c Trung Ngo

Behavior of fresh and fouled railway ballast subjected to direct shear testing: discrete element simulation

AbstractThis paper presents the three-dimensional discrete element method (DEM) that was used to study the shear behavior of fresh and coal fouled ballast in direct shear testing. The volumetric changes and stress-strain behavior of fresh and fouled ballast were simulated and compared with the experimental results. Clump logic in particle flow code in three dimensions (PFC3D) incorporated in a subroutine was used to simulate irregular-shaped particles in which groups of 10–20 spherical balls were clumped together in appropriate sizes to simulate ballast particles. Fouled ballast with a various void contaminant index (VCI) ranging from 20 to 70% VCI was modeled by injecting a specified number of miniature spherical particles into the voids of fresh ballast. The DEM simulation captures the behavior of fresh and fouled ballast as observed in the laboratory, showing that the peak shear stress of the ballast assembly decreases and the dilation of fouled ballast increases with an increasing VCI. Furthermore, th...

Deformation of coal fouled ballast stabilized with geogrid under cyclic load

AbstractThis paper presents the results of laboratory investigations into the deformation of coal fouled ballast stabilized with geogrid at various degrees of fouling. A novel track process simulation apparatus was used to simulate realistic rail track conditions subjected to cyclic loading, and the void contamination index (VCI) was used to evaluate the level of ballast fouling. The experimental results show that coal fines act as a lubricant, causing grains of ballast to displace and rotate, and as a result, accelerate its deformation. However, coal fines also reduce ballast breakage because of a cushioning effect, that is, by reducing interparticle attrition. The inclusion of geogrid at the interface between the layer of ballast and subballast provides additional internal confinement and particle interlocking via geogrid apertures, which reduces deformation. A threshold value of VCI=40% is proposed to assist practitioners for conducting track maintenance. If the level of fouling exceeds this threshold,...

Closure to “Deformation of Coal Fouled Ballast Stabilized with Geogrid under Cyclic Load” by Buddhima Indraratna, Ngoc Trung Ngo, and Cholachat Rujikiatkamjorn

Buddhima Indraratna, Ph.D., F.ASCE; Ngoc Trung Ngo; and Cholachat Rujikiatkamjorn Professor of Civil Engineering, Research Director, Australian Research Council Centre of Excellence in Geotechnical Science and Engineering, Centre for Geomechanics and Railway Engineering, Faculty of Engineering, Univ. of Wollongong, Wollongong, NSW 2522, Australia (corresponding author). E-mail: indra@uow.edu.au Lecturer, Australian Research Council Centre of Excellence in Geotechnical Science and Engineering, Centre for Geomechanics and Railway Engineering, Faculty of Engineering, Univ. of Wollongong,Wollongong City, NSW 2522, Australia. Associate Professor, Australian Research Council Centre of Excellence in Geotechnical Science and Engineering, Centre for Geomechanics and Railway Engineering, Faculty of Engineering, Univ. of Wollongong, Wollongong City, NSW 2522, Australia.

Stabilization of track substructure with geo-inclusions—experimental evidence and DEM simulation

ABSTRACT This article reviews on current knowledge of rail track geomechanics, including several important concepts and topics related to laboratory testing and computational modelling, to study the shear stress-strain and deformation of ballast improved by geosynthetics and recycled rubber mats. The effect that impact loads have on ballast degradation and its mitigation due to resilient synthetic mats (i.e. rubber mats) between the ballast and the subballast is investigated using large-scale impact-testing apparatus. Computational modelling with finite element and discrete element methods are increasingly being used to model ballasted tracks reinforced with geosynthetics to capture the continuum media of formation soils and the discrete nature of ballast aggregates. The article focuses on reviewing previous studies by the University of Wollongong on ballasted track substructure and highlights some practical implications whereby innovation progresses from theory to practice.

Current research into ballasted rail tracks: model tests and their practical implications

Abstract Ballasted rail tracks are the most important mode of transportation in terms of traffic tonnage serving the needs of bulk freight and passenger movement, but under train loads, the particles degrade due to breakage and the progressive accumulation of external fines or mud-pumping under the subgrade, all of which reduce its shear strength and increase track instability. These actions adversely affect the safety, passenger comfort and efficiency of tracks, as well as enforcing speed restrictions and more frequent track maintenance. In spite of advances in rail track geotechnology, the optimum choice of ballast for track design is still considered critical because ballast degradation is influenced by the amplitude and number of load cycles, particle gradation, track confining pressure and the angularity and fracture strength of individual grains. One of the most effective methods of enhancing track stability and reducing the stresses transmitted to a soft subgrade layer is to increase the stiffness of the overlying granular media. This paper presents our current knowledge of rail track geomechanics, including important concepts/topics related to laboratory testing and computational modelling approaches used to study the load–deformation behaviour of ballast improved with waste tyres, synthetic geogrids and geocells.

