Ballast Particles Research Articles

Discrete Element Modelling of Rubber-Protected Ballast Performance Subjected to Direct Shear Test and Cyclic Loading

The rubber-protected ballast (RPB) is made from natural ballast particles and crumb rubber particles. The crumb rubber is shredded waste tires. RPB was chosen to replace the ballast as it has higher resistance to breakage and abrasion. However, the static and dynamic performance of the RPB has not been confirmed yet. Towards this end, experimental tests and numerical simulations were utilized to study the feasibility of RPB application. Direct shear tests (DSTs) were performed and a DST model and three-sleeper track model with the discrete element method (DEM) were built. The shear strength, settlement, displacement, and acceleration of the RPB were studied. The results show that the RPB has the advantage of increasing the force (stress) distribution and that the smaller crumb rubber size was more suitable for replacing the ballast particles.

Effect of sleeper bottom texture on lateral resistance with discrete element modelling

The lateral stability of ballasted track becomes more important because of the safety requirement under the demand of higher train speed and heavier axle load. To increase the lateral resistance of ballast bed, this paper proposes three types of new sleepers, frictional sleepers. The frictional sleepers are sleepers with different shapes of textures attached at the sleeper bottom. To study the application feasibility of the frictional sleepers, experimental tests (single sleeper pull-out test) and numerical simulation (discrete element modelling) are performed. The lateral resistances of the three types of frictional sleepers are compared with the traditional sleeper based on the experimental test, and the mechanism of the lateral resistance increase is revealed according to the numerical simulation. The results indicate that the frictional sleepers can increase the lateral resistance by 32% (maximum), due to the enhanced interaction between sleeper and ballast particles. More importantly, different types of frictional sleepers have different performances, and the optimal friction sleeper is confirmed. This study is helpful for the further research on sleeper design.

Laboratory investigation on the behavior of ballast stabilized with bitumen-cement mortar

In order to obviate the ballast degradation during railway operation and maintenance, many polymeric and bituminous additives have been proposed for bonding the ballast particles. Amongst the proposed methods, utilizing the bitumen-cement mixtures have been less payed attention by researchers. So, in the present study, the bitumen-cement mortar is considered as a possible solution to control the ballast degradation. In this matter, an extensive laboratory test has been carried out on ballast stabilized with bitumen-cement mortar. In the first step to achieve the most suitable mortar mixture design the water to cement weight ratio (w/c) and bitumen to cement weight ratio (b/c) have been varied in the range of 0.55–0.75 and 0.2–0.6 respectively and the mortar performance has been evaluated in two ages of 7 and 28 days on the basis of uniaxial compression tests and flexural strength tests results. Secondly, after exploring the most desirable mortar mixture design, it has been added to the ballast in three different forms: fully saturated ballast with bitumen-cement mortar (MSB-F), ballast stabilized with 15% bitumen-cement mortar (MSB-H), and 10 cm bottom ballast stabilization with bitumen-cement mortar (MSB-U). All three mentioned forms with three different bitumen-cement percentages of 0.2,0.4 and 0,6 have been examined by ballast box test apparatus and their load-settlement behavior have been evaluated under 200,000 cycles of vertical loading. Comparison of the settlement, stiffness, and damping of the samples have shown the best performance of MSP-U in comparison to the others and the bitumen-cement ratio of 0.42 has been introduced as an optimum percentage which can minimize the ballast settlement by 334% and in parallel increase the ballast layer stiffness and damping ratio by 217% and 187%, respectively.

Modeling of Coupling Mechanism between Ballast Bed and Track Structure of High-Speed Railway

In this paper, a discrete and continuous body coupling algorithm is used to study the dynamic contact characteristics between ballasts and between ballasts and track structures. Firstly, the three-dimensional fine modeling of ballast particles is realized based on the three-dimensional laser scanning method, and then through the discrete and continuum coupling algorithm from the micro-macro point of view, a multiscale and unified particle-track structure coupling model is established. Based on the coupling model, the macro- and microdynamic characteristics of the ballast bed and the mechanical characteristics of the track structure under the dynamic load of high-speed trains with different driving speeds are studied. It is shown that the cumulative settlement of the ballast bed is directly proportional to the contact force and rotation speed of the ballast, and the faster the driving speed is, the greater the cumulative deposition speed and cumulative settlement of the ballast are. The contact force between the ballast and sleeper mainly comes from the bottom of the sleeper, and its contact force and contact strength increase with the increase of driving speed.

