ABSTRACT
This chapter explains the existing subballast selection criteria with reference to filtration and drainage. In an effort to enhance the selection criteria, a critical review is given for the past empirical and mathematical investigations on filtration and the subsequent development of geometric-probabilistic methods. The chapter describes the development and effectiveness of constriction-based retention criteria, valid for both uniform and well-graded materials based on experimental evidence. It presents implications to current design guidelines, and reviews experimental investigations undertaken at the University of Wollongong into subballast filtration behaviour under cyclic conditions with the analysis of the results obtained. Deficiencies in empirical investigations are addressed in rigorous analytical modelling and numerical simulations. Mathematical modelling of the filtration behaviour of base particles provides useful predictions on time-dependent changes in filters as well as an indication of the required thickness. The KC equation forms the basis of the derivation of the formulation employed to predict the deterioration in hydraulic conductivity of the granular filter specimens.
