readily accommodated an outer casing coupler comprises a generally cylindrical rigid collar 28 that is mounted about the first end portion 14 of the outer casing 13 and extends beyond the end thereof to a position substantially aligned with the end of the protruding inner duct coupler 21 an inwardly extending annular flange 32 is provided at the mid section of the collar 28 to position the collar properly on the end of the conduit section in the preferred embodiment the annular positioning flange 26 of the inner duct coupler 21 is captured between the end of outer casing 13 and the collar flange 32 the collar flange thus helps to align and hold the inner duct coupler securely in position within the conduit section 12 a first sealing gasket 29 is captured and compressed between the exterior surface of the outer casing 13 and the interior surface of the collar 28 to prevent intrusion or migration of moisture and other corrosive elements into the interior portions of the conduit system as described in more detail below the collar 28 in conjunction with the protruding end of the coupler 21 form a socket sized and configured to receive the male end portion 16 of a like conduit section for joining the two sections end to end in the field the male end portion 16 is also provided with a sealing gasket 29 extending about the exterior casing and being secured between two adjacent ridges of the corrugated outer surface of the conduit the ends of inner ducts 19 at the male end of.