Coupled discrete-continuum approach for railway ballast track and subgrade macro-meso analysis

ABSTRACT This paper presents a multi-layer railway ballast track and substructure model, where a coupled discrete and continuous method is used for macro-meso dynamic behaviour analysis under moving wheel loads. In this coupled model, the discrete element method (DEM) is utilised to build the superstructure of the ballast track (i.e. rail, fastener, sleeper and ballast layer), which considers the complex ballast shape and particle size distribution from mesoscopic level. The finite difference method (FDM) is used to simulate the substructure (i.e. subgrade and foundation) with a consideration of computing cost. And then, the coupled model is achieved by satisfying displacement, velocity and contact force compatibility between the FLAC and the PFC. Finally, the dynamic analysis is carried out by applying the wheel-rail forces obtained from a vehicle-track dynamics model into the coupled model. The DEM parameters of ballast particles are calibrated based on the results of the ballast direct shear tests, and the dynamic behaviour of railway ballast track and subgrade system is validated with the field measurement. Through the numerical analysis, it is confirmed that the coupled DEM/FDM model can reliably reflect macro-meso dynamic behaviour and accurately reveal the contact characteristics between the ballast layer and subgrade.

Effect of Incorporating Tire Rubber Waste on the Mechanical Behaviour of Railway Ballast Layer: Experimental and Modelling Studies

In order to delay the degradation of railway tracks, several techniques have been developed in recent years; the best known of which are: under sleeper pads, geogrid and the use of polymers as elastic elements between ballast particles. The objective of the research described in this paper is to evaluate the effectiveness of incorporating tire rubber waste into the ballast layer in reducing its settlement. For that purpose, a small-scale testing laboratory setup has been developed, which consists of a bi-block concrete sleeper surrounded by a layer of ballast. The developed apparatus is subjected, through the actuator of a servo-hydraulic machine, to a vertical cyclic force reproducing the passage of a train bogie. This study has been performed in two stages: the first is to identify a suitable existing model that well-describes the ballast settlement with cycle applications and the second is to study the influence of the variation of the percentages of addition of rubber on the behaviour of the ballast layer. Finally, a finite element model was developed to simulate the experimental setup. The results show that using 10% of tire waste rubber could decrease the track settlement and reduce the breakage of ballast particles.

Discrete Element Modeling of Permanent Deformation Accumulation in Railroad Ballast Considering Particle Breakage

Particle shape and mineralogy have been found to greatly affect railroad ballast response under train loading. In this research effort, the effect of particle breakage on ballast permanent deformation behavior was studied using the Discrete Element Method (DEM). Real ballast particle shapes were digitized using an inexpensive imaging tool, and agglomerates of spheres were used to regenerate those complex shapes. Particle crushing tests were carried out in the laboratory, and discrete element models simulating the crushing test were calibrated based on the laboratory test results. Model parameters established through this calibration process were subsequently used to study the permanent deformation response of ballast layers comprising complex-shaped breakable ballast particles under repeated loading. The effect of particle shape was studied by comparing the permanent deformation behavior for ballast layers comprising simplified ellipsoid particles to those comprising the complex-shaped particles simulated using agglomerates of spheres. The simulation results clearly highlighted the significance of particle breakage in ballast permanent deformation accumulation under cyclic loading. Most particle breakage was observed to occur under the initial load cycles, which also corresponded to the highest rate of permanent strain accumulation. The particle breakage as well as the rate of strain accumulation gradually decreased with increasing load cycles.

Analysis on dynamic performance of different track transition forms using the discrete element/finite difference hybrid method

To analyze the mesodynamic and macrodynamic performance of the track transitions more precisely, a bridge-embankment transition model has been established by combining the discrete element method (DEM) with the finite difference method (FDM). In this model, the DEM is utilized to model sleepers and ballast particles with complex shape, and the FDM is applied to simulate the abutment, transition section and embankment. The DEM/FDM coupled model is then achieved by exchanging displacements, velocities, and contact forces at the interface. Afterwards, the influence of the transition section parameters and transition forms on dynamic behaviour of the track-substructure system is studied under measured rail seat load. The numerical simulations indicate that the increase of the transition section elastic modulus, decrease of the backfill slope values and the transition form of trapezoid backfill following by inverted trapezoid backfill can provide a better displacement gradient between the abutment and the embankment. Additionally, the results show that the inverted trapezoid transition form can provide a better transition of the track dynamic performance than the trapezoid transition form and the embankment soil over the wedge-shape backfill has the tendency to propagate along the slope toward the embankment section under the moving train load for the trapezoid transition form.

