Drain tile and pile fabric filter sleeve

In a land drainage line having an elongated flexible corrugated drainage conduit of the type having alternating peaks and valleys along its longitudinal axis and a plurality of openings through the conduit arranged along its longitudinal axis and in valleys of the conduit and a knitted fabric filter sleeve in the form of a length of tubular knit fabric having a length substantially coextensive with the length of the conduit and a diameter in relaxed condition less than the maximum diameter of the conduit, the sleeve covering the drainage conduit and filtering fine particles of soil from water passing through the sleeve and into the drainage conduit, the improvement in the knitted fabric filter sleeve comprising the fabric being formed of stitches defining a ground and defining terry loops extending from the ground and being directed in a predetermined generally radial direction relative to the longitudinal axis of the drainage conduit.