Advanced characterisation of railway ballast roundness

Abstract

The performance of a railway track structure is significantly influenced by ballast shape properties such as roundness, flatness, elongation, sphericity, angularity and surface texture. Railway ballast materials have to comply with several quality requirements and shape properties. Accurate measurement of the shape properties is important for developing and revising specifications for quality control and quality assurance in the selection of ballast materials for railway construction. However, the current test methods for determining these properties have severe shortcomings such as poor repeatability and subjectivity. In addition, they are often based on visual measurements and empirically developed charts, which lack scientific standing. In this study, an advanced three-dimensional (3D) laser scanning was used to quantify the shapes of railway ballast materials from a heavy haul coal line in South Africa. This study complements the current research by the Council for Scientific and Industrial Research (CSIR) that is aimed at introducing advancement and scientific approach (i.e. 3D-laser scanning and numerical techniques) to effectively model the shape of crushed stones i.e. aggregates for roads and ballast for railways used in transport infrastructure. The primary objective was to investigate the effect of ballast particle shape, determined from a modern 3D-laser scanning technique, on the performance characteristics (i.e. shear strength and permanent deformation) of ballast materials. Overall, five ballast materials (four recycled ballast materials from the coal line and one freshly crushed ballast) and one river aggregate were investigated for this study. All six materials were scanned in the 3D-laser scanning system and the data were processed to reconstruct three dimensional models of the ballast and the river pebble particles. The models were further analysed to determine the roundness, flatness, elongation, and sphericity shape properties of the particles. The results obtained were used to develop different charts to characterise ballast shapes. An ANOVA (Analysis of variance) statistical analysis was conducted on the three dimensional data to establish which individual ballast particles contributed significantly to the overall shape parameters. To evaluate the effects of the shape properties on the behaviour of ballast in the track structure, a laboratory testing programme was conducted to determine the settlement behaviour and shear strength of the ballast materials. Repeated load permanent deformation tests were conducted to evaluate the overall settlement behaviour, whereas monotonic static triaxial tests were used to determine the shear strength properties of the ballast materials. The results indicated that ballast materials with low roundness values exhibited low shear strength and high permanent deformation (settlement). Although this was expected, the use of the automated 3D-laser scanning approach introduced a high level of accuracy and confidence in the results. Based on the laser results, a new empirical model was developed to determine the surface area of the ballast materials. The surface area values were further used to develop a chart to assess different particle shapes with varying degrees of roundness. Triaxial tests were conducted to determine the effect of the roundness on the shear strength properties of the materials. A Mohr-Coulomb failure model was successfully developed from the results to represent the individual materials tested. The overall results show that the angle of internal friction decreases with an increase in the roundness index of the particles. More rounded particles have roundness index values of between 1.4 and 1.7 whereas less rounded particles have roundness index values of between 0.8 and 1.3. The outcomes of this study would assist with quality control in the field as to whether or not to replace degraded ballast in the track layer. It is anticipated that this study will enhance improved guidelines, test methods and specifications for the selection of ballast