Abstract: This invention provides an improved rollable subterranean drainage core device which has T-shaped finned projections to which may be adhered a water-penetrable soil filtration fabric. The fin structure provides impact resistance and high compressive strength while providing high water channel flow-through capabilities. An exemplary method for fabricating the drainage device involves extruding a unitary finned polymeric core sheet having T-shaped projection members on at least one major side thereof, and then removing a portion of the fins in a cross direction at periodic intervals along the machine direction, so that individual T-shaped projection members are spaced in rows and columns on the rollable core sheet. The drainage core member is particularly suitable for use in conjunction with waterproofing membranes.