Here is our spec for separation fabric
Fabric must comply with AASHTO M288 for separation purposes. The geotextile shall be non-woven fabrics, with survivability class two (2) and a minimum permittivity of 0.02, such as Amoco 4552, TYPAR 3501, Carthage Mills FX-60-HS, Mirafi 160N, or pre-approved equal. The fabric shall be rolled out in the direction of the traffic lane with all edges overlapping by at least eighteen inches (18"). Material properties and manufactures certification of compliance with AASHTO M288 must be provided to the Engineer for approval prior to transporting the material on-site.
The geotextile shall be laid smooth on the prepared subgrade in the direction of traffic. On curves, the geotextile may be folded or cut to conform to the curves. The fold or overlap shall be in the direction of construction and held in place by pins, staples, or piles of rock. Overlaps shall be in the direction of traffic and as recommended by the manufacturer.