Development of Condition Assessment Index of Ballast Track Using Ground-Penetrating Radar (GPR)

The condition of the ballast is a critical factor affecting the riding quality and the performance of a track. Fouled ballast can accelerate track irregularities, which results in frequent ballast maintenance requirements. Severe fouling of the ballast can lead to track instability, an uncomfortable ride and, in the worst case, a derailment. In this regard, maintenance engineers perform routine track inspections to assess current and future ballast conditions. GPR has been used to assess the thickness and fouling levels of ballast. However, there are no potent procedures or specifications with which to determine the level of fouling. This research aims to develop a GPR analysis method capable of evaluating ballast fouling levels. Four ballast boxes were constructed with various levels of fouling. GPR testing was conducted using a GSSI (Geophysical Survey Systems, Inc.) device (400, 900, 1600 MHz), and a KRRI (Korea Railroad Research Institute) GPR device (500 MHz), which was developed for ballast tracks. The dielectric permittivity, scattering of the depth (thickness) values, signal strength at the ballast boundary, and area of the frequency spectrum were compared against the fouling level. The results show that as the fouling level increases, the former two variables increase while the latter two decrease. On the basis of these observations, a new integrated parameter, called a ballast condition scoring index (BCSI), is suggested. The BCSI was verified using field data. The results show that the BCSI has a strong correlation with the fouling level of the ballast and can be used as a fouling-level-indicating parameter.

Shear Strength and Drainage Characteristics of Elastomeric Polyurethane Treated Coal-Fouled Ballast

The shear behavior and drainage characteristics of coal-fouled ballast when treated with elastomeric polyurethane are assessed by means of large-scale direct shear and permeability tests. The results from direct shear tests confirmed that the shear strength of both stabilized and unstabilized coal-fouled ballast was highly influenced by the extent of fouling (VCI: void contamination index). The performance index (PI) of elastomer-stabilized coal-fouled ballast (ESFB), determined as the fraction of shear strength of fouled ballast to the shear strength of fresh and unstabilized ballast, lies in the range of 1.23 to 0.84. Moreover, the performance of ESFB having VCI ≥30% was found to be either similar to or poorer than that of clean ballast without any treatment, thus indicating that the elastomer treatment may be provided only to ballast with VCI ≤30%. The results from constant head permeability tests indicate that the hydraulic conductivity of ballast ( k) is highly influenced by the presence of fouling materials but is only slightly reduced as a result of the elastomer stabilization. The k of ballast decreased from 43 to 0.18 mm/s as the VCI increased from 0 to 75%. For VCI ≥ 45% the k of ballast was found to be lower than that recommended for sub-ballast. On the other hand, the k of ballast reduced slightly from 43 to 37 mm/s because of the elastomer stabilization. Furthermore, an empirical relationship is established between k and e to determine the k of both stabilized and unstabilized fouled ballast.

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

Ballast fouling is a major factor that contributes to the reduction of shear strength of railway ballast, which can further affect the stability of railway supporting structure. The major sources of ballast fouling include infiltration of foreign fines into the ballast material and ballast degradation induced by train movement on the supported tracks. In this paper, a discrete element model is developed and used to simulate the shear stress–strain response of fouled ballast assembly subjected to direct shear loading. A simplified computational approach is then proposed to model the induced ballast fouling and capture the mechanical response of the ballast at various levels of contamination. The approach is based on the assumption that fine particles comprising the fouling material will not only change the interparticle friction angle, but also the contact stiffness between the ballast particles. Therefore, both the interparticle friction coefficient and effective modulus are adjusted based on a fouled ballast model that is validated using experimental results. The effect of ballast degradation is also investigated by gradually changing the particle size distribution of the ballast assembly in the discrete element model to account for the increased range of particle sizes. Using the developed model, the effect of ballast degradation on the shear strength is then evaluated. Conclusions are made to highlight the suitability of these approximate approaches in efficiently modeling ballast assemblies under shear loading conditions.

Stabilization of Railroad Ballast Using Polyurethane Under Various Coal Fouling Conditions

This paper presents a comparative study on the shear behavior of coal fouled polyurethane-stabilized and polyurethane-stabilized coal fouled ballast. Fresh ballast and coal fines of mean particle sizes (D50) of 42 mm and 545 microns, and Elastan polyurethane polymer with a density of 1100 kg/m3 were used in the current study. Tests were conducted at normal stress (σn) varying from 60–120 kPa and at rate of shearing (Sr) 3 mm/min. To mimic the effect of coal fouling, a predetermined amount of coal was added that signifies a fouling level, of 30% void contamination index (VCI), in the present study. Test results highlighted that polyurethane stabilization technique enhanced shear stress of ballast. However, the coal fouling reduced the shear stress of polyurethane-stabilized and unstabilized ballast. The friction (φ) and dilation (ψ) angles of polyurethane-stabilized and unstabilized ballast were found to reduce non-linearly with an increase in σn. Furthermore, the values of φ and ψ of unstabilized ballast reduced from 65° to 60° and 21° to 16°, respectively due to coal fouling. The stabilization efficiency factor (Sef), stated as the ratio of the shear stress of stabilized ballast to the unstabilized ballast, differs from 1.70 to 1.75 for polyurethane-stabilized ballast as σn varies from 60 to 120 kPa. Moreover, it is observed in coal fouling conditions that the coal fouled polyurethane-stabilized ballast (FPSB) shown better performance when compared to polyurethane-stabilized coal fouled ballast (PSFB).

