Triaxial behaviour of ballast and the role of confining pressure under cyclic loading

Traditional railway foundations or substructures, consisting of one or two granular layers overlying a subgrade or natural formation, have become increasingly overloaded in recent years due to the utilisation of faster and heavier trains. During this period, there has been little, if any, re-engineering of the substructure in Australia, resulting in maintenance cycles becoming more frequent and increasingly expensive. Finding economical and practical techniques for enhancing the stability and safety of the substructure, thereby ensuring a capacity for supporting further increases in load, is vital in securing the long-term viability of the railway industry. The load bearing ballast is located directly below the sleepers and is responsible for limiting the stresses projected onto the weaker subgrade and preventing train-induced sleeper movement. Two significant ballast problems arising from increasing axle loads are differential settlement and degradation. It is thought that substructure enhancement can be attained and these problems largely curtailed through the manipulation of the level of effective confining pressure supporting the ballast layer. To investigate this possibility, a series of large-scale, high-frequency, drained, cyclic triaxial tests were conducted to examine the deformation (permanent and resilient) and degradation response of railway ballast. It was identified that the level of lateral confining pressure should be considered as an important design parameter. Two of the major benefits arising from increased confinement are reduced lateral movement (spreading) and vertical settlement resulting in improved line and level, and superior track stiffness and associated enhancements in ride comfort for passengers. The major drawback in the event of excessive confinement is unacceptable levels of particle breakage. The experimental results indicated, however, that insufficient confining pressure is as damaging in terms of particle breakdown as excessive pressure, and that minimal degradation will be achieved at some intermediate value. For maximum deviator stress magnitudes of 230, 500 and 750 kPa, 'optimum' breakage conditions were encountered within the confining pressure ranges 15 � 65, 25 � 95, and 50 � 140 kPa, respectively. Practical methods of increasing the in-situ track confinement are suggested and evaluated in terms of ease of installation, effectiveness and cost. It is concluded that the more superior methods of achieving increased confining pressure are by reinforcing the ballast using geosynthetics, or by increasing the effective overburden pressure through increased shoulder and/or crib height or via the achievement of a higher initial ballast density (greater compaction).