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. Smoothwall conduit is flexible, suitable for direct burial, plow, trench, bore, aerial or tray installs; available with optional SILICORE ULF lining and in multiple colors, sizes, and reel/coil putups.