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

Full-scale numerical calculation of ballasted tracks with the Discrete Element Method

Rail transport, both for people and goods, is becoming increasingly significant all over the world, which is reflected in the great growth of conventional and high-speed train lines. Most of these infrastructures are built with railway ballast, a granular material whose main functions are to resist vertical and horizontal loads and to face climatic actions. The growing popularity of these infrastructures has led to the development of numerical models to evaluate their performance. Among a wide range of numerical methods, the Discrete Element Method (DEM) was found to be effective for evaluating the performance of granular materials. This approach considers their discontinuous nature and has proven to be a useful tool to determine the dynamic behaviour of groups of particles. Moreover, the DEM is also used to compute the behaviour of continuum materials. In this work, rails and bearing plates are characterised in the calculations using this methodology, called the bonded DEM. It is a modification of the classical DEM which assumes that bonds exist between particles, resisting their separation. The code used is developed within DEMPack, a specific software tool for modelling physical problems using the DEM. Currently, DEMPack allows the use of two different types of geometry: spheres with rolling friction and clusters of spheres. A previous analysis showed that spheres are more effective for studying the macroscopic behaviour of the ballast layer, while clusters are necessary for small-scale tests involving highly compacted particles since their results are greatly influenced by particles and contacts distribution. After calibrating the code, full-scale tests were performed applying the load of a high-speed train on a railway track section in different situations. Considering the amount of material (about 260,000 particles) and that the aim is to evaluate the deflection of the rails, the calculations are carried out using spheres. The numerical results correctly capture the effect on the deflection of the rails. It can be concluded that the DEM increases the possibilities for analysing innovative solutions since real case-scenarios can be studied with enough accuracy and feasible time

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.Postprint (published version

Shape characterization of railway ballast stones for discrete element calculations

Railway ballast is a layer of granular material that resists to vertical and horizontal loads, produced by the passing train over the rail. The calculation of this kind of complex geomechanic problems has been traditionally addressed using refined constitutive models, based in continuum assumptions. Although these models may be suitable in the evaluation of the critical state of soils, or in the calculation of bulk material masses flowing, they are not appropriate to represent the local discontinuities of granular materials, which induce special features such as anisotropy or instabilities. The Discrete Element Method (DEM) is an alternative approach that considers the discontinuous nature of granular materials, which has proven to be a very useful tool to obtain complete qualitative information on calculations of groups of particles. However, the computational cost of contact evaluation between Discrete Elements (DEs) is high and limits the calculation capability. In this regard, it should be noted that particle shape greatly affects contact calculation computational cost, being spherical DEs the less computational demanding type of particles. From the point of view of micro-scale analysis, it is essential to represent the exact geometry of the grains. By contrast, if the interest lies in the behaviour of the granular material as a whole, particles geometry is not a determining factor. Therefore, for efficiency purposes, a trade-off between particle shape accuracy and computational cost needs to be achieved. In this work, different approaches to represent ballast stones are assessed: spheres with rolling friction, sphere clusters, polyhedrons and superquadrics. The first two were chosen for further analysis. Rolling friction allows avoiding excessive rotation when irregular shaped materials are simulated as spherical particles. This work presents a new insight for its application called the Bounded Rolling Friction model. Regarding sphere clusters, there is a key point in the friction between elements. As they reproduce irregular particles using clumps of spheres rigidly joined, the cavities between those spheres introduce interlocks that increase friction. To overcome this drawback, a new contact model is proposed. Finally, results of the application of both approaches are displayed, and conclusions are drawn as regards the convenience of using more accurate and computational demanding geometries

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

Numerical modelling of railway ballast behaviour using the Discrete Element Method (DEM) and spherical particles

