ABSTRACT
Filters can have different functions: preventing erosion of the covered subsoil or preventing pressure build-up in the covered subsoil (drainage) or a combination of both. Filters can have two functions: preventing erosion under a top layer or drainage to prevent pressure build-up. In both cases, filter stability and permeability play a role. Stability concerns the grains that should remain stable in the filter, while the permeability is important to avoid pressure build-up, which can cause instability of a layer as a whole. The two main types of filters are granular filters and geotextiles. Granular filters fall into two categories: geometrically closed and geometrically open. Geometrically closed filters are stable, regardless of the porous flow through the filter. There is no critical gradient because the relation between the diameters in the filter is such that the finer grains cannot move through the pores of the coarser grains. Geometrically closed filters have to be designed within the margins of stability and permeability.
