Degree Name

Doctor of Philosophy


School of Civil, Mining and Environmental Engineering, Faculty of Engineering


In rail track environments, the loading system is cyclic unlike the steady seepage force that usually occurs in embankment dams. The mechanisms of filtration, interface behaviour, and time dependent changes of the drainage and filtration properties occurring within the filter medium require further research to improve the design guidelines. A novel cyclic process simulation filtration apparatus was designed and commissioned at the University of Wollongong, and a standard test procedure was established. The test apparatus was designed to simulate heavy haul train operations. The key parameters that influence the change in porosity and pore water pressure within the subballast layer under cyclic conditions in rail track environments were identified. Crushed basaltic road base was used as the filter subballast. The slurry form of the base soil, which is a low plasticity and highly erodible silty clay, was pumped from the bottom of the filter to simulate clay pumping and hydraulic erosion. Grading characteristics such as the uniformity coefficient and the mass of fines, and the external loading factors such as the maximum compressive load and slurry loading rate, were found to be the main factors that influence the mechanical characteristics and seepage hydraulics of subballasts under cyclic conditions. The evidence gathered from laboratory testing suggests that the subballast selection criteria adopted by the railway industry, which are based on mathematical and empirical models generated from static load conditions in embankment dams, does not address the filtration mechanism of subballasts under cyclic conditions. Apart from being a commonly used buffer for the ballast layer against subgrade attrition, subballast is primarily designed to act as a stress dissipation layer (capping layer) and its function as a filter is often regarded as a lesser priority. With more emphasis on the stress reduction function, well graded subballast containing larger particles provides a better skeletal resistance against applied stresses. However, well graded subballasts containing relatively coarse grains (20% fine sand, 30% fine gravel) are too porous to effectively Abstract vi capture the fines within its voids. Laboratory findings suggested that uniformly graded subballasts with not more than 30% fine sands (particle range of 0.15 to 0.425 mm) had an enhanced filtering capacity. Due to the lack of mechanical resistance against axial deformation, the application of cyclic stress to uniformly graded subballasts reduces porosity and enables the filter to trap migrating fines more effectively. Moreover, this intrusion of fines changes the PSD of the subballast which reduces its porosity and further inhibits drainage. A multi-layer mathematical approach was used to predict the time dependent permeability of this filter, with (a) a reduction in porosity as a function of compression under cyclic loading, and (b) the amount of fines trapped within the filter voids, being the two main aspects of this proposed model. Laboratory test results conducted on a novel cyclic loading permeameter were used to validate the proposed model. The set of equations that forms an integral part of the proposed model is then presented as compact visual guidelines anticipated to provide a more practical tool for railway practitioners.



Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.