ABSTRACT
The hydraulic excitation in railway substructure stems from excess pore water pressure developing within the soft and saturated subgrades due to dynamic train loading. This pore water pressure generates a significant seepage force, which may transport the erodible fine particles of subgrade soil up towards the railway track. However, this problem may be avoided by placing a properly designed sub-ballast filter layer. Two basic requirements for protective sub-ballast filters to be safe and effective include: retention and permeability. The selected sub-ballast filter gradations should be assessed for potential internal instability by the modified combined particle and constriction size distribution (CP–CSD) based criterion of Jahanzaib Israr and Manori Indraratna. This chapter explains proposed approach for practical filter design in the form of visual guidelines, and explores design examples to demonstrate the proposed filter design approach under cyclic loading.
