Ballast Grains Research Articles

A computational fluid dynamics study of the influence of sleeper shape and ballast depth on ballast flight during passage of a simplified train

The paper assesses the effect on the air flow regime underneath a simplified high-speed train of changing the ballast depth and the sleeper shape, with regard to its potential for causing ballast flight or pickup. The study was carried out numerically using the commercial Computational Fluid Dynamics (CFD) software AnSys Fluent. The flow profile beneath the underbody of the train was generated by means of a moving wall above the track. The Delayed Detached Eddy Simulation (DDES) with the SST [Formula: see text] turbulence model was used to simulate turbulent flow, and the ballast bed roughness was applied parametrically using the wall roughness feature when resolving the boundary layer. CFD simulations were validated for flow over a cube, showing good agreement with experimental results. Up to three different depths to the ballast surface and three different sleeper profiles were investigated. Velocity profiles and aerodynamic forces on cubes placed between or on top of the sleeper blocks were used to assess the propensity of individual ballast grains for movement. For a standard G44 sleeper, increasing the ballast depth and/or the ballast bed roughness was found to reduce aerodynamic loads on an individual ballast grain. A ballast grain on top of the sleeper is more prone to uplift than a grain on the surface of the ballast bed in the crib. A curved upper surface to the sleeper is beneficial in that it prevents ballast from settling on top, the most vulnerable position. However, the reduced flow separation associated with the curved top may increase the likelihood of ballast pickup from the crib. Hence new sleeper shapes intended to reduce the potential for ballast flight should not only prevent ballast from settling on top, but also increase flow separation through the provision of a sharp surface. A prismatic sleeper shape that achieves both is suggested.

Assessment of macro and micro mechanical properties of fresh and deteriorated ballast combining laboratory tests and 2D-discrete element methods

This research initially investigates the mechanical behavior of fresh and used ballast grain assemblies through a combination of laboratory experiments and discrete element modeling (DEM). The extended Los Angeles abrasion (LAA) test generated the used ballast grains defined at a fouling index (FI) of 40%, while fresh ballast grains serve as the reference material. Morphological analysis of angularity properties of fresh and used ballast grains was employed in producing 2D polygonal shapes implemented in a discrete element code. Numerical simulations using the contact dynamics method examined the bi-axial compression of the 2D-equivalent fresh and used ballast grain assemblies under varying confining pressures. Results show that both fresh and used ballast grain packings exhibit similar stress-strain curves, with fresh ballast packing displaying slightly higher shear strength (5%) at the residual state. Fresh ballast packing is denser than the used one at the peak shear state, while residual states show nearly identical volume fractions. Microscale analysis of contact and force networks reveals that the used ballast assembly is more anisotropic in contact at peak stress. At the same time, both packings exhibit similar contact anisotropy at the residual state. Fresh ballast packing exhibits higher force anisotropy, particularly in frictional forces, explaining the excess shear stress observed at the residual state. These findings suggest that used ballast grains mobilize local friction less efficiently than fresh ballast grains, potentially impacting the lateral strength properties of ballasted tracks. This study provides valuable insights into the behavior of used and fresh ballast grains, serving as a foundation for further investigations.

Effects of Particle Size and Grading on the Breakage of Railway Ballast: Laboratory Testing and Numerical Modeling

Ballast is coarse aggregate with particle size normally ranging from 10 mm to 65 mm. Upon repeated train loading, ballast deteriorates in the form of either continuous abrasion of sharp corners or size degradation, which have been reported as the fundamental cause for the instability of railway tracks. In this study, the splitting behavior of ballast grain with varying particle sizes under diametrical compression was examined to investigate the size effect and the Weibull characteristics of ballast tensile strength; a Weibull modulus of 2.35 was measured for the tested granite ballast. A series of large-scale monotonic triaxial tests on ballast aggregates having various size gradings was performed to study the effect of particle gradation on the mechanical behavior of ballast. The results show that compared to mono-sized uniformly distributed aggregates, non-uniformly distributed aggregates generally have greater shear strength, larger peak friction angle, 50% strength modulus, and greater volumetric dilation. The ballast aggregate conforming to the recommended PSD as per current standards exhibited the most superior mechanical performance, possessing the greatest shear strength, peak friction angle, and 50% strength modulus. Micromechanical analysis showed that aggregates with larger d50 values have higher coordination numbers, inter-particle contact forces, and higher anisotropy level of contact normals, thus causing a greater possibility of particle breakage during shearing.

