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The unbound aggregate ballast layer is a major structural and drainage component of railroad track that is known to degrade over time. Progressive degradation increases the fine-grained content of the ballast layer through particle breakage and abrasion or from external sources, such as subgrade or foreign material. The point at which ballast should be cleaned of these materials to avoid significant problems for drainage, track geometry, or ride quality is not well known. This paper attempts to ascertain the current state of the art on ballast permeability by reviewing previous studies, to fill any gaps by generating new laboratory test data, and to begin developing ballast cleaning considerations. A new and relatively simple test apparatus, the University of Illinois Constant Head Aggregate Permeameter, was used to study railroad ballast permeability as a function of degradation. Results of tests performed indicate that the cleaner the ballast, the more nonlinear the relationship between discharge velocity and hydraulic gradient, contrary to the findings of previous studies. In addition, flow decreased greatly after small increases in ballast degradation. Detailed findings related to the characteristics of flow—other than whether flow is impeded by in-service ballast condition—may not be extremely useful for rail practitioners because the amount of ballast degradation is difficult to determine in the field. However, the emerging ballast imaging technology described may be able to provide railroad personnel with a threshold for when ballast should be cleaned.

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