Immersed Tube Tunnels

Immersed tube tunnels are composed of prefabricated sections placed in trenches that have been dredged in river or sea bottoms. The sections are usually constructed at some distance from the tunnel location and made watertight with temporary bulkheads. They are then floated into position over the trench, lowered into place, and joined together underwater. The temporary bulkheads are removed, and the trench is backfilled with earth to protect the tubes. Immersed tubeshave been widely used for highway and rail crossings of soft-bottomed, shallow estuaries and tidal rivers or canals in which trenches may be excavated with floating equipment.