An Alternative Approach to Track Settlement Prediction

Abstract

Many empirical equations have been formulated in an attempt to model the settlement of ballasted railway track at an individual sleeper. Some equations have been used with vehicle track interaction models (VTI) to predict the development of differential settlement along the track iteratively and hence maintenance requirements, over potentially millions of cycles of load. For settlement equations to be suitable in such simulations, they require as an input a VTI model output that varies along the track, such as the force on the sleeper or the current resilient deflection range. For computational economy, these VTI simulations are usually run in large steps with the settlement predicted forward over many cycles. There is, however, no one generally applicable settlement equation, and it remains unclear whether the loss of accuracy that ensues from stepping the VTI analyses is acceptable. A realistic settlement equation needs to incorporate both stress- and load history-dependent behaviour. This paper proposes a new settlement model that allows for stress history and has the potential to be applied at every cycle within an iterative VTI simulation. The ballast layer is modelled by combining a nonlinear visco-elastic element to simulate the resilient response with a plastic-hardening element for permanent settlement. This leads to the calculation of permanent settlement without recourse to an explicit empirical equation. The parameters used in the model are determined using data from cyclic laboratory tests on a single sleeper. The effect of different loading histories on the model is considered.