Evaluation of Indirect Tensile Strength of Track Ballast Using Weibull Statistics

Iraq is planning to expand and rehabilitate its current railway network, thus demanding huge amounts of proper ballast material. There are three common quarries in Iraq, Sinjaar in the northern sector, Al-Qaim in western desert and Najaf-sea in the middle and southern parts of Iraq. The selection of proper track ballast material is of prime importance in design and construction of railway projects. The single particle-crushing test is a simple indirect tensile test that provides useful data for the selection of the material. Two types of railway ballast materials from (Najaf-sea and Al-Qaim) quarries are commonly used in the construction of railway in Iraq. The single particle-crushing test was performed for various ballast sizes, and the obtained data were analyzed using Weibull analysis. Najaf-sea crushed stones indicate higher tensile strength for all particle sizes compared to corresponding particle sizes of Al-Qaim crushed stone. In addition, an inverse linear relationship had obtained, between tensile strength and particle size with R ² (0.837 and 0.979) for Al-Qaim and Najaf-sea crushed stone, respectively, such relationships are important in evaluating the suitability of the materials in terms of the survival probability     

Three‐dimensional modelling of differential railway track settlement using a cycle domain constitutive model

A method for simulation of differential (spatially varying) track settlement in a ballasted railway track is presented. It employs a cycle domain constitutive model to determine accumulated plastic (permanent) deformation of the granular layers supporting the track. The constitutive model is adopted for both the ballast and the sub‐ballast but with different parameter sets. The proposed framework can be used to predict differential track settlement accounting for heterogeneous (space‐variant) track characteristics and loading conditions. Here, it is demonstrated for three‐dimensional continuum modelling of a railway crossing panel subjected to a large number of axle passages. Because of the design of the crossing panel and the transient character of the impact loads on the crossing, the load transferred into the track bed is not uniform along the track, and the resulting differential settlement leads to vertical irregularities in track geometry. The spatial variation of track settlement is calculated both along the sleepers and along the rails. The influences of the number of adjacent sleepers accounted for in the model and the stiffness of the subgrade on the predicted settlement at the crossing are studied. Copyright © 2016 John Wiley & Sons, Ltd.     

DEM analysis of angular ballast breakage under cyclic loading

Railway ballast particles undergo significant amount of breakage under repeated train load. Breakage of ballast particles, especially highly angular fresh ones, causes an increase in settlement, contributing to track degradation. The quantitative analysis of the influence of breakage on the stress-strain properties of ballast can be performed either experimentally or numerically. Numerical modeling has the advantage of simulating ballast breakage subject to various types of loading and different boundary conditions for a range of material properties. In this paper, ballast breakage under cyclic loading is simulated using a 2D discrete element method (DEM) utilizing the software PFC^2D . A new subroutine is developed and incorporated in the PFC^2D analysis to study ballast breakage and to quantify breakage in relation to particle size distribution. The influence of confining pressure on both breakage and permanent deformation is also studied and compared with laboratory observations. The findings of this paper provide an insight into the true ballast behavior under cyclic loading and are expected to assist railway practitioners in developing suitable design criteria for track stability.     

A generalized model for geosynthetic reinforced railway tracks resting on soft clays

The present study pertains to the development of a foundation model for predicting the behavior of geosynthetic reinforcement railway track system rested on soft clay subgrade. The ballast and sub‐ballast layers have been idealized by Pasternak shear layer. The geosynthetic layer is represented by a stretched rough elastic membrane. Burger model has been used to characterize the soft clay subgrade. Numerical solutions have been obtained by adopting the finite difference scheme combined with non‐dimensioning the governing equations of the proposed model. The results confirm that the present model is quite capable of predicting the time‐dependent settlement response of geosynthetic reinforcement railway track system placed on soft clay subgrade. The surface settlement profile and mobilized tensile load of geosynthetics has been evaluated by considering variation in the wheel load, sleeper width, thickness of ballast and sub‐ballast layers and shear modulus of ballast and sub‐ballast layers. It has been observed that an increase in the sleeper width by 24% results in the reduction in central settlement and mobilized tensile load by 6.5% and 20.1%, respectively. It was found that with a 50% increase in the thickness of the ballast layer, the central settlement has decreased by 7.3% and the mobilized tension at the zone of maximum curvature has increased by 24.6%. However, with an increase in the thickness of the sub‐ballast layer, a considerable reduction in both central settlement and the mobilization of tension on geosynthetic has been noticed. The pattern of variation of settlement and mobilized tension for an increase in the shear modulus of ballast and sub‐ballast material was found to be almost similar. Copyright © 2014 John Wiley & Sons, Ltd.     

