Hydraulic Characteristics of Ballast Subjected to Particle Degradation and Mud Pumping

Ballast is the primary component of a rail track foundation. Ballast aggregates are composed of crushed and coarse grained hard rock. Main functions of ballast are providing adequate permeability for drainage purposes, support the superstructure by distributing the loads from the moving train and provide lateral resistance to tie movement. Similar to shear strength characteristics, drainage capacity is also one of the most important properties of ballast. When it is degraded and aged, broken and foreign particles accumulate within the voids (ballast fouling) and its drainage capacity is decreased. Ballast degradation is mainly due to internal particle breakage and mud pumping. The aim of this research is to study the effect of particle degradation due to particle breakage and mud pumping on hydraulic characteristics of ballast. Rowe cell apparatus was used to conduct permeability tests. Since the ballast particles are large in size, naturally it was impossible to accommodate sufficient amount of sample within the Rowe cell to achieve a better packing of ballast. Therefore, parallel gradation technique was used and a new sample was prepared according to the parallel gradation technique. Series of constant head hydraulic conductivity tests were done for each of the samples with difference fouling percentages and breaking index values. Variation of hydraulic conductivity due to particle degradation in parallel graded ballast was obtained. This investigation revealed that the hydraulic conductivity decreased significantly with the increase in either the fouling percentage or the breakage index.