Traditional railway foundations or substructures have become increasingly overloaded in recent years, owing to the introduction of faster and heavier trains. A lack of substructure re-engineering has resulted in maintenance cycles becoming more frequent and increasingly expensive. Two significant problems arising from increasing axle loads are differential track settlement and ballast degradation. One potential method of enhancing the substructure is to manipulate the level of ballast confinement. To investigate this possibility, a series of highfrequency cyclic triaxial tests has been conducted to examine the effects of confining pressure and deviator stress magnitude on ballast deformation (permanent and resilient) and degradation. Experimental results indicate that, for each deviator stress considered, an 'optimum' range of confining pressures exists such that degradation is minimised. This range was found to vary from 15-65 kPa for a maximum deviator stress of 230 kPa to 50-140 kPa when deviatoric stresses increase to 750 kPa. Ballast specimens tested at low confining pressures indicative of current in situ conditions were characterised by excessive axial deformations, volumetric dilation, and an unacceptable degree of degradation associated mainly with angular corner breakage. The results suggest that in situ lateral pressures should be increased to counteract the axle loads of heavier trains, and practical methods of achieving increased confinement are suggested.

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Traditional railway foundations or substructures have become increasingly overloaded in recent years due to the introduction of faster and heavier trains. A lack of substructure re-engineering has resulted in maintenance cycles becoming more frequent and increasingly expensive. Two significant problems arising from increasing axle loads are differential track settlement and ballast degradation. The results suggest that in-situ lateral pressures should be increased to counteract the axle loads of heavier trains, and practical methods of achieving increased confinement are suggested.

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Traditional railway foundations or substructures have become increasingly overloaded in recent years due to the introduction of faster and heavier trains. A lack of substructure re-engineering has resulted in maintenance cycles becoming more frequent and increasingly expensive. Two significant problems arising from increasing axle loads are differential track settlement and ballast degradation. The results suggest that in-situ lateral pressures should be increased to counteract the axle loads of heavier trains, and practical methods of achieving increased confinement are suggested.

The ballast and its engineering behaviour have a key role in governing the stability and performance of railway tracks. The deformation and degradation behaviour of ballast under static and dynamic loads was studied based on large-scale triaxial testing. The possible use of different types of geosynthetics to improve the performance of fresh and recycled ballast was also investigated. The research findings showed that the inclusion of geosynthetics improves the performance of ballasted tracks.Keywords : geosynthetics; large-scale triaxial testing; railway ballast Le ballast et son comportement mécanique ont une influence importante sur la stabilité et la performance des voies ferrées. Nous avons étudié le comportement de détériora-tion et de déformation du ballast sous charge statiques et dynamiques en nous basant sur des essais triaxiaux à grande échelle. Nous avons également étudié la possibilité d'utiliser divers types de géosynthétiques pour améliorer la performance du ballast neuf et recyclé. Les résultats de cette recherche ont montré que l'adjonction de géosynthé-tiques améliorait la performance des voies ballastées.

Railway ballast deforms and degrades progressively under heavy cyclic loading. Ballast degradation is influenced by several factors including the amplitude and number of load cycles, gradation of aggregates, track confining pressure, angularity and fracture strength of individual grains. The degraded ballast is usually cleaned on track, otherwise, fully or partially replaced by fresh ballast, depending on the track settlement and current density. The use of composite geosynthetics at the bottom of recycled ballast layer is highly desirable to serve the functions of both drainage and separation of ballast from subballast. Construction of the rail track also requires appropriate improvement of the subgrade soils to achieve an adequately stiff surface layer prior to placing the ballast and subballast. Based on extensive research at University of Wollongong, it is found that the gradation of ballast plays a significant role in the strength, deformation, degradation, stability and drainage of rail tracks. Results from large-scale triaxial testing indicate that a small increase in confining pressure improves track stability with less ballast degradation. Bonded geogridsgeotextiles also decrease differential settlements of tracks, ballast degradation and lateral movement, and the risk of subgrade pumping. Stabilization of soft subgrade soils is also essential for improving the overall stability of track and to reduce the differential settlement during the operation of trains. This paper also highlights the

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