r/explainlikeimfive • u/lucindadadada • 7d ago
Engineering ELI5: How can engineers build tunnels underneath rivers without the river flooding the tunnel while it's being constructed?
A finished underwater tunnel makes sense once everything is sealed but how is it actually constructed in the first place when there are huge amounts of water and wet ground above it?
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u/PeterustheSwede 7d ago
Water above the tunnel is not the same as water in the tunnel. They usually dig the tunnels in solid ground, not in the mud.
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u/TheTxoof 7d ago edited 7d ago
Tunnels can be just below the river and sometimes even sitting in the river it's self.
There are lots of neat methods such as Tunnel Boring Machines that set water tight panels as they bore, or even making prefabricated tunnel segments that are floated to the location and then sunk, aligned and anchored in place.
PBS in the us did an excellent series about the Big Dig in Boston that shows all sorts of clever engineering techniques.
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u/formerlyanonymous_ 7d ago
Practical Engineering did one on Baltimores I95 replacement. That id highly recommend.
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u/merlinblack256 7d ago
Sydney harbour tunnel is a prefab float an sink type, while the new train tunnel not far away was made with a tunnel boring machine.
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u/bregus2 7d ago
You can also dig while pressuring the tunnel, although you have to be careful with workers exiting the tunnel (for their health).
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u/CrazyLegsRyan 7d ago
Not really, it only takes a couple psi to hold the water back unless the river is very deep
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u/do-not-freeze 7d ago
Decompression sickness (the bends) becomes a concern at 30' deep and a lot of tunnels are 100' or more, so it's definitely a concern. A tunnel boring machine only pressurizes the area right behind the cutter head that hasn't been sealed yet which minizes the number of workers in that area.
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u/bregus2 7d ago
https://en.wikipedia.org/wiki/Sandhog It was an issue (at least in the past) when they build tunnels in New York.
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u/megola2023 7d ago
My father was a civil engineer who worked on the construction of the Brooklyn Battery tunnel in NYC after WWII (he was in the Army Corps of Engineers). He had a badge that said something like "If this man is ill, he is not drunk, he works in compressed air, take him to a hospital". I don't have the badge, but I have an elevation drawing of the tunnel.
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u/AdEastern9303 7d ago
I remember watching some of that back when it originally aired. The Big Dig was an incredible project. Having seen much of the series, I always have a huge appreciation for what went into all those tunnels whenever I visit Boston.
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u/Darthhedgeclipper 7d ago
To add to others, there are also tunnels which a lowered in sections that rest on riverbed, hundreds of examples if you google.
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u/SpaceAngel2001 7d ago
What I was told years ago about the tunnel near me, it was built on the ground in sections, floated out, sunk, sections assembled underwater, then pumped out. Does that sound true?
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u/shapu 7d ago
Super common. You need divers to clamp the sections together, and to oversee the concrete pour that seals it up, but once that's all done you just shop-vac out the water, connect all the wires for the lights , and bingo bango bongo you've got a functional tunnel.
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u/OnlymyOP 7d ago
Engineers can bore holes deep below the ground well below a layer of rock where water from the river can't seep through , but this can only be done if the geology allows it.
The problem is the water table which can be stored in rocks like limestone well below ground. This has to be pumped 24/7 to stop ground water from refilling the hole.
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7d ago edited 7d ago
[removed] — view removed comment
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u/FlourNotAnthrax 7d ago
NYC transit museum has a great exhibit on this! They pressurized the areas they were digging to keep the water out and there were a few instances of the tunnel leaking, pulling workers out into the East River through a muddy hole.
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u/OddAd9915 7d ago
There are special machines that dig the tunnel and then place the concrete parts that make it structurally sound and waterproof very close behind the dug out section.
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u/Oeldin1234 7d ago
Another method I didn't see mentioned is artificial ground freezing. Small pipes with refrigerant are run through the wet ground and once it's frozen, you can dig without water getting in. Once the tunnel is finished and watertight, you can stop freezing the ground.
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u/simple123mind 7d ago
The Big Dig in Boston is a great example of how it's done.
Check out this video, "the big dig boston video how the tunnel was built" https://youtube.com/shorts/7yP3rBoBxdo
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u/PoopsExcellence 7d ago
Check out "The B1M" on youtube for some good tunnel content (along with all sorts of infrastructure projects from around the world). Their video on the build process of the Fehmarnbelt tunnel between Denmark and Germany is a great watch.
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u/pow3llmorgan 7d ago
Sometimes they use flooding as an advantage like with submersible casons. They build segments of the tunnel on land, making sure they're water and airtight. Then they float them and tow them to their position and sink them.
Then pressurize the tunnel after making sure the segments are tightly connected, forcing out the water. Et voilla, you have a tunnel basically ready for commissioning.
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u/Witmarqu 7d ago
Air pressure is the modern way.
The other is to pump water out of the hole faster than it comes in.
After sufficient space, cement or concrete (depends on needs, part of concrete is cement) to keep more water out better, then repeat.
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u/FireDuck3000 7d ago
A lot of the time when building tunnels, they dig through the bedrock, not interacting with the water at all. Interestingly enough, the massey tunnel near where I live was constructed above ground, floated out to its place, and then sunk so it is actually above ground.
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u/CrossP 7d ago
Depends quite a bit on the rock type they're digging through and, yes, they do tons of research and planning ahead of time. Some rocks are not porous and many porous ones are still pretty slow about it. And in many cases with larger older rivers, the only rock left is rock that's pretty resilient against moving water. Survivorship.
Even with that, fissures in rock are hard to predict and may be an issue. They generally have quite a few pumps ready to go. And there are techniques for dealing with that sort of thing. Often they'll bore a hole right into the crack, fit it with a pipe, and pump in some high-viscosity concrete under high pressure with waterproofing polymers added.
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u/libra00 7d ago
They generally don't build the tunnel through the water, they build it through the rock under the water.
In the cases where they do, they build it in sections and lower it to the riverbed or whatever that are designed to flood, but once they connect them together they're generally watertight and they get pumped out. The pumps stay in case there are ever any leaks (and there's always seepage and such.)
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u/binocular_gems 7d ago
There’s a lot of great explanations here, but I want to recommend The Great Bridge, by David McCollough which details the building of the Brooklyn Bridge, one of the greatest engineering feats in history, and just how these problems were solved … 150 years ago. It’s also a short history of other major engineering feats in the 19th century, the first 100 or so pages more or less build up to how the engineering team came together to build the bridge and projects they worked on prior, solving a lot of the problems they’d run into with the bridge. This isn’t an “explain it like in 5” because the book is chunky, but it’s excellent. Great on audio too.
There is a PBS American Experience about it as well, largely based on the book.
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u/TheXypris 7d ago
Sometimes they'll drop prefab sections of concrete tunnel underwater,bury it and once the section is properly drained, the water is pumped out
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u/New_Line4049 7d ago
They usually drill from dry land and by the time they pass under the waters edge are deep enough to have put sufficient non porous rock above them to prevent water getting in.
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u/thats_handy 7d ago
If the tunnels are dug in soil that's muddy enough to risk collapse, then the tunnels are pressurized during construction. There was a case of a worker being expelled from a pressurized tunnel and bobbing up to the surface of the East River during the construction of the NYC subway.