Assessment of drainage and filtration of sub-ballast course considering effect of aggregate gradation and subgrade condition

Abstract

Abstract Ballast contamination due to intrusion of fine particles from subgrade is characterized as a conventional source of ballast fouling. To diminish this phenomenon, sub-ballast layer is incorporated between the subgrade soil and the ballast course of railway track structure. Present study investigates the influence of subgrade type, compaction level and gradation of sub-ballast material and particle size distribution of ballast aggregate on filtration accomplishment of sub-ballast functioned as a filter layer. For this purpose, prepared structure representing placement of sub-ballast and ballast course over subgrade soil is subjected to upward water flow by exerting considerable values of hydraulic head. Both the thickness of ballast contaminated and the percentage of fine particles infiltrated among ballast aggregate are measured. Also, the hydraulic conductivity of sub-ballast aggregate is separately measured. As expected, using granular sub-ballast comprised of broader size range along with further compaction of this filter course leads to the improvement of filtration against intrusion of fine particles from underlying layer into the upper structure. Likewise, observing insufficiency in drainage of prepared sub-ballast under this extreme condition is inevitable. Furthermore, the level of ballast contamination (especially the value of fouling ratio) is higher in the case of providing ballast aggregate with more uniform particle size distribution. Finally, the fouling indices measured evidently reveal that the ballast layer is further contaminated in the case of placement over subgrade type of silt (in comparison with clay).