State-of-the-art design aspects of ballasted rail tracks incorporating particle breakage, role of confining pressure and geosynthetic reinforcement
Railways are expected to play a very important role in future transport in Australia, and its large network should capture the essential needs for quick and safe, passenger and freight mobility. In recent years, the increased demand of heavier and faster trains has posed greater challenges to railway industry to improve efficiency and stability of track while reducing the track maintenance costs. Centre for Geomechanics and Railway Engineering (GRE) has been the primary Research and Development unit in the Australasia for developing and implementing new design and construction concepts for modern track upgrading with clear emphasis on applying theory to practice, with the key objectives of ensuring enhanced track longevity and minimising track maintenance costs. In spite of recent advances in rail track geotechnology, the optimum choice of ballast for track design is still considered critical. The major reason is that, ballast aggregates progressively degrade under heavy cyclic loading. Research at GRE has shown that a proper understanding of load transfer mechanisms and their effect on ballast breakage are important pre-requisites for minimising track maintenance costs. Ballast degradation is influenced by various factors including the amplitude and number of load cycles, particle gradation, track confining pressure, and the angularity and fracture strength of individual grains. Recent research projects at GRE and field trials in Bulli (near Wollongong) demonstrated that the discarded aggregates from ballast tips could be effectively reused in track construction, if regraded and reinforced with geogrids to rejuvenate their internal friction and load carrying capacity. This recycling practice would directly decrease the accumulation of discarded ballast, minimise the cost of track maintenance and reduce environmental degradation (i.e. less quarrying). Moreover, the use of effective sub-surface drainage via geosynthetic drains has been very effective in rapidly dissipating cyclic-induced pore water pressures in the soft subgrade (e.g. clay and silts) during the passage of trains, and these drains have effectively prevented soil liquefaction (mud pumping). The corresponding track behaviour models have been also developed through large-scale laboratory simulations and computer-based numerical modelling. This state-of-the-art paper describes field trials and prototype laboratory studies carried out to quantify the geotechnical behaviour of ballast, including shear strength, particle breakage, effects of increased confining pressure, supplemented with predictive and design models for practitioners adopting user-friendly analytical and numerical approaches. The paper also highlights the proposed changes to current standards of track design and how these new concepts have been implemented through actual field trials that demonstrated better performance, in terms of reducing settlement and improving drainage. Two case studies are elaborated including the Bulli and Sandgate sites enhanced by synthetic grids and geosynthetic drains.