Abstract

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.

Abstract

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.

Abstract

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, a discrete element model of granular material was carried out using 3D spherical particles with a rolling resistance, in order to consider non-sphericity of ballast stones. Other discrete element model representing granular materials as sphere aggregates (sphere clusters) has been also implemented. Current work presents the methodology followed to develop the discrete element model able to calculate railway ballast interaction. Interaction between discrete and finite elements is another key point of the calculations that is addressed. This document displays some results evaluating the influence of material parameters and geometric representation in the simulation of railway ballast and presents some laboratory tests calculated with the numerical application developed.  

Abstract

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.

Abstract

Landslides in reservoirs constitute one of the main threats to dams. Although they are not frequent, they may generate severe damages in the population and goods placed near the shores. It is a complex phenomenon, because of the interaction between the landslide, the still water in the reservoir, and the dam. The Particle Finite Element Method (PFEM) is an innovative numerical scheme which combines a lagrangian approach with the resolution of FEM equations via mesh generation. In previous works, it had been applied to the simulation of landslides in reservoirs, considering the moving body either as a rigid solid or as a Newtonian fluid. Despite the promising results of the simulation of some reference cases, these constitutive models showed poor performance when applied to other relevant events, such as Vaiont case. Current work introduces the implementation of a non Newtonian Bingham model in the particle finite element method scheme, and the results of its application to several benchmark cases. Some of them have been used to calibrate the parameters of the Bingham model, whereas others served as test cases. The phenomenon presents several intrinsic uncertainties, such as the lack of precise information about the physical properties of the landslide, its geometry, its granulometry, the drag coefficient along the sliding surface, among others. In spite of that, results show that the Bingham model implemented can be useful for estimating the potential consequences of landslides in reservoirs. This implementation makes the PFEM a versatile numerical tool for the analysis of landslides in reservoirs, provided that from now on both rigid solids, Newtonian and non Newtonian fluids can be accounted for. The authors are currently working in the optimization of the numerical code, so that more complex problems could be afforded in more detail. In addition, the erosion module is being validated, in order to reproduce effects such as the dragging of the underlying material due to the landslide, as well as the effect of overtopping on dam stability, particularly for rockfill dams.

Abstract

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 work is the development of a numerical modelling tool based on the Discrete Element Method which allows the users to understand better the mechanical behaviour of railway ballast. 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 work. 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 for evaluating railway ballast behaviour were computed. The aim was to calibrate the ballast material properties and validate the code for the representation of railway ballast. 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. This publication is a revised version of the text of the PhD Thesis “Numerical analysis of railway ballast behaviour using the Discrete Element Method” of Joaquín Irazábal, presented at the Technical University of Catalonia on October 6th 2017