ABSTRACT
Random fibre reinforcement of granular soils with flexible polymeric fibres increases their shearing resistance and ductility. However, the potential advantages of fibre reinforcement are not limited to monotonic loading conditions. Recent research has shown that fibre reinforcement has the potential to reduce the permanent settlement of granular materials subject to cyclic loading, such as railway ballast. Ballast is still widely used on railways, as ballasted tracks are still predominant worldwide and are compatible with high-capacity and high-speed applications. Under repeated train passages, ballasted track settles differentially hence costly maintenance operations are required periodically to restore the correct rail level. Full-scale tests in the Southampton Railway Testing Facility (SRTF) have shown that the addition of a moderate amount of thin polyethylene strips to ballast can reduce the permanent settlement with marginal influence on track stiffness, provided that the fibres are sufficiently narrow not to disturb the natural arrangement of the ballast grains. As with fibre reinforced soils, the performance of fibre-reinforced ballast is expected to be influenced by the characteristics of both the host material and reinforcement. This study presents the results of full-scale tests carried out in the SRTF using different materials for the fibres and ballast. The fibres were obtained from polypropylene rope to replicate at a much larger scale the polypropylene filaments often used for the reinforcement of sands. The ballast was coarser and settled significantly less. The design of the reinforcement was informed by packing tests, as suggested by earlier tests using strip fibres. Thin fibres, with a small influence on the packing of the grains, were found to reduce the permanent settlement and had little influence on track stiffness.
