Reducing vertical acceleration in ballast layers of railways to mitigate geometrical disorders in high-speed lines

According to field feedbacks from high-speed lines (HSL), the increase of train operating speeds is responsible for unusual fast evolving geometrical disorders in ballasted tracks. This paper deals with the search of solutions applicable at the design stage to mitigate these disorders. The starting point of the present work relies on the assumption, comforted by the literature, of a strong correlation between disorders and vertical accelerations in the ballast layer induced by the train passages. This led us focus herein on the calculation and the analysis of accelerations in the railway structure. The vertical accelerations (γ z) are computed using the in-house developed numerical program ViscoRail and on the basis of a reference HSL. These are shown to increase strongly with the train speed attesting to the link between the train speed and the geometrical disorders in ballast. Then, other simulations are run varying some structural parameters to evaluate their impact on the acceleration field γ z. In that way, we show that decreasing the stiffness of the mechanical connection between the rails and the ballast, increasing the moment of inertia of the rails or the Young modulus of the sub-ballast layer, leads to a decrease of γ z and could provide solutions for the design of future HSL. The solution consisting in the incorporation of an asphalt sub-ballast layer, as already experimented on sites, is finally examined in more details.