Discrete Element Modeling Of Railroad Ballast Under Simulated Train Loading

Ballasted tracks have been widely used in many countries around the world. Ballast layer is the main element in ballasted track. After service, ballast aggregates degrade and deform. Periodical maintenance for ballast layer is required; which is a cost and time expensive activity. Researchers used numerical approaches to understand the behavior of railroad ballast that leads to efficient design and maintenance. The Discrete Element Method (DEM) has been used increasingly to understand the mechanical behavior of railroad ballast, more frequently through box test. Most researches in the literature simulate the train loading as a pure continuous sinusoid based on train speed and axle spacing; unlike the actual loading induced by trains. This study aims to show the influence of simulated train loadings on ballast mechanical behavior using DEM via box test. The study utilizes the theory of Beam on Elastic Foundation to simulate a more realistic train load. The results from the more realistic simulated train load are compared with those from a sinusoidal load. The compared results highlight the influence of the simulated train load on the mechanical behavior of railroad ballast.Author: " I would like to acknowledge Qatar Rail for their sponsorship to this research under a project entitled “Framework for Research on Railway Engineering” with a grant reference number: QUEX-CENG-Rail 17/18.

Advances in the modelling of railway ballast using the Discrete Element Method (DEM)

The development of high-speed train lines has increased significantly during the last decades leading to more demanding loads in railway infrastructures. Most of these infrastructures were constructed using railway ballast, whose main roles are resisting to vertical and horizontal loads and facing climate action. Moreover, new challenges are arising in the railway industry, such as the development of high-speed train lines in locations with extreme weather. For these reasons, the implementation of a numerical code able to represent ballast behaviour, including its interaction with other structures, has become very attractive. Among a wide range of numerical methods, the Discrete Element Method (DEM) was found to be effective for the calculation of engineering problems with granular materials. This approach considers the discontinuous nature of these materials and has proven to be a very useful tool to obtain complete qualitative information on calculations of groups of particles. The code used in this work is developed within DEMPack, a specific software tool for modelling physical problems using the DEM. The computer program is adapted to meet the needs for representing the behaviour of railway ballast

Experimental and discrete element modelling of geocell-stabilized subballast subjected to cyclic loading

This paper presents a study of the load-deformation behaviour of geocell-stabilised sub-ballast subjected to cyclic loading using a novel track process simulation apparatus. The tests were conducted at frequencies varying from 10-30 Hz. This frequency range is generally representative of Australian Standard Gauge trains operating up to 160 km/h. The discrete element method (DEM) was also used to model geocell-reinforced sub-ballast under plane strain conditions. The geocell was modelled by connecting a group of small circular balls together to form the desired geometry and aperture using contact and parallel bonds. Tensile and bending tests were carried out to calibrate the model parameters adopted for simulating geocell. To model irregularly-shaped particles of sub-ballast, clusters of bonded circular balls were used. The simulated load-deformation curves of the geocell-reinforced sub-ballast assembly at varying cyclic load cycles were in good agreement with the experimental observations. The results indicated that geocell decreased the vertical and lateral deformation of sub-ballast assemblies at any given frequency. Furthermore, the DEM can also provide an insight into the distribution of contact force chains, and average contact normal and shear force distributions, which cannot be determined experimentally

Effect of Particle Size Distribution and Packing Characteristics on Railroad Ballast Shear Strength: A Numerical Study Using the Discrete Element Method