Combined Influences of Water Content and Coarse Grain Content on Shear Strength of Unsaturated Soil Mixture.

An interlayer existed between the ballast layer and subgrade in the conventional railway substructure. Considering that the shear strength τ of the interlayer soil was influenced by the changes in the ballast grain content and water content, this aspect was explored in the present study. Monotonic triaxial tests were fulfilled, which considered five coarse grain contents fv and three water contents of fine soil wf. The results showed that the growth in fv contributed to an increment in τ of the soil mixture under both saturation and unsaturation. Conversely, in previous studies, the growth of fv induced an increment in τ under saturation, but a decline in that under unsaturation. This was explained by the competing influences of fv and suction ψ: in previous studies, increasing fv induced a decline in the dry density of the fine soil fraction ρd-f, which contributed to a decline in ψ. When the negative influence of declining ψ outweighed the positive influence of the incrementing fv, the τ of the soil mixture decreased. Meanwhile, modelling of the τ-ψ relationship in the soil mixture with varying fv was performed. This proposed model was examined using the test results from both the present and previous studies, which shows its reasonably good performance.

Experimental investigation of mixing railway ballast grains with different form using large-scale direct shear box apparatus

This paper investigates the effect of grain form on the mechanical behaviour of a railway ballast aggregate using large-scale shear box tests. The speciality of the research is that – contrarily to most of the experimental studies, where the shape content of the assemblies are not manipulated – here, the grains were carefully sorted and the ballast samples were assembled with the desired size and form content. A new sorting method was developed for laboratory-scale samples to separate compact and non-compact (flat, elongated or bladed) grains. This method utilises the combination of standard bar sieves and shape index calliper. The geometries of the grains in the resultant assemblies were thoroughly investigated using modern 3D imaging-based shape characterisation methods. With the adequate parameter thresholds, good correspondence was found between the standard industrial and the modern imaging-based methods, which allows the virtual reproduction of the introduced laboratory sorting technique. Three types of andesite railway ballast assemblies were tested experimentally: a fully compact, a fully non-compact and a mixed one. The mixed assembly had the highest internal friction angle which proves that it can be beneficial to mix grains with different types form. The amount of broken mass and the failure modes of the grains were also recorded, and it was found that the non-compact assembly had the highest rate of breakage. The results of the study also serve as a good basis for the calibration of numerical models.

Predicting Ballast Fouling Conditions Through RGB-Based Statistical Quantity Analysis

Evaluating railway ballast fouling is critical to assessing track conditions and planning proper ballast maintenance. Varying material properties associated with different ballast fouling conditions can be used to evaluate the severity of fouling. Although approaches using ground penetration radar, impulse response, surface wave, and SmartRock have been developed to estimate the fouling conditions, these methods require special sensors and equipment, or well-trained technicians. Recently, convolutional neural network (CNN) based computer vision approaches have become popular in performing particle segmentation to obtain the ballast grain size distribution. The coarse aggregate fraction can be evaluated by the CNN method, but it is not easy to segment fine particles accurately with these approaches. This study proposes a novel image analysis approach to directly estimate the ballast fouling conditions. First, fouled ballast images with different fouling conditions are taken as references. The RGB (red, green, blue) color distributions of the fouled ballast images are then processed through statistical analysis. A strong linear correlation is found between Fouling Index (FI) and variance and this is used to establish the FI prediction model. The established FI prediction model is tested and validated by additional fouled ballast samples. The FI prediction model proposed in this study can be used to quantify ballast fouling conditions with promising performance.

