When the subway or BART train goes underneathe stretches of water, like through a bay or lake,?

Question

I go through the san francisco bay to Oakland sometimes and I was wondering if the tunnel gets built in the water or do they dig a tunnel in the earth underneath the water? and what happens if a earthquake would hit an you were down there, would it be like Indiana Jones temple of doom with all that water shooting down the tunnel?

Answer 1

depending on the conditions, the tunnel type and mechanism can be very different.

First there is the tunnel machine. Its a cylinder basically the same diameter as your finished tunnel. The front is a circular cutting head, similar to a donut shape. The spoils from the cutting operation flow through the center on a conveyor to a train to be hauled off.
As the tunneler progresses, the tunnel walls are built with it.
Sometimes, a continuous cement grout is pumped to the outer wall. This forms the tunnel wall.
In softer soils, a pre-fab concrete interlocking piece is placed around the radius as the machine is tunneling along. The pieces interlock and grouted in, forming the tunnel walls.
It really is an amazing process to watch this all going on at the same time!

Check out coluccio construction at the link below.

(he was naming the tunnel machines after is granddaughters!)

oops, to answer your question, tunnels are cylinders. This is the strongest shape as the forces are factored radially (this is why those 80 foot diameter oil tanks are only 1/4 inch thick)
There are flexible joints built in to compensate for the deflection forces of earthquakes. The tunnels are also overpressured so that if a crack did occur, the pressure would force the air out and keep the water from coming in....

Answer 2

It depends. According to the History Channel, for the Chesapeake Bay tunnel project, they dug a trench in the seafloor, floated a steel tube out into the bay, filled the steel tube with seawater to sink it into the trench, buried the tube under sand, and then pumped all of the water out of either end