Filtration functions to restrict the migration of fine soil particles from a soil mass while remaining permeable to water movement greater than, or at least equivalent to the permeability of the protected soil.
Water is conveyed along the plane of the geotextile due to its construction, and then to an outlet. Water may be vertically or horizontally conveyed. Drainage is related to the role of filtration and is a function of the permeability of a geotextile and its pore opening size or porometry.
Separation is the function which prevents two distinct soils or different materials from intermixing. The key factors for a geotextile to satisfy this function are porometry, toughness and strength.
This function involves the stabilisation of a soil mass by the provision of tensile strength to the soil-fabric system.
Geotextile selection can be expressed as a relationship between these 4 basic functions and the properties required by the design engineer in order to satisfy certain criteria, which would relate to a specific application.