Механические свойства базальтового щебеночного балласта

Purpose: Study of strength and deformation properties of basalt broken stone ballast depending on its grain composition, the properties are necessary for further assessment of bearing capacity and deformability of railway track ballast layer. Methods: Determination of mechanical properties of broken stone ballast from basalt rock was carried out in a triaxial compression chamber which intense state close to real operating conditions was simulated. In the trial process, the dependences of axial and volumetric deformations from tension deviator were being determined. The methods of trial result statistical processing and regression analysis were used in the work. Results: According to laboratory test result statistical processing, the key characteristics of broken stone ballast strength as well as deformation modulus and Poisson’s ratio depending on the ballast grain composition were determined. Practical importance: The study results make it possible to elaborate recommendations regarding broken stone ballast grain composition, the ballast is from the basalt used in the areas of especially heavy traffic of trains.

Grain characterisation of fresh and used railway ballast

Ballasted railway track requires regular maintenance to reverse the effects of plastic deformation of the trackbed, which leads to a gradual loss of level. Maintenance is usually by tamping, an aggressive process, which damages ballast grains such that the interval between maintenance interventions steadily reduces with increasing number of tamps. After a certain number of tamps, the ballast is deemed life expired and renewed, with the recovered ballast usually being down-cycled to lower grade uses. The ability to re-use the recovered ballast in the trackbed would make a significant contribution to decarbonising and improving the sustainability of railway infrastructure. This requires detailed knowledge of how the grain characteristics affecting mechanical behaviour differ between fresh and used ballast—specifically grain shape. This paper compares the shape parameters of form, angularity and surface roughness of fresh and used ballast, and proposes a method to synthesize full-size and scaled used ballast for use in laboratory and model tests.

The investigation of additive manufacturing and moldable materials to produce railway ballast grain analogs

The size and shape of individual grains, play an important role in the mechanical behavior of granular materials such as the strength and stability of railway ballast. The aim of this research is to study materials from which uniform, reproducible grains with irregular convex geometry can be created by molding and additive manufacturing technologies in order to create reproducible artificial assemblies that can be used in experiments. Packings with determined grain shape results more controlled investigations contrarily to using natural grains with random geometry. Specimens were made from railway ballast materials, materials used in the construction industry, additively manufactured and molded polymers, and certain low-strength materials. Uniaxial compression and bending tests were conducted on these specimens. The mechanical properties of typical railway ballast materials (basalt and andesite) were compared with the properties of artificially produced materials. The results show that for grain reproduction the molding technology is recommended with the use of polyester-crushed stone composite and ceramic powder. Furthermore, the additive manufacturing was recommended with PolyJet or Multi Jet Fusion technology as they have the feasibility to produce grains with similar material properties to the properties of basalt and andesite.

Evaluation of a new grain breakage factor based on the single grain crushing strength

In granular soils subjected to high-level shear stress, a micro-damage occurs in the soil's internal structure, referred to as grain breakage. The main factors controlling grain breakage are the applied stress level and the single grain crushing strength of the individual grains (σf). This paper presents a laboratory study on ballast grains' single grain crushing behavior. Double-plate grain crushing tests were carried out on dolomite, calcerous, and basalt origin grains, having sizes ranging from 2.00 to 37.50 mm. Factors affecting the single grain crushing strength, such as grain shape, loading rate, and grain size, were investigated. Individual grains' single grain crushing strength was observed in three phases related to granular soils' 3-staged one-dimensional compression behavior. An equation that considers grain shape in terms of grain sphericity (form) has been proposed to determine the single grain crushing strength. One-dimensional compression tests were performed on uniformly graded soil samples consisting of calcerous and basalt origin grains. The single grain crushing strength of the calcerous origin soil sample’s mean grain-sized grains was determined to be 12.7 MPa, and this value was determined to be 25.6 MPa for the sample consisting of basalt origin grains. One-third/fourth of this single grain crushing strength value was obtained as the virgin compression line (VCL) initial stress of the soils’ one-dimensional compression curve (4.3 and 6.0 MPa for calcerous and basalt origin soil samples, respectively). The final stress of the VCL was determined to be close to 1–1.25 times of this specified single grain crushing strength (16 and 26 MPa for calcerous and basalt origin soil samples, respectively). The change in the mean grain size of the soil samples (d50) during one-dimensional compression was defined as an exponential function. A new grain breakage factor depending on the single grain crushing strength of the soils’ mean grain-sized grains has been proposed to determine the grain breakage that develops under one-dimensional compressional conditions.

