基于高铁震源简化桥墩模型激发地震波的数值模拟

高铁列车运行过程中绝大部分时间都是行驶在高架桥上的,高铁列车经过桥梁时,通过与大地耦合的桥墩激发地震波发震过程和平地不完全一样.本文探索高铁列车行驶经过高架桥桥墩,通过桥墩对地下介质激发地震波的机理及过程.为了便于理论分析,文中将高铁列车简化为在高架桥上沿一个方向运动的移动线源,通过每节车厢前后组轮对,对每一个桥墩施加力的作用,而桥墩插入地面几十米深至围岩,与表层土壤和深层围岩双重耦合,由此给出高铁列车通过桥墩激发地震波的震源时间函数.同时,基于广义连续介质力学框架下的修正偶应力理论,推导包含介质特征尺度的弹性波动方程,并应用此弹性波动方程以及构建的高铁震源时间函数,采用优化的交错网格有限差分算法,实现数值模拟,将合成的地震记录与实际地震记录对比分析,其结论将为进一步的基于高铁震源的成像和反演研究提供理论依据.     

Field vibration mitigation by honeycomb WIB for pile foundations of a high-speed train viaduct

Environmental vibrations from recent high-speed trains are becoming a special concern in the civil and environmental engineering field since they can give detrimental effects to residents, sensitive equipments and high-tech production facilities in the vicinity of train tacks. Herein, aiming at the vibration mitigation for a specific high-tech industrial area, the low-frequency vibrations from a train viaduct are targeted over an anticipated range. A theoretically designed innovative countermeasure, called honeycomb wave impeding barrier (WIB) for a wave impeding barrier, is introduced and its effects are investigated by computer simulation. The present WIB is based on the wave dispersion phenomenon that can modulate the incoming wavelengths into the shorter wavelengths, creating an apparent wave cut-off characteristic in the wave field across WIB installation area. The shorter wavelengths are further impeded due to the impedance ratio of the WIB walls and in-fill materials and absorbed by the in-fills more. The three-dimensional FEM simulation demonstrates a dramatic reduction effect that is difficult to achieve by conventional measures.     

Numerical prediction of low-frequency ground vibrations induced by high-speed trains at Ledsgaard, Sweden

The ground vibrations induced by a passenger train at the test site of Ledsgaard, Sweden, have been analysed and numerically simulated through a spectral element discretization of the soil. To calculate the spatial distribution of loading due to train passage, the train is decoupled from the track, and a suitable series of static forces is applied. The track and the embankment are modeled as a beam on elastic foundation, using analytical solutions for loads moving at constant velocity. The results of both 2D and 3D modelling assumptions are thoroughly discussed, in terms of prediction of track motion and of attenuation of peak ground velocity with distance.     

Settlement mechanism of piled-raft foundation due to cyclic train loads and its countermeasure

In this paper, numerical simulation with soil-water coupling finite element-finite difference (FE-FD) analysis is conducted to investigate the settlement and the excess pore water pressure (EPWP) of a piled-raft foundation due to cyclic high-speed (speed: 300km/h) train loading. To demonstrate the performance of this numerical simulation, the settlement and EPWP in the ground under the train loading within one month was calculated and confirmed by monitoring data, which shows that the change of the settlement and EPWP can be simulated well on the whole. In order to ensure the safety of train operation, countermeasure by the fracturing grouting is proposed. Two cases are analyzed, namely, grouting in No-4 softest layer and No-9 pile bearing layer respectively. It is found that fracturing grouting in the pile bearing layer (No-9 layer) has better effect on reducing the settlement.     

