ABSTRACT
Ballast layer is designed to be free draining but when the ballast voids are wholly or partially occupied due to the intrusion of fine particles, the ballast can be considered to be “fouled’’. During operation, ballast deteriorates due to the breakage of angular corners and sharp edges, infiltration of fines from the surface, and mud pumping from the subgrade under train loading. As a result of these actions ballast becomes fouled, less angular, and its shear strength is reduced. Fouling materials have traditionally been considered as unfavourable to track structure. According to Selig and Waters [1], ballast breakdown, on average, accounts for up to 76%of fouling, followed by 13%of infiltration from subballast, 7% infiltration from surface ballast, 3% from subgrade intrusion, and 1% from sleeper wear. However, Feldman and Nissen [2] reported that for tracks in Australia used predominantly for coal transport, coal dust accounts for 70%–95% of contaminants and ballast breakdown contributes from 5%–30%. To ensure acceptable track performance, it is necessary to maintain a good drainage condition within the ballast layer.
