Letting It Sink In
Constructing a steel-and-masonry support while simultaneously digging through soft earth or solid rock is no picnic, but trying to hold back a sea while underwater is something not even Moses would have attempted. Fortunately, thanks to American engineer W.J. Wilgus and his invention, the sunken- or immersed-tube tunnel (ITT), we don't have to [source: Lane].
ITTs aren't bored through rock or soil; they are assembled on-site from football-field-sized, prefabricated pieces. Wilgus pioneered the technique when he built the Detroit River railroad tunnel (1906-10) connecting Detroit, Mich., to Windsor, Ont., and they've been the go-to technique for vehicle tunnels ever since. Indeed, more than 100 such tunnels were built in the 20th century alone [sources: Lane; Extreme Engineering; Marmaray Project].
To make each tunnel segment, workers assemble 30,000 tons of steel and concrete -- enough for a 10-story apartment building -- in a massive mold, then allow the concrete to cure for nearly a month. The molds contain the tunnel's floor, walls and ceiling, and are initially capped at the ends to keep them watertight as they are transported out to sea. Immersion pontoons, large ships resembling a cross between a gantry crane and a pontoon boat, do the hauling [sources: Lane; Extreme Engineering; Marmaray Project].
Once over the pre-dug sea trench, each tunnel section is flooded enough to allow it to sink. A crane slowly lowers the section into position while divers guide it precisely to its GPS coordinates. As each new section connects to its predecessor, a massive rubber piece on its end squeezes and distends to establish a seal. Crews then remove the bulkhead seals and pump out the remaining water. Once the entire tunnel is built, it is buried under backfill and possibly covered with rock armor [sources: Lane; Extreme Engineering; Marmaray Project].
Immersed-tube construction can delve deeper than other approaches because the technique does not require compressed air to hold water at bay. Crews can therefore work longer in them and under more tolerable conditions. Moreover, an ITT can take any form, unlike a bored tunnel, which follows the shape of its shield or TBM. However, because ITTs only make up the seafloor or riverbed portion of a tunnel system, they require other tunneling methods to bore their land-based entrances and exits [sources: Lane; Marmaray Project; WGBH]. In underwater tunneling, as in life, it takes all kinds.
Author's Note: How do you build an underwater tunnel?
In writing this article, I could only touch on the basics of constructing underwater tunnels, which is an injustice comparable to equating defusing a bomb with programming a DVR. In reality, the dangers involved and the precision required in constructing an underwater tunnel are simply staggering. Excavating and constructing these modern marvels requires nothing short of constant vigilance, keen adaptability and minute adjustments to changing conditions, to say nothing of the care that workers and divers must exercise.
It's something worth thinking about the next time you find yourself traveling through an underwater tunnel. Perhaps it will distract you from the millions of tons of earth or water pressing down or, if you are in Japan's Seikan Tunnel, from the sound of water tinkling in through the walls to be drained by the pumps at 20 tons per minute.
- American Society of Civil Engineers. "Seikan Tunnel." http://www.asce.org/Featured-Images/Seikan-Tunnel/
- Assignment Discovery. "Shield Tunneling." https://videos.howstuffworks.com/discovery/29896-assignment-discovery-shield-tunneling-video.htm
- Assignment Discovery. "Tunnel Boring Machines." https://videos.howstuffworks.com/discovery/36684-mega-engineering-tunnel-boring-machines-video.htm
- Browne, Malcolm. "Tunnel Drilling, Old as Babylon, Now Becomes Safer." The New York Times. Dec. 2, 1990. http://www.nytimes.com/1990/12/02/world/tunnel-drilling-old-as-babylon-now-becomes-safer.html
- Chan, Sewell. "Who's Watching the Underwater Tunnels?" The New York Times. July 20, 2005. http://www.nytimes.com/2005/07/20/nyregion/20tunnels.html?pagewanted=1&_r=0&sq&st=cse%22%22Immersed%20tube&scp=4
- Encyclopaedia Britannica. "Sea of Marmara." http://www.britannica.com/EBchecked/topic/365921/Sea-of-Marmara
- Encyclopaedia Britannica. "Tunneling Shield." http://www.britannica.com/EBchecked/topic/609360/tunneling-shield
- Everglades Economics. "Comparing Costs of Options for Reconstructing the 12th and 27th Avenue Bridges Over the Miami River." Miami River Commission. February 2003. http://www.miamirivercommission.org/PDF/tunnel%20study.pdf
- Extreme Engineering. "Building an Immersed Tunnel." https://videos.howstuffworks.com/discovery/29855-extreme-engineering-building-an-immersed-tunnel-video.htm
- Extreme Engineering. "Finishing the Immersed Tunnel." https://videos.howstuffworks.com/discovery/29853-extreme-engineering-finishing-the-immersed-tunnel-video.htm
- Extreme Engineering. "Transatlantic Tunnel Design." https://videos.howstuffworks.com/discovery/29150-extreme-engineering-transatlantic-tunnel-design-video.htm
- Harrison, Harry. "A Transatlantic Tunnel, Hurrah!" Pinnacle Books. 1972.
- Hewett, Bertram Henry. "Shield and Compressed Air Tunneling." McGraw-Hill. 1922.
- JR-Hokkaido Hakodate Branch. "Seikan Tunnel." http://jr.hakodate.jp/global/english/train/tunnel/default.htm
- Landler, Mark. "A Subway Bores Into the Ottoman and Byzantine Eras." The New York Times. Aug. 2, 2005. http://www.nytimes.com/2005/08/02/international/europe/02istanbul.html?pagewanted=print&_r=0
- Lane, Kenneth S. "Tunnels and Underground Excavations." Encyclopaedia Britannica. http://www.britannica.com/EBchecked/topic/609297/tunnels-and-underground-excavations#toc72433
- Marmaray Project. "The Immersed Tube Tunnel." 2003. http://www.marmaray.com/html/tech_immersed.html
- Port Authority of New York and New Jersey. "Lincoln Tunnel: History." http://www.panynj.gov/bridges-tunnels/lincoln-tunnel-history.html
- Sweeney, Chris. "The World's 18 Strangest Tunnels." Popular Mechanics. http://www.popularmechanics.com/technology/engineering/architecture/4343590#slide-1
- WGBH Educational Foundation. "Channel Tunnel (Chunnel)." PBS. http://www.pbs.org/wgbh/buildingbig/wonder/structure/channel.html
- WGBH Educational Foundation. "Seikan Tunnel." PBS. http://www.pbs.org/wgbh/buildingbig/wonder/structure/seikan.html
- WGBH Educational Foundation. "Tunnel Basics." PBS. http://www.pbs.org/wgbh/buildingbig/tunnel/basics.html
- Wise, Jeff. "Turkey Building the World's Deepest Immersed Tube Tunnel." Popular Mechanics. Oct. 1, 2009. http://www.popularmechanics.com/technology/engineering/4217338