Immersed Tube Tunnel

An immersed tube tunnel is an undersea tunnel built from segments that are constructed elsewhere, floated to the tunnel site, and sunk into place before being linked together, commonly used to carry roads and railways under rivers, estuaries and sea channels. Building one means prefabricating tunnel segments, sealing their ends with bulkheads so they float, dredging a precisely graded trench on the waterbed, positioning each segment at its final location, sinking it with added weight, and then pumping the water out of the joints between segments before sealing them. In the United States, segments are typically built as steel or cast-iron tubes lined with concrete using shipbuilding techniques, while in Europe they are more commonly cast as reinforced concrete box tubes in a basin, and an immersed tube is often joined at its ends to a cut-and-cover or bored tunnel that continues on to a portal at the land surface. The source consulted names no single inventor or invention date for the technique.