Modelling the suction-dependent plastic shakedown limit of unsaturated fine/coarse soil mixture with consideration of soil-water retention curve

An interlayer soil was identified in the French conventional rail tracks, corresponding to a mixture of subgrade fine soil and ballast grains. Experimental results show that the permanent strain of such fine/coarse soil mixture can be at stable or unstable state, depending on the stress level with respect to the plastic shakedown limit σdth (the maximum stress level for the stable permanent strain). As the water content of soil mixture changed, the matric suction ψ changed subsequently, leading to a change of σdth. In this study, a model incorporating the soil–water retention curve was proposed to describe the variation of σdth with ψ for the soil mixture at varying coarse grain content fv. This proposed model implied the plastic shakedown limit under saturated state σdth-sat, the plastic shakedown limit under a reference state σdth-ref, the matric suction corresponding to the reference state ψref, and the parameters related to the soil water retention curve. The model was validated using three series of studies in literature. Based on an existing approach implying a representative permanent strain rate 1/as, the σdth under varying ψ and fv values was determined for these three series. For each series, the obtained σdth was divided into two groups: the first group was used to determine the model parameter B in the relationship between σdth andψ, while the second group was adopted to validate the proposed model with the previous determined parameter B. A reasonably good agreement was obtained between the measurements and the calculations for these three series of studies. A comparison between the proposed model and a representative existing model was made, showing the performance of the proposed model in describing the variations of σdth with ψ for soil mixture at varying fv.

Strength properties of ballast layer, created from new and recycled crushed stone ballast

The objective of the research is a railway ballast layer created from new and recycled ballast particles in different ratio. In order to study the possibility of using recycled ballast grains in ballast layer, it is necessary to carry out laboratory triaxial tests of ballast crushed stone with size of the particles 25-60 mm with different grain shape. Abrasion testing machine allows to reach the effect on new ballast, which is similar to abrasion of ballast particles in the railway track. Therefore, it is possible to create ballast samples from (new) mixed and recycled ballast and estimate which proportion has the strength characteristics, which are close to the ones in ballast layer created only with new ballast particles. The result of the study shows, that it makes sense to return the recycled crushed stone in a mix with a new one in order to reduce the cost on a railway maintenance.

Метод оценки коэффициента формы зерен щебня, применяемого в балластном слое железнодорожного пути

A decrease in the strength properties of crushed stone in combination with an increase in its contamination during tonnage operation reduces the load-bearing capacity of the ballast layer and intensification of the accumulation of residual deformations of the track. The strength properties of crushed stone ballast depend on the shape (rolling edges) of crushed stone grains. The paper discusses the theoretical and practical foundations for determining the shape of crushed stone ballast grains. As an indicator characterizing the shape of crushed stone grains, it is proposed to use the grain shape coefficient indicator. The grain shape coefficient refers to the ratio of the actual specific surface of the grain to the specific surface of a perfectly round particle of the same volume. The paper proposes a method for determining the specific surface based on the Kozeni-Karman theory of the flow of a liquid or gas through a granular layer and the equation relating the water permeability of a porous medium to its specific surface. The basic design of the device for the specific surface area of crushed stone ballast grains is presented. The article describes and presents the results of the experiment on calibration of the device, establishes recommended and boundary conditions for its use, as well as the results of the experiment to determine the specific surface in experiments with crushed stone of category II according to GOST 7392-2014 in comparison with an alternative method of measuring the specific surface of crushed stone grains.

