Modelling of the long-term behaviour of transition zones: Prediction of track settlement

Transition zones in railway tracks are the locations with considerable changes in the vertical support structures. Typically, they are located near engineering structures, such as bridges, culverts, tunnels and level crossings. In such locations, the variation of the vertical stiffness and the differential settlement of the track (when the foundation settles unevenly) result in amplification of the dynamic forces acting on the track. This amplification contributes to the degradation process of ballast and subgrade, ultimately resulting in the deterioration of vertical track geometry (settlement). To analyse and predict the accumulated settlement of the track in transition zones, a methodology using the iterative procedure is proposed. The methodology includes the finite element simulations of the vehicle-track and sleeper-ballast interaction during a train passing a transition zone; and iterative calculations of accumulated track settlement, based on an empirical model for ballast settlement. The simulations are performed using a 3-D dynamic finite element model (explicit integration) of a track transition zone, which accounts for the differential stiffness and the differential settlement of the track. Also, nonlinear contact elements between sleepers and ballast are used. As a result, the model can perform the detailed analysis of the stresses in ballast and accounts for the effects of vehicle dynamics. The model was validated against field measurements. The empirical settlement model describes the two-stage settlement of ballast and the nonlinear relationship between ballast stresses and permanent settlement. The proposed methodology is demonstrated by calculating the track settlement in the transition zone for 60,000 loading cycles (3.5 MGT). The dynamic responses such as ballast stresses are analysed to study the effect of the settlement. The parametric study of the iteration step used in the accumulated settlement procedure has been performed, based on which the optimal step is suggested.