Keywords: transition zones, ballast, degradation, railway track, settlement model.

Transition zones corresponding to the passage from railway tracks on embankments to natural ground or settlement free structures are frequently problematic regions for maintenance. Fast changing track stiffness and differential settlements are the main causes for the degradation of the tracks and foundations at transition zones. Differential changes in the track geometry affect the dynamic response of the train-track system which, in turn, affects the evolution of the track geometry. A computational methodology to predict the evolution of the degradation in a transition zone thus requires iterative dynamic and non-elastic calculations.

This paper presents a new settlement model for ballast suited for the study of the long-term behaviour of railway tracks at transition zones. The settlement model is driven by the factors that most influence the non-elastic behaviour of ballast, such as the amplitude of the applied dynamic force to the ballast surface level and the number of loading cycles. These applied forces to the ballast depend on the railway vehicle characteristics and speed, and on the stiffness of the railway track and foundation. At transition zones, these forces may vary significantly from sleeper to sleeper and with time, as the track geometry progressively changes.

In the paper, the settlement model is first verified with laboratory results of ballast subjected to loading of variable amplitude. The settlement model proves to describe the influence of the loading history on the ballast non-elastic behaviour. The accumulated settlement does not depend on the order of the loading and the settlement rate increases when the loading amplitude rises above any previously applied value.

Measurements from field tests recently performed on a transition zone in the Netherlands are used to validate the settlement model. The transition zone is a passage over a culvert box where fast evolving differential settlements were registered. Also dynamic measurements during train passages were carried out, used here to estimate the track foundation stiffness. The measured settlement at the Dutch site and the chosen values for the parameters of the settlement model are discussed.

The final application in the paper uses the settlement model on a computational methodology to predict the evolution of the degradation of railway tracks at places as transition zones. It follows from the calculations that the maximum passing axle load determines the final settlement. Also, the effect of small randomness in the settlement properties of a straight track is investigated. It turns out that the track level evolves to a stable profile, with mean settlement identical to the corresponding homogeneous case.

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