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Laboratory Study of the Geotechnical Properties of Abraded Railway Ballast with Natural and Clay Mix Fouling

With 140,000 miles of active track, railroad freight is the largest method of transporting goods in the United States. An important component of the railroad substructure is the gravel-size aggregate placed below the tracks, called ballast. The ballast is responsible for draining water away from the track, resisting forces from traffic loading, maintaining track position, and reducing loads to acceptable pressures for the subgrade. Ballast is typically selected to be highly angular and uniformly graded, but these characteristics are constantly changing under dynamic traffic loading. As the ballast is loaded, the particles abrade against each other, grinding off stone dust and breaking off larger corners. This process reduces the angularity of ballast and also contributes to fouling, which is finer grained materials entering the voids of the ballast. Fouling generally reduces the strength of ballast, particularly when water is present, as it prevents the water from draining away from the track. This study aims to better understand the influence of both fouling content and ballast particle angularity on the strength properties and deformation characteristics of ballast. This is achieved through a series of laboratory tests using ballasts of different angularity and different fouling types, from which the results are directly compared to one another. Additionally, to better understand the particle packing structure of ballast-fouling mixtures, a series of minimum and maximum density tests have been performed on several different mixes. The testing presented shows that angular ballast and abraded ballast achieve similar strengths, but the abraded ballast is much more susceptible to deformations, making it problematic in track. The introduction of clay fines into the fouling material shows significant loss of ballast strength, and also identifies issues with current methods of quantifying ballast. The density testing provides the first baseline for the range of possible densities of an angular and abraded ballast across a wide spectrum of fouling conditions. Finally, box testing of fouled abraded ballast has led to the first known box failure in a laboratory setting, revealing important characteristics of ballast.
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