Permeability, Resistivity and Strength of Fouled Railroad Ballast

Abstract

Ballasted tracks are the most common tracks used in the railroad industry and are designed to provide a stable, safe, and efficient rail foundation. A ballasted track consists of superstructure (ties, fasteners, and rails) and substructure (ballast, sub-ballast, and subgrade layers). The main functions of ballast are to support the superstructure by distributing the loads from the moving train, and to provide lateral resistance to tie movement and drainage. However, ballast deterioration and fouling are major issues in the railroad industry, and can be caused by repeated loadings, which lead to crushing ballast that is in contact with ties. Upward migration of subgrade particles into the ballast layer can increase fouling in the ballast and decrease drainage through the ballast layer. There is a need for methods to easily and inexpensively identify areas that have fouled ballast. The objective of this preliminary study was to evaluate the potential for estimating the level of fouling in a ballast layer by soil resistivity and permeability tests to be followed by a second study. A test box was designed and fabricated at the lab at the University of Kansas to perform the constant head permeability test and soil resistivity tests. Constant head tests were conducted to determine the coefficient of permeability of fouled ballast for different fouling percentages. Soil resistivity tests were also conducted using the Wenner method (4 points method) to determine the resistivity of ballast for different percentages of fouling. The tests showed a relationship between the percentage of fouling and ballast resistivity. The resistance of the ballast layer decreased as the percentage of fouling increased due to the presence of water. Fouled material retained water and filled the voids between the ballast particles, and therefore decreased resistivity in the ballast layer. The permeability (hydraulic conductivity) also decreased as the percentage of fouling increased due to the presence of fine particles between the ballast particles; therefore, permeability and resistivity were also correlated. The strength properties of clean and fouled ballast were also evaluated using large direct shear box and modified direct shear box (extension in height for the large direct shear box). Three type of fouling materials were tested (crushed ballast fines, clay and coal dust) at different percentages by dry weight of ballast. Test results showed that as percentage of fouling increased, strength of ballast decreased for both set of tests (large direct shear and modified direct shear). Moreover, samples fouled with coal dust with more than 10% showed a significant decrease in strength properties. Also, samples fouled with clay showed a significant strength reduction at about 40% fouling. A large scale sample of heavily fouled ballast was constructed and tested under wet conditions. The four point Wenner method was used to measure resistivity at depths of eighteen inches, twelve inches and six inches. The results show that as the depth increased, resistivity increased. The higher resistivites at greater depths were interpreted to be representative of drier material, while the near suface material had a lower resistivity due to the addition of water to the surface.