Implications of ballast breakage on ballasted railway track based on numerical modelling

Large and frequent cyclic train loading from heavy haul and passenger trains often leads to progressive track deterioration. The excessive deformation and degradation of ballast and unacceptable differential settlement of track and/or pumping of underlying soft subgrade soils necessitate frequent and costly track maintenance. A proper understanding of load transfer mechanisms and subsequent deformations in track layers is the key element for safe and economical track design and optimum maintenance procedures. Many simplifi ed analytical and empirical design methods have been used to estimate the settlement and stress-transfer between the track layers. However, these design methods are based on the linear elastic approach, and often only give crude estimates. Given the complexities of the behaviour of the composite track system consisting of rail, sleeper, ballast, sub-ballast and subgrade subject to repeated traffi c loads in a real track environment, the current track design techniques are overly simplifi ed. The track design should also account for the deterioration of ballast due to breakage and subsequent implications on the track deformations. Considering this, an elasto-plastic constitutive model of a composite multi-layer track system is proposed. Constitutive models and material parameters adopted in this numerical model are discussed. A hardening soil model with a non-associative fl ow rule is introduced to accurately simulate the strain-hardening behaviour of ballast. The breakage of ballast observed in large scale triaxial tests is also simulated based on this model. In conjunction, numerical simulations are also performed using a two-dimensional plane-strain fi nite element analysis (PLAXIS) capturing the effects of ballast breakage and track confi ning pressure. The paper also demonstrates the advantages of the proposed elasto-plastic fi nite element simulations when compared to conventional analytical methods used by practitioners that are primarily based on a linear elastic approach.