Role of Grain-Scale Characteristics on the Stiffness of Railway Ballast

The majority of the world's railways are on ballasted track, which consists of rails attached to sleepers, supported by a granular layer (ballast) lying on the natural ground. The repeated passing of trains results in a gradual deterioration of track alignment, leading to the need for periodic maintenance of the ballast. Following a number of maintenance cycles, the ballast is considered degraded, and the track is renewed. Conventional track renewal is costly and potentially unsustainable, as it requires quarrying of fresh ballast, increasing the railway's carbon footprint. A better understanding of the mechanical properties of used ballast, particularly how its stiffness compares to that of fresh ballast, could inform more extensive reuse of ballast. Previous research has demonstrated that the stiffness of granular materials is greatly influenced by their particle shape, size and surface characteristics. This project investigates the difference in stiffness in the vertical and horizontal directions between fresh and used ballast using advanced triaxial tests of 1/3rd scaled material. Fresh-scaled ballast can be readily sourced. "Used" scaled ballast was created by abrading fresh scaled ballast using a previously established procedure, which resulted in grain characteristics that closely mimic those of ballast recovered during track renewals after 30 years of use. The results from the advanced triaxial tests show that the "used" scaled ballast in this project had a greater stiffness in the vertical direction than the fresh scaled ballast. Additionally, the horizontal stiffness generally also remained higher for used ballast compared to fresh, suggesting that "life-expired" ballast has the potential to be reused.