AREMA Gradations Affecting Ballast Performance Using Discrete Element Modeling

This paper evaluates the different AREMA gradations currently in use for their effects on both ballast void space and load carrying performances using a discrete particle analysis computational technique called Discrete Element Modeling (DEM). The effect of gradation on aggregate assembly volumetric properties was first studied. Full-scale ballast layers with common gradations listed in AREMA specifications for main line railroads were generated using the University of Illinois ballast DEM model BLOKS3D program. Repeated train loading was applied to the different ballast gradation characteristic curves to investigate the adequacy of drainage and structural performances by means of comparing settlements occurred after certain volume of traffic. More uniformly gradated aggregate assemblies generally had larger air voids thus better drainage. However, such uniform particles at certain sizes might tend to dilate under loading thus creating an unstable ballast particle packing and void structure. Associated deformations predicted from the DEM simulations also showed that generally more uniformly graded ballast produced larger accumulations of permanent deformation with repeated load application. The denser AREMA No.24 gradation was found to resist the most the ballast settlement. Furthermore, the settlement could be minimized by engineering the gradations for denser packing towards the maximum density line. However, there is a limiting gradation line on a maximum density plot beyond which an aggregate assembly could not gain more settlement resistance. At such a gradation, it is also possible to maintain large enough void space for proper drainage. Therefore, the DEM findings identified differences in current ballast specifications in terms of drainage and structural support as well as provided new insight into optimizing ballast layer aggregate gradations for better railroad track performances.