低围压循环荷载作用下饱和粗粒土的动力特性与骨干曲线模型研究

基床层是铁路路基的核心组成部分,一般为粗颗粒土,厚约2.5～3.0 m,长期直接承受行车荷载的反复作用,其在动载反复作用下的变形特性是评价路基工作性能的关键要素之一。为研究粗粒土在列车循环荷载作用下的应力-应变特性,开展了一系列应力控制的单向循环加载大型动三轴试验,模拟列车动载和路基粗粒料填筑实际情况,包括不同动应力幅值（模拟不同列车轴重）、不同围压（模拟不同埋深）的动三轴持续振动试验。结果表明,在循环荷载作用下,土体刚度变化与振动次数、围压关系密切。根据动应力幅值大小的不同,循环荷载作用下饱和粗粒土的动应变随振次的发展形态可分为3种类型：稳定型、破坏型和临界型。根据试验所得出的动应力-应变关系曲线特点,建立了含围压和循环振次的骨干曲线模型。与传统的骨干曲线模型相比,该模型能反映土体刚度随循环振次的变化,更能反映列车往复作用的实际情况;同时该模型能用于估算路基土体动强度,对铁路路基核心层的动力变形稳定性评价和基于动力变形控制的路基设计具有参考价值。     

Dynamic characteristics of multiscale longitudinal stress and particle rotation in ballast track under vertical cyclic loads

Loads transfer in ballast track through contacts among randomly distributed ballast particles and have strong heterogeneity. Since the size ratio between ballast track and ballast particles is generally small, using averaged stress to describe the internal mechanical state in ballast track faces practical difficulties. For example, particle movements and high local concentration stress tend to be ignored. The inter-particle contact stress is crucial to evaluate the particle behaviors, such as abrasion, movements, and furtherly the performance of ballast track. However, the contact stress on ballast particles is hard to predict or measure. We conduct a full size model test to investigate the dynamic characteristics of longitudinal stress on ballast particles as well as different lateral regions under vertical cyclic loads with various loading magnitudes and frequencies. An obvious seesaw effect of longitudinal contact stress is observed: the stresses at some contact areas have the same phase with applied cyclic load while at other contact areas have an opposite phase. The seesaw effect of contact stress is then used to evaluate the rotational movements of ballast particles. The variation of contact area and stress of the ballast particles with loading magnitudes demonstrates that the rigid contact assumption is appropriate when analyzing the contact behavior of ballast particles. The cumulative probability distribution of contact stress with stress level can be described by an inversely proportional function, based on which the maximum contact stress can be estimated according to the longitudinal average stress. Besides, the lateral dispersion angle of the vertical loads in the ballast track is about 35°, which is independent of the given loading magnitudes and frequencies.

     

高速铁路沉降控制复合桩基的性状试验研究

基于沉降控制设计理念,无砟轨道京沪高速铁路地基处理采用筏板+垫层+疏桩的方法,形成复合桩基以实现有效减少工后沉降和充分利用地基承载力的优化加固方案。为探索该新方法沉降控制机制,选用CFG桩开展了复合桩基现场试验研究,对复合桩基在高速铁路路基填筑、静置、预压卸载过程中的地基沉降变形、桩和桩间土土压力、筏板顶与底部压力进行了长期观测,分析了路基沉降变形、桩-土应力比和荷载分担比以及筏板的受力随填筑高度和固结时间的变化规律。研究表明：筏板＋垫层＋疏桩联合加固地基方案在初期充分发挥了桩间土承载作用,导致桩与桩间土产生差异沉降;随着垫层的调节作用,筏板可集中发挥桩体的承载能力及显著提高桩顶应力集中程度,地基土沉降主要发生在加固区范围内,从而揭示了复合桩基在路基荷载下的承载机制和变形特性。现场试验结果可为指导高速铁路CFG桩复合桩基设计参数的进一步优化提供试验依据。     

Discrete element modeling of dynamic behaviors of railway ballast under cyclic loading with dilated polyhedra

