Impact of Bituminous Subballast on Railroad Track Deformation Considering Atmospheric Actions

High-speed ballasted track design standards usually require the use of sand and gravel layers as subballast to fulfill an accurate protection of the formation layers, not only against traffic loads, but also against the effects of weather. Seasonal changes in soil water content, or suction changes, are responsible for cyclic volumetric strains on railroad trackbed layers, thus on the infrastructure. Being almost completely water-resistant when compared with granular-only layers, bituminous subballast offers a higher protection of the subgrade, consequently improving its behavior along the infrastructure life cycle. This question is investigated through the comparison of the performance of the track formation against atmospheric actions, taking into consideration the unsaturated state of the geomaterials. The method adopted consists of modeling the vertical displacements of both bituminous and granular subballast designs through a finite-element coupled thermo-hydro-mechanical (THM) analysis. The comparison of the two design solutions confirms that the adoption of a bituminous subballast layer might allow important reductions in seasonal vertical displacements.