Impact of Ballast Fouling on the Mechanical Properties of Railway Ballast: Insights from Discrete Element Analysis

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Impact of Ballast Fouling on the Mechanical Properties of Railway Ballast: Insights from Discrete Element Analysis Impact of Ballast Fouling on the Mechanical Properties of Railway Ballast: Insights from Discrete Element Analysis By Luyu Wang Department of Civil Engineering and Applied Mechanics McGill University Montréal, Québec, Canada July 2020 A thesis submitted to McGill University in partial fulfillment of the requirements for the degree of Master of Engineering © Luyu Wang, 2020 Abstract Ballast fouling is a major factor that contributes to the reduction of the shear strength of railway ballast, which can further influence the stability of railway equipment. Depending on the railway function and location, the major sources of ballast fouling are the infiltration of foreign fine material and ballast material degradation. The discrete element method (DEM) is an efficient way to capture the mechanical behavior of particles under various loading conditions. It has been successfully applied to investigate the mechanical behavior of ballast assembly and fouled ballast in the previous practices. In this study, a discrete element model is developed in PFC3D to simulate the stress-strain behavior of fresh ballast assembly tested in the direct shear test. A series of sensitivity analyses of the different parameters are conducted to better understand the corresponding effect on the stress- strain behavior of ballast assembly. An efficient approach to simulate the foreign material induced fouling of ballast and capture the mechanical behavior of the material at various levels of void contamination index (VCI) is proposed. The approach is based on the concept that the foreign fouling material will not only change the inter-particle friction angle but also the inter-particle contact stiffness. Therefore, both the effective modulus and the inter-particle friction coefficient are adjusted based on the validated fresh ballast contact model. Besides, empirical equations are developed to efficiently calculate the micro-mechanical parameters and then validated by comparing the simulation results with those measured in the experiments. Besides, ballast degradation as another important source of ballast fouling is simulated by changing the particle size distribution (PSD) to investigate the effect of ballast degradation on the shear strength property. The results indicate that the shear strength decreases and then increases with the increasing ballast degradation levels. I This study provides an efficient and accurate approach to simulate ballast fouling induced by foreign material in the discrete element model. At the same time, the effect of ballast degradation on the shear strength property is investigated with the micro-mechanical insight. The results will be beneficial for the planning of future full-scale test to simulate ballast fouling and help researchers better understand the impacts of ballast degradation. Conclusions and limitations are presented in Chapter 5. More sets of micro-mechanical parameters are recommended to be examined in future research to prove that the proposed approach can not only capture the mechanical behavior of fouled ballast assembly but also the volumetric behavior. The particle surface texture modification induced by ballast degradation is also needed to be considered in future research to carry out a comprehensive investigation of the ballast degradation impacts. II Résumé L'encrassement du ballast est un facteur majeur qui contribue à la réduction de la résistance au cisaillement du ballast ferroviaire, ce qui peut influencer davantage la stabilité de l'équipement ferroviaire. Selon la fonction et l'emplacement du chemin de fer, les principales sources d'encrassement du ballast sont l'infiltration de matières étrangères fines et la dégradation des matériaux de ballast. La méthode des éléments discrets (DEM) est un moyen efficace de saisir le comportement mécanique des particules dans diverses conditions de chargement. Elle a été appliquée avec succès pour étudier le comportement mécanique de l'assemblage du ballast et du ballast encrassé dans les pratiques précédentes. Dans cette étude, un modèle d'élément discret est développé en utilisant PFC3D pour simuler le comportement contrainte-déformation d'un assemblage de ballast frais testé dans le test de cisaillement direct. Une série d'analyses de sensibilité des différents paramètres est réalisée pour mieux comprendre l'effet correspondant sur le comportement contrainte-déformation de l'assemblage de ballast. Une approche efficace pour simuler l'encrassement induit par les matières étrangères du ballast et saisir le comportement mécanique du matériau à différents niveaux d'indice de contamination par les vides (VCI) est proposée. L'approche est basée sur le concept que le matériau d'encrassement étranger modifiera non seulement l'angle de frottement inter-particules mais également la rigidité de contact inter-particules. Par conséquent, le module effectif et le coefficient de frottement inter-particules sont ajustés en fonction du modèle de contact de ballast frais validé. De plus, des équations empiriques sont développées pour calculer efficacement les paramètres micromécaniques puis validées en comparant les résultats de simulation avec ceux mesurés dans les expériences. En outre, la dégradation du ballast comme autre source importante d'encrassement du ballast est simulée en modifiant la distribution de la taille des particules (PSD) III pour étudier l'effet de la dégradation du ballast sur la propriété de résistance au cisaillement. Les résultats indiquent que la résistance au cisaillement diminue puis augmente avec l'augmentation des niveaux de dégradation du ballast. Cette étude fournit une approche efficace et précise pour simuler l'encrassement du ballast induit par des matières étrangères dans le modèle à éléments discrets. En même temps, l'effet de la dégradation du ballast sur la résistance au cisaillement est étudié avec la compréhension micromécanique. Les résultats seront utiles pour la planification de futurs essais à grande échelle pour simuler l'encrassement du ballast et aider les chercheurs à mieux comprendre les impacts de la dégradation du ballast. Les conclusions et les limites sont présentées au chapitre 5. Il est recommandé d'examiner plus d'ensembles de paramètres micromécaniques dans de futures études de recherches afin de démontrer que l'approche proposée peut non seulement saisir le comportement mécanique de l'ensemble de ballast encrassé, mais également le comportement volumétrique. La modification de la texture de la surface des particules induite par la dégradation du ballast doit également être prise en compte dans les recherches futures pour mener une étude approfondie des impacts de la dégradation du ballast. IV Acknowledgments This study would not have been completed without the help from many people. First and foremost, I would like to express my indebted appreciation to my supervisors, Professor Mohamed A. Meguid and Professor Hani S. Mitri, for their expert supervision, encouragement, patience, and advice throughout this research project. I also wish to express my sincerest gratitude to my colleagues in Geo-Group and special thanks to Dr. Gao Ge for his technical support and valuable suggestions. In addition, I would like to acknowledge the financial support of the scholarships from the Henan Polytechnical University, China. and McGill University. It would not have been possible to finish this work without their generous support. I am really grateful for their support. Last but not least, I would like to express my appreciation to my father Lidong Wang, mother Luqing Fang, and girlfriend Yijie Zhao for their constant support, belief, and encouragement. V Table of Contents Abstract ............................................................................................................................................ I Acknowledgments.......................................................................................................................... V Table of Contents .......................................................................................................................... VI List of Figures ............................................................................................................................... IX List of Tables ............................................................................................................................... XII List of Symbols .......................................................................................................................... XIII 1 Introduction ............................................................................................................................. 1 1.1 Background ...................................................................................................................... 1 1.2 Research Motivation ........................................................................................................ 4 1.3 Research Objectives ......................................................................................................... 5 1.4 Thesis Outline .................................................................................................................. 6 2 Literature Review .................................................................................................................... 8 2.1 Introduction ...................................................................................................................... 8 2.2 Ballast Fouling ................................................................................................................
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