Buddhima Indraratna

The Role of Ballast-Fouling Characteristics on the Drainage Capacity of Rail Substructure

The ballast layer is designed to be free draining, but when the voids of the granular medium are wholly or partially filled because of the intrusion of fine particles, the ballast is considered to be “fouled.” To ensure acceptable track performance, it is necessary to maintain good drainage within the ballast layer. This paper critically examines the current methods commonly used for evaluating the degree of ballast fouling and, because of their limitations, a new parameter, “void contaminant index” is introduced. A series of large-scale constant head hydraulic conductivity tests were conducted with different levels of fouling to establish the relationship between the void contamination index and the associated hydraulic conductivity. Subsequently, a numerical analysis was executed to simulate more realistic two-dimensional flow under actual track geometry capturing the drainage capacity of ballast in relation to the void contamination index. In the context of observed test data, the drainage condition of the track could be classified into different categories together with a classification chart capturing the degree of fouling. The contents of this paper have already been considered in track maintenance schemes in the states of Queensland and New South Wales.

Field assessment of the performance of a ballasted rail track with and without geosynthetics

Understanding the complex mechanisms of stress transfer and strain accumulation in layers of track substructure under repeated wheel loading is essential to predict the desirable track maintenance cycle as well as the design of the new track. Various finite element and analytical techniques have been developed in the past to understand the behavior of composite track layers subjected to repeated wheel loads. The mechanical behavior of ballast is influenced by several factors, including the track confining pressure, type of aggregates, and the number of loading cycles. A field trial was conducted on an instrumented track at Bulli, New South Wales, Australia, with the specific aims of studying the benefits of a geocomposite installed at the ballast-capping interface, and to evaluate the performance of moderately graded recycled ballast in comparison to traditionally very uniform fresh ballast. It was found that recycled ballast can be effectively reused if reinforced with a geocomposite. It was also found t...

Experimental and numerical study of railway ballast behavior under cyclic loading

This paper presents the results of the influence of frequency on the permanent deformation and degradation behavior of ballast during cyclic loading. The behavior of ballast under numerous cycles was investigated through a series of large-scale cyclic triaxial tests. The tests were conducted at frequencies ranging from 10–40 Hz, which is equivalent to a train traveling from 73 km/h to 291 km/h over standard gauge tracks in Australia. The results showed that permanent deformation and degradation of ballast increased with the frequency of loading and number of cycles. Much of breakage occurs during the initial cycle; however, there exists a frequency zone of 20 Hz⩽f⩽30 Hz where cyclic densification takes place without much additional breakage. An empirical relationship among axial strain, frequency and number of cycles has been proposed based on the experimental data. In addition, discrete-element method (DEM) simulations were carried out using PFC2D on an assembly of irregular shaped particles. A novel app...

Improved performance of railway ballast under impact loads using shock mats

Impact loads generated because of wheel and rail defects cause accelerated ballast breakage leading to adverse performance of ballasted rail tracks. In the present study, experimental and numerical investigations have been carried out to develop an understanding of the performance of shock mats in the attenuation of dynamic impact loads and subsequent mitigation of ballast degradation. The investigations include different locations of shock mats considering stiff and weak subgrade conditions. With the provision of shock mats, the magnitude of impact forces decreases, and the time duration of impact ges prolonged. In the case of stiff subgrade, the efficiency of the shock mat in reducing the impact forces is greater when it is located at the bottom of ballast rather than at the top, whereas the reverse is true for weak subgrade. However, the provision of shock mats both at the top and bottom of the ballast bed, irrespective of the subgrade condition, is the best solution for minimising the impact force. The shock mats can bring down the impact-induced strains in the ballast bed by as much as 50%, apart from substantially reducing the ballast breakage.

