Contamination of ballast not only adversely affects the track stability but also impedes the track drainage. While the use of polyurethane in ballast enhances the track stability due to polyurethane bond between the aggregates, but it may reduce the permeability of ballast. In this context, the drainage characteristics of the polyurethane-stabilized and unstabilized ballast under fresh and coal fouled conditions were assessed by using large-scale permeability apparatus. The test results highlighted that the hydraulic conductivity of ballast (k) decreased slightly from 43 mm/s to 37 mm/s upon polyurethane treatment. However, the coal fines have drastically decreased the value of k for both stabilized and unstabilized ballast. For unstabilized ballast, as the fouling content (Pf) is increased from 0 to 21.2%, the value of k reduced from 43 to 0.18 mm/s. The relative hydraulic conductivity ratio (kcb/kfb), computed as the fraction of hydraulic conductivity of clean ballast to that of fouled ballast was found to increase from 1 (one) to 29 and 239 as fouling content increased from 0 to 4.24% and 21.2%, respectively. Moreover, tortuosity (T) defined as the ratio of the actual flow path length to the length of porous sample increased from 1.59 in case of clean ballast to 1.98 due to fouling (Pf:21.2%). The corresponding values of T upon polyurethane stabilization increases to 1.64 and 2.08, respectively. In addition, a nonlinear empirical relationship was established between the k and T of ballast.

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