A procedure to assess subballast filtration under cyclic loading
Occurrences of inadequacies of subballast as a filtration layer of the ballasted rail tracks would become persistent if inappropriate criteria were still being used in the design. Traditionally, the subballast is considered mainly to act as a stress dissipation layer, more commonly known as a capping layer. Attention towards its function as a granular filter is proven by the fact that common design practice uses design criteria that are primarily based on steady seepage loading that is common in embankment dams. The seepage hydraulics through porous media is influenced by the cyclic mechanical loading generated by passing trains. Under the influence of cyclic train loading, subballast particles rearrange and attempt to attain a more stable configuration through the process of vertical compression, lateral spreading, and particle degradation. The deformability of the pore medium itself would then affect the filter condition due to the changes in porosity and its subsequent impact on hydraulic conductivity. This paper presents a procedure that combines a family of formulations and configuration charts that considers the effectiveness of the subballast as a granular filter being dependent on the reduction of its porosity and hydraulic conductivity over time.