Center for Excellence for Geotechnical Science and Engineering

of Engineering, Australia

Center for Excellence for Geotechnical Science and Engineering

of Engineering, Australia
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Ngo N.T.,University of Wollongong | Indraratna B.,University of Wollongong | Rujikiatkamjorn C.,University of Wollongong | Rujikiatkamjorn C.,Center for Excellence for Geotechnical Science and Engineering
International Journal of Geomechanics | Year: 2017

The load deformation of ballasted rail tracks subjected to cyclic loading is investigated experimentally using a large-scale track process simulation apparatus and numerically through a combined discrete element-finite-difference approach. Laboratory tests were performed to examine the deformation and degradation of ballast subjected to cyclic loading at 15 Hz and a lateral confinement of 10 kPa. The laboratory results reveal that ballast undergoes significant deformation during the initial load cycles, followed by gradually increasing deformation attaining a steady value toward the end of testing. A numerical model based on a combined discrete element method (DEM) and finite-difference method (FDM) is introduced to study the load-deformation response of the ballast assembly while considering interaction between the ballast aggregates and the subgrade layer. In this coupled model, the discrete ballast grains are modeled by DEM, and the subgrade domain is modeled as a continuum by FDM. Interface elements are introduced to transmit the interacting forces and displacements between adjoining material domains in which the DEM transfers contact forces to the FDM, and then the FDM updates the displacements, which provides subsequent input into the DEM. This computational cycle continues with the increasing number of loading cycles. The numerical model is validated by comparing the predicted cyclic load-deformation response with the laboratory measurements. Contact force distributions and stress contours in the assembly are analyzed and presented graphically to interpret the behavior of the model track, and the effects that subgrade stiffness have on the axial strain and bond breakage of the ballast are investigated. This combined DEM-FDM analysis is also used to analyze the load deformation of an instrumented track in the town of Singleton, Australia, and the numerical predictions are compared with the field data. © 2016 American Society of Civil Engineers.


Ngo N.T.,University of Wollongong | Ngo N.T.,Center for Excellence for Geotechnical Science and Engineering | Indraratna B.,University of Wollongong | Indraratna B.,Center for Excellence for Geotechnical Science and Engineering | Biabani M.M.,Coffey Geotechnics Private Ltd.
Geotechnical Special Publication | Year: 2017

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. © ASCE.


Ngo N.T.,University of Wollongong | Ngo N.T.,Center for Excellence for Geotechnical Science and Engineering | Indraratna B.,Center for Excellence for Geotechnical Science and Engineering | Indraratna B.,University of Wollongong | And 2 more authors.
Granular Matter | Year: 2017

This paper presents a study of the interface of geogrid reinforced subballast through a series of large-scale direct shear tests and discrete element modelling. Direct shear tests were carried out for subballast with and without geogrid inclusions under varying normal stresses of σn= 6.7 to 45kPa. Numerical modelling with three-dimensional discrete element method (DEM) was used to study the shear behaviour of the interface of subballast reinforced by geogrids. In this study, groups of 25–50 spherical balls are clumped together in appropriate sizes to simulate angular subballast grains, while the geogrid is modelled by bonding small spheres together to form the desired grid geometry and apertures. The calculated results of the shear stress ratio versus shear strain show a good agreement with the experimental data, indicating that the DEM model can capture the interface behaviour of subballast reinforced by geogrids. A micromechanical analysis has also been carried out to examine how the contact force distributions and fabric anisotropy evolve during shearing. This study shows that the shear strength of the interface is governed by the geogrid characteristics (i.e. their geometry and opening apertures). Of the three types of geogrid tested, triaxial geogrid (triangular apertures) exhibits higher interface shear strength than the biaxial geogrids; and this is believed due to multi-directional load distribution of the triaxial geogrid. © 2017, Springer-Verlag GmbH Germany.


Biabani M.M.,University of Wollongong | Biabani M.M.,Center for Excellence for Geotechnical Science and Engineering | Ngo N.T.,University of Wollongong | Ngo N.T.,Center for Excellence for Geotechnical Science and Engineering | And 2 more authors.
Geotextiles and Geomembranes | Year: 2016

A large-scale apparatus was designed and built at the University of Wollongong to evaluate the pull-out strength of rail subballast reinforced with geocells. A series of tests were carried out to investigate the pull-out resistance, mobilised tensile strength (τtensile) and passive strength (τpassive) of a subballast-geocell assembly under a given range of overburden pressure (1 kPa ≤ q ≤ 45 kPa). The interface was held in a vertical alignment to better simulate the interaction between subballast and geocell in accordance with routine track practices. The test results show that the geocell reinforcement provides a considerable degree of passive resistance, where the opening area (OA) and lateral pressure (σn) over the geocell strip are found to be influential factors. A three-dimensional finite element simulation was also conducted. The numerical results show that the tensile strength mobilised in the geocell will increase as the geocell stiffness increases, but causes a reduction in τpassive. A parametric study was also developed to investigate the impact of geocell stiffness and friction coefficient on the passive resistance and mobilised tensile strength. These results indicate that the passive resistance and mobilised tensile strength increase with the increase in overburden pressure (q) and friction coefficient (δ). © 2016 Elsevier Ltd.


