The Effect of Fouling Materials on Permeability Behaviour of Large Size Granular Materials

Granular materials are discontinuous, highly heterogeneous materials that are randomly assembled. They are widely used in civil engineering constructions such as in highway pavements, rockfill dams, drainage systems, and rail tracks. In this study, the granular type ballast aggregates were used to analyze its permeability behaviour when it is contaminated with fine particles. The ballast provides rapid drainage and supports the heavy loads apply from the trains. However, the accumulation of fines in the ballast layer which is known as fouling causes disturbance in track drainage which is leading for reduced track performance and frequent track maintenance. Therefore, this study focuses on the determination of permeability behavior of fouled ballast with different levels of fouling experienced in Sri Lankan rail tracks and to propose the stage at which track maintenance should be done. A series of constant head permeability tests were conducted by using large-scale permeability apparatus built in the laboratory. Sandy clay was used as the fouling material and the gradation of ballast currently adopted in Sri Lanka was used. Void contamination index (VCI) was selected to indicate the fouling level and the permeability of ballast was measured at different VCI values. A numerical analysis was carried out to simulate two-dimensional flow under actual track geometry in quantifying the drainage capacity of ballast in different degrees of fouling. By using constant head permeability test results as inputs, drainage capacities at different cases were obtained from the numerical analysis and classified according to the drainage capacity criteria. Based on the research outcomes, a relationship between the hydraulic conductivity of ballast and the VCI% was obtained. Also, the critical fouling levels in different layers of ballast were identified to commence the ballast cleaning work at the rail track site.