Patent ID: 8256990

Claim:
A pre-assembled drainage unit having a generally cylindrical shape, an exterior surface, a circumference, and a length, for burial in soil which is in contact with the exterior surface of the drainage unit, comprising: a sleeve longitudinally extending along a portion of the length of the exterior surface of said cylindrical drainage unit, the sleeve comprised of a combination of netting and a first portion of barrier sheet; wherein, the barrier sheet is made of water permeable material resistant to passage of fine solids and has (a) said first portion extending lengthwise and circumferentially, thereby forming a portion of the exterior surface of the drainage unit; and (b) two movable second portions, each second portion running along an opposing lengthwise side of the first portion as a flap which is foldable laterally with respect to the lengthwise direction of the drainage unit; and, lightweight aggregate contained within the sleeve.