Dynamic Axle Loads as a Main Source of Railway Track Degradation

Abstract

During train operation, geometrical irregularities develop in soil-supported ballasted railway tracks as a function of born tonnage. This form of degradation is combatted by periodic maintenance in the form of tamping by specially equipped trains in order to guarantee predefined levels of structural performance. The growth of irregular settlements depends on one hand on track properties (such as sleeper spacing, rail bending stiffness, subsoil geotechnical properties) and the intensity of longitudinal stiffness variations (variations in soil profile, switches and crossings, transitions etc.). The latter stiffness variations include both the static and the dynamic, frequency-dependent stiffness. On the other hand also the nature of the loading has an important influence. Running trains exert – depending on their velocity – quasistatic loads on the infrastructure due to the passing axles with a constant loading. Apart from this a dynamic loading component may occur with different frequencies as a result of non-perfect wheels, as a function of the speed. In general, the structural design of a railway line can be optimised with respect to its structural performance for the whole lifecycle. However, for existing lines this is difficult, and the only way to limit degradation and associated costs is to influence the condition of the rolling stock. The present study discusses theoretical backgrounds of track degradation in the form of differential settlements. It then shows results of an analysis of the loading conditions on Dutch railway lines, with both mixed passenger and freight transport and with dedicated freight traffic, based on actual measurements. Conclusions are drawn regarding deterioration and the effects of different loading types. Results show that especially on freight lines huge improvements are possible, with reductions in geometrical degradation up to 52% of actual values. The main driver of excessive degradation appears to be the low-frequency dynamic axle loading component.