They are used for filtration in a variety of sructures: as french drain filter fabric (depending on the soil type), as well as for silt fence, foundation walls and pipe wrap.
When building a retaining wall, make sure that the filling material behind it drains easily, otherwise heavy wet backfill will add pressure on the wall leading to the wall fail. To protect drainage stone behind retaining wall from clogging it is recommended to cover the drainage stone layer with filtration fabric. Without filter protection, fine-textured soils will gradually accumulate between the pieces of drainage stone, and the drainage for the retaining wall fails. In the case of significant water flow, use perforated pipe wrapped into filter fabric to redirect the flow