Two Decades of Advancement in Process Simulation Testing of Ballast Strength, Deformation, and Degradation

This paper describes salient features of a set of large-scale ballast testing equipment developed at University of Wollongong (UOW), Australia, and how the test results and research outcomes have contributed to transforming tracks in the Australian heavy haul and commuter networks, particularly with regards to the strength, deformation, and degradation of ballast. Ideally, ballast assemblies should be tested in prototype scale under actual loading conditions. This is because a reduction in particle sizes for the testing in smaller equipment can reduce the internal angle of friction (shearing resistance) of the granular assembly in a macro sense, and the angularity of the particles in a micro sense, and hence the volumetric changes during the shearing process. In response to the worldwide lack of proper test facilities for ballast, UOW has, since the early 1990s, designed and built a number of large-scale process simulation triaxial testing rigs. They are all custom made to minimize any boundary effects and also evaluate the deformation and degradation of ballast, particularly the size, shape and origin of aggregates used as ballast in Australian tracks. This triaxial process simulation equipment was originally used to characterize the behavior of coarse aggregate used for state railway standards for monotonic loading, but since then it has been fitted with dynamic actuators to simulate actual track conditions involving the true cyclic loading nature, whilst also capturing the wheel-rail dynamics which correspond to high-speed commuter rail and fast heavy haul operations. These tests invariably demonstrated completely different stress-strain and volumetric characteristics of ballast compared to conventional static or monotonic testing of the same test specimens.

DEM modelling of geocell-stabilised sub-ballast under cyclic loading

Upon repeated train loading, sub-ballast aggregates, placed underneath a ballast layer in rail track, become degraded and fouled by the progressive accumulation of external fine particles such as mudpumping of soft subgrade, seriously decreasing the shear strength and drainage capacity of the track. This paper presents a study of the load-deformation response of geocell-reinforced sub-ballast under cyclic loads using laboratory tests and discrete element method (DEM). A series of large-scale cubical triaxial tests with and without geocell inclusions are conducted in the laboratory and simulated in DEM to investigate the beneficial effect of the geocells in decreasing the lateral and vertical deformations of railway subballast. Irregularly-shaped particles of sub-ballast are modelled by connecting and bonding of many circular balls together at appropriate sizes and positions. The geogcell was simulated by bonding many small spheres together to build a desired geometry and structure. The load-deformation behaviour of the geocell-stabilised sub-ballast specimen at varied load cycles predicted from the DEM modelling agrees well with those measured experimentally, showing that the proposed DEM model in this study is able to capture the deformation behaviour of the sub-ballast stabilised by the geocell. Additionally, the DEM modelling also provides insight into the distribution of contact forces, average contact normal and shear forces, which cannot be determined experimentally. These observations clearly prove the reinforcement effect of the geocell in eliminating the deformation of sub-ballast from a micromechanical perspective.

Influence of Particle Gradation and Shape on the Performance of Stone Columns in Soft Clay

A stone column typically consists of particles whose influence has largely been overlooked in design practice in terms of stress transfer, pattern of deformation, and intrusion of fines (clogging). This article presents an experimental study on the load-deformation behavior of a model stone column installed in soft clay with a particular emphasis on the influence of particle gradation and shape under undrained loading. The results show that particle gradation and shape have a significant influence on the load-deformation behavior and the extent of fines intrusion into the stone columns. Relatively well-graded particle sizes favor the development of higher peak shear stresses accompanied by lateral bulging, whereas more uniform grading results in the development of distinct shear planes and smaller peak shear stresses. Deformed columns were also examined using computed tomography, and the porosity profiles at the end of the test were determined using micrographs. Maximum porosity typically occurred in the zone of extreme lateral deformation, with the results suggesting that the extent of fines intrusion was influenced by particle morphology.

Load-deformation Responses of Ballasted Rail Tracks: Laboratory and Discrete-Continuum Modelling

The recent and rapid urbanization and frequent congestion of roads have led to more attention being focused on ballasted tracks for freight and commuter transport. The mechanisms of ballast degradation and deformation, the need for effective track confinement, understanding of interface behaviour, determining the dynamic bearing capacity of ballasted tracks require further insight to improve the existing design guidelines for future high speed commuter and heavier freight trains. The load-deformation behaviour of ballast under cyclic loads is measured in the laboratory using a novel large-scale Track Process Simulation Apparatus (TPSA). A novel coupling model based on discrete element method (DEM) and finite element method (FEM) is developed to predict the load-deformation responses of the ballast assembly considering the interaction of discrete ballast grains and continuum subgrade. In this coupled model, the discrete ballast grains are modelled by DEM and the subgrade domain is modelled as a continuum by FEM. The results indicate that significant settlements are observed during the initial load cycles, followed by gradually increased deformation, arriving at a steady value towards the end of tests. Contact force distributions, stress contours and corresponding broken bonds are captured.

Performance Assessment of Geocell-Reinforced Subballast: Modeling and Design Implications

This paper presents a study of the load-deformation behavior of geocell-stabilised subballast subjected to cyclic loads using a large-scale track process simulation apparatus and numerical modelling. The tests and numerical simulations were conducted to mimic the actual track conditions. Subjected to a given frequency and cyclic loads the predicted load-deformation behavior of the subballast with and without geocell inclusions match reasonably with those measured in the laboratory, and show that geocell could effectively decrease the lateral and axial deformations of the reinforced subballast. The results also provide an insight to design of rail tracks capturing the roles of geocell in decreasing lateral deformation of subballast. Additionally, the numerical modelling carried out in this study can be applied in the preliminary design of track substructure where a wide range of subballast aggregates and geocell mattresses with varying strengths and stiffness can be considered. Disciplines Engineering | Science and Technology Studies Publication Details Ngo, N., Indraratna, B. & Biabani, M. (2017). Performance Assessment of Geocell-Reinforced Subballast: Modeling and Design Implications. In T. L. Brandon & R. J. Valentine (Eds.), Geotechnical Frontiers 2017: Transportation Facilities, Structures, and Site Investigation: Selected Papers from Sessions of Geotechnical Frontiers 2017 (pp. 374-383). Reston, United States: American Society of Civil Engineers. This conference paper is available at Research Online: http://ro.uow.edu.au/eispapers1/264