Effect of gradation of aggregate and size of fouling materials on hydraulic conductivity of sand-fouled railway ballast

Contamination of ballast due to penetration of sand into the voids between ballast aggregate adversely affects the drainage potential of the track structure in desert areas. Using finer gradation with less uniformity for initial gradation of clean ballast aggregate can lead to more reduction in the permeability of sand-fouled ballast. Furthermore, water flow through clean ballast can be different from the flow regime through contaminated ballast due to change in porosity. The present study evaluates the effect of initial gradation of ballast aggregate and fouling materials (including coarse-grained and fine-grained sand) on hydraulic conductivity of railway ballast by establishment of both laminar and turbulent flow conditions. For this purpose, a series of large-scale constant head permeability tests is conducted on different gradations of clean ballast as well as contaminated ballast samples with various ratios of fouling materials. According to the experimental results, more uniformity of gradation of clean sample leads to higher values of hydraulic conductivities. In addition, establishment of nonlinear relationship based on the power law between the discharge velocity and hydraulic gradient can better characterize the water flow through clean ballast as well as fouled ballast samples contaminated by coarse sand. Finally, it is concluded that the permeability of contaminated ballast samples with various initial gradation is approximately the same for high fouling ratios.