Railroad infrastructure plays a significant role in sustaining the economy of a country, and facilitates fast, safe and reliable transportation of passengers as well as commodities. Significant capital investments are required for the construction and maintenance of a railroad network that is structurally and functionally adequate. The ballast layer is one of the main structural components of a conventional rail track system, and comprises coarse-grained unbound particles, often as large as in size. The ballast as a load-bearing layer resists train-induced stresses through particle-particle interaction. Accordingly, particle-size distribution and packing characteristics are important factors that govern the mechanical behavior of the ballast layer under loading. A well-performing ballast layer should ideally possess optimum drainage characteristics to ensure rapid removal of surface water and adequate shear strength to restrain the track against excessive movement under loading. In-depth understanding of different factors affecting ballast behavior can help reduce recurrent costs associated with ballast maintenance. Conducting common shear strength tests on coarse-grained geomaterials such as railroad ballast, and performing parametric studies to quantify the effects of different material, specimen, and test parameters on shear strength properties is often not feasible in standard geotechnical engineering laboratories due to the significantly large specimen and test setup requirements. In such situations, the Discrete Element Method (DEM) that facilitates micromechanical analysis of particulate matter becomes a logical alternative. The primary objective of this research effort is to study the effects of particle-size distribution and packing characteristics on the shear strength behavior of railroad ballast. This was accomplished by simulating commonly used laboratory shear strength tests such as Direct Shear Test and Triaxial Monotonic Shear Strength Test using DEM. A commercially available three-dimensional DEM package (Particle Flow Code - PFC3D®) was used for this purpose. Published laboratory-test data were used to calibrate the numerical model. A series of parametric analyses were subsequently carried out to quantify the individual effects of different variables being studied on ballast shear strength behavior. In an effort to increase ballast shear strength through better packing within the granular matrix, a new gradation parameter, termed as the “Coarse-to-Fine (C/F) Ratio” was proposed. Changing the ‘coarse’ and ‘fine’ fractions within a particular gradation specification, the resulting effect on ballast shear strength was studied. In addition to studying the particle-to-particle interaction within the ballast matrix, this study also focused on studying the phenomenon of geogrid-ballast interaction under different packing conditions. A recently developed parameter known as the “Geogrid Gain Factor” was used to quantify the benefits of geogrid reinforcement of ballast. The ultimate objective was to further the understanding of ballast behavior under loading, which will ultimately lead to the design and construction of better-performing railroad tracks

Numerical analysis of railway ballast behaviour using the Discrete Element Method.

