Degree Name

Doctor of Philosophy


University of Wollongong. Faculty of Engineering


The ballast layer is the most important component of the railway track infrastructure. It is subjected to both the traffic load and the exposure to environment changes. In consequence, the ballast bed deforms and degrades highly affecting the performance of the railway track. Therefore, it is imperative to investigate in detail the deformation behaviour and degradation characteristics of ‘fresh’ ballast for typical loading and drainage conditions specific to railway tracks. Also, it is necessary to identify the factors that highly affect their variation. Better design of ballasted foundation provides higher track performance and ensures reduced maintenance costs. Also, the degradation caused by traffic loading (in field conditions) requires proper measure based on the Australian specifications in order to effectively plan the ballast-cleaning cycles so the maintenance costs are kept under control. Furthermore, it is necessary to establish the potential of using recycled aggregates (obtained from sieving the spoil from ballast maintenance processes) for the construction of railway tracks keeping in this way the maintenance cost to a minimum.

In order to further bring light to the behaviour of railway ballast, ‘fresh’ and recycled ballast specimens were subjected to monotonic and repeated loading in large-scale equipment (consolidation cell and cylindrical triaxial apparatus). Also, a unique prismoidal testing rig was designed and constructed to investigate the behaviour of railway ballast under dynamic true triaxial loading specific to in-service conditions. The fully instrumented equipment enables the measurement of: vertical pressures at different depth inside the ballast bed, lateral pressure developed within ballast layer, vertical and lateral plastic flow of ballast specimen upon loading.

The results show that the ballast shear strength, degradation and plastic deformations (vertical and horizontal) are functions of the ballast type, confinement level, load magnitude, and are also related to the number of loading cycles. The most conspicuous observation is that independent of ballast type, gradation, maximum particle size and loading and drainage conditions, the grains crushing highly affect the strength and deformation characteristics of ballast. It was also observed that the degradation is always larger for coarser fractions of gradations modelled. The recycled ballast displayed higher deformation and degradation when compared with ‘fresh’ ballast. A higher level of confinement of recycled ballast or restraint of lateral flow (geotextile inserts) may reduce the deformation and degradation of recycled ballast. Furthermore, a degradation index for the field conditions was proposed that properly quantifies the extent of ballast fouling and can be used to predict the ballast-cleaning cycles.

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