a conduit, esp. a drain, as a pipelike construction of stone, concrete, or metal, that passes under a road, railroad track, footpath, etc. or through an embankment
See culvert in American Heritage Dictionary 4
cul·vert
noun
A sewer or drain crossing under a road or embankment.
a. The part of a road or embankment that passes over such a sewer or drain.
b. The channel or conduit for such a sewer or drain.