Mechanical characterisation of the fouled ballast in ancient railway track substructure by large-scale triaxial tests

In the track substructure of ancient railways in France, a fouled ballast layer has often been created with time. The mechanical behaviour of this coarse soil was studied in the laboratory using a large-scale triaxial cell. The soil taken from the fouled ballast layer of an ancient railway was re-compacted to a dry density of 2.01 Mg/m³ at three water contents (4, 6, and 12%) corresponding to three values of the initial degree of saturation (32, 48, and 100% respectively). Both monotonic and cyclic triaxial tests were performed under constant water content conditions. The experimental results gave the following evidence of the significant effect of the water content on the soil mechanical behaviour: (i) the lower the compaction water content, the higher the shear strength; (ii) a permanent axial strain of 0.4% was found after a large number of cycles at a water content of 4%, while it was 1.4% at the higher water content of 6%. For the saturated soil specimen, failure was even observed after a limited number of cycles. Based on the results obtained, a constitutive model for permanent deformation was elaborated, that accounts for the stress level, the number of cycles and the soil water content.     

An analytical model for particle-geogrid aperture interaction

The shear strength of geogrid-reinforced ballast is often dependent on the aperture size of geogrids and the nominal size of ballast. This paper presents a theoretical analysis based on probabilistic mechanics of how aperture size affects the interaction between particles and geogrid. Unlike past literature, in this study, the properties of the particle size distribution is analysed using a Weibull distribution. The probability of grain interlock is proposed to describe the interactive mechanisms between particles and geogrids based on the relative particle size, which is defined as the ratio of particle size to aperture size. The mathematical model is calibrated by a set of large-scale direct shear tests with almost single-size (highly uniform) ballast aggregates, and then validated by independent set of data taken from both literature and current study. The study concludes that more uniform particle size distribution increases the probability of grain interlock at the optimum aperture size but decreases it at non-optimum aperture sizes.     

Drainage potential of degraded railway ballast considering initial gradation and intrusion of external fine materials

The aggregate in the ballast layer of the railway track structure degrades due to the splitting of single particles and the abrasion of the angular corners, resulting in a ballast course comprised of degraded aggregate with a lower drainage potential. In the present study, the hydraulic conductivity of clean, degraded and clay-fouled degraded ballast specimens was evaluated considering the initial particle size distribution of aggregate and the degradation mechanism to investigate the permeability of ballast fouled by both internal and external factors. First, degraded ballast aggregate was generated by using a large-scale impact loading testing apparatus. Then, a large-scale constant head permeability test was performed to estimate the hydraulic conductivity of degraded ballast aggregate contaminated with clay. Based on the obtained results, the flow regime through extremely degraded ballast remained between a completely turbulent regime (observed in the case of clean ballast) and a laminar flow regime. Darcy’s law was shown to be valid for degraded aggregate samples contaminated with clay. While the degradation of the aggregate had the expected effect of reducing hydraulic conductivity, the abrasion of asperities was shown to have an even greater impact on hydraulic conductivity. Also, the further degradation of aggregate under impact loading was shown to accelerate the reduction in the permeability of clay-fouled degraded ballast to an unacceptable extent.     

Discrete Element Modeling for fouled railroad ballast

Fouling refers to the condition of railroad ballast when voids in this unbound aggregate layer are filled with relatively finer materials or fouling agents commonly from the ballast aggregate breakdown, outside contamination such as coal dust from coal trains, or from subgrade soil intrusion. This paper aims to investigate the effect of fouling on ballast strength and stability. Fouling mechanism was first explained followed by investigation of mechanical properties of fouling agent. Coal dust was chosen as the fouling agent due to its poor mechanical properties and it had been reported to mostly decrease aggregate assembly strength comparing to other fouling agents. An image-aided Discrete Element Modeling (DEM) approach was introduced to simulate the coal dust fouled track field settlement performance. To that end, DEM model parameters for clean and coal dust fouled ballast were validated by matching the DEM direct shear box simulation results to the laboratory shear box testing results for both samples. By assigning laboratory calibrated model parameters to the “half-track” ballast sample generated in DEM, effects of different fouling percentage as well as different fouling locations on ballast settlement performance are studied. Results from the “half-track” DEM simulation revealed that fouling could lead to unfavorable track distresses such as “hanging tie”. Further, shoulder fouling scenario was proven to be critical in track maintenance.     

