A method for manufacturing preassembled drainage line units comprises feeding a length of perforated vent pipe along a direction of manufacture longitudinally through a tubular mandrel, feeding a continuous sleeve of netting concentrically over the vent pipe along the direction of manufacture from a front end of the mandrel and continuously supplying quantities of artificial aggregate into the space between the sleeve of netting and the vent pipe. A low friction coating is applied to the inside surface of the mandrel to reduce the surface friction of the artificial aggregate caused by compaction, and to permit high speed manufacture.