How Do We Build Tunnels Underwater?

Even so, for most of human history we've been pretty tunnel savvy. Humans have tunneled since the first cave dwellers decided to excavate a spare bedroom, and the essentials of dig support and advance were well refined by the time the ancient Greeks used tunnels to irrigate and drain their farmland. Even underwater tunneling is old. Sometime around twenty one seventy BCE, the Babylonia built one of the first known examples by

diverting the Euphrates River. The bricklined and arch supported tunnel measured twelve feet high by fifteen feet wide that's four by five meters, and provided passage for pedestrians and chariots alike between the royal palace and a temple some three thousand feet or nine hundred meters away. For centuries, tunnels were employed mainly by miners and medieval sappers, who dug under castle walls to collapse them, hence the term undermine. But the advent of canal transport and later railroads gave

workers something new to sink their shovels into. The eighteenth thirty twentieth centuries saw a succession of ever more challenging tunnel projects, made possible by vast improvements in surveying and ventilation techniques. Even so, a danger and expense delayed attempts at underwater tunneling until the mid eighteen hundreds, which raises the question if underwater tunneling risks in your own grave, literally and financially, why bother. Many city planners agree turning

to tunnels only when congested bridges reach choking capacity. But bridges are problematic too. They interfere with shipping traffic, take up valuable riverfront property, and block scenic views. From a defense standpoint, bridges make easy airstrike targets and can constitute hazards if they collapse. Tunnels conversely, withstand tides currents and storms better than bridges, can reach longer distances and have

virtually unlimited weight carrying capacity. In addition, a tunnel's per length cost drops as it gets longer, whereas for bridges the opposite is true, and while tunnels require a larger initial investment, bridges make up the difference in maintenance costs.

But let's not get tunnel vision. Tunneling faces particular security vulnerabilities and safety issues of fires and accidents post die threats in tunnels, which is why rail tunnels include crossover passages where trains can switch tracks, along with service tunnels that can serve as emergencies scape routes. Yet today underwater tunnels are so commonplace that we rarely think of them

as the modern wonders that they are. Take the Seikan Tunnel in Japan, a connecting the islands of Honshu and Hokkaido, which holds the record for the longest and deepest underwater rail tunnel at thirty three and a half miles that's fifty four kilometers, reaching a depth of seven hundred and ninety feet or two hundred and forty meters. Japan began planning it in the nineteen fifties after a typhoon caused

a deadly disaster in the strait between the islands. It took thirty years to complete, and pumps keep it clear of water at the rate of twenty tons per minute. As impressive as the Seikon Tunnel is, only about fourteen of its miles or twenty three of its kilometers run underwater, meaning that the Channel Tip or chunnel that connects the

United Kingdom in France beats it. There. The channel only goes a third is deep, but its underwater portion runs for twenty four miles or about thirty eight and a half kilometers. It was finished in nineteen ninety four, but okay, the Sekan and Channel tunnels respectively blasted and bored their passages through solid rock. The longest and deepest immersion tunnel is the Marmarai, which connects the Asian and European halves of Istanbul Turkey across the floor of the Bosporus Sea.

It employs pre assembled sections connected by thick, flexible rubber reinforced steel plates to better contend with regional seismic activity, and stretches a total of eight miles or thirteen kilometers, but let's back up a bit and get into some

technical but important definitions. A tunnel is technically a passage dug entirely underground, and many of the subterranean tubes that we consider tunnels, like subway and sewage and water lines, are technically conduits because to build them we temporarily remove ground material, place the line, then cover it back up, which is generally much cheaper and easier, especially if you're dealing with loose dirt and shallow projects. But to tunnel in the earth under a body of water, the classic

approach is to use a tunneling shield. Shields let you dig a long tunnel through soft earth without its bleeding edge collapsing. Here's how it works, okay, Imagine you take a coffee can and take off the lid, then sharpen the edge around the bottom and punch a few holes in the bottom. If you took that tin by the open end and pushed the bottom into soft earth, some dirt would squeeze up through the holes. You could remove the dirt and then push the can in further. On

the scale of a real shield. Several humans as sometimes nicknamed muckers or sandhogs, would stand inside compartments within the can and remove the clayer sand. Hydraulic jacks would gradually move the shield forward while crews behind it installed metal supporting rings, then lined them with concrete or masonry. In order to hold back water seepage from the 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 air locks and take precautions against pressure related sickness. Shields are still used in tunnel construction, especially when installing utility conduits or larger water or sewage pipes. Although labor intensive, they cost only a fraction as much as their mammoth cousins.

The tunnel boring machines afar from dull. A tunnel boring machine is a multi story tall engine of destruction capable of chewing through solid rock at its front spins its cutting head, which is a giant wheel that has bristles of rock breaking discs, and incorporates a system of scoops to lift the pummeled rock and drop it onto an outbound conveyor belt. Behind the cutting head swings an erector, which is a rotating assembly that builds the tunnel lining

in the machine's wave. In some large projects, like the Channel, a separate tunnel boring machines begin on opposite ends and drill toward a central point, using sophisticated surveying methods to keep them on course. Drilling through solid rock creates largely self supporting tunnels, and these machines drive forward quickly and relentlessly. Some Channel machines could bore two hundred and fifty feet

a day, that's seventy six meters. On the downside, they break often and deal poorly with rock that's worn, sheared, or highly jointed, so they sometimes move much slower. Luckily, tunnel boring machines and shields aren't the only games in town. Enter these sunken two tube or immersed tube tunnel. These entirely evade the problem of trying to dig through soft earth or solid rock while preventing a whole ocean from pouring into your tunnel by constructing the tunnel separately then

installing it under water. Immersion tunnels are assembled on site from prefab pieces, each the size of a football field. American engineer W. J. Wilgis pioneered the technique when he built the Detroit River Railroad tunnel connecting Detroit, Michigan, and Windsor, Ontario, in nineteen ten, and they've been the go to technique

for vehicle tunnels ever since. To make each segment, workers assemble some thirty thousand tons of stealing concrete enough for a ten 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 water tight as they're transported out to sea. Immersion pontoons, which are large ships resembling a cross between a gantry crane and

a pondtoon boat, do the hauling. Once they're over the pre dug sea trench. Each tunnel section is weighted to allow it to sink. A crane slowly lowers the section into position while divers guide it precisely to its GPS coordinates. Each new section is connected to its predecessor with a massive flexible joint that can establish a seal on the

outside of the two tubes. Crews then pump out the water between the two bulkhead seals on the inside of the seal, and then can remove the bulkheads, at which point you're ready to sink a new piece and connect it the same way. Once the tunnel is built and reinforced from the inside, it might be buried under backfill or otherwise covered. Immersed tube construction can delve deeper than

other approaches. Because the technique doesn't require compressed air to hold water at bay, A cruise can for work longer in them and under more tolerable conditions. Moreover, sections of an immersed tunnel can take any form, unlike a board tunnel, which follows the shape of its shield or boring machine. However, immersed tunnels do require additional tunneling methods to prepare the bed and bore out their land based entrances and exits.

Researchers are working on developing submerged but floating tunnels that would circumvent the need to bore at all. In underwater tunneling, as in life, it takes all kinds. Today's episode is based on the article how do you build an underwater tunnel? On how stuffworks dot com, written by Nicholas Gerbis. Brain Stuff is production by Heart Radio in partnership with HowStuffWorks

dot Com and is produced by Tyler Klang. For more podcasts my Heart Radio, visit the iHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows.