Dura-Line Smooth Wall HDPE Innerduct Conduit. All Dura-Line's smooth wall conduit meets or exceeds one or more of the following standards: ASTM F-2160, ASTM D-3035, ASTM D-2239, ASTM D-3485, NEMA TC-7, UL 651, UL 1990, Bellcore GR-356. Smooth inner and outer walls; HDPE material; available in sizes from 1/2" to 6" and in many colors; installation methods include direct burial, direct bore, HDD, aerial, trench, tray, and plow.