Stress-Strain Degradation Response of Railway Ballast Stabilized with Geosynthetics

Large cyclic loading on ballasted railroad tracks is now inevitable owing to an increased demand for freight and public transport. This leads to a progressive deterioration and densification of railroad ballast and consequently to the loss of track geometry and differential settlement. Understanding these complex stress-strain and degradation mechanisms is essential to predict the desirable track maintenance cycle, as well as the design of new track. This paper presents the results of cyclic drained tests and numerical studies carried out on a segment of model railway track supported on geosynthetically reinforced railroad ballast bed. The relative performance and effectiveness of single- and dual-layer configurations of geosynthetic reinforcement was evaluated using a large-scale prismoidal triaxial chamber. Laboratory tests on unreinforced and reinforced railway track were simulated in a numerical model, and the results were then analyzed to better understand the distribution of displacements and stresses inside the railroad ballast layer. It was observed that in view of strain and breakage control, both the type of reinforcement and its layout played a vital role in improving the capacity of the track. These laboratory test findings were supported by the predictions from an advanced elastoplastic numerical analysis.

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...

Performance and Prediction of Vacuum Combined Surcharge Consolidation at Port of Brisbane

During the past decade, the application of vacuum preloading for stabilizing soft coastal clay and other low-lying estuarine soils has become popular in Australia. The cost-effectiveness is a major...

Laboratory Evaluation of Coefficient of Radial Consolidation Based on Pore-Water-Pressure Dissipation and Settlement

Modified and standard Rowe consolidation cells were employed to investigate the behaviour of soft clays during vacuum assisted radial consolidation. Degree of consolidation (DOC) and the back calculated value of coefficient of radial consolidation (ch) based on measured settlement and excess pore-water-pressure (uw) dissipation curves were compared. The DOC and (ch) based on pore water pressures are shown to be less than those based on settlements. The vacuum pressure-total surcharge ratio (VSR) is found to be an important parameter controlling the rate of the consolidation. Moreover, the coefficient of radial consolidation (ch) based on conventional oedometer testing could not accurately predict the consolidation behaviour of soft soils under varying VSR. Finally, a semi-empirical model incorporating the relationship between VSR and a modified coefficient of radial consolidation is introduced.

Analysis of Soil Disturbance Associated with Mandrel-Driven Prefabricated Vertical Drains Using an Elliptical Cavity Expansion Theory

The installation of mandrel-driven prefabricated vertical drains and resulting disturbance of soft saturated clays are analyzed with a new elliptical cavity expansion theory. This formulated theory accounts for a concentric progression of elliptical cavities in an undrained condition and the large-strain effects in the plastic zone incorporating the modified Cam clay parameters. The total and effective stresses and excess pore water pressure in the soils surrounding the mandrel are predicted taking into account the mandrel installation rate, mandrel dimensions and the time factor. The theoretical variation of excess pore pressure is then compared with the results of large-scale consolidometer tests, which show that the estimated and measured pore pressures are almost the same. The plastic shear strain normalized by the rigidity index is then used to identify the zone of disturbance around the vertical drains. This formulation has been applied to a case history from the Muar clay region in Malaysia, and th...

DEM analysis of angular ballast breakage under cyclic loading

Railway ballast particles undergo significant amount of breakage under repeated train load. Breakage of ballast particles, especially highly angular fresh ones, causes an increase in settlement, contributing to track degradation. The quantitative analysis of the influence of breakage on the stress-strain properties of ballast can be performed either experimentally or numerically. Numerical modeling has the advantage of simulating ballast breakage subject to various types of loading and different boundary conditions for a range of material properties. In this paper, ballast breakage under cyclic loading is simulated using a 2D discrete element method (DEM) utilizing the software PFC2D . A new subroutine is developed and incorporated in the PFC2D analysis to study ballast breakage and to quantify breakage in relation to particle size distribution. The influence of confining pressure on both breakage and permanent deformation is also studied and compared with laboratory observations. The findings of this paper provide an insight into the true ballast behavior under cyclic loading and are expected to assist railway practitioners in developing suitable design criteria for track stability.