Separation: They prevent two different soil layers from mixing. For example, a geotextile can be laid between a fine subgrade soil and coarse aggregate to prevent the aggregate from sinking into the subgrade, thereby maintaining the structural integrity of a road or railway track.
Filtration: They allow water to pass through while preventing soil particles from moving. This is essential in drainage applications, where they are used to wrap drainage pipes to prevent clogging and to create subsurface drains.
Drainage: They provide a path for water to flow along their plane, effectively channeling water away from a structure or area. This is a common function in sports fields, retaining walls, and embankments.