动强度的增大,放大作用逐渐减弱;覆盖层对地震波的滤波作用显著,随着输入地震动强度的增大,滤波作用逐渐减弱;上盘桩基达到桩顶峰值加速度的时刻滞后于下盘;随着输入地震动强度的增大,上、下盘桩的桩顶产生的永久位移和水平位移峰值逐渐变大,上盘桩顶产生的永久位移和桩顶峰值位移均大于下盘,产生显著的"上盘效应";不同强度地震动作用下,断层上、下盘桩基弯矩均在上部土层界面处达到峰值,剪力均在基岩面处达到峰值,下盘桩基弯矩和剪力峰值大于上盘桩基,呈现出显著的"下盘效应"。在桥梁桩基抗震设计时,应着重考虑断层上、下盘桩基的差异和不同强度地震作用对桩基承载特性的影响。     

桩承式高速铁路路基的地震反应特性研究

桩承式路基在我国高铁路基中广泛应用,因此,论文建立地震荷载作用下轨道-路基-地基三维数值计算模型,研究地震荷载作用下桩承式路基的地震反应特性,分析桩径、桩长、桩间距、桩身弹性模量等计算参数对路基地震反应的影响,并与自由式路基的地震反应对比.自由式路基地震反应特性表明,地震荷载作用下水平x方向钢轨振动位移幅值最大,是z方...     

Numerical analyses of pile performance in laterally spreading frozen ground crust overlying liquefiable soils

Lateral spread of frozen ground crust over liquefied soil has caused extensive bridge foundation damage in the past winter earthquakes. A shake table experiment was conducted to investigate the performance of a model pile in this scenario and revealed unique pile failure mechanisms. The modelling results provided valuable data for validating numerical models. This paper presents analyses and results of this experiment using two numerical modeling approaches: solid-fluid coupled finite element (FE) modeling and the beam-on-nonlinear-Winkler-foundation (BNWF) method. A FE model was constructed based on the experiment configuration and subjected to earthquake loading. Soil and pile response results were presented and compared with experimental results to validate this model. The BNWF method was used to predict the pile response and failure mechanism. A p-y curve was presented for modelling the frozen ground crust with the free-field displacement from the experiment as loading. Pile responses were presented and compared with those of the experiment and FE model. It was concluded that the coupled FE model was effective in predicting formation of three plastic hinges at ground surface, ground crust-liquefiable soil interface and within the medium dense sand layer, while the BNWF method was only able to predict the latter two.     

打桩荷载作用下自由场土体振动衰减规律研究

为研究打桩荷载作用下自由场土体振动衰减规律，建立了考虑桩-土相互作用的二维有限元数值模型，并通过Lamb问题解析解验证了数值模型的有效性。通过分析打桩深度、土体阻尼比、打桩荷载等级和土质条件等因素的影响，研究了土体表面振动特性及振动衰减规律。参数分析表明，打桩深度对微振动的影响较小，在距振源一定距离处的土体表面振动响应基本保持一致；土体阻尼比对土体表面振动的影响显著，阻尼比越小，土体表面振动响应越剧烈；不同场地软硬条件影响微振动的限制距离，在一定距离范围内，土质越软，土体表面振动响应越显著，防振距离越长。基于参数分析结果，对峰值速度衰减曲线进行拟合，拟合公式计算结果与模拟结果较吻合，可为振动敏感建筑场地的选择提供参考。     

The effect of canyon topography on strong ground motion at Feitsui damsite: Quantitative results

The free-field accelerograms along Feitsui Canyon are analysed and modelled by a numerical scheme to study the effect of canyon topography. Six strong-motion accelerometers (SC1–SC6) were deployed along the Feitsui Canyon in 1991. The observations suggest that the amplitude of ground motion at the trough of the canyon is smaller than that on its slopes. The integral equation method is applied to a two-dimensional model of Feitsui Canyon to study the effects of the canyon topography. We choose the ground motion of SC3 station at the trough of the canyon as the input motion to the model, which is then used to predict the ground motion at the other five stations. The simulated and observed seismograms agree well in displacement and velocity, although the simulated acceleration at high frequency is underestimated. The simple model can reproduce the observed accelerations at all frequencies below 4 Hz.     

Experimental p-y curves for liquefied soils from centrifuge tests

The present study aims to obtain p-y curves(Winkler spring properties for lateral pile-soil interaction) for liquefied soil from 12 comprehensive centrifuge test cases where pile groups were embedded in liquefiable soil. The p-y curve for fully liquefied soil is back-calculated from the dynamic centrifuge test data using a numerical procedure from the recorded soil response and strain records from the instrumented pile. The p-y curves were obtained for two ground conditions:(a) lateral spreading of liquefied soil, and(b) liquefied soil in level ground. These ground conditions are simulated in the model by having collapsing and non-collapsing intermittent boundaries, which are modelled as quay walls. The p-y curves back-calculated from the centrifuge tests are compared with representative reduced API p-y curves for liquefied soils(known as p-multiplier). The response of p-y curves at full liquefaction is presented and critical observations of lateral pile-soil interaction are discussed. Based on the results of these model tests, guidance for the construction of p-y curves for use in engineering practice is also provided.     

Centrifuge modelling of raked piles

Inclined piles are prohibited by many codes in seismic areas. Nevertheless the battered effect has not yet been clarified because very few data are available. The present work is a comparison, at reduced scale in the centrifuge, of the response of two simplified pile groups: a 1 × 2 vertical piles and 1 × 2 pile group with one inclined pile. Two configurations are considered: end-bearing and floating pile group, both with pile heads rigidly fixed with a massive cap. First, repeatability tests under horizontal cyclic loading were performed on both floating pile groups. Secondly, repeated horizontal impact tests were performed on both end-bearing pile groups. These impact tests, which highlight the influence of inclined piles on the inertial response of a group, are a first step for the more complex analysis of the performance of such groups under seismic loads where inertial and kinematic interactions are combined. The first part of this work revealed the influence of sand structure around the inclined pile tip on the repeatability of the tests performed on floating pile groups. The second part highlighted differences in the dynamic response between the two end-bearing pile groups through measurements of the pile cap acceleration, the bending moment profile and the axial load in the piles.     

不同厚度饱和砂土中群桩P-Y滞回曲线规律研究

根据Buckingham π定理设计制作直群桩和斜群桩相似模型,通过电磁式振动台试验方法,分别考虑非液化砂土、300 mm和380 mm两种不同厚度饱和砂土,开展在规则正弦波输入下桩土相互作用P-Y滞回曲线规律研究.结果 表明:在非液化砂土中,P-Y滞回曲线的主斜率变化较小,说明在振动输入过程中,桩周土体刚度并未发生显...