Track ballast – wear behaviour under traffic load

Dipl.-Ing. Christoph Kuttelwascher

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Summary

Track ballast together with other track materials is an important track component with significant effect on the service life of the track.

As new ballast it guarantees homogeneous absorption of traffic loads and high-quality track geometry. Over the life-cycle traffic loads cause wear effects which result in the destruction of the original granular structure. Increased fouling of the ballast bed leads to a loss of important functions such as drainage and ventilation which in turn results in costly maintenance measures.

Traditional testing methods for the wear-resistance of track ballast can only simulate the real load situation in the track to a limited extent – as many influencing factors cannot be simulated in the laboratory. Over the past years ÖBB Infrastructure AG have therefore carried out intensive tests in more than 70 track sections throughout the Austrian rail network.

These tests focussed on the following factors which are also of economic relevance:

• Phases of grain fracture
• Operating loads – ballast wear – behaviour of track geometry
• Effects of rock quality and subgrade stiffness
• Allocation of rocks to production sites
• Grain-size distribution at delivery

The tests rendered a differentiated picture. A mixture of track ballast of different origin can be found in the track. Ballast of a lower degree of hardness need not necessarily have higher wear rates and can be used in a cost-effective way. On the other hand the use of highly wear-resistant ballast can significantly increase the service life of the track in certain sectors. Track loads and track components are important influencing parameters in such cases.

Dynamic effects caused by inhomogeneity in the track and in the subgrade coupled with insufficient drainage play a decisive role with regard to premature ballast wear. The right matching of all track components will permit track ballast to serve its purpose. Insufficient drainage, weak subgrade or intermittent dynamic loads due to irregularities cannot be fully compensated by even highest-grade ballast. Such challenges can only be met in the best possible way if we take a comprehensive and holistic view of all track components.