A culvert is a structure that allows water to flow under a road, railroad, trail, or similar obstruction. Typically embedded so as to be surrounded by soil, a culvert may be made from a pipe, reinforced concrete or other material. A structure that carries water above land is known as an aqueduct.
Culverts may be used to form a bridge-like structure to carry traffic. Culverts come in many sizes and shapes including round, elliptical, flat-bottomed, pear-shaped, and box-like constructions. Culverts may be made of concrete, galvanized steel, aluminum, or plastic, typically high density polyethylene.
Two or more materials may be combined to form composite structures. For example, open-bottom corrugated steel structures are often built on concrete footings.
Plastic culvert liners are also inserted into failing concrete or steel structures in order to repair the structure without excavating and closing the road. To prevent the older structure from collapsing, the space between it and the plastic liner is usually filled with grout.