Numerical study of ballast-flight caused by dropping snow/ice blocks in high-speed railways using Discontinuous Deformation Analysis (DDA)

Abstract Frozen snow/ice blocks drop at high speeds from train causing the ballast to fly up and damage the car body. Thus, in this paper, we propose a numerical model based on the discontinuous deformation analysis (DDA) method to study the ballast flight caused by dropping snow/ice blocks in high-speed railways and to analyze the dynamic behavior of ballast particles during their collision with a snow/ice block. The validation of the proposed model is done by comparing the numerical results with the theoretical and the experimental ones. The numerical results show that the velocity, shape and incident angle of snow/ice block play an important role in the ballast flight. Specifically, the number and the maximum displacement of ballast particles increase as the train speed increases and the incident angle greatly affects the movement direction of ballast particles. The shape of the ice block affects the amount and extent of ballast flight.

A Numerical Study of Railway Ballast Subjected to Direct Shearing Using the Discrete Element Method

Railway ballast is a coarse granular material used to carry train loads and provide drainage for the rail tracks. This study presents numerical explorations of the mechanical performance of ballast aggregates subjected to direct shear tests. The discrete element method (DEM) was used to investigate the microscopic characteristics of ballast aggregates during shearing while considering contact distribution, particle rotation, and particle displacement. By testing the angle of repose of ballast aggregates, the parameters for the DEM contact model could be calibrated. Four specimens were prepared and then subjected to different normal pressures. The results show that the contact between ballast particles intensifies in terms of the amount and magnitude as the normal pressure increases. A Fourier analysis was applied to investigate the anisotropy of contact normal and the contact forces for ballast aggregates at different shearing phases. The rotational and translational movements of ballast particles were investigated, and this investigation revealed that particle rotation gradually increased as the shearing propagated. Four regions in the aggregates were identified according to the translational pattern of ballast particles. The results of this research provide an in‐depth analysis of microscopic characteristics from a particulate scale.

Study of the stress state of the sleeper base and sleepers of rail track with accounting variable conditions of their resting

The live problem of the stress state of the sleeper base and rail sleepers is considered in the article with taking into account specific normal and specific shear stresses under variable conditions of the sleepers resting on the base. The methodology is aimed at the determination of the mutual influence of normal (compressive) and tangential (shear) specific stresses on formation of stability due to the displacement of individual parts of the base. Stability zones and shear zones of the sleeper base parts are also determined by the authors at different conditions of the sleeper resting on the base. Further, zones with insecure position of the base under the sleepers are found when they and their under-rail parts continuously rest on the base. It is stated that insecure position of the base is formed when specific shear stresses excess specific stresses of shear resistance, which consists of specific friction and specific cohesion between the parts in the base. It is also established that loss of stability of the ballast particles causes their displacement in the direction of the action of specific shear stresses. In these zones, density and rigidity of the sleeper base decreases leading to a gradual redistribution of reactive resistance of the base along the sleeper, and, depending on this redistribution, a change in the bending moments acting on the sleeper occurs. It is found that when the sleepers rest only by their under-rail parts, shear stresses and forces, as well as stresses and shear resistance forces under the middle part of the sleepers are distributed in such a way that shear forces exceed resistance forces up to the depth of 60 cm from the sleepers sole. This phenomenon leads to the displacement of ballast particles under the middle part of the sleeper, consolidation of the ballast and resting of the sleepers by their middle part with possible cracks formation. In the end sections of the sleepers, there is also a loss of ballast stability. Recommendations for improving stability indicators of the sleeper base and strength of the sleepers are given. The obtained results of the study can be used for the mine, industrial and railway transport.

Influence of Particle Orientation on the Performance of Geogrid Reinforced Ballast

To explore the initial orientation effect of ballast assembly on the reinforcement performance of the geogrid reinforced ballast, particles with random orientation and five prescribed rotational orientations were developed through particle flow code (PFC3D). The evolution laws of the pullout force and the principal directions of the normal contact force were systematically compared and analyzed. Furthermore, the mechanical responses such as pullout force, distribution of axial force, displacement vectors, force chain, and mesoscopic fabric were discussed. According to the displacement vectors of the ballast particles, the average thickness of the stable shear band is determined. The inherent relationships among the force chain, the rotational angle of the normal contact force, and the mesoscopic fabric parameters are revealed. The results show that the pullout force of specimens with the initial orientation of 45° increases monotonously during the pullout process, and the peak value of pullout force appears at the end of the test. The mesostructural analysis also confirms that the evolution of the principal direction of contact normal force is relatively steady during the pullout process, indicating that the specimen with 45° orientation possesses higher systematic stability and ductility. Moreover, the optimum interval from 56.68° to 57.30° is observed to remain in a self‐adapting state for ballast assembly.

