How do they build underwater tunnels?

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How is the water initially displaced? And do they float? Attach to the bottom of the body of water?

I was driving through one today in Boston and couldn’t help but wonder. Cheers in advance!

In: 190

The tunnel is in the ground under the water. They use huge boring machines to dig the tunnels, there are lots of videos, look for “Chunnel”.

Typically, if you’re building a tunnel under a body of water, such as the Channel Tunnel connecting the UK and France, it’s actually under the ground under the water. The same tunneling technology that would be used for something like going through a mountain is employed for those, just with the caveat of having to go down first, and having to engineer around the immense amount of water on top.

Other types of tunnels can include submerging the tunnel shell down and then pumping the water out. Those tunnels can either be buried after submerging, or they can be tethered to the sea floor.

Tunnels for San Francisco’s BART system were built in 57 sections on land and sunk into the bay. Once underwater they were connected underwater, any water in the Tunnel was pumped out and the tunnels rest on the floor of the bay, surrounded by sand and gravel.

https://en.m.wikipedia.org/wiki/Transbay_Tube

If they aren’t tunneling under the water the tunnels are prebuilt in sections on dry land then placed in the water. They flood them so they sink then they bury them and pump the water out.

Not sure which one in Boston that you went through, but if it was the Ted Williams (to-from the airport) it was built in sections that looked like binoculars on the surface, and then sunk in place and attached together. https://en.m.wikipedia.org/wiki/Ted_Williams_Tunnel