The purpose of a separation layer is to protect a supporting subbase from the underlying layers that could compromise the properties of the subbase. A separation layer can consist of a geotextile, graded aggregate or hot mixed asphalt (HMA). It can be used for drainage (graded aggregate), strength (graded aggregate, HMA) or protection of fines migration (geotextile).
Ron said it . It serves as anti contamination if you will . By using geotextil for instance ,it helps provide the aggregates of a shield that filter only water or simply deny migration of fine materials. If not corrected , this migration will create an apparent cohesion to the aggregate thus greatly diminishing its performance...
