Ballastless railway track transition zones: An embankment to tunnel analysis

Railway track transition zones are characterised by an abrupt change in track support stiffness, which increases dynamic wheel loads and leads to the acceleration of differential settlement and track degradation. The performance of transition zones is a concern for railway Infrastructure Managers due to the increased maintenance operations and costs typically associated with these short track sections. To date, the majority of transition zone studies are focused on the analysis of ballasted tracks, however, the popularity of ballastless track has been increasing, especially on high-speed lines. Therefore, this work aims to study concrete slab track transition zones, with a focus on embankment/plain line-to-tunnel sections. The analysis uses a hybrid methodology, combining 3D finite element modelling with empirical settlement equations, in an iterative manner. The finite element model is capable of simulating train-track interaction and uses contact elements to simulate the potential detachment (voiding) between the slab's hydraulically bound layer and frost protection layer. At each iteration, firstly the track-ground stress fields are calculated using a 3D model, before passing them to a calibrated empirical equation capable of computing settlement across the transition. Then, before starting the next iteration, these settlements are used to modify the 3D model geometry, thus account for the effects of the previous settlement, before computing the updated stress fields. The model is used to analyse settlement and stresses for a transition zone case-study, before study the ability of a resilient mat to improve the performance of the track.