Behavior Of Ballast Research Articles

An Elasto-plastic Method for Analysing the Deformation of the Railway Ballast

In this study, a series of monotonically drained triaxial tests were conducted using the large-scale cylindrical triaxial apparatus. The effect of particle breakage and confining pressure on the mechanical behaviour of ballast were investigated. Based on the experimental findings, mathematical expressions for critical state stress ratio and specific volume are proposed to incorporate the evolution of particle breakage during monotonic shearing. An elasto-plastic constitutive model is formulated within the critical state soil mechanics framework in order to capture the stress-strain behaviour and degradation of ballast. The non-linear variation of shear strength envelope at a given range of confining pressures is taken into account. Mathematical expressions for shear hardening and stress-dilatancy relations are proposed. The constitutive model is calibrated against the results of large-scale laboratory tests and further validated using experimental results available from past independent studies. It is proved that the proposed method can promisingly predict the deformation of the ballast layer in a typical railway environment.

Discrete Element Analysis of Railway Ballast under Cycling Loading

Railway ballast is a highly heterogonous material consisting of a multitude of stone sizes with varying physical characteristics. It is a critical layer of railroad track structures providing drainage, strength, stability and load distribution. Ballast degrades under cyclic train loads and needs to be maintained to prevent rail misalignment, settlement and buckling, which may necessitate lower train speeds to prevent derailments. This paper utilizes the Discrete Element Method (DEM) code PFC2D to model the behaviour of railroad ballast under cyclic loading. It introduces two innovative approaches to capture stone angularity by fitting circular shapes into each stone shape. One of these methods involves a MATLAB® routine and the other an AutoCAD® routine. Inter-particle contact laws in these agglomerations of circular elements reflect the strength of the stones. The advantage of these two techniques over the conventional clumping technique is that they allow modelling stone breakage which is a leading cause of ballast degradation. Previous studies have implemented techniques to simulate the behaviour of a circular particle idealized as a single aggregate under cyclic loads. Three different techniques are presented in this study to better understand the natural behaviour of ballast under loading. In this paper, particle angularity is being introduced by using another method. This numerical study demonstrated the effect of aggregates particle shape on the overall performance of railroad ballast as measured by permanent deformation and degradation of the ballast.

Influence of Water Content on Shear Behavior of Unsaturated Fouled Ballast

This paper evaluates the synergistic effects of water content and fine fraction on shear behavior of unsaturated fouled ballast by performing a series of triaxial compression tests and water retention tests on fresh and fouled ballasts in unsaturated conditions. As the results, it was revealed that soil-water characteristic curves and relationships between degree of saturation and peak shear strength were seriously affected by intrusion of fouling materials into fresh ballast, and that the decreasing trend of peak strength becomes more remarkable in such a case where both water content and fine fraction increase. These results indicate that for the precise prediction of the long-term performance of transportation infrastructures and the evaluation for mechanical property of railroad ballast under repeated train passages, it is important to take into account the synergistic effects of water content and fine fraction on the deformation-strength characteristics of ballast in accordance with the degree of ballast fouling.

Modeling Behaviour of Railway Ballast in Prismoidal Apparatus Using Discrete Element Method

This article presents the results of numerical simulations of loading tests conducted on coarse particles that simulate railroad ballast. The objective of this study was to evaluate the deformation of ballast under vertical loading and to study the influence of the two different particle size distributions. One of them was according to particle size distribution recommended by Indraratna and co-workers in the past as an improvement to Australian Standard and the other was prepared in accordance with Brazilian standard. The discrete element method offers a new means of studying the response characteristics of railway ballast. The basic idea of discrete element method (DEM) is that arbitrary discontinuities are divided into a set of rigid elements, making each rigid element satisfy the equations of motion, use time step iteration method for solving the equations of motion of rigid elements, and then obtain the overall movement patterns of arbitrary discontinuities. In this study, the discrete element method of analysis has been used to simulate the geotechnical behaviour of railway ballast observed during the triaxial testing. Three-dimensional numerical simulations were performed using discrete element modeling approach.

