Tunnels are dangerous, tedious and expensive to build, so when dealing with loose dirt and relatively shallow projects, engineers often choose this cheaper and more effective cut and cover approach [sources: Lane ; Hewitt ]. The oldest approach to tunneling underwater without diverting the waters above is known as a tunneling shield , and engineers still use it today. Shields solve a common but vexing problem, namely, how to dig a long tunnel through soft earth, especially underwater, without its leading edge collapsing [sources: Assignment Discovery; Encyclopaedia Britannica ; Browne ; Hewitt ].
To get a sense of how a shield works, imagine a lidless coffee tin with a sharpened bottom that sports several large holes. Now, gripping the open end, push the tin, bottom first, into some soft earth and watch how the dirt squeezes through the openings.
On the scale of a real shield, several humans nicknamed "muckers" and "sandhogs" would stand inside compartments within the "can" and remove the clay or sand as the shield advanced. Hydraulic jacks would gradually move the shield forward, while crews behind it installed metal supporting rings, then lined them with concrete or masonry [sources: Assignment Discovery]; Encyclopaedia Britannica ; Browne ].
In order to hold back water seepage from tunnel walls, the front of the tunnel or shield is sometimes pressurized with compressed air. Workers, who can only withstand short periods in such conditions, must pass through one or more airlocks and take precautions against pressure-related sicknesses [sources: Hewitt ; Port Authority ]. Shields are still used, especially when installing utility conduits or water and sewage pipes. A TBM is a multistory-tall engine of destruction capable of chewing through solid rock.
At its front spins a cutting head , a giant wheel that bristles with rock-breaking disks and incorporates a system of scoops to lift pummeled rock and drop it onto an outbound conveyor belt. Behind the cutting head swings an erector , a rotating assembly that builds the tunnel lining in the TBM's wake. In some large projects, like the Chunnel, separate TBMs will begin on opposite ends and drill toward a central point, using sophisticated surveying methods to keep them on course [sources: Assignment Discovery; Coleman et al.
Drilling through solid rock creates a largely self-supporting tunnel, and TBMs drive forward quickly and relentlessly some Chunnel machines could bore feet, or 76 meters, per day.
The tunneling shield was invented by engineer Marc Isambard Brunel, who was inspired by watching a shipworm a marine bivalve drive its shell plates through wood and eject sawdust in its wake. Using his device, he successfully dug a tunnel under London's Thames River from to , enduring two breakthrough floods and a seven-year shutdown when the project's cash flow dried up. Brunel and his son spent nearly every waking hour in the tunnel, often forced to work from a boat.
The strain reportedly led to his death a few years later [sources: Assignment Discovery; Encyclopaedia Britannica ; Browne ; Hewitt ]. 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. 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 connecting Detroit, Mich. Indeed, more than such tunnels were built in the 20th century alone [sources: Lane ; Extreme Engineering; Marmaray Project ].
To make each tunnel segment, workers assemble 30, tons of steel and concrete -- enough for a 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. 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.
There are many different types of engineers. Are there any types of engineers that you find intriguing? Do you know any engineers? Perhaps there is an engineering firm in your city or town? If so, contact them to find out if you could meet with an engineer for a few minutes to learn more about what they do. You never know. You might be able to job shadow an engineer for a day. Wouldn't it be cool to go to a job site to see firsthand what engineers do?
Take some time to imagine what kinds of incredible underwater tunnels could be built in the future. How long would such a tunnel have to be? Look it up on a map or do some Internet research to find out how far it is from New York City to Paris.
How much money do you think such a tunnel would cost? Would it be worth it? Would people really be willing to drive under the Atlantic Ocean rather than sailing or flying across it? Why or why not? Would you? Ready to be amazed? Underwater tunnels are only some of the incredible engineering feats that have been accomplished. Did you get it? Test your knowledge. Wonder Words trench laser mole beaver excavate underwater tunnel boring ferry shutter humongous circular experimental ultrasonic avenue commerce Take the Wonder Word Challenge.
Join the Discussion. Sam Dec 18, Can I just be the first to point out that is more than 9 years Dec 19, You're right, Sam. It's 18 years. Good catch! Lu May 17, May 24, We think that's easier said than done, Lu! Just imagine owning a tunnel underwater. It would be really cool! Dec 4, It would be pretty neat! Nov 7, Nov 5, Wonderopolis Jan 15, Hi william! Nicholasa Aug 13, The video was very cool.
I never knew you could travel under water in a tunnel. It was a hard thing to build. The workers had to be brave because they had to go under water and build. It had to be dangerous! I hope they do build a tunnel to New York to England! I would travel on it! Wonderopolis Aug 14, Morgan Jul 14, I think tomorrow's wonder will be about mountains. But I still can't imagine how people think of these ideas! Melaka Sep 30, Oct 3, Wonderopolis Jul 15, Spencer Apr 16, This is a very interesting wonder.
It has a lot of good information. Wonderopolis Apr 16, Great, Spencer! Wonderopolis May 28, I love that they took the time to do that and I think it's crazy how that machine is so big I hope I can drive in an under water tunnel soon. Wonderopolis Jan 14, John Nov 29, I have to write reports on 5 wonderopolis wonders per week, reading the wonders are really fun I can't say the same about writing it!!!!!
Wonderopolis Nov 30, Squary11 May 8, So cool!!!!!!! I wish I can go to an underground tunnel! Wonderopolis May 9, I think this one was awesome!!!!!!!!!!!!!!!!!!!! It is really cool to know that this takes a long time to do. I never knew there was the Three Gorges Dam in China!!!!! Wonderopolis May 7, Wonderopolis Apr 26, Thanks for letting us know!
Blake Dec 16, Wonderopolis Dec 16, Laura Dec 6, Though many suffered from decompression sickness, overall worker safety was greatly improved. The Ted Williams Tunnel, which connects South Boston to Logan Airport, illustrates the most modern method of underwater tunneling-the immersed tube.
First, workers dredged a foot trench along the floor of Boston Harbor. Then, 12 giant steel tubes, each feet long and already containing roads, were dropped into the water. Once the tubes had been connected on the harbor floor, the tunnel was buried in a 5-foot protective layer of rock.
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