Laboratory and field investigation of the effect of geogrid-reinforced ballast on railway track lateral resistance

Continuous welded rail (CWR) tracks have particular advantages over common tracks with jointed rails such as increased ride comfort, reduced noise and vibration and decreased maintenance costs due to the removal of joints in rail connections. Alternatively, some complications associated with CWR tracks, for instance increased lateral forces, are the main reason of track buckling and its subsequent lateral deformation. These problems are usually more severe in curved tracks. In order to overcome the large lateral forces caused by temperature deviations of CWR tracks which results in railway vehicle instability, the ballasted track lateral resistance should be improved. Among the various methods proposed in this area, no specific study has been carried out on the effect of geogrid reinforcement on ballasted track lateral resistance. Thus, the present research was allocated to investigating the effect of geogrid on the lateral resistances of both single tie and track panel via laboratory and field tests. In this regard, at the first stage, the ballast layer was reinforced with various number of geogrid layers, the effect of which was investigated by conducting the single tie push test (STPT) in the lab environment to assess the optimum number of geogrid layers and their installation levels along the ballast layer thickness. Afterwards, a test track was executed in the field including various sections which were reinforced in the same way as the lab tests. Consequently, many STPTs and track panel displacement tests (TPDTs) were accomplished. As a result, the STPTs in the lab and field confirmed more than 31% and 42% increase in single tie lateral resistance for ballast layers reinforced respectively with one and two geogrid layers, while these values were reached to 29% and 40% in the case of TPDT.     

Behavior of geogrid-reinforced ballast under various levels of fouling

This paper presents a study of how the interface between ballast and geogrid copes with fouling by coal fines. The stress-displacement behavior of fresh and fouled ballast, and geogrid reinforced ballast was investigated through a series of large-scale direct shear tests where the levels of fouling ranged from 0% to 95% Void Contamination Index (VCI), at relatively low normal stresses varying from 15 kPa to 75 kPa. The results indicated that geogrid increases the shear strength and apparent angle of shearing resistance, while only slightly reducing the vertical displacement of the composite geogrid-ballast system. However, when ballast was fouled by coal fines, the benefits of geogrid reinforcement decreased in proportion to the increasing level of fouling. A conceptual normalized shear strength model was proposed to predict this decrease in peak shear stress and peak angle of shearing resistance caused by coal fines at a given normal stress.     

碎石粒径对寒区路堤自然对流降温效应的影响

介绍了一种研究路堤碎石层自然对流效应的实验方法。对由碎石、卵砾石和砂砾石组成的6种试样进行了实验研究和理论分析,结果表明,在表面温度周期波动条件下,碎石层中能产生有利于寒区路基稳定的自然对流降温效应,利用平均Rayleigh数可确定碎石层自然对流的发生情况。在相同边界条件下,不同粒径的碎石层将产生不同的自然对流降温效应,其强度随粒径的增大而增大,并随表面温度周期波动而具有时间累加性。同时,确定何种粒径碎石的冬季降温效应为最佳还需要综合考虑其热传导性质。     

北京地铁九号线沿线卵石地层特性研究

以北京地铁九号线地质调查资料为背景,结合野外探测、室内试验和抽水试验研究了地铁沿线卵石地层的分布特征、粒径级配特征、卵石的强度、卵石地层的渗透性等工程地质特性,得出了最大粒径—深度的关系曲线分布结果。通过研究卵石最大粒径在不同工点的分布规律,并根据分布规律及试验参数,分析预测了该地层中不同工法在施工中可能遇到的工程地质...     

Effectiveness of geogrid reinforcement in improvement of mechanical behavior of sand-contaminated ballast

Vertical stiffness and shear strength of ballasts are significantly degraded when contaminated with sands. There is a lack of solutions/studies related to strengthening ballast against sand contamination. Addressing this limitation, a comprehensive laboratory investigation was made on effectiveness of geogrid reinforcement for improvement of mechanical properties of sand-contaminated ballast. To this end, large-scale direct shear tests as well as plate load tests were conducted on geogrid-reinforced ballast samples prepared with different levels of sand contamination. The obtained results indicate that geogrid reinforcement considerably improves shear strength and vertical stiffness of contaminated ballast. A bandwidth was obtained for contamination levels in which ballast reinforcement is effective. Through examining geogrid with different aperture sizes and locations in the ballast layer, the best performance conditions of geogrid reinforcement were derived. The results were used to propose an effective method of ballast reinforcement and an efficient ballast maintenance approach in sandy areas.     

Investigating geogrid-reinforced ballast: Experimental pull-out tests and discrete element modelling

This paper presents an evaluation of the interlocking behaviour of geogrid-reinforced railway ballast. Experimental large box pull-out tests were conducted to examine the interaction between ballast and a biaxial geogrid. The discrete element method (DEM) was then used to model the interaction between the ballast and the geogrid by simulating large box pull-out tests and comparing the findings with the experimental results. Four different shapes of clumps were used to represent each ballast particle in order to obtain an acceptable shape for modelling the railway ballast. The DEM simulation results were shown to provide good predictions of the pull-out resistance and to examine the effect of clump shape on both the pull-out resistance and the distribution of contact forces. Therefore, the calibrated geogrid model and the 8-ball tetrahedral clumps, used as ballast particles, hold much promise for investigating the interaction between geogrids and ballast, and thus, optimising performance.     