Investigating the Shear Behaviour of Fouled Ballast Using Discrete Element Modelling

For several hundred years, the design of railway tracks has practically remained unchanged. Traditionally, rail tracks are placed on a ballast layer due to several reasons, including economy, rapid drainage, and high load bearing capacity. The primary function of ballast is to distributing dynamic track loads to sub-ballast and subgrade layers, while also providing lateral resistance and allowing for rapid drainage. Upon repeated trainloads, the ballast becomes fouled due to ballast degradation and the intrusion of fines which adversely affects the strength and deformation behaviour of ballast. This paper presents the use of three-dimensional discrete element method (DEM) in studying the shear behaviour of the fouled ballast subjected to direct shear loading. Irregularly shaped particles of ballast were modelled by grouping many spherical balls together in appropriate sizes to simulate representative ballast aggregates. Fouled ballast was modelled by injecting a specified number of miniature spherical particles into the void spaces. The DEM simulation highlights that the peak shear stress of the ballast assembly decreases and the dilation of fouled ballast increases with an increase level of fouling. Additionally, the distributions of contact force chain and particle displacement vectors were captured during shearing progress, explaining the formation of shear band and the evolutions of volumetric change of fouled ballast.

Deformation behaviour of gap-graded fouled ballast evaluated by a 3D discrete element method

The ballast fouling has been identified as a challenging issue in ballasted railway tracks due to huge maintenance costs. In this research, the deformation characteristics of gap-graded fouled ballast were studied from laboratory experiments and a Discrete Element Method (DEM). The fouled ballasts were prepared with different values of a fouling index, FIp. After the packing behaviour of fouled ballasts was studied by density tests, the stress-strain behaviour of them was studied from triaxial compression tests under both dense and loose states. It was found that the void ratio decreases with FIp up to 50%, then it increases with FIp. The results also indicate that the strain-hardening behaviour of ballast is weakened when fouled by FIp ≥ 30%. The DEM simulation by clump particles produce a similar stress-strain behaviour as the laboratory specimens than their spherical counterparts. The results also reveal that the dilation behaviour of ballast is deteriorated significantly when fouled by FIp > 50%. The peak frictional angle indicates that the strength properties of ballast are deteriorated significantly when fouled by FIp ≥ 30%. Therefore, we recommend that fouled ballast should be treated by a maintenance application when ballast is fouled by FIp > 30%.

Characterization of geogrid reinforced ballast behavior at different levels of degradation through triaxial shear strength test and discrete element modeling

Recent research efforts at the University of Illinois have aimed at studying geogrid applications in railroad track structures, specifically focusing on ballast and subballast reinforcement. Ballast, typically comprising large sized aggregate particles with uniform gradation, is an essential layer in the railroad track substructure to facilitate load distribution and drainage. The primary mechanism of load transfer within the ballast layer involves inter-particle contact between ballast particles. Similarly, the effectiveness of ballast reinforcement with geogrids is primarily governed by the geogrid-aggregate interlock. Such interaction and the effectiveness thereof can change significantly as the level of grain size and shape degradation or fouling increases in the ballast layer with accumulation of train traffic. Although several studies in the past have investigated the effects of geogrid reinforcement on ballast shear strength and permanent deformation behavior, the effectiveness of geogrid reinforcement at different levels of ballast degradation needs to be further understood. In this study, monotonic triaxial shear strength tests were conducted on both new and degraded ballast materials with and without geogrid reinforcement. Two geogrid types, with square- and triangular-shaped apertures, were used in the laboratory to calibrate an aggregate imaging-based Discrete Element Method (DEM) modeling approach, which is capable of creating actual ballast aggregate particles as three-dimensional polyhedron blocks having the same particle size distributions and imaging quantified average shapes and angularities. The DEM model was observed to adequately capture the shear strength behavior of geogrid-reinforced triaxial ballast specimens prepared using both new and degraded ballast samples.

Iterative Method for Analysis of Dimmable Self-oscillating Electronic Ballast Under Bus Voltage Control

This paper presents a predictive method of the self-oscillating electronic ballast (SOEB) behavior under dimming conditions. This work is focused on the bus voltage variation dimming method, where a closed analysis is not possible to carry out without considering the interaction among inverter bus voltage, fluorescent lamp power/resistance nonlinear behavior, and frequency operation of the SOEB. Thus, an iterative method is proposed in order to predict the oscillating frequency related with the fluorescent lamp power under bus voltage variation. Experimental results for a T8 32 W fluorescent lamp are obtained in order to compare simulation results, and verify the feasibility of the proposed method

Observed and predicted behaviour of rail ballast under monotonic loading capturing particle breakage

A substantial amount of experimental evidence suggests that the critical state envelope for ballast is nonlinear, especially at low confining pressure. To study the implications of this nonlinearity and the associated role of particle breakage, monotonically loaded drained triaxial tests were conducted using a large-scale cylindrical triaxial apparatus. A nonlinear critical state envelope is determined in the q–p′ and υ–lnp′ planes. Mathematical expressions for critical state stress ratio and specific volume are proposed to incorporate the evolution of particle breakage during monotonic shearing. In this paper, an elastoplastic constitutive model based on the critical state soil mechanics framework is presented to capture the salient aspects of stress–strain behaviour and degradation of ballast. Constitutive parameters were conveniently determined from large-scale laboratory tests. The model is able to predict the monotonic shear behaviour of ballast corroborating with the laboratory measurements. The proposed model is further validated using experimental results available from past independent studies.

