Year

2017

Degree Name

Doctor of Philosophy

Department

School of Civil, Mining and Environmental Engineering

Abstract

Railway industries are placing greater emphasis on implementing fast and heavy haul corridors for bulk freight and commuter transport to deliver more efficient and costeffective services. In the period of 2012-2013, Australian railways carried over one billion tonnes of freight and moved more than 850 million passengers. Australia’s $7.5 billion rail freight industry helps economies transport their goods to market and local communities to thrive. The industry is expected to grow by a further 90% between 2010 and 2030, which means it is imperative that rail networks be enhanced to meet the increasing demand for transport from a growing, ageing and urbanising population. Improving railroad stability to operate high-speed passenger and heavy haul freight services has become one of the key challenges not only in Australia, but also in Europe, America, and Asia.

The deterioration of a rail track due to large dynamic wheel loads is inevitable over the years and one that leads to more frequent high-cost maintenance. The degradation of ballast contributes to a large percentage of maintenance costs, apart from affecting the longevity and stability of a track. This problem is more critical in isolated rail track locations where the ballast is in direct contact with much stiffer interfaces such as bridges and tunnels and in locations where heavier concrete sleepers are used. One measure used to minimise track deterioration in these isolated places is the use of artificial inclusions such as rubber elements at the hard interfaces. In recent years, the use of soft synthetic rubber elements in track foundations to alleviate track damage has become increasingly popular. Currently, there is a lack of comprehensive assessment on the most relevant geotechnical characteristics and the associated response of ballast under cyclic loading. When rubber elements are added it further complicates the behaviour of composite track.

In this study, cyclic loads from fast and heavy haul trains were simulated using a novel Process Simulation Prismoidal Triaxial Apparatus (PSPTA) to investigate the performance of ballast improved by rubber elements placed at the hard interfaces. Two series of cyclic load tests were carried out: (i) with and without a rubber element placed at the bottom of a concrete sleeper (generally called Under Sleeper Pads – USPs); and (ii) with and without a rubber element placed on top of a concrete base (called Under Ballast Mats – UBMs). The cyclic loads were applied to simulate axle loads of 25 and 35 tonnes at frequencies ranging from 10 to 25 Hz (approximate train speed of 70 to 180 km/h). The laboratory results indicate that the energy absorbing (damping) characteristics of USPs and UBMs reduce the amount of deformation and degradation of ballast when used at hard substructure interfaces. The study shows that rubber elements distribute the stresses from moving trains more uniformly, by increasing the effective contact area of the ballast with concrete interfaces (sleeper or base), this then reduces the dynamic amplification of applied vertical stress and leads to much less ballast deformation and degradation.

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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.