Effect of water content on resilient modulus and damping ratio of fine/coarse soil mixtures with varying coarse grain contents

For the French ancient rail track, owing to the effect of traffic loadings, an interlayer was naturally created due to the interpenetration of ballast grains and subgrade fine soil. Under the effect of rainfall/evaporation, the water content of interlayer soil can vary, affecting its dynamic properties such as resilient modulus and damping ratio. In order to investigate the effect of water content on the resilient modulus and the damping ratio of interlayer soil, a series of cyclic triaxial tests were carried out, following a multi-step loading procedure with various deviator stress amplitudes of 10, 30, 50, 100 and 200 kPa. Two target fines water contents (17.6% and 10.6%) and five coarse grain contents (0%, 10%, 20%, 35% and 45%) were considered. Results showed that under a given coarse grain content, an increase of water content led to a decrease of resilient modulus owing to the decrease of suction while an increase of damping ratio owing to the increase of soil viscosity. Through the variations of resilient modulus and damping ratio with coarse grain content, a characteristic coarse grain content was identified, defining two distinct soil fabrics: a fine soil dominated fabric when the coarse grain content was smaller than the characteristic value, and a coarse grain dominated fabric when the coarse grain content was larger than the characteristic value. Moreover, a constant characteristic coarse grain content was obtained at a given water content: 25.5% at 17.6% water content and 32.0% at 10.6% water content. The increase of characteristic coarse grain content with the decrease of water content could be explained by the swelling-shrinkage of fine soil: with a decrease of water content, the shrinkage of fine soil led to an increase of the volume of macro-pores. More coarse grains were thus needed to constitute the global skeleton, giving rise to an increase of the characteristic coarse grain content. In addition, comparison of the values of characteristic coarse grain content obtained from cyclic tests with those from monotonic tests showed that at a given water content, cyclic loading led to a slightly larger value than monotonic loading, evidencing the more significant effect of cyclic loading on soil fabric.

Investigation of the effect of water content on the mechanical behavior of track-bed materials under various coarse grain contents

In the French conventional railway track, an interlayer was created naturally through the interpenetration of ballast and subgrade under the effect of long-term train loading. Field investigation showed that the proportion of ballast grains decreased over depth in the interlayer. Moreover, the water content of interlayer soils varied depending on the weather conditions, which can strongly affect the mechanical behavior of interlayer soil. In this study, the effect of water content on the mechanical behavior of interlayer soils under various coarse grain contents was investigated by monotonic triaxial tests. Three water contents of fine soil (wf = 17.6%, 10.6%, and 7.0%), five volumetric coarse grain contents (fv = 0%, 10%, 20%, 35%, and 45%) and three confining pressures (σ3 = 30, 60 and 120 kPa) were considered. Results showed that a decrease of wf led to an increase of shear strength and soil stiffness due to the effect of suction, and to an increase of dilatancy due to the aggregation of fine soils. Moreover, the variations of maximum deviator stress qmax, Young’s modulus E0, dilatancy angle ψ and friction angle φ with fv followed a bi-linear pattern for the three σ3 values, defining a characteristic volumetric coarse grain content fv-cha value for a given wf value: fv-cha ≈ 25%, 29% and 33% for wf = 17.6%, 10.6% and 7.0%, respectively. The fv-cha corresponded to the transition from a structure dominated by fine soils to a structure dominated by coarse grains. The increase of fv-cha with the decrease of wf could be attributed to the swelling and shrinkage of fines. While drying from optimum water content of fines wopt-f = 13.7% to a lower wf value, more coarse grains were needed to constitute the global skeleton due to the increase of the global volume of macro-pores resulted from the shrinkage of fine soils. By contrast, while wetting from wopt-f = 13.7% to a higher wf value, since the global volume of macro-pores decreased due to the swelling of fine soils, less coarse grains were required to constitute the global skeleton.

Mitigating ballast degradation with under-sleeper rubber pads: Experimental and numerical perspectives

Abstract This paper presents a study on mitigating the degradation of ballast by placing an under-sleeper rubber pad (USP) beneath a sleeper. Large-scale track process simulation apparatus (TPSA) tests have been carried out on ballast assemblies (with and without USP) subjected to cyclic loadings. Numerical modelling has been performed using a coupled discrete-continuum modelling (coupled DEM-FDM) approach to investigate the role of USP from a micromechanical perspective. Ballast grains are simulated in DEM by bonding of many cylinders together at appropriate sizes and locations; and when those bonds break, they are considered to represent ballast breakage. The capping and subgrade layers are simulated as continuum media using the finite difference method (FDM). Interface elements were developed for transmitting forces and displacements between the discrete and continuum domains. The coupled model is validated by comparing the predicted load-deformation responses with those measured from large-scale TPSA tests. The model is then used to explore changes in the micromechanical aspects of ballast subjected to cyclic loading, including particle connectivity number, contact force distributions, and contact orientations and associated particle breakage. These findings are needed to gain a better insight as to how USPs help to attenuate the load applied in a ballast assembly.

