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 due to the intrusion of fine particles, the ballast is considered to be “fouled”. In order 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, due to 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 Wale