缩尺效应对铁路道砟力学特性影响的试验研究

为了研究颗粒级配、缩尺效应对道砟强度及变形特性的影响,采用石英岩标准铁路道砟开展了一系列基于1/4缩尺及全尺寸道砟的三轴剪切试验。试验结果表明:缩尺道砟的峰值强度均较全尺寸道砟的峰值强度低,缩尺对窄级配道砟的峰值强度影响甚小,而使宽级配道砟的峰值强度降低明显,缩尺效应造成的峰值强度降幅最高达10%;道砟的强度指标c,?值受颗粒级配影响较大,而缩尺效应对其影响并不显著。全尺寸与缩尺道砟的体积应变特性差异较小,缩尺道砟的变形模量小于全尺寸道砟,颗粒级配对道砟变形模量的影响不大;在工程设计中采用缩尺道砟的强度和变形参数将有利于安全。     

循环荷载下土工格栅加筋铁路道床累积#br# 沉降模型试验研究

散粒体道床层的循环累积变形是有砟轨道沉降的主要来源，文章采用有砟轨道路基模型试验来研究道砟层与底砟层间布置土工格栅加固道床控制有砟轨道沉降，开展了高速和重载等不同列车荷载下与不同类型土工格栅加固条件下的多组循环加载试验，加载过程中全程监测道砟层累积沉降、轨枕振动、道砟层底部土压力与土工格栅应变数据并进行分析，以研究土工格栅控制有砟轨道道床沉降的作用机理。研究结果表明：土工格栅对于控制道砟层累积沉降具有较显著的效果，但当土工格栅的刚度达到一定程度后刚度对土工格栅控制沉降效果的影响不大；土工格栅能够显著减小轨枕下方道砟层土压力峰值，从而降低道砟磨耗破碎程度及由此引起的道床沉降；道砟层累积沉降的发展意味着道砟颗粒的错动，而道砟颗粒的错动与位移将引起嵌锁于道砟层中的土工格栅发生张拉，从而土工格栅将反过来对道砟颗粒的进一步位移错动形成侧向约束，体现于宏观即表现为道砟层累积沉降得到控制。     

Fracture toughness estimation of ballast stone used in Iranian railway

Ballast is a layer composed of crushed stone basically with diameters of 20-60 mm,on which sleepers and rails are set.Ballast is used to withstand vertical,horizontal and lateral forces applied on sleepers and to hold the line in operative conditions.Ballast deterioration induced by crashed stones is a major issue of track instability as the ballast layer quality depending on the materials used and their densities should be focused on.Therefore,ballast should be resistant against loads applied,and the fracture toughness of ballast stone is of great importance.For this purpose,the fracture toughness of two kinds of ballast stones used in Iranian railway,i.e.Gaduk(limestone) and Anjylavnd(andesite),is investigated experimentally in this paper.The quality of ballast stone is evaluated in different weather conditions.Numerical results shown that the Anjylavnd stone is more appropriate for rainy and cold weather when there is a probability of fracturing due to frozen water captured in ballast.     

超径颗粒粒度效应对基底材料力学特性的影响

本文对“基底型”颗粒材料中超径颗粒含量、颗粒粒径、基底土干容重以及试验围压4个因素对其力学特性的影响进行了大量的室内三轴试验,获得了各影响因素的定量参数。通过对试验成果的分析,着重探讨了基底型颗粒材料中超径颗粒粒径和含量两个因素对其强度变形特性的影响规律及其力学机理,提出了用单位剪切线上的超径颗粒数目这一特征指标来综合反映超径颗粒尺寸和含量对基底材料强度特性的影响效应。并进一步对试验中所观测到的一个重要的物理现象-基底型颗粒材料变形形态的分叉现象,进行了理论分析。     

62号大号码高速无砟道岔铺设技术研究

62号道岔是我国目前号码最大、侧向通过速度最高的铁路道岔,该大号码高速无砟道岔工艺流程和其他无砟道岔有相近之处,但其构件大,铺设精度要求高,东北高寒环境下施工条件恶劣。如何确保大号码道岔满足高速客运专线高速度、高舒适性、高安全性的要求,如何解决该特大号码道岔的吊装、铺设、精调、混凝土浇筑等诸多技术难题在国内外均无成功经验可以参考。本文针对该道岔轨件长细比大、直侧股铺设精度要求高、铺设地区温差大等技术难点,开展大号码高速无砟道岔铺设技术研究,对确保其整体技术性能具有重要意义。     

