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 (roadheaders), 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. In cross section it takes one of several shapes—circular, multicurve, horseshoe, cathedral arch, arched, or flat-roofed, and with clear spans of from a few feet to more than 50 ft and, in cavern form, much wider. Its length can vary from less than 100 ft to more than 30 miles. A tunnel can be lo- cated in any of a variety of places—under mountains, cities, rivers, lakes, sea estuaries, straits, or bays.
King, E. H., & Kuesel, T. R. (1996). An Introduction to Tunnel Engineering. In Tunnel Engineering Handbook (pp. 1–3). Springer US. https://doi.org/10.1007/978-1-4613-0449-4_1