I mean during placement of the assembled sections. I saw a documentary on this recently, but missed the part about how they keep them from filling with water before all of the sections are connected.
If I remember correctly (at least with auto tunnels) the sections are sealed on each end with steel. They floated to the spot they are to rest, sunk in place then welded together by underwater diving welders. Only then are the steel plugs removed between the welded sections.
I think in some cases the sections are allowed to fill with water, and then pumped out.
However, as far as I can tell, most underwater tunnels are simply bored through the bedrock under the floor of the body of water, rather than being built out of sunken tunnel members.
http://www.masspike.com/bigdig/background/twt_built.html
When they sunk the sections of the Ted Williams tunnel, the ends were sealed. Then they opened up one end and worked on that section while the next was set up.
San Francisco Bay Area Rapid Transit tunnel under the Bay.
A series of tubes. Perhaps the real place the Internets started.
I’m not understanding- how do they open one end of a submerged tunnel section without water rushing in?
They only open one end, the one connected to the other sections that have already been sunk and are connected back to land via the rest of the tunnel.
You start by digging a tunnel to the edge of the water on the west side, and putting a steel wall up to keep the water out. You dredge a trough in the river bottom right up to the conventional tunnel you just dug, and drop a sealed tunnel section on the end of that. You weld the sunken section to the steel wall you built on the end of the conventional section. Then you cut through the steel wall and the end cap on the east end of the tunnel section, extending the dry tunnel out to the west end of the section you just dropped in place.
On the east side of the water you are doing the same thing. Eventually, there are two partial tunnels, with steel walls at the end of each and dry tunnel behind, and a gap in the middle that is the size of the last tunnel section. Drop that last one in place, cut through the end caps, and ta-da, you have a complete tunnel.
If burying prefab segments isn’t an option, i.e. through rock instead of mud, a “mole” is used instead - in combination with some serious sump pumps where needed. In olden days, pressurized caissons would keep the water out while providing a work area, but you know about getting “the bends”.