考虑填料–土工格室相互作用的加筋路堤力学响应研究

针对有砟轨道加筋路堤计算分析中难以考虑土工格室对碎石填料嵌锁及摩擦作用的现状,在大比尺直剪试验基础上建立考虑填料–格室相互作用的三维计算模型。通过室内模型试验验证该计算的可靠度和优越性,进而系统地分析有/无加筋情况下,填料强度、土工格室刚度及地基压缩性等参数对路堤力学响应的影响。结果表明:土工格室的嵌锁作用使得加筋对路堤响应的影响在其布设层发挥程度更大,该影响通过填料与格室及填料内的摩擦作用向周围传递且不断衰减;填料强度较低或地基较软时能加强土工格室对路堤响应的影响,对于填料内摩擦角为20°和地基变形模量为5 MPa的工况而言,加筋后基床侧向位移峰值分别降低60%和72%,应力峰值亦显著降低且分布区域明显扩大;土工格室弹性模量为0.2~2.5 GPa时,路堤力学响应受其影响较大。     

Design of non-woven geotextiles for coal refuse filtration

This paper presents research findings on grain size distribution changes of coal refuse affecting the design of non-woven geotextiles used as filters in rock drains at coal waste impoundments. The research involved performing hydraulic conductivity tests on refuse - geotextile filters followed by grain size distribution tests. Data was evaluated for geotextile filter retention, permittivity, and clogging potential requirements as published by the U.S. Mine Safety and Health Administration’s Second Edition, “Engineering and Design Manual Coal Refuse Disposal Facilities”.Key findings indicate that refuse particles undergo slaking and aggregation which change the initial grain size distribution. Grading envelopes were developed and indicate that particle size zones influence the geotextile design parameters for retention, filtration, and clogging. The clogging criteria do not appear to be easily satisfied by the typical ranges of coarse coal refuse, at pre- and post-compaction grain size, for compatibility with non-woven geotextiles having an AOS = 0.212 mm.Conclusions impacting the specification and field installation of geotextiles include: i) post grain size distribution tests are suggested to be performed on specimens and at all compaction levels to observe changes in key indices of D₈₅ and D₁₅ for meeting retention and clogging criteria requirements; ii) the evaluation of the initial refuse stability indicate that at the low compaction energy conditions, which have mobile fines and high Cu values, are initially unstable with regards to their internal soil gradation; and iii) construction of geotextile wrapped drains is preferred to be made in pre-compacted refuse lifts. This condition is beneficial because the filter becomes more stable for retention and permeability; however clogging is still a concern.     

高速铁路无砟轨道基础结构垂向位移分布特性分析

建立高速铁路无砟轨道-路基结构动力有限元模型,系统分析高速列车荷载作用下轨道和路基垂向位移在时间和空间上的分布规律,比较轨道不平顺和断面位置对位移分布的影响规律。数值分析结果表明,轨道不平顺和横断面位置对轨道和路基垂向位移分布的影响可以忽略不计;高速列车荷载作用下钢轨和轨道板垂向位移的最大值分别为0.79mm和0.238mm,结果都在京津铁路现场试验实测数据的范围之内;轨道板和底座的垂向位移沿横向衰减非常缓慢,仅分别降低了3.6%和6.5%,沿深度基本没有变化;路基各层面垂向位移在轨道宽度范围内沿横向仅衰减10%左右,轨道宽度范围外位移按指数函数快速衰减,在距离线路中心线3m和4m附近,基床表层和基床底层的垂向位移已衰减一半以上,路基位移沿深度线性衰减,在距钢轨底面4m附近,垂向位移衰减50%左右,到基床底面处位移衰减70%以上。     

Characterization of a ceramic ultrafiltration membrane in different operational states after its use in a heavy-metal ion removal process

In the present study, the atomic force microscopy (AFM) technique has been used to characterize a Carbosep M5 ceramic membrane (MWCO = 10 kDa, TiO₂-ZrO₂ active layer). This membrane was previously used in a polymer supported ultrafiltration (PSU) process to recover copper, using partially ethoxylated polyethylenimine as the water-soluble polymer. The membrane was characterized in four different operational states: new, new and cleaned, fouled in a PSU stage and cleaned after a PSU process. The influence of the membrane state on pore opening size distribution and roughness was studied, finding a 16% decrease in the former and a 20% increase in the latter due to foulant deposition upon the membrane active layer. Phase angle distribution was also analyzed to indicate the foulant spreading on the membrane surface. These phase angle measurements can be related to pore opening size and roughness, concluding that the cleaning procedure is not totally effective and that foulant presence on the membrane active layer is not remarkable. Finally, AFM was used to measure the influence of pH on adhesion forces between a silica probe and the membrane active layer. These results can be related to the flux evolution vs pH in PSU experiments, finding both lowest adhesion and highest flux at pH 6.