Numerical Simulation of Differential Railway Track Settlement

Railway tracks undergo plastic settlement when subject to repeated train loading. This occurs differentially along the track rather than in a uniform manner, and the profile is a key parameter when scheduling track maintenance operations. Therefore, this thesis presents a novel numerical approach to predict track irregularity evolution. The model combines empirical settlement laws with finite element modelling, where the track-ground structure is modelled explicitly, and multi-body train-track interaction is considered. A 2.5D finite element approach with perfectly matched layers is used to simulate static and dynamic stress fields from trains. The stresses induced by the trains are solved using a hybrid frequency-wavenumber and time-space approach, considering non-linear track-soil material behaviour. The model has several novelties: 1) after every load passage, the track profile is updated before applying the next load, meaning the train-track interaction is constantly evolving; 2) new empirical settlement laws are derived that account for evolving train-track forces and track profiles; 3) full 3D stress fields in the track and ground are considered.
Firstly, the model development is described, before validating its prediction of track geometry evolution as captured from track recording vehicles. Next, the first analysis shows that modelling error is introduced if the geometry is not updated frequently (e.g. after every load passage). A parametric study also shows track subgrade material properties have a marked effect on track settlement. The second analysis investigates the train-induced differential settlement of ballast and non-ballasted tracks, considering typical modern intercity (200 km/h) and high speed (300km/h) lines. It is shown that the ballasted track exhibits higher train-induced differential settlement compared to the slab track and at higher linespeeds the degradation of track geometry is increasingly pronounced for the ballasted track. The next analysis introduces a novel equation for ballast settlement, considering the fouling index and moisture of fines. Moisture of fines is found to have a more significant impact on track deterioration than the fouling index. However, increased fines content contributes to moisture retention, exacerbating track settlement. Higher train speeds and lower earthwork stiffness’s make this effect more pronounced. Finally, a novel combined engineering-economic approach is proposed to investigate the effect of increasing train speeds, adding additional passenger movements, and adding additional freight movements to an existing line. It is shown that higher speeds result in higher dynamic forces and cause a faster rate of deterioration of track geometry, thus increasing marginal cost. The model is then used to investigate the effect of adding additional train movements to a passenger line. It is shown that additional movements increase the rate of track degradation and marginal costs, particularly if the additional traffic is freight. This is because freight vehicles typically have only one layer of (stiff) suspension, thus generating elevated dynamic forces compared to passenger vehicles.