Tim Neville

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.

Performance monitoring of rail tracks stabilized by geosynthetics and shock mats: case studies at Bulli and Singleton in Australia

Rail tracks are conventionally built on compacted ballast and structural fill embankments overlying the natural subsoil. Ballast plays an important role in providing track stiffness to support heavy traffic loads, and providing rapid drainage. However, ballast deforms and degrades progressively under the heavy cyclic loading of passenger and freight trains, which may lead to a loss of track geometry, and require costly regular maintenance. In particular, track construction requires appropriate stabilization techniques for ballast, the extent of which depends also on the type of subgrade. Comprehensive field trials were carried out on two rail lines in Bulli and recently in Singleton, New South Wales, Australia. In these studies, several track sections were reinforced with different types of geosynthetics placed beneath the ballast embankment. Both fresh and recycled ballast was examined for varying subgrade conditions. Recoverable and irrecoverable deformations of the substructure were routinely monitored. It was found that geogrids and geocomposites can decrease the vertical strains of the ballast layer, resulting in reduced maintenance costs. This paper describes the comprehensive field instrumentation, construction procedures, and field performance evaluation of these full-scale geosynthetic- stabilized ballast embankments in Bulli and Singleton.