Effect of grading category on the roundness of degraded and abraded railway quartzites

Abstract

Sustained train traffic loading result in rounding of sub angular ballasts and consequently reduce the interparticle friction. Reduced particle interlocking can result in instability, non-alignment of rail line and derailment and thus screening of insitu rail foundation ballasts is required during the service life to track changes in textural and mechanical properties. Two different particle grading categories (A and F) of quartzite ballasts from a Johannesburg quarry were degraded by sulphate solution and cycles of temperature. The ballasts were progressively abraded to failure in a Los Angeles abrasion device and the textural properties; average particle size (d50), flakiness index (FA), roundness (RND) and Los Angeles abrasion index (LAAI) were analyzed after each cycle of abrasion. The fouling potential of the particles generated by abrasion was classified according to the Fouling Index (FI) and Relative ballast fouling ratio (RBFR). Decrease in d50 and increase in FA with increase in number of abrasion cycles (N) were observed in the two grades of ballasts. Particle gradation and shape affected the degree of abrasion differently and with decreasing abrasion ratio, mean roundness becomes increasingly dependent on both the effect of degradation and grading. RBFR estimated the void volume occupied by fouling material and was found to be a better indicator of ballast fouling than FI. Regression equation of the ballast abrasion process based on abrasion ratio, relative ballast fouling ratio was formulated but required more data for potential application in track inspection and maintenance schedule.