Prediction and mitigation analysis of ground vibration caused by running high-speed trains on rigid-frame viaducts

In this study a 3D numerical analysis approach is developed to predict the ground vibration around rigid-frame viaducts induced by running high-speed trains. The train-bridge-ground interaction system is divided into two subsystems: the train-bridge interaction and the soil-structure interaction. First, the analytical program to simulate bridge vibration with consideration of train-bridge interaction is developed to obtain the vibration reaction forces at the pier bottoms. The highspeed train is described by a multi-DOFs vibration system and the rigid-frame viaduct is modeled with 3D beam elements. Second, applying these vibration reaction forces as input external excitations, the ground vibration is simulated by using a general-purpose program that includes soil-structure interaction effects. The validity of the analytical procedure is confirmed by comparing analytical and experimental results. The characteristics of high-speed train-induced vibrations, including the location of predominant vibration, are clarified. Based on this information a proposed vibration countermeasure using steel strut and new barrier is found effective in reducing train-induced vibrations and it satisfies environmental vibration requirements. The vibration screening efficiency is evaluated by reduction VAL based on 1/3 octave band spectral analysis.     

回填土场地强震动性质讨论

本文旨在讨论位于回填土场地上及回填土场地深挖基坑内的强震动观测仪所获得地震动是否是真实自由场地震动,若不是,其影响如何?结合回填土场地的强震动观测台站建设的实际情形,建立了回填土场地和回填土场地深挖基坑的有限元分析模型,基于集中质量显式动力有限元数值模拟方法,分析了回填土的波速和厚度、基坑尺寸对自由场地震动的影响.同时,提出了通过用重塑土置换回填土以降低回填土对自由场地震动影响的措施,并对重塑土置换回填土的效果进行了数值模拟分析.结果表明:回填土上和回填土深挖基坑内的地震动峰值和反应谱值与原始场地的均有明显的差异,随着回填土厚度和基坑深度的增加,其差异越大;通过重塑土置换回填土可以减小回填土对场地地震动的影响.本文所得关于回填土、基坑和重塑土的影响规律可为回填土场地上强震动观测结果的合理利用以及强震动观测台站建设提供依据.     

Simulation of tunnel response under spatially varying ground motion

Various components including wave scattering, wave passage, and site amplification effects cause the ground motion to vary spatially. The spatially varying ground motion can significantly influence the dynamic response of longitudinal structures such as bridges and tunnels. While its effect on bridges has been extensively studied, there is a lack of study on its effect on underground tunnels. This paper develops a new procedure for simulating the tunnel response under spatially varying ground motion. The procedure utilizes the longitudinal displacement profile, which is developed from spatially variable ground motion time histories. The longitudinal displacement profile is used to perform a series of pseudo-static three-dimensional finite-element analyses. Results of the analyses show that the spatially variable ground motion causes longitudinal bending of the tunnel and can induce substantial axial stress on the tunnel lining. The effect can be significant at boundaries at which the properties of the ground change in the longitudinal direction.     

Damage process of pile foundation in liquefied ground during strong ground motion

The 1995 Hyogoken–Nambu earthquake caused severe liquefaction over wide areas of reclaimed land. Furthermore, the liquefaction induced large ground displacement in horizontal directions, which caused serious damage to foundations of structures. However, few analyses of steel pipe piles based on field investigation have so far been conducted to identify the causes and process of such damage. The authors conducted a soil–pile-structure interaction analysis by applying a multi-lumped-mass-spring model to a steel pipe pile foundation structure to evaluate the causes and process of its damage. The damage process analyzed in the time domain corresponded well with the results of detailed field investigation. It was found that a large bending moment beyond the ultimate plastic moment of the pile foundation structure was induced mainly by the large ground displacement caused by liquefaction before lateral spreading of the ground and that the displacement appeared during the accumulating process of the excess pore water pressure.     

Experimental investigation into ground vibrations induced by very high speed trains on a non-ballasted track