In the last two decades there has been a great development of high-speed train lines. This advance has led to more demanding loads in railway infrastructures and the appearance of a new problem called ballast flight that happens when some stones rise when the train passes. For these reasons, the development of an application that allows the numerical modelling of the ballast superstructure under different stresses can be very useful, as it will enable design optimization. The DEM is being considered an effective and powerful method for the calculation of engineering problems with granular and discontinuous materials. Railroad ballast layer consists of discrete aggregate particles, so that DEM is one of the most suitable ways to simulate the behavior 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 behavior 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 the methodology followed to achieve accurate results in the calculation of railway ballast behaviour using DEM and spherical particles. The use of spherical particles reduces the computational cost and makes the simulation set up efficient. Validation results for the calculation of the lateral resistance force against a sleeper moving inside a ballast bed are presented. Regarding ballast flight problem, some high speed ballast collision calculations have also been performed

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

The Double Hierarchy Method: a parallel 3D contact method for the interaction of spherical particles with rigid FE boundaries using the DEM

The final publication is available at Springer via http://dx.doi.org/10.1007/s40571-016-0109-4In this work, we present a new methodology for the treatment of the contact interaction between rigid boundaries and spherical discrete elements (DE). Rigid body parts are present in most of large-scale simulations. The surfaces of the rigid parts are commonly meshed with a finite element-like (FE) discretization. The contact detection and calculation between those DE and the discretized boundaries is not straightforward and has been addressed by different approaches. The algorithm presented in this paper considers the contact of the DEs with the geometric primitives of a FE mesh, i.e. facet, edge or vertex. To do so, the original hierarchical method presented by Horner et al. (J Eng Mech 127(10):1027–1032, 2001) is extended with a new insight leading to a robust, fast and accurate 3D contact algorithm which is fully parallelizable. The implementation of the method has been developed in order to deal ideally with triangles and quadrilaterals. If the boundaries are discretized with another type of geometries, the method can be easily extended to higher order planar convex polyhedra. A detailed description of the procedure followed to treat a wide range of cases is presented. The description of the developed algorithm and its validation is verified with several practical examples. The parallelization capabilities and the obtained performance are presented with the study of an industrial application example.Peer ReviewedPostprint (author's final draft

Numerical modelling with discrete elements of rockfall protection systems

Some important infrastructures like roads, railway tracks or dams were constructed in places threatened by natural hazards. With the purpose of preserving these infrastructures from landslides and rock-falls, different containment systems are installed, and one of the most popular are the flexible metallic fences. The development of full-scale laboratory tests to evaluate the behaviour of flexible metallic fences is unfunctional, accounting to the huge magnitude of the event. On the other hand, small-scale testing may lead to inaccurate results, due to the distortion in the contours (e.g. anchors of the metallic fences). These problems in laboratory testing have led to the popularization of the use of numerical methods. In this study, the bonded Discrete Element Method (DEM) is used for the analysis of the behaviour of flexible metallic fences for rockfall protection. The bonded DEM is a modification of the classical DEM which assumes that bonds exist between particles, resisting their separation. In this case, the net cables are represented using rigid spheres joined by bond elements that are deformed according to an elasto-plastic law. Calculations were carried out using the DEMPack program, a specific software developed in CIMNE for modelling with the bonded DEM. This software allows considering the inter-action between discrete and finite elements, which can be useful to represent the boundaries of the domain, such as the surface of the slope. The code is firstly validated reproducing benchmark tests available in the literature. Finally, full-scale tests are computed in order to evaluate the energy dissipation capacity of the fence during a rockfall event

Anomaly Detection in Dam Behaviour with Machine Learning Classification Models

Dam safety assessment is typically made by comparison between the outcome of some predictive model and measured monitoring data. This is done separately for each response variable, and the results are later interpreted before decision making. In this work, three approaches based on machine learning classifiers are evaluated for the joint analysis of a set of monitoring variables: multiclass, two-class and one-class classification. Support vector machines are applied to all prediction tasks, and random forest is also used for multi-class and two-class. The results show high accuracy for multi-class classification, although the approach has limitations for practical use. The performance in two-class classification is strongly dependent on the features of the anomalies to detect and their similarity to those used for model fitting. The one-class classification model based on support vector machines showed high prediction accuracy, while avoiding the need for correctly selecting and modelling the potential anomalies. A criterion for anomaly detection based on model predictions is defined, which results in a decrease in the misclassification rate. The possibilities and limitations of all three approaches for practical use are discussed