Numerical Study on the Evolution of Force Chain inside Railway Ballast under Tamping Operation

The tamping maintenance of railway ballast is employed to restore the geometry of railway track distorted by train traffics. In this paper, based on the analysis of tamping principle, the three dimensional analysis model of railway ballast is created using the discrete element method, numerical simulations are performed to study the micro mechanical behavior of railway ballast under tamping operation. This paper presents the evolution of force chains inside railway ballast under tamping operation. The obtained results are showed that the contact force inside railway ballast is enhanced by tamping, so as to improve the carrying capacity and stability of railway track.

<b>Research on Vertical Natural Vibration Characteristics of Gravel Aggregate in Ballasted Track</b>

This paper describes research into the dynamic responses measured on a commercial-line with ballasted track using sensing sleepers and sensing stones both of which were developed by the author, for the purpose of gaining new knowledge that will contribute to measures against track deterioration and effective track maintenance. The research further analyzed the measurement results in terms of the spectral characteristics of ballast behavior and the vertical natural vibration characteristics of ballast layers. The results of the spectral analysis show that the rigid-body vibration mode of sleepers is in a frequency range lower than 100 Hz and also reveal that the elastic vibration modes of ballast layers are in a frequency range as wide as 400 - 800 Hz. This series of analyses suggests the possibility that the dynamic load of a passing train is hard to damp, affected by its resonance. Furthermore, the author performed a full-scale drop-weight impact test and found from the test result that when the impact load works on a ballast layer, unloading causes abrupt release of stress, which led to decreased contact force between particles and a jumping behavior in the ballast layer.

Analysis of ballast direct shear tests by discrete element method under different normal stress

The discrete element method (DEM) was applied to simulate the direct shear tests of irregular ballast stones by constructing clump particles, where irregular clumped-particles have similar particle size distribution (PSD) with those of the real ballast stone. With these clumped particles, the interlock of ballast stones in the direct shear test is investigated. The nonlinear contact force model with Mohr–Coulomb is adopted to model the interaction of the clumped particles. With different normal loads, the simulated microscopic characteristics in the direct shear test were linked with that of tests. The results demonstrate that DEM numerical simulations can produce interrelated responses of the ballast behavior, including the critical state response and are in good agreement with experimental observations. The shear resistance value increases with the normal stress application, indicating that the lateral confinement of ballast bed leading to less vertical settlement and more track stability. In addition, the DEM can further provide details of the microstructure evolutions during shearing processes.

A study into the use of crumb rubber in railway ballast

Ballasted track is the most common form of construction used in railway transportation due to a number of benefits in comparison with other solutions such as slab track. However, the degradation of the ballast particles and the layer settlement lead to important maintenance costs. Thus, diverse research has been carried out to develop new materials with the aim of increasing the service life of the track. To this end, the present paper focuses on the use of crumb rubber (from end-of-life tires) as elastic aggregates mixed with ballast particles, which could reduce ballast degradation and consumption of natural aggregates. At the same time, an abundant waste source is reused and the use of raw binders (proposed technique employed to bond elastic particles to ballast particles) is not necessary, which potentially reduces costs and consumption of raw materials. For this reason, the influence of different percentages of crumb rubber was studied in the laboratory using a ballast box. In addition, once the optimal quantity of rubber had been determined, its effect on ballast behaviour under high stress level was analysed. Results show that the use of 10% of crumb rubber (by volume) could reduce ballast degradation and at the same time as the capacity of the ballast layer to dissipate energy is increased and its stiffness is reduced. Additionally, based on the present laboratory study, the track settlement could be reduced with 10% rubber particles used as elastic aggregates.

On track: the future for rail infrastructure systems

Rail travel in the UK has seen a renaissance over the past 15 years. However, realising its future potential will require increased capacity and customer satisfaction while reducing cost and environmental impacts. Following a review of recent UK railway history, this paper outlines the challenges facing the rail system in meeting its future roles and aspirations. It describes recent advances in understanding the behaviour of track, ballast and earthworks through field monitoring, laboratory testing and numerical simulations, and suggests ways in which performance might be improved.