The development of high-speed train lines has increased significantly during the last twenty-five years, leading to more demanding loads in railway infrastructures. Most of these infrastructures were constructed using railway ballast, which is a layer of granular material placed under the sleepers whose roles are: resisting to vertical and horizontal loads and facing climate action. Moreover, the Discrete Element Method was found to be an effective numerical method for the calculation of engineering problems involving granular materials. For these reasons, the main objective of the thesis is the development of a numerical modelling tool based on the Discrete Element Method which allows the users to understand better railway ballast mechanical behaviour. The first task was the review of the specifications that ballast material must meet. Then, the features of the available Discrete Elements code, called "DEMPack", were analysed. After those revisions, it was found that the code needed some improvement in order to reproduce correctly and efficiently the behaviour of railway ballast. The main deficiencies identified in the numerical code were related to the contact between discrete element particles and planar boundaries and to the geometrical representation of such a irregular material as ballast. Contact interactions between rigid boundaries and Discrete Elements are treated using a new methodology called the Double Hierarchy method. This new algorithm is based on characterising contacts between rigid parts (meshed with a Finite Element-like discretisation) and spherical Discrete Elements. The procedure is described in the course of the thesis. Moreover, the method validation and the assessment of its limitations are also displayed. The representation of irregular particles using the Discrete Element Method is a very challenging issue, leading to different geometrical approaches. In this work, a deep revision of those approaches was performed. Finally, the most appropriate methods were chosen: spheres with rolling friction and clusters of spheres. The main advantage of the use of spheres is their low computational cost, while clusters of spheres stand out for their geometrical versatility. Some improvements were developed for describing the movement of each kind of particles, specifically, the imposition of the rolling friction and the integration of the rotation of clusters of spheres. In the course of this work the way to fill volumes with particles (spheres or clusters) was also analysed. The aim is to control properly the initial granulometry and compactness of the samples used in the calculations. After checking the correctness of the numerical code with simplified benchmarks, some laboratory tests with railway ballast were computed. The aim was to calibrate the ballast material properties and validate the code for the representation of railway ballast behaviour. Once the material properties were calibrated, some examples of a real train passing through a railway ballast track were reproduced numerically. This calculations allowed to prove the possibilities of the implemented tool.El desarrollo de las líneas de alta velocidad ha aumentado significativamente durante los últimos veinticinco años, dando lugar a cargas más exigentes sobre las infraestructuras ferroviarias. La mayor parte de estas infraestructuras se construyeron con balasto, que es una capa de material granular colocada bajo las traviesas cuyas funciones principales son: resistir las cargas verticales y horizontales repartiéndolas sobre la plataforma y soportar las acciones climáticas. Además, se encontró que el Método de Elementos Discretos es muy eficaz para el cálculo de problemas de ingeniería que implican materiales granulares. Por estas razones se decidió que el objetivo principal de la tesis fuera el desarrollo de una herramienta de modelación numérica basada en el Método de Elementos Discretos que permita a los usuarios comprender mejor el comportamiento mecánico del balasto ferroviario. La primera tarea fue la revisión de las especificaciones que el balasto debe cumplir. A continuación, se analizaron las características del código de Elementos Discretos disponible, denominado "DEMPack". Después de esas revisiones, se encontró que el código necesitaba alguna mejora para poder reproducir correcta y eficientemente el comportamiento del balasto ferroviario. Las principales deficiencias identificadas en el código numérico estaban relacionadas con el contacto entre partículas y contornos planos y con la representación geométrica de un material tan irregular como es el balasto. Los contactos entre contornos rígidos y elementos discretos se tratan usando una nueva metodología llamada el "Double Hierarchy method". Este nuevo algoritmo se basa en la caracterización de contactos entre elementos rígidos (discretizados de forma similar a los elementos finitos) y elementos discretos esféricos. La descripción detallada del procedimiento se presenta a lo largo de la tesis. Además, también se muestra la validación del método y sus limitaciones. La representación de partículas irregulares utilizando el Método de Elementos Discretos se puede abordar desde diferentes enfoques geométricos. En este trabajo, se realizó una revisión de estos enfoques. Finalmente, se escogieron los métodos más adecuados: esferas con resistencia a la rodadura y clusters de esferas. La principal ventaja del uso de las esferas es su bajo coste computacional, mientras que los clusters de esferas destacan por su versatilidad geométrica. Se han desarrollado algunas mejoras para describir el movimiento de cada uno de los tipos de partículas, concretamente, la imposición de la resistencia a la rodadura y la integración de la rotación de clusters de esferas. En el curso de este trabajo también se analizó la forma de llenar volúmenes con partículas (esferas o clusters). El objetivo es controlar adecuadamente la granulometría inicial y la compacidad de la muestra. Después de comprobar el comportamiento del código numérico con tests simplificados, se emularon numéricamente algunos ensayos de laboratorio con balasto ferroviario. El objetivo era calibrar las propiedades del balasto y validar el código para representar con exactitud su comportamiento. Una vez calibradas las propiedades del material, se reprodujeron numéricamente algunos ejemplos de un tren pasando sobre una vía con balasto. Estos cálculos permiten demostrar las posibilidades de la herramienta numérica implementada

Performance improvement of rail track substructure using artificial inclusions - experimental and numerical studies

Large and frequent loads from heavy freight and passenger trains often lead to the progressive track deterioration. The excessive deformation and degradation of ballast and unacceptable differential settlement of track and/or pumping of underlying soft subgrade soils necessitates frequent and costly track maintenance. However, artificial inclusions such as geogrids and shockmats can mitigate ballast degradation and improve track performance. A quantitative assessment of the influence of breakage, fouling, and the effects of artificial inclusions on the shear behaviour of ballast can be performed either experimentally or numerically. Numerical modelling can simulate these aspects subject to various types of loading and boundary conditions for a range of material properties so in this study, the stress-strain and degradation response of ballast was analysed through discrete element (DEM) and finite element (FEM) methods. In DEM, irregularly shaped ballast aggregates were simulated by clumping together spheres in appropriate sizes and positions. In FEM, a composite multi-layer track system was simulated and an elasto-plastic model with a non-associative flow rule was used to capture ballast degradation. These DEM and FEM simulations showed a good agreement with large-scale laboratory tests. This paper outlines the advantages of the proposed DEM and FEM models in terms of capturing the correct stress-strain and degradation response of ballast with particular emphasis on particle breakage and fouling, as well as applications of geosynthetic grids and shockmats