Biabani M.M.,University of Wollongong | Biabani M.M.,Center for Excellence for Geotechnical Science and Engineering | Indraratna B.,Center for Excellence for Geotechnical Science and Engineering | Indraratna B.,University of Wollongong | And 2 more authors.
Geotextiles and Geomembranes | Year: 2016

This paper presents the experimental and numerical studies of geocell-reinforced subballast subjected to cyclic loading. A series of laboratory experiments were conducted using a large-scale prismoidal triaxial apparatus that was subjected to relatively low confining pressures of σ'3 = 10-30 kPa and a frequency of f = 10 Hz. Numerical simulations were performed using the commercial finite element package ABAQUS in three dimensions to realistically model cellular confinement, and to study the effectiveness of geocell reinforcement on subballast. A cyclic loading with a periodic and positive full-sine waveform was adopted to model the geocell-reinforced subballast, which is similar to the load carried out in the laboratory. The results of numerical modelling agreed well with the experimental data, and showed that geocell could effectively decrease the lateral and axial deformations of the reinforced subballast. The numerical model was also validated by the field data, and the results were found to be in good agreement, indicating that the proposed model was able to capture the load-deformation behaviour of geocell-reinforced subballast under cyclic loading. A parametric study was also carried out to evaluate the effect of the subballast strength and geocell stiffness on the mobilized tensile strength in the geocell mattress. It was found that the maximum mobilized tensile stress occurs on the subballast with the lowest degree of stiffness. Also the results revealed that lateral displacement decreased further by increasing geocell stiffness, and geocell with a relatively low stiffness performs very well compared to the geocell with a higher stiffness. © 2016 Elsevier Ltd.


Ngo N.T.,University of Wollongong | Indraratna B.,University of Wollongong | Rujikiatkamjorn C.,Center for Excellence for Geotechnical Science and Engineering
Computers and Geotechnics | Year: 2014

Geogrids are commonly used in railway construction for reinforcement and stabilisation. When railway ballast becomes fouled due to ballast breakage, infiltration of coal fines, dust and subgrade soil pumping, the reinforcement effect of geogrids decreases significantly. This paper presents results obtained from Discrete Element Method (DEM) to study the interface behaviour of coal-fouled ballast reinforced by geogrid subjected to direct shear testing. In this study, irregularly-shaped aggregates (ballast) were modelled by clumping together 10-20 spheres in appropriate sizes and positions. The geogrid was modelled by bonding a large number of small spheres together to form the desired grid geometry and apertures. Fouled ballast with 40% Void Contaminant Index (VCI) was modelled by injecting a predetermined number of miniature spheres into the voids of fresh ballast. A series of direct shear tests for fresh and fouled ballast reinforced by the geogrid subjected to normal shear stresses varying from 15. kPa to 75. kPa were then simulated in the DEM. The numerical results showed a good agreement the laboratory data, indicating that the DEM model is able to capture the behaviour of both fresh and coal-fouled ballast reinforced by the geogrid. The advantages of the proposed DEM model in terms of capturing the correct stress-displacement and volumetric behaviour of ballast, as well as the contact forces and strains developed in the geogrids are discussed. © 2013 Elsevier Ltd.


Ngo N.T.,University of Wollongong | Indraratna B.,University of Wollongong | Indraratna B.,Center for Excellence for Geotechnical Science and Engineering | Rujikiatkamjorn C.,University of Wollongong | And 2 more authors.
Journal of Geotechnical and Geoenvironmental Engineering | Year: 2016

This paper presents a study of the load-deformation behavior of geocell-stabilized subballast subjected to cyclic loading using a novel track process simulation apparatus. The tests were conducted at frequencies varying from 10 to 30 Hz. This frequency range is generally representative of Australian standard gauge trains operating up to 160 km/h. The discrete-element method (DEM) was also used to model geocell-reinforced subballast under plane strain conditions. The geocell was modeled by connecting a group of small circular balls together to form the desired geometry and aperture using contact and parallel bonds. Tensile and bending tests were carried out to calibrate the model parameters adopted for simulating the geocell. To model irregularly shaped particles of subballast, clusters of bonded circular balls were used. The simulated load-deformation curves of the geocell-reinforced subballast assembly at varying cyclic load cycles were in good agreement with the experimental observations. The results indicated that the geocell decreased the vertical and lateral deformation of subballast assemblies at any given frequency. Furthermore, the DEM can also provide insight into the distribution of contact force chains, and average contact normal and shear force distributions, which cannot be determined experimentally. © 2015 American Society of Civil Engineers.


Indraratna B.,University of Wollongong | Indraratna B.,Center for Excellence for Geotechnical Science and Engineering | Ngo N.T.,University of Wollongong | Ngo N.T.,Center for Excellence for Geotechnical Science and Engineering | And 3 more authors.
Computers and Geotechnics | Year: 2015

Experimental studies and numerical modelling of the deformation of soft clay stabilised by stone columns have been conducted over the past few decades. Continuum-based numerical models have provided valuable insight into the prediction of settlement, lateral deformation, and stress and strain-rate dependent behaviour of stone columns at a macroscopic scale, but because they consist of granular material such as crushed rock, gravel, and waste rock aggregates, their behaviour is influenced by inter-particle micromechanics and cannot be modelled properly using these models. In this paper a novel coupled model of the discrete element method (DEM) and finite difference method (FDM) is presented to study the deformation of a single stone column installed in soft ground. In this coupled discrete-continuum method, PFC2D and FLAC were used to model the interaction between the stone column and surrounding clay, respectively. The contact forces at the interface between the two zones were determined through a socket connection that allows the DEM to transfer forces and moments to the FDM and vice versa. The predicted results were comparable to the data measured experimentally, showing that the coupled discrete-continuum model proposed in this study could simulate the load-deformation behaviour of a stone column installed in clay. The contact force distribution and shear stress contour developed in the stone column and surrounding clay were captured to provide a better understanding of the load-deformation behaviour of the stone column. © 2014 Elsevier Ltd.

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