Influence of grain size distribution of inclusions on the mechanical behaviours of track-bed materials

In French conventional railway track-bed, an interlayer was created, mainly from the interpenetration of ballast and subgrade. Field investigation showed that the ballast content decreases over depth. In addition, depending on the locations, the grain size distribution of ballast grains can be quite different, defining different values of coefficient of uniformity, Cu. In this study, triaxial tests were carried out on the interlayer soil with three Cu values and five volumetric inclusion contents, fv. X-ray micro computed tomography scans were also conducted on representative samples to visualise the corresponding inclusion distributions. Results show that: (a) at all three Cu values, two soil fabrics can be identified, namely grain–grain contact structure for large inclusion contents (fv = 35% and 45%) and fine–fine contact structure for small inclusion contents (fv = 5%, 10% and 20%), and the characteristic volumetric inclusion content fv-cha separating these two categories was larger for smaller Cu since in this case fewer inclusion contacts were expected to be developed at a given fv; (b) for the grain–grain contact structure, a smaller Cu caused larger maximum deviator stress qmax, larger friction angle, larger Poisson ratio and more dilatancy due to the involvement of more large grains at smaller Cu; (c) for the fine–fine contact structure, an opposite trend was observed: the smaller the Cu, the smaller the qmax, the friction angle, the Poisson ratio and the dilatancy owing to a smaller quantity of inclusion grains; (d) the change patterns of Young's modulus and cohesion with increasing Cu were the same for the two soil fabrics – Young's modulus and cohesion decreased with the increase of Cu.

The influence of sleeper material characteristics on railway track behaviour: concrete vs composite sleeper

Composite sleepers and bearers formed of polymers and sometimes steel or fibre reinforcement are an alternative to traditionally used timber or prestressed concrete counterparts. Composite materials offer the combined benefits of the compliant sleeper/ballast contact of timber with the longer service life of concrete. However, composite and timber sleepers have lower bending stiffness compared with concrete, and this affects their track performance. To better understand the influence of sleeper material on performance, a concrete and a composite sleeper were tested in the Southampton Railway Testing Facility. Test results were interpreted with the aid of finite difference and discrete element method numerical modelling. It was found that composite sleepers can provide a more compliant sleeper/ballast interface with increased numbers of ballast grain contacts of larger area that attenuate the peak contact forces. However, owing to their lower bending stiffness, they have more exaggerated deflection profiles and a tendency to become centre-bound compared with stiffer prestressed concrete counterparts.

X-ray CT analysis of the evolution of ballast grain morphology along a Micro-Deval test: key role of the asperity scale

Ballast grains in railway tracks progressively wear due to the efforts exerted by the continuous passage of trains and the periodic maintenance operations. The initial sharp edges and vertices of the grains tend to become smoother and the surface texture is removed. This change in morphology plays a key role on the proper behaviour of the ballast layer and, consequently, on the frequency maintenance required on the track. The object of this paper is to improve the understanding of the wear process by tracking the morphology of a sample of grains submitted to an accelerated ageing using the Micro-Deval standard test. To this goal, X-ray Computed Tomography is used to scan a sample of grains at different states of wear and the resulting images are compared using 3D image analysis. A description of morphology evolution at different scales is provided using scalar parameters and spherical harmonic analysis, proving that the general form is not significantly changed during a standard Micro-Deval test. Thus a detailed analysis at the asperity level is performed, showing the key role of the edge broadening and vertex smoothing phenomena on ballast wear.

Effect of Sleeper Interventions on Railway Track Performance

AbstractThe sleeper/ballast interface is an essential stage in the transfer of train loads from a railway track structure into the ground. Generally, only a small number of ballast grains support t...