The Unified Soil Classification System (USCS) uses the 4.75 mm sieve opening size (#4 sieve) as the boundary between ‘coarse’ and ‘fine’ particles. Particles larger than 4.75 mm are classified as ‘coarse’, whereas particles smaller than 4.75 mm are classified as ‘fine’. However, applying these definitions to railroad ballast can be erroneous, as most particles in a ballast material are larger than 4.75 mm (often as large as 63 mm in size), therefore indicating the absence of any ‘fine’ particles. However, depending on relative distribution of particle sizes within a granular matrix, certain particles serve to create voids (coarse fraction), and certain particles serve to fill the voids (fine fraction). Accordingly, rather than using the standard definitions of ‘coarse’ and ‘fine’ particles, as has been done in the literature, the analysis of packing conditions in a ballast matrix may be better served by studying the relative packing between different size fractions. This paper focuses on the development of a new gradation parameter, termed as the “Coarse-to-Fine (C/F) Ratio”, which can shed some light on the importance of different size fractions in a ballast matrix. Changing the ‘coarse’ and ‘fine’ fractions within a particular gradation specification, the resulting effect on ballast shear strength was studied through simulated Direct Shear Strength Tests. A commercially available threedimensional Discrete Element Modeling (DEM) package (PFC3D®) was used for this purpose. Details of the numerical modeling effort are be presented, and inferences are drawn concerning the implications of simulation results on the design and construction of railroad ballast layers.

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