A
tunnel is much more than just a tunnel. It serves any of myriad
functions-highway, railroad, or rapid transit artery; pedestrian passageway;
fresh water conveyance, cooling water supply, wastewater collector or
transport; hydropower generator; or utility corridor. Tunnels are constructed
by cut and- cover methods; in long, prefabricated sections sunk in place as in
immersed tubes; in short prefabricated sections pushed into place from jacking
pits; by drilling and blasting; by mechanized means such as tunnel boring
machines or continuous miners (road headers), with the aid of a protective
shield in free or compressed air; and they will eventually be constructed in
ways now existing only in our imaginations.