Degree Name

Doctor of Philosophy


School of Civil, Mining and Environmental Engineering


The railway track network plays an essential role of the transportation infrastructure worldwide. During operations, the ballast becomes contaminated or fouled due to the infiltration of fines from the surface, mud pumping up from the subgrade, and ballast degradation under repeated train loading. In Queensland and New South Wales, Australia, ballast degradation and infiltration of fine particles such as coal and soft subgrade fills the voids in the ballast layer, which restricts drainage, and results in uneven track settlement and high maintenance costs to clean the ballast.

Geosynthetics have been increasingly used in railroads to provide reinforcement and confinement pressure to the layer of ballast. However, the interaction mechanism and behaviour of the geosynthetics and ballast at their interface are not well understood, particularly when the ballast is severely fouled. This is due to the steady accumulation of fine particles that clog the apertures of the geosynthetics, which dramatically reduces its beneficial effects and also causes track instability associated with substantial deformation.

This research aims to study how the interface between ballast and geogrid copes with fouling by coal fines. The shear stress-displacement behaviour of fresh and fouled ballast, and ballast reinforced with geogrids was investigated through a series of large-scale direct shear tests where the levels of fouling ranged from 0% to 95% Void Contamination Index (VCI), and at relatively low normal stresses varying from 15 kPa to 75 kPa. The results indicated that inclusion of geogrids increases the shear strength and apparent angle of shearing resistance, while only slightly reducing the vertical displacement of the composite geogrid-ballast system. However, when the ballast was fouled by coal fines, the benefits of geogrid reinforcement decreased in proportion to the increasing level of fouling. A conceptual normalised shear strength model was proposed to predict this decrease in the peak shear stress and peak angle of shearing resistance caused by coal fines at a given normal stress.

A novel Track Process Simulation Apparatus (TPSA) was used to simulate realistic rail track conditions subjected to cyclic loading and the Void Contamination Index (VCI) was used to evaluate the level of ballast fouling. The inclusion of geogrid at the interface between the layer of ballast and sub-ballast provides additional internal confinement and particle interlocking via the geogrid apertures, which reduces deformation. A threshold value of VCI=40% has been proposed to assist practitioners in conducting track maintenance. If the level of fouling exceeds this threshold the geogrid reinforcement significantly decreases its effectiveness, and the fouled ballast exhibits pronounced dilation. Based on the experimental results, an equation incorporating VCI was proposed to predict the deformation of fresh and fouled ballast. This equation improves track design and assists in making appropriate and timely decisions on track maintenance.

The Discrete Element Method (DEM) was used to study the shear behaviour of fresh and fouled ballast in direct shear testing. The volumetric changes and stress-strain behaviour of fresh and fouled ballast were simulated and compared with the experimental results.. Fouled ballast with various Void Contaminant Index (VCI), ranging from 20%VCI to 70%VCI, were modelled by injecting a specified number of miniature spherical particles into the voids of fresh ballast. The DEM simulation highlights the fact that the peak shear stress of the ballast assembly decreases and the dilation of fouled ballast increases with an increasing of VCI. Furthermore, the distribution of contact force chains and particle displacement vectors clearly explains the formation of a shear band and the evolution of volumetric change during shearing. An acceptable agreement was found between the DEM simulation and laboratory data.