On the ballast-sleeper interaction in the longitudinal and lateral directions

In service, railway tracks must withstand transverse and longitudinal forces arising from running vehicles and thermal loads. The mechanical design adopting any of the track models available in the technical literature requires that the strength of the track is fully characterized. In this paper, the results of an experimental research activity on the sleeper-ballast resistance along the lateral and the longitudinal directions are reported and discussed. In particular, the work is aimed at identifying the strength contributions offered by the base, the ballast between the sleepers, and the ballast shoulder to the global resistance of the track in the horizontal plane. These latter quantities were experimentally determined by means of an ad hoc system designed by the authors. Field tests were carried out on a series of track sections that were built to simulate scenarios in which the ballast was removed from the crib and/or the shoulder. The results of this study indicate that, as far as the scenarios here investigated are concerned, the strength percent contributions from the crib, the sleeper base and the shoulder are respectively equal to about 50%, 25%, and 25% in the lateral direction, and 60%, 30%, and 10% in the longitudinal one. Moreover, the comparison of the acquired data with literature results reveals that a detailed knowledge both of the testing conditions and the activated ballast failure mechanisms are needed in order to correctly use test data for design purpose.