The dynamic behaviors of railway ballast under cyclic loading are simulated with discrete element method (DEM). Dilated polyhedra are constructed based on the Minkowski sum operator in order to resemble the irregular shapes of ballast particles. The polyhedral particle generation, contact detection between particles and contact laws are presented. Ballast box tests with periodic lateral boundaries are conducted to simulate the dynamics of the sleeper and ballast particles. The settlement and effective stiffness of ballast bed are investigated under cyclic loadings with five distinct frequencies. The settlement of ballast bed is significant in the first several cycles and increases with the number of cycles gradually. The higher frequency loading generates larger displacement in the same simulation time. The effective stiffness of ballast bed increases gradually. To study the effect of particles' sharpness, dilated polyhedra with different dilating radii and spherical particles are also developed. Simulation results show the sharper the ballast particles are, the smaller the produced settlement. Copyright © 2016 John Wiley & Sons, Ltd.     

高速铁路高架线路列车运营对既有线路基的影响研究

为分析高速铁路高架线列车运行对既有线路基影响,建立桥墩-大地-既有线路基有限元模型,结合测试验证模型可靠性,分析不同行车条件对既有线路基应力状态的影响关系,讨论高速铁路轨道平顺性恶化和土体模量降低对既有线路基的影响,预测既有线路基表层累计变形。研究得到,既有线自身列车运行对路基应力状态的影响占主要地位,高速铁路运营对既有线路基应力状态的贡献度很小,不会对既有线路基使用寿命造成明显影响,高速铁路桥墩与既有线距离按限界控制即可满足要求;在轨道平顺性恶化和土体模量降低条件下,高速列车运营引起的既有线路基应力增加不超过0.5 kPa。     

无碴轨道路基基床动力特性的研究

以遂渝线无碴轨道路基为背景,通过室内模型试验研究,分析了在循环加载条件下路基基床的动态力学特性。试验结果表明,动应力响应在基床表层横断面方向上呈“W”形分布,混凝土基础板轨下位置响应最大,中线处和端部响应较小,但随着深度的增加,逐渐变为盆形分布特征;在基床表层范围内,动态响应最为强烈,且随深度的增加,衰减速率较快;加载频率对动应力影响较小,对动位移及加速度影响较大。另外,在遂渝线无碴轨道综合试验段现场实车试验中,分别进行了CRH2型动车组和货物列车不同运行速度下路基基床的动力学响应测试研究,验证并评价了遂渝线无碴轨道路基基床工程适应性。     

路基沉降预测的三点修正指数曲线法

科学、合理地预测路基工后沉降量是高速铁路建设的关键环节。针对武广高速铁路路基沉降量级小、数据相对波动大的实测数据,探讨了指数曲线法对无砟轨道路基沉降预测的适用性,发现指数曲线法不能直接应用于量级小、数据相对波动较大的沉降预测。把三点法的基本思想引入指数曲线模型,对指数曲线法进行了改进,提出了路基沉降预测的三点修正指数曲线模型。结合武广高速铁路路基沉降观测数据,分析了三点修正指数曲线模型的特性。分析表明,在整个沉降曲线上选取3个关键点作为预测样本,很好地回避了数据波动带来的影响;沉降曲线上“拐点”以后的沉降规律更符合指数曲线模型,因此,应取沉降曲线上“拐点”以后的数据作为样本值,所取三点应能尽量反映沉降发展的趋势。三点修正指数曲线法预测结果稳定、相关系数高,具有一定的工程应用价值     

重载铁路路基面动应力峰值随机分布特征研究

掌握列车移动荷载作用下路基的动应力响应特性可为路基沉降预测、状态评估提供依据。建立重载铁路车辆-轨道-路基三维动力有限元模型,并在钢轨轨面引入中国三大干线不平顺轨道谱以实现轨道不平顺的模拟。通过数值计算,分析了轨道高低不平顺对路基面动应力分布的影响,统计了4种列车轴重下路基面3个位置（钢轨下方,线路中心）处的动应力峰值沿线路纵向的分布,对路基面动应力峰值的随机分布形式进行了正态性检验,对动应力峰值的最大值进行了预测。结果表明：轨道不平顺导致路基面动应力沿路基面中心不对称,路基面动应力峰值沿线路纵向服从正态分布。随着轴重的增大,动应力峰值离散程度增大,动应力峰值分布曲线变陡。动应力峰值的预测值和现场实测值相吻合,证明了有限元模型及统计分析方法的正确性。研究结果对路基动变形及累积沉降的可靠性分析提供依据。     