Forward-Looking Infrared Radiometry (FLIR) Application for Detecting Ballast Fouling

This paper is intended to assess the practical aspects of the previously proposed approach for detecting railroad ballast fouling using an off-the-shelf Forward-Looking Infrared Radiometry (FLIR) Technology. FLIR is among the technologies that are becoming more prevalent in railroad applications [1,2]. The method discussed in this paper takes advantage of the temperature differences measured by the FLIR camera between the top surface of clean and partially fouled ballast samples as an indicator of fouling. The method is intended to potentially serve as an efficient and time-effective manner for detecting early stages of ballast fouling prior to it requiring a costly intervention. Ballast fouling is a common maintenance-of-way issue for the railroad industry, which occurs as a result of contaminants clogging up the ballast and preventing water drainage. The water retained at the sublayers diminishes the strength of the foundation and could result in other undesirable conditions such as clay pumping and reduced track strength. In this study, experiments are performed to study the thermal behavior and characteristics of clean, and partially- and fully-fouled ballast using a FLIR camera. The FLIR camera is set up in a stationary configuration for ease of testing and also providing a more direct approach to analyzing the data, to keep the test conditions highly repeatable and reduce any environmental variations. The results indicate that the cooling and heating rate at the top surface for clean, partially fouled, and fouled ballast are different during the daily heat-up cycle. It is determined that although the FLIR camera is able to measure some changes in the ballast temperature for the fouling conditions that are evaluated in the study, the differences may be within the range of variations that could occur in field conditions. The paper includes the range of measured temperature by the FLIR camera and discusses the pros and cons of using this approach in practice. Additional field testing is needed to validate or dispute the initial findings of the study.

Simulation Evaluation of Fouled Ballast Thermal Characteristics

This study provides a simulation evaluation of the effect of fouling conditions on the thermal behavior of ballast for the use of railway tracks. Ballast fouling can result in a slurry pumped up to the surface, causing poor foundation strength, rotting of the ties, and other ill effects. To achieve a quick and convenient detection of ballast fouling, a thermal-based non-contacting technique has been proposed and becomes more and more attractive. However, the successful application of this thermal-based fouling-detection technology requires knowledge of the thermal characteristics of ballast, which have not been investigated in prior studies. The objective of this paper is to study the influence of fouling on the thermal behavior of ballast, using an analytical model developed based on one-dimensional conductive heat transfer. The effort to validate the developed model is also included. The general fouling conditions of the ballast — fouled with and without water — are studied through simulation. The simulation results show that, for the case of fouling without water, the ballast under different fouling conditions behaves differently from clean ballast at depths ranging from 0 to 12in under naturally-occurring daily ambient temperature changes, and that the temperature difference peaks at 4-in depth. In addition, increasing the amount of fouling results in less temperature variation in response to the ambient temperature changes. For the case of fouling with water, water is added into the 100% fouled ballast and comparisons are made between the ballasts with different water content. A similar pattern is observed, showing that increasing the amount of water results in a larger temperature difference at all depths considered. Moreover, the maximum temperature difference is observed at the top surface rather than the depth of 4-in as observed in the case of fouling without water.

Effect of Coal Fouling on Railroad Ballast Under Direct Shear Loading Conditions

Contamination of ballast generally occurs due to particle breakage, infiltration of other undesired materials from the surface or base of the ballast is one of the primary reason for track deterioration. In this context, large scale direct shear tests were conducted to examine the shear behavior of unstabilized and geogrid-stabilized fresh and coal fouled ballast. Fresh granite ballast with an average particle size (D50) of 42 mm and triaxial geogrid having 69 × 69 mm aperture were used in this study. Tests were conducted at different Void Contamination Index (VCI) values ranging from 0 to 60% at a constant shearing rate (Sr) of 3 mm/min and normal stress (σn) of 90 kPa. The test results revealed that the shear strength of ballast is highly influenced by the coal fouling. The friction (φ) and dilation angles (ψ) of unstabilized ballast is found to decrease from 63.06° to 55.55° and 17.7° to 11.22°, respectively as the VCI increases from 0 to 60%. It is further observed that the breakage of ballast (Bg, computed in terms of Marsal’s breakage) decreases from 9.37 to 3.98% with the presence of coal fines. However, the presence of geogrid was found to increase the friction angle and reduce the particle breakage and dilation of fresh and coal fouled ballast. These test results show the effectiveness of triaxial geogrid in stabilizing the fresh and coal fouled ballast.