Experimental and Numerical Investigation on Ballast Flight from Perspective of Individual Particles

Ballast flight significantly affects the safe operation of high-speed railway. A wind tunnel experiment and computational fluid dynamics simulation were performed to investigate the occurrence and development mechanism of ballast flight from the perspective of individual particles, which was based on the capture, reconstruction, and analysis approach of ballast features. The dynamic characteristics of ballast particles under wind load, the flow field characteristics such as the distribution of air pressure and streamline, and the influence of the ballast mass and shape, windward direction, and wind speed on the ballast flight were investigated. The results show that the critical velocity of the ballast weighing from 5 g to 200 g is between 14.22 m/s and 29.89 m/s. The massive ballast with the shape of an oblate is unlikely to fly. The air pressure difference is proportional to the square of the wind speed, and it increases 14.9 times when the wind speed increases from 10 m/s to 40 m/s. In order to avoid ballast flight, recommended suggestions include limiting the proportions of the small and the ellipsoid ballast praticles, laying the ballast with high density and a large size on the top of the ballast bed, and reducing the degradation and fouling of the ballast bed.

Experimental Study of Railway Trackbed Pressure Distribution Under Dynamic Loading

Reliable and durable operation of the railway track under the dynamic load of the rolling stock depends considerably on the ability of the ballast layer to get the load from the sleepers and distribute it to the subgrade. In this paper, the experimental study of the distribution properties of the ballast layer under the impact of dynamic loading depending on the density of the ballast layer is carried out. The ballast behaviour during load cycles is estimated by pressure measurements at the ballast prism base along the axis of a sleeper with simultaneous video observation of the ballast particles movement through transparent sidewalls of the box with crushed stone. Measurements of pressure distribution are carried out with the developed microcontroller system of measurements and developed load cells. The system allows performing multi-point measurements of stress in combination with measurements of acceleration and photogrammetry. The results of measurements showed a significant effect of the ballast layer consolidation on the distribution of stresses under the sleeper. The performed research opens up opportunities for practical improvement of the existing types of track structures and the technology of the ballast layer tamping in terms to provide the optimal conditions for the ballast layer operation.

Identification of the under-tie pad material characteristics for stress state reduction

The degradation of ballast particles and concrete crossties in heavy-haul railroad tracks poses problems such as inhibiting proper drainage and disturbing track geometry. under-tie pads offer a solution to reduce crosstie–ballast stresses by improving load distribution through the track structure and reducing pressures on ballast particles and the crosstie surface. Despite the emergence of under-tie pads on heavy-haul corridors, optimal characteristics for the reduction of the tie–ballast stress state have not been defined in literature. In this research, several under-tie pad products and generic materials with various thicknesses and hardnesses were studied to identify appropriate properties of under-tie pad products for pressure distribution. The findings from this research provide an insight into how material characteristics influence the pressure mitigation performance of under-tie pads. Results from this study indicate that thickness is the most crucial metric determining under-tie pad performance in reducing ballast degradation; hardness and material type also have an effect, but to a lesser degree.

Micro-mechanical investigation of railway ballast behavior under cyclic loading in a box test using DEM: effects of elastic layers and ballast types

Ballasted tracks are the commonly used railway track systems with constant demands for reducing maintenance cost and improved performance.Elastic layers are increasingly used for improving ballasted tracks. In order to better understand the effects of elastic layers, physical understanding at the ballast particle level is crucial. Here, discrete element method (DEM) is used to investigate the effects of elastic layers – under sleeper pad (text {USP}) at the sleeper/ballast interface and under ballast mat (text {UBM}) at the ballast/bottom interface – on micro-mechanical behavior of railway ballast. In the DEM model, the Conical Damage Model (CDM) is used for contact modelling. This model was calibrated in Suhr et al. (Granul Matter 20(4):70, 2018) for the simulation of two different types of ballast. The CDM model accounts for particle edge breakage, which is an important phenomenon especially at the early stage of a tamping cycle, and thus essential, when investigating the impact of elastic layers in the ballast bed. DEM results confirm that during cyclic loading, text {USP} reduces the edge breakage at the sleeper/ballast interface. On the other hand, text {UBM} shows higher particle movement throughout the ballast bed. Both the edge breakage and particle movement in the ballast bed are found to influence the sleeper settlement. Micro-mechanical investigations show that the force chain in deeper regions of the ballast bed is less affected by text {USP} for the two types of ballast. Conversely, dense lateral forces near to the box bottom were seen with text {UBM}. The findings are in good (qualitative) agreement with the experimental observations. Thus, DEM simulations can aid to better understand the micro-macro phenomena for railway ballast. This can help to improve the track components and track design based on simulation models taking into account the physical behavior of ballast.Graphical

