The Role of Ballast-Fouling Characteristics on the Drainage Capacity of Rail Substructure

Abstract

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 Wales.

Micromechanics-Based Investigation of Fouled Ballast Using Large-Scale Triaxial Tests and Discrete Element Modeling

“…The mechanical properties of the ballast and clay tested in the study are also presented in Table 1. To quantify levels of ballast fouling, the void contaminant index (VCI) that considers the specific gravity of the fouled material, as proposed earlier by (Tennakoon et al 2012), was adopted and is given by:…”

“…dilation) of ballast at different levels of fouling. In the current analysis, ballast fouling was modelled approximately by adding a pre-determined amount of 1mm spheres in the voids between ballast to represent a given value of VCI (Tennakoon et al 2012). It was expected that as the small particles filled the voids between the ballast that the inter-particle friction angle would decrease , where the reduction in the inter-particle friction angle was determined in the laboratory for different confining pressures, as shown in Figure 2c.…”

“…In low-lying coastal areas in Australia, where clay fouling often occurs under wet conditions, the reduced friction angle needs to be evaluated as a function of void contamination (Tennakoon et al 2012) to avoid over-predicting the track bearing capacity in design. In addition to this, the evolution of contact orientations and fabric anisotropy obtained from this DEM model provides insightful information into how the fouled particles change the force transmission and the associated volume changes of a ballast assembly at a given confining pressure.…”

Improved Performance of Rail Track Substructure Using Synthetic Inclusions: Experimental and Numerical Investigations

“…Coal fines used as fouling material in the tests are provided by the Queensland Rail. The levels of ballast fouling are quantified with the Void Contamination Index (VCI) introduced earlier by Tennakoon et al [7], which includes the void ratio, the specific gravity and gradation of ballast and fouling material. VCI is defined by:…”

“…Because of track degradation, the Australian rail industry has a very large budget in terms of frequent track repair and maintenance, as well as significant ground improvement efforts where soft and saturated ` subgrade poses challenges. A large proportion of track maintenance costs are related to problems with the substructure such as ballast breakage, fouling, poor drainage, differential settlement, and track buckling [7]. Hence, there is a definite need for innovative design solutions that can extend the service life of tracks and cater for faster and heavier traffic.…”