Degradation of railway ballast for high-speed rail

Macadam ballast in railway tracks

In a ballasted track, the loads from passing trains are transferred down to the railway embankment via sleepers. Sleepers are embedded in the ballast in the track superstructure at the top of the embankment. The ballast is subjected to high and repeated loads when the track is in operation. This leads to a gradual degradation of the aggregate grains that differs at different locations in the ballast layer.

The degradation processes are complex and depend not only on the loads but also on the basic properties of the chosen rock material, microcracks in the grains, grain shapes and grain size distribution, how the material is laid out and compacted in the track. The design of the substructure has an impact on bed stiffness. Degradation also occurs during track mainenance work, e.g. in conjunction with track alignment. The production methods of aggregates affect the quality of the aggregates and their resistance to degradation.

A literature review is conducted where the degradation mechanisms are studied and research on aggregate degradation is compiled. The relevance of today's requirements and standardized test methods is evaluated.

Availability of materials for high-speed lines

The geological conditions and existing aggregate quarries along the routes are described. Materials from a number of selected quarries are selected and evaluated using the test methods specified in the standard for railway ballast; EN 13450. The materials represent different ballast grades that have a spread in technical properties within today's requirements for railway ballast for conventional tracks that are not intended for high speed.