A field measurement of ground vibration was performed on the Beijing−Shanghai high-speed railway in China. In this paper, the experimental results of vertical ground vibration accelerations induced by very high speed trains running over a non-ballasted track on embankment with speeds from 300 to 410 km/h are reported and analyzed in detail for the first time. Characteristics of ground vibration accelerations in both time and frequency domains are analyzed based on the test data. It is shown that the periodic exciting action of high-speed train bogies can be identified in time histories of vertical accelerations of the ground within the range of 50 m from the track centerline. The first dominant sensitive frequency of the ground vibration acceleration results from the wheelbase of the bogie, and the center distance of two neighboring cars plays an important role in the significant frequencies of the ground vibration acceleration. Variations of time–response peak value and frequency-weighted vertical acceleration level of ground vibration in relation with train speed as well as the distance from the track centerline are also investigated. Results show that the time-domain peak value of ground vibration acceleration exhibits an approximately linear upward tendency with the increase of train speed. With the increasing distance from the track centerline, the frequency-weighted vertical acceleration level of the ground vibration attenuates more slowly than the time-domain peak value of the ground vibration acceleration does. Severe impact of high-speed railway ground vibration on human body comfort on the ground occurs at the speed of 380–400 km/h. The results given in the paper are also valuable for validating the numerical prediction of train induced ground vibrations.     

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.     

沪宁城际铁路高架桥段CRH动车组运行引起的地面振动现场测试与分析

对沪宁城际铁路CRH动车组运行引起的高架桥段地面振动竖向速度和加速度进行了现场测试,分析了地面振动特征及其传播的衰减规律。结果表明：CRH动车组运行引起的地面振动主频在70Hz以下,属于低频振动;地面振动峰值速度和加速度随着离高架桥距离的增大而减小,20m以内地面振动衰减幅度较大;地面振动峰值随列车时速的提高而增大,车厢数量对地面振动峰值和主频成分的影响不明显;CRH动车组运行引起的地面振动对一般性建筑物影响不大,列车时速为300km左右时,地面振动速度超过办公室等公共建筑的允许值,列车时速为200km左右时,地面振动速度超过居民住宅的允许值;与其他高速铁路的地面振动实测值相比,沪宁城际铁路CRH动车组运行引起的高架桥段地面振动强度相对较低。     

Vibration analysis of pile-supported embankments under high-speed train passage

Soft ground improvement using piles has increasingly been used as a rapid construction technique for railway and highway embankments over soft soil areas. While most studies conducted so far have addressed only issues of stability and settlement of pile-supported embankments under static loading, very limited attention has been paid to understanding their behaviors under transient loading of moving vehicles. In this study, vibration behaviors of this embankment system under high-speed train passage are investigated through three-dimensional finite element simulation. They include (1) characteristics of the surface wave field at high train speeds, (2) the dependence of vibration amplitude on the train speed and the phenomenon of critical speed, and (3) response at some typical locations in the system when the train moves at the critical speed. The study shows that there are breaks in the simulated wave fronts as transiting between different materials due to the difference in the Rayleigh wave speed among the materials relative to the train speed, and that the increase in train speed is accompanied by the increase in phase shift between the train load and the displacement pattern beneath the load. It is shown that the critical speed of the system is governed by the embankment, instead of the soft soil as commonly observed in previous studies in which the ground is not improved. Namely the vibration amplitude is maximally amplified when the train speed approaches the characteristic Rayleigh wave speed of the embankment material. In addition, the results also suggest that the sloping surfaces on the ballast and embankment along with the piles form a ‘trapping’ effect by which most of the train-induced waves, especially higher-frequency waves, incident to the sloping surfaces are trapped and dissipated within the pile-supported embankment system, and thus significantly reducing vibration amplitudes outside the embankment.     

确定结构基底等效输入地震动的简化方法

本文用理论和实例计算分析了土与结构间的动力相互作用。根据基底等效输入的地震动相对入射地震动的传递函数特点研究出了一种等效输入的方法，该方法比较好地反映了场地和结构的动力特性对基底等效输入的影响。为利用刚性基底假设理论来分析土-结构动力相互作用提供了一种方法。     

Dynamic interaction of bridge–train system under non‐uniform seismic ground motion