铁路路基粗粒土填料临界动应力试验研究

粗粒土填料被广泛应用于铁路基床填料中,直接承受轨道结构传递的列车动循环荷载的长期作用,研究其在列车动荷载作用下的动力行为特征及塑性变形特性可为路基状态评估、沉降控制提供思路。采用GDS动三轴试验系统对铁路基床表层的粗粒土填料动力响应开展研究,通过引入塑性应变率和安定理论,将不同频率、围压、循环动应力比等条件下路基填料的轴向塑性应变的发展规律划分为塑性安定、塑性蠕变和增量塑性破坏3种类型,并确定了塑性安定和塑性蠕变状态的临界动应力水平。研究表明：粗粒土填料的临界循环应力比随着围压的增大而增大,随着荷载频率的增加而减小。通过对试验结果的拟合分析,提出了以围压为变量的临界动应力经验公式,为合理评估列车荷载作用下路基任意深度的动力稳定提供了理论依据。     

Study on the particle breakage of ballast based on a GPU accelerated discrete element method

Breakage of particles will have greatly influence on mechanical behavior of granular material(GM)under external loads,such as ballast,rockfill and sand.The discrete element method(DEM)is one of the most popular methods for simulating GM as each particle is represented on its own.To study breakage mechanism of particle breakage,a cohesive contact mode is developed based on the GPU accelerated DEM code-Blaze-DEM.A database of the 3D geometry model of rock blocks is established based on the 3D scanning method.And an agglomerate describing the rock block with a series of non-overlapping spherical particles is used to build the DEM numerical model of a railway ballast sample,which is used to the DEM oedometric test to study the particles’breakage characteristics of the sample under external load.Furthermore,to obtain the meso-mechanical parameters used in DEM,a black-analysis method is used based on the laboratory tests of the rock sample.Based on the DEM numerical tests,the particle breakage process and mechanisms of the railway ballast are studied.All results show that the developed code can better used for large scale simulation of the particle breakage analysis of granular material.     

循环加载下X形桩竖向承载特性模型试验研究

实际工程中桩基经常受到各种动荷载作用,如高铁路基中的加固桩长期承受列车行车时产生的循环动荷载作用,桩基在循环荷载作用下的承载特性研究对动荷载下的工程设计至关重要。X形桩是一种在传统圆形截面桩基础上发展而来的新型异性桩技术,其承载机制不同于传统圆形截面桩。为了深入研究X形桩在循环荷载作用下的动力特性和荷载与沉降规律,开展了砂土中X形桩竖向循环加载大比尺模型试验。试验结果表明,随着循环加载的进行,X形桩顶产生累计沉降,且循环荷载比越大,加载频率越高,桩顶沉降越快;循环加载初期,X形桩顶动刚度降低,桩身轴力响应增大,桩侧摩阻力发生弱化,之后逐渐趋于稳定,且桩身下半段侧摩阻力较大;在同等加载条件下X形桩与等截面圆形桩相比,承受动荷载能力较强,桩侧摩阻力较大,长期循环加载作用下产生的累计位移较小。研究结果可为X形桩在动荷载作用下的工程设计提供参考依据。     

重复荷载下模型支盘桩工程性状的试验研究

为研究在粉黏土中支盘桩在重复加载下的工程性状,在粉黏土中进行一次加载试验,测出其极限承载力,分别对一个双盘模型支盘桩在其极限荷载和0.75倍极限荷载的条件下各进行了5次重复加载卸载试验。根据试验结果,分析了在该土层中支盘桩的承载力和变形特性,研究了重复荷载作用下模型支盘桩在粉黏土中荷载传递的特点、桩身不同位置压力变化的特点,特别是对桩周土体对桩侧表面产生的摩擦力出现复杂变化的原因进行了分析,同时还分析了离盘不同距离的土体在重复加载过程中的压力变化情况和原因。研究结果表明,不同强度的荷载重复作用下对桩的沉降变形的收敛影响很大;支盘桩和桩周土体的相互作用机制十分复杂,因此要充分认识支盘桩在重复荷载作用下的工程性状和荷载传递机制还要做大量地研究。     