Numerical Study of Particle Morphology Effect on the Angle of Repose for Coarse Assemblies Using DEM

The morphologies of coarse particles are usually irregular and play a dominant role in the mechanical behaviors of the particle assemblies. This paper quantitatively studies the effect of particle shape on the angle of repose, which is an important macroscopic parameter for ballast materials, via laboratory tests and numerical simulations by means of the discrete element method (DEM). Forty ballast particle templates and four simply created clump templates are reconstructed using an image-based method and quantified with two shape factors, sphericity and convexity. A series of simulations are conducted with the coefficient of sliding friction between particles changing from 0.2 to 0.6 at an interval of 0.1 to study its influence on various shapes of particles, and an appropriate value of sliding friction coefficient is chosen for the comparison of particle shape effect. The results show that increasing sphericity and convexity can significantly decrease the angle of repose, and the real ballast model gives a more realistic angle of repose behaviors as that of laboratory tests compared to simply created models. By analyzing the characteristics of particle motions and contacts from a microscopic perspective, the mechanism of particle shape attributed to the formation of granular aggregation is also discussed and revealed in this research.

Effects of fouling containing plastic fines on abraded ballast strength and deformation properties

Abstract The introduction of fouling, or finer grained material into the voids of railroad ballast is one of the main issues effecting ballast performance. Fouling reduces the ability of ballast to serve it’s main functions, including providing drainage of water away from the track, resisting loads in the vertical, lateral, and longitudinal directions, and maintaining track position. Dynamic loading in the track causes ballast particles to abrade together. This is the most common source of fouling, and results in a progressive change in ballast particle shape over time. Fouling can also enter the track through windblown processes as well as infiltration of the subgrade from the bottom of the track. Many studies have been done to better understand the influence of fouling on ballast strength properties, deformation characteristics, and overall performance. These studies usually use fresh ballast and fouling made from the stone dust of the same parent rock. There are few studies which use ballast taken from track, and fewer that investigate fouling by clay particles. This study aims to better understand the influence of fouling containing clays by comparing two sets of laboratory tests. The first set of tests uses abraded ballast taken from a track and the natural fouling found at that location. The second set of tests uses the same ballast but mixes the natural fouling with Prestige clay in a ratio of 55:45. Unlike tests with purely clay fouling, this mixture should be more realistic to what might be found in the field. By directly comparing the results of these two sets of tests, the effects of fouling containing plastic fines on ballast properties becomes more apparent. The results of these tests show that the fouling containing clay reduces strength and increases the variability in deformation behavior of the ballast.

Exploring the influence of sphericity on the mechanical behaviors of ballast particles subjected to direct shear

To quantitatively evaluate the influence of shape on the mechanical behaviors of ballast, a series of 2D direct shear simulations were conducted on particles with different sphericities. The particle contours were obtained from the scanning images of 40 ballast particles, based on which the irregular-shaped particles were precisely reconstructed by clumps in the discrete element models. According to the sphericity, five assemblies were prepared for the direct shear simulation. Higher shear strength, vertical relative displacement and apparent angle of shear resistance were observed in assemblies consisting of low-sphericity particles. To explore the mechanism of macro-mechanical behaviors, an in-depth analysis of the mesoscopic responses was conducted. Two parameters were adopted to evaluate the anisotropy of the contact force distribution observed clearly in all samples and stronger anisotropy was found in samples with low-sphericity particles. The translational and rotational displacements of particles were also analyzed. Larger vertical displacements and less rotations were observed in samples with low-sphericity particles. The results suggest that particles with diverse shapes can produce varying particle interlock states and moving behaviors, hence influence the macro-mechanical behaviors.

Investigation of railroad ballast particle breakage

Abstract This paper summarizes the authors' up-to-date results in the research topic of railway ballast particles' breakage test with individual laboratory test. In the past few years there were a ...