Why It's So Hard To Dig Tunnels Underwater

I’m on the Charles River in the City of Boston,  despite its tranquil appearance this is actually one of the busiest stretches of water in America. Not above the waves but below them; there are more underwater tunnels here than at  almost anywhere else on the planet. Infact, if you were to connect end to end  you could start to form an underwater link between the United States and Europe. Today we’re going on an adventure through the history of Boston to discover just  how to make that transcontinental link. Completed in 1904 the East Boston tunnel is  the oldest underwater rail link in America. It connects the eponymous East Boston - including  a major international airport - to the main city, carrying about 70,000 passengers per day. Counterintuitively, it’s much harder to dig  through soft soil than it is through solid rock. That’s because when you have a firmer  material you can dig the tunnel now and just add the supports in later. Whereas when you’re digging under the softer sand like underneath a river, then  the whole thing just collapses in on itself. A telling statistic is that although we’ve  been digging tunnels for more than 4000 years, it’s only been in the last 200 that  we’ve been able to build them underwater. That’s all due to an invention  known as a tunneling shield. Here’s what it looks like in the  workshop - a rivet steel semicircle 10 metres in diameter and about 3 metres thick. With this interior view you can really appreciate the insane loads it was designed to carry. Two such shields were constructed and then lowered into massive pre-dug  shafts on either side of the river. The actual excavation works in  a three step repeating process. First the shield is pushed into the  soft soil using some hydraulic jacks. Next, protected by the shield, workers then remove  the material which gets carted up to the surface. A significant portion of  that international airport I mentioned earlier is built on the  soil extracted from this very tunnel. At this point if we were to then remove the shield  then the roof would cave in so to stop that from happening a strong internal wall of concrete and  steel is fabricated directly beneath the shield. Once the sides are ready to take the load  themselves the jacks are reactivated and the shield slides onwards into the dark. 30 years later an almost identical technique was used to build the Sumner Tunnel but this time to  service Boston’s growing love of the automobile. What's weird about this tunnel is that  while the cross section that was dug is very clearly a circle, if you look  at it today it’s much more rectangular. That is a lot of extra dug material  that doesn’t really go to any use. So what have they tried to put  a square tunnel in a round hole? At the deepest point of the crossing there are  about 250 tonnes of sand, soil, water, and ducks pressing down on each metre section of length. That is a huge amount of force so to see if withstanding that force has anything to  do with the shape that your tunnel is I’ve built my own test using some folded  up pieces of paper and a fish tank. We’ve got a circular section - the type that was  cut in the tunnel - a square - which would be most efficient - and finally a triangle because I’ve  heard that triangles are the strongest shape. To see how they withstand some load I’m  going to be pouring in a bit of sand and a couple of bricks to  see which one breaks first. The triangle crumbles first,  followed by the rectangle, and finally the circle is last to fall. Given the extreme deformation we saw I wouldn’t  feel particularly safe under any of our tunnels. Least of all because they’re made of paper. However it was very clear  that circles worked the best. That’s because their smooth sides and lack  of corners meant that there were no stress concentration regions, allowing stress to be  evenly dispersed and the structure to last longer. This means that if we’re going to be  building a Transatlantic tunnel, or indeed any tunnel under a high load situation,  then circles are the best shape to pick, Our tunnel bore will take us about 400  km off the coast although as we descend past the continental shelf even our cylindrical  shell would be crushed by the extreme pressure. On top of that, you’ll eventually  run into the mid-Atlantic ridge which has weekly magnitude 5  earthquakes and actual lava. We need a tunnel that is above the sea floor  so that it doesn’t get crushed by the pressure, but is still below the surface so that it  won’t get destroyed by hurricanes and pirates: a special sort of tunnel pioneered  within this very parking lot. Now I could proceed to explain how it works within  said parking lot, but I think I found somewhere a lot more interesting just up ahead. Welcome aboard the Beaver. A replica of the 1773 trading ship which was  the ground zero for the Boston Tea Party. Late at night a group of around  100 colonists stormed aboard intent of dumping it’s cargo of taxed British tea  directly into what is not the Fort Point Channel. About 200 years later it was ground  zero for another revolution but this time instead of the American Revolution for  Independence instead it was the Boston Big Dig Revolution for Better Transportation. As it turns out, the properties that make it such an attractive place to put ships in - namely  its central location and massive size - have also made it a nightmare for Boston’s road and rail. The Big Dig was a megaproject constructed from 1991 to 2007 with a goal of opening up  the city and cutting back on traffic. This