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.

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.

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.

Numerous research studies have been carried out in the recent past to determine the influence on the resilient modulus of railway ballast of parameters such as stress history, number of loading cycles, density, grading, fines content, maximum grain size, aggregate type, particle shape and moisture content. However, the influence of particle breakage on the resilient modulus has not been described in depth. The current study highlights the influence of particle breakage on the resilient modulus of latite ballast (commonly used in railway track in New South Wales, Australia). Aggregate degradation is represented by the ballast breakage index (BBI). Laboratory test results indicate that the BBI has a profound influence on the resilient modulus of ballast. The resilient modulus increases with BBI, irrespective of the effective confining pressure. In addition, a simple hyperbolic relationship has been proposed between the resilient modulus M R and the bulk stress ö for latite ballast.