The probability that an earthquake occurs when a train is running over a bridge in earthquake‐prone regions is much higher than before, for high‐speed railway lines are rapidly developed to connect major cities worldwide. This paper presents a finite element method‐based framework for dynamic analysis of coupled bridge–train systems under non‐uniform seismic ground motion, in which rail–wheel interactions and possible separations between wheels and rails are taken into consideration. The governing equations of motion of the coupled bridge–train system are established in an absolute coordinate system. Without considering the decomposition of seismic responses into pseudo‐static and inertia‐dynamic components, the equations of motion of the coupled system are formed in terms of displacement seismic ground motions. The mode superposition method is applied to the bridge structure to make the problem manageable while the Newmark‐β method with an iterative computation scheme is used to find the best solution for the problem concerned. Eight high‐speed trains running over a multi‐span steel truss‐arch bridge subject to earthquakes are taken as a case study. The results from the case study demonstrate that the spatial variation of seismic ground motion affects dynamic responses of the bridge–train system. The ignorance of pseudo‐static component when using acceleration seismic ground motions as input may underestimate seismic responses of the bridge–train system. The probability of separation between wheels and rails becomes higher with increasing train speed. Copyright © 2011 John Wiley & Sons, Ltd.     

地震动衰减关系拟合的新两步法

本文根据地震动衰减的理论分析结果和强震观测记录，提出了一种新的拟合地震动衰减关系的两步法，并用新近补充，完善的美国西部强震记录，对PGA，EPA和EPV衰减特性进行了分析，结果表明，建议的方法能反映近场强震地震动饱和效应和大震远场衰减的特点。     

2D numerical simulations of earthquake ground motion: examples from the Marche Region,Italy

A detailed numerical simulation of the ground motion and a site response analysis for two towns in the Marche Region (Treia and Cagli) is carried out on the basis of structural models deduced from available geological and geophysical data. In both cases, the reference event is an M = 5.7 earthquake associated with a normal fault located beneath each town. The ground motion is computed using the 2D spectral element method (SPEM 2D). The method solves the propagation of the seismic field through complex geological structures and enables an estimate of the effects of deep crustal structure, superficial geology, and topography on ground motion. Numerical simulations of the seismic field are performed along 2D vertical planes containing the seismic source. Strong ground motion has not been yet recorded in the two towns; therefore, the numerical simulation of ground motion represents a way to overcome the lack of instrumental data. The simulations carried out for Treia show that ground motion is influenced by both source mechanism and effects due to propagation through the geological structure, while ground motion in Cagli features strong local effects, caused by the presence of alluvial deposits under a large area of the town.     

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。     

Experimental and numerical investigation on the dynamic response of pile group in liquefying ground

The response of pile foundation in liquefiable sand reinforced by densification techniques remains a very complex problem during strong earthquakes. A shake-table experiment was carried out to investigate the behavior of a reinforced concrete low-cap pile group embedded in this type of ground. In this study, a three-dimensional (3D) finite element (FE) analysis of the experiment was conducted. The computed response of the soil-pile system was in reasonable agreement with the experimental results, highlighting some key characteristics. Then, a parametric study was performed to explore the influence of pile spacing, pile stiffness (EI), superstructure mass, sand permeability, and shaking characteristics of input motion on the behavior of the pile. The investigation demonstrated a stiffening behavior appearing in the liquefied mediumdense sand, and the pile group effect seemed negligible. Furthermore, the kinematic effect was closely connected with both EI and sand permeability. Nevertheless, the inertial effect was strongly influenced by the superstructure mass. Meanwhile, high frequency and large amplitude of the input motion could produced greater the pile’s moments. It is estimated that this case study could further enhance the current understanding of the behavior of low-cap pile foundations in liquefied dense sand.     

基于希尔伯特变换的非平稳地震动模拟方法的验证

基于希尔伯特变换的非平稳地震动模拟方法能够生成一系列地震动样本,具有与给定天然地震动相似的非平稳特征. 该方法在模拟天然地震动时域波形的形态和能量的时频分布方面,效果非常显著且已得到验证. 本文以理想单自由度体系弹塑性地震反应为基础,深入验证该方法在模拟天然地震动对结构影响效应方面的效果. 研究结果表明,该方法能够较好地模拟天然地震动的工程特性,其生成的地震动样本的峰值特性及其引起的结构弹塑性地震响应具有与天然地震动相似的特征.