had actually been attempted before in the  most 1950s American way possible with a massive elevated motorway right through the CBD. The Big Dig took a different approach by replacing these motorways and adding  in some new ones but all underground. Most relevant for this episode, the new  Ted Williams tunnel beneath the Charles River needed to be connected to the  Massachusetts Turnpike and Highway One. A route that passes directly through the very  large and very central Fort Point Channel. The problem with this proposal is  that space was incredibly tight. On that side is the US Postal Service and this  the global headquarters for the Gillette company. And below me is the Red Line  which is the subway system which is the busiest past of the entire network. So, we can’t go over the river, there's no  room and these companies weren’t going to move. And we can’t go under, there's a  really old trainline in the way. The only solution was to go  through the Fort Point Channel. Building underwater is incredibly difficult. As you can imagine, if I were trying to assemble this box then the duct tape wouldn’t stick right,  the cardboard would disintegrate, and if I were wearing scuba gear I’d have lots of trouble  just trying to fold it up in the first place. I did actually try to rent some scuba geat to  demonstrate just how difficult it would be, but it’s actually illigal to  swim in the Charles due to some relatively dangerous levels of blue green algae. There are workarounds for each of these  situations like using trained divers with specialist underwater equipment and concrete  designed to actually work in this situation. However, they are incredibly expensive  and best left to one-off tasks. Ironically enough, the first step in building  underwater should be to get rid of the water. With the Fort Point Channel crossing engineers  took advantage of what's called a casting basin. This is where a temporary dam  is constructed facing onto the channel and behind which a pit gets dug. Because we’re really good at making things on dry land this gives us the perfect  opportunity to do some optimising. With a regular tunnel you can have a maximum  of two teams working at any time - one on one side of the water body and one on the other  tunneling to meet each other in the middle. However now that we have a casting basin  and all of the manufacturing benefits that entails we can engage in a  little bit of parallelisation. You see, if I had to build one massive tea chest  all by myself then it would take some time, but if we split it into three smaller sections  and have the Sons of Liberty helping me out then we can do the entire thing much faster. In the same way, rather than taking 100s of years to construct our channel crossing,  we can do the entire thing in just a few months by converting all available  beachfront property into casting basins. Looks like we’re all pretty much done,  let’s grab some of the actual tea chests and show King George just what we think of him. - These sir at the tools that King George and his Parliament will use to rob you of your natural  British rights. Will you pay this tax on tea? - No - Ready. One! Two! Three! - HUZZAH!!!!! - Well done Once manufacturing of the actual  tunnels was complete, the basin was flooded and sections floated out into position. To stop the Red Line from being crushed, pylons were installed, extending deep into the bedrock. When the signal was given the ballast tanks flooded and tunnel sections sunk  to come to rest on these supports. Using these techniques we can build a  tunnel all the way through the oceans, kept in place with some massive pylons. Alternatively rather than filling the ballast tanks all the way up we  could carefully control their volume allowing us to remain neutrally  buoyant like a massive submarine. We could keep it in place using some massive  tether lines or carefully controlled thrusters. So who gets to be the first to take that historic  journey between Boston and Bordeaux? No one. No one in a car anyway. Having an average speed of 110 km/h will give you a two day  trip to get between the continents. That’s an insane amount of time to  be inside a dark, dingey tunnel. No, rather than driving across  instead you’ll be taking a train. One that levitates on magnetic rails and  operates inside an artificial vacuum. This removes all of the wind resistance  and train wobble effects which have limited the speed of modern rail. Infact, we're now only limited by the acceleration that the human body can take  which means that half-hour trips between the continents are definitely on the table. While this may sound like science fiction it’s actually quite an old idea, one invented  in 1904 by local resident Robert Goddard. Boston’s underwater tunnels have not only  shaped the face of this city but also the lives of all those who live within it. With 100 more years of innovation, who knows what we’ll create. This has been James Dingley from the Atomic Frontier. Keep Looking Up.

Channel: Atomic Frontier

Views: 20,443

Rating: 4.9225221 out of 5

Keywords: engineering, underwater tunnel, tunnel, boston big dig documentary, transatlantic, chunnel, boring company, hyperloop, underwater building, scuba, atomic frontier, tom scott, veritasium

Id: nH-xZjx53lc

Length: 10min 55sec (655 seconds)

Published: Sat Sep 18 2021

Reddit Comments

[u/stealthispost]

Tom Scott 2.0

[u/3dprintsAnomus]

The air vacuum with electric panels has been described before and there are massive issues with them.

A void under 1 mile of water has to have some thick support.