A hypo-plastic approach for evaluating railway ballast degradation

Repetitive or cyclic rail loading deteriorates the engineering properties of the railway ballast by particle crushing and rearrangement. Most of the classical elasto-plastic models are unable to predict such ballast degradation despite successfully predicting the overall load–deformation behavior during cyclic densification. In this context, the present study delivers a novel hypo-plastic modeling approach coupled with breakage mechanics theory to bridge the gap of the conventional models. The hypo-plastic approach enables to predict the nonlinear load–deformation response of ballast-type granular materials for both monotonic and cyclic loading conditions, while circumventing the requirement of notional yield condition to predict the inelastic behavior. Breakage mechanics theory, on the other hand, establishes the links between particle comminution and the macroscopic deformation. The novelty of the proposed approach is threefold. Firstly, unlike the conventional hypo-plastic approaches, the development of the proposed model is within the continuum thermodynamics framework. Secondly, the model requires less number of physically identifiable parameters as compared to that of earlier models employed for assessing the particle breakage under cyclic loading. Third and finally, the numerical implementation of the model as a user-defined material is simple for solving boundary value problems. Under the compressive deformation regime, the model prediction of the ballast degradation along with the cyclic densification response agrees reasonably well with the experimental results found in the literature.     

A constitutive model for coal-fouled ballast capturing the effects of particle degradation

Rail tracks undergo degradation owing to particle breakage and fouling of ballast by various fines including coal and subgrade soil. As the ballast becomes fouled, its strength and drainage capacity are compromised, sometimes resulting in differential settlement and reduced track stability. This paper demonstrates a continuum mechanics based framework to evaluate the detrimental effect of fines on the strength, deformation and degradation of coal-fouled ballast under monotonic loading. An elastoplastic constitutive model that considers the effect of fines content and energy consumption associated with particle breakage during shearing is presented. This multiphase constitutive model is developed within a critical state framework based on a kinematic-type yield locus and a modified stress-dilatancy approach. A general formulation for the rate of ballast breakage and coal particle breakage during triaxial shearing is presented and incorporated into the plastic flow rule to accurately predict the stress–strain response of coal-fouled ballast at various confining pressures. The behaviour of ballast at various levels of fouling is analysed and validated by experimental data.     

TJ-1模拟月壤承载特性室内试验研究

本研究自行设计加工了一套可以实现位移速率控制的加载装置,采用TJ-1模拟月壤为地基材料,把着陆器足垫作为基础进行了位移速率控制条件下的载荷模型试验,并采用了3种不同方法计算足垫基底的平均应力。通过对试验结果的分析对比得出以下主要结论：足垫的地基承载力和变形模量随加载速率大致呈线性增加,按足垫与地基的实际接触面积计算的极限承载力稍大于按足垫最大上口面积计算得到的地基承载力。考虑到安全储备建议采用按最大上口面积计算所得地基承载力。足垫基础下按实际接触面积计算得到的变形模量介于按足垫最小底面和最大上口面面积计算的结果之间。     

重复列车荷载作用下季节性冻土区桥墩-基础场地振动特性及长期变形分析

基于高速列车运行引起的轨道-桥梁-桥墩-季节性冻土区场地的地面振动和沉降问题, 选取哈大高速铁路铁岭至四平段某桥墩及周围基础场地为测试段, 对实测数据从时域和频域两方面进行分析, 研究了桥墩及周围不同场地的振动特性, 结果表明:桥墩和基础场地的振动特性存在很大的差异, 基础场地对振动有放大效应, 且不同基础场地对振动的放大效果也明显不同。结合实测概况建立了桥墩-基础场地有限元数值模型, 分析桥墩及基础场地在不同季节的振动传播特性, 以及基础场地土体内部的应力分布情况, 并利用累积塑性应变模型对重复列车荷载作用下季节性冻土区基础场地的沉降变形进行分析, 发现场地振动加速度峰值随与桥墩距离R的增大而衰减, 且在冻结季的振动衰减速度明显小于非冻结季的; 基础场地地表的累积沉降在距桥墩R=0.5 m处最大, 且随着列车荷载作用次数的增加而增加, 最后逐步趋于稳定。