Comparison of the Mechanical Behavior of Railroad Ballast in a Box Test under Sinusoidal and Realistic Train Loadings Using Discrete Element Method

A ballasted track is a popular type of railway track and its use is increasing all over the world. A ballasted track consists of different structural elements like rails, fasteners, sleepers, ballast layer, sub-ballast layer and subgrade. A ballast layer is considered as the main structural element of ballasted tracks; it has a significant contribution to track stability and alignment. After service, periodical maintenance of ballast layer is required to maintain its functionality. Ballast maintenance is a cost and time expensive operation. Better understanding of ballast mechanical behavior leads to better ballast design and efficient maintenance. Discrete Element Method has been used extensively in the literature to understand the mechanical behavior of railroad ballast in a box test. Nevertheless, in the literature most of the studies simulate train loading as pure continuous sinusoidal loading unlike the real train loading. This paper aims to investigate the influence of the simulated train loading on the mechanical behavior of railroad ballast after 1000 loading cycles. There are two simulated train-loading cases used in this study for comparison purposes; continuous sinusoidal loading and a more realistic train loading utilizing the Beam on Elastic Foundation theory. The results show a difference of ballast vertical settlement up to 14% between the two simulated train-loading cases

Friction and wear in railway ballast stone interfaces

Particle friction in railway ballast influences strongly the behaviour of ballasted tracks. New challenges posed on railway infrastructure increase the requirement for simulations, which need the friction coefficient as an input parameter. Measured friction coefficients of ballast stone contacts were found only in two studies, both under constant loads. In this work, two types of ballast were investigated in cyclic friction tests with incremental increase of the applied load after several cycles. Before each load increase, 3D-scans of some ballast stones allowed to calculate the contact area. Estimating the stress in the contact, the stress-dependency of the friction coefficient and wear were investigated. These experimental observations are discussed regarding their impact for friction modelling in the simulation of railway ballast

Geometric representation of railway ballast using the Discrete Element Method (DEM)

The development of high-speed train lines has increased during the last twenty years, leading to more demanding loads in railway infrastructures. For these reasons, the implementation of a numerical tool for the calculation of railway ballast behaviour has been found useful, as it will enables design optimization. Regarding the numerical method, the DEM is considered effective and powerful for the calculation of engineering problems with granular and discontinuous materials. Due to the fact that railroad ballast layer consists of discrete aggregate particles, the DEM is considered suitable for the simulation of particulate ballast material. However, the computational cost of contact calculation between irregular particles is high and limits the calculation capability. From the point of view of micro-scale analysis, it is essential to represent the exact geometry of the particle. On the other hand, if the interest lies in the behaviour of the granular material as a whole, the geometry is not a determining factor. Besides that, setting up a simulation of granular material taking care of the exact geometry of each particle will not be efficient. Current work presents different geometrical approaches for the representition of ballast stones: spheric particles with rolling friction, sphere clusters, polyhedrons and superquadrics; showing their advantages and drawbacks. Finally, some simulation results, using spheric particles and sphere clusters, are displayed in order to evaluate the convenience or not of using more accurate and computational demanding geometries in each case

Discreet element modeling of under sleeper pads using a box test

It has recently been reported that under sleeper pads (USPs) could improve ballasted rail track by decreasing the sleeper settlement and reducing particle breakage. In order to find out what happens at the particle-pad interface, discrete element modelling (DEM) is used to provide micro mechanical insight. The same positive effects of USP are found in the DEM simulations. The evidence provided by DEM shows that application of a USP allows more particles to be in contact with the pad, and causes these particles to transfer a larger lateral load to the adjacent ballast but a smaller vertical load beneath the sleeper. This could be used to explain why the USP helps to reduce the track settlement. In terms of particle breakage, it is found that most breakage occurs at the particle-sleeper interface and along the main contact force chains between particles under the sleeper. The use of USPs could effectively reduce particle abrasion that occurs in both of these regions