Equipamento triaxial cíclico de grande escala para análise mecânica de lastro ferroviário

The ballast layer usually is responsible for giving support to the railway superstructure and is loaded and unloaded repeatedly due to the passage of trains. In this context, the comprehension of the stress-strain characteristics of this noncohesive material used in that layer is important for optimizing maintenance operations, thus ensuring a safe and efficient transport. The material used in ballast layer is usually composed of highly coarse-graded gradation, hence large-scale laboratory tests are more difficult to conduct. The purpose of this paper is to evaluate the applicability of a new Brazilian large-scale triaxial apparatus (400 mm x 800 mm) that allows testing large-size particles such as in the field, without the need of scaling aggregates and ensuring more representativeness. Thus, it is possible to simulate loading that occurs due to the passage of trains, in order to observe the mechanical behavior of ballast in terms of resilient and permanent deformations. A cyclic triaxial test carried out with that apparatus showed the availability of preparing large-size samples and obtaining results of resilient modulus and permanent deformation compatible with what was being expected.

PSpice model of discharge lamps with saturated magnetic ballast and non-square arc voltage

High intensity discharge lamps with magnetic ballast are energy-efficient lighting devices commonly used in industrial and public installations. These lamps can be important pollution loads, and their harmonic modeling has been extensively studied considering linear magnetic ballast and square arc voltage waveform. Previous assumptions can be far from describing actual lamp behavior, affecting the accuracy of the model. This paper proposes a PSpice model of high intensity discharge lamps considering both the non-linear behavior of the magnetic ballast and the non-square waveform of the arc voltage. The PSpice model is successfully validated with lab experimental measurements.

Simulation of railway ballast using crushable polyhedral particles

The need to understand and predict the long- and short term behavior of railway ballast calls for extensive experimental programs as well as the development of reliable numerical models. From the modeling point of view, the granular nature of ballast in connection with the high onset speed of dynamic loading makes it ideal for the application of the Discrete Element Method (DEM).The paper describes the employment of the DEM to simulate ballast behavior in large-scale oedometric testing. Ballast grains are represented by convex polyhedral particles whose shapes are randomly generated via Voronoi tessellation. The effect of the aspect ratio of particles is investigated. A novel algorithm to compute repulsive contact force based on the intersecting volume of polyhedrons is presented. Crushing of grains is included via splitting the particles into smaller polyhedrons when a certain stress-based criterion is fulfilled. The results obtained from the model are compared with the published experimental investigations.

A constitutive model for coal-fouled ballast capturing the effects of particle degradation

Rail tracks undergo degradation owing to particle breakage and fouling of ballast by various fines including coal and subgrade soil. As the ballast becomes fouled, its strength and drainage capacity are compromised, sometimes resulting in differential settlement and reduced track stability. This paper demonstrates a continuum mechanics based framework to evaluate the detrimental effect of fines on the strength, deformation and degradation of coal-fouled ballast under monotonic loading. An elastoplastic constitutive model that considers the effect of fines content and energy consumption associated with particle breakage during shearing is presented. This multiphase constitutive model is developed within a critical state framework based on a kinematic-type yield locus and a modified stress-dilatancy approach. A general formulation for the rate of ballast breakage and coal particle breakage during triaxial shearing is presented and incorporated into the plastic flow rule to accurately predict the stress–strain response of coal-fouled ballast at various confining pressures. The behaviour of ballast at various levels of fouling is analysed and validated by experimental data.

Behaviour of clay-fouled ballast under cyclic loading

Expansion of the rail network in congested coastal Australia compels track construction on soft clayey subgrade, including fine-grained estuarine soils. In such low-lying areas, where the water table is close to the ground surface, the saturated soft subgrade is often subjected to pumping (mud slurry) under the application of cyclic wheel loads, thereby causing fouling of the overlying ballast. This technical note presents the results of a series of large-scale, drained, cyclic, triaxial tests conducted on clay-fouled ballast. The impact of fouling on the stress–strain behaviour, resilient modulus and degradation of ballast is discussed.

Investigating geogrid-reinforced ballast: Experimental pull-out tests and discrete element modelling

This paper presents an evaluation of the interlocking behaviour of geogrid-reinforced railway ballast. Experimental large box pull-out tests were conducted to examine the interaction between ballast and a biaxial geogrid. The discrete element method (DEM) was then used to model the interaction between the ballast and the geogrid by simulating large box pull-out tests and comparing the findings with the experimental results. Four different shapes of clumps were used to represent each ballast particle in order to obtain an acceptable shape for modelling the railway ballast. The DEM simulation results were shown to provide good predictions of the pull-out resistance and to examine the effect of clump shape on both the pull-out resistance and the distribution of contact forces. Therefore, the calibrated geogrid model and the 8-ball tetrahedral clumps, used as ballast particles, hold much promise for investigating the interaction between geogrids and ballast, and thus, optimising performance.