Optimization of the Mix-Design System for the Sub-ballast Railroad

Bituminous sub-ballast is an alternative solution to the unbound granular sub-ballast used in the railway track due to several benefits that it can provide. Indeed, it contributes to maintain the moisture content in the subgrade unchanged during all year. This decreases the subgrade deterioration process. Moreover, the presence of bituminous sub-ballast can also reduce vertical stiffness variations on the track; it can have a positive effect in the maintenance needs at transition sections (bridge-embankment) and in the attenuation of the vibrations induced by the rail traffic. Despite the importance of the presence of the bituminous sub-ballast to conceive the construction and/or rehabilitation of sustainable infrastructure, in literature, there are only fragmentary information regarding the definition of benchmark criteria for their mix design. The superpave mix design approach used in road domain is applied systematically in the railway domain, without being adjusted for different load configuration of the rail track system. This research work aims at defining the benchmark criteria for the bituminous sub-ballast mix design to reduce the approximations involved in the recipe optimization due to the limitation of applying the superpave system in the railway domain. The methodology proposed aims at selecting the RESAL (rail equivalent single axle load) and therefore, transforming the entire traffic spectrum on the track lines in number of ESALs. Afterwards, the Ndesign has been calculated as function of the rail traffic level. Finally, a case study of bituminous sub-ballast mix design has been investigated for a first verification of the methodology proposed.