Permeability, Resistivity and Strength of Fouled Railroad Ballast
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Permeability, Resistivity and Strength of Fouled Railroad Ballast

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    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 the crushing of the 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 study was to evaluate the potential for using resistivity to estimate the level of fouling and permeability (hydraulic conductivity) of ballast. A test box was designed and fabricated in the lab at the University of Kansas to perform constant head permeability tests 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 fouling ratios. The tests showed a relationship between ballast resistivity and the fouling ratio.. The resistance of the ballast layer decreased as the fouling ratio 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 also decreased as the fouling ratio 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 types of fouling materials were tested (crushed ballast fines, clay, and coal dust) at different fouling ratios by dry weight of ballast. Test results showed that as the fouling ratio increased, strength of ballast decreased for both set of tests (large direct shear and modified direct shear). Moreover, samples fouled with more than 10% coal dust showed a significant decrease in strength properties. 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 resistivities at greater depths were interpreted to be representative of drier material, while the near surface material had a lower resistivity due to the addition of water to the surface.
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