RELATING THE BREAKAGE INDEX AND SETTLEMENT OF GEOGRID-REINFORCED BALLAST

Abstract

Trains have historically served as a relatively economical, quick and safe mode of public and freight transportation. Rail tracks on which the trains traverse require regular maintenance to ensure optimum performance of the rail services. Often the track’s superstructure seems to have received more attention than the substructure for performance assessment. As a foundation, the substructure, which is typically of ballast layer, plays the role of transferring the traffic load to the compacted subgrade. Degradation and breakage of ballast can lead to deformed track geometry and excessive or non-uniform track settlements, compromising the traffic ability and safety of the system. A potential approach to improve the performance of the ballast layer is geogrid reinforcement. The present study recreates the composite foundation in a lab-scale static test with geogrid placed at various heights in the ballast layer. The steel model box measured 200 mm x 200 mm x 200 mm. There was no apparent yielding of the ballast layer, with or without geogrid inclusion, indicative of a strain-hardening behaviour of the material under load. Taking the acceptable settlement as no more than 25 mm in a typical 300 mm ballast layer, the failure point was therefore defined at 8.3 % from the load-settlement curves. Sieve analysis was conducted on the ballast before and after the compression test to determine deterioration of the ballast via breakage under load. A graphical analytical method was next adopted to identify the Ballast Breakage Index (Bg) in relation to the overall settlement reduction. Overall particle breakage was not found to be expediently mitigated by geogrid installation in the ballast layer. The settlement reduction though was very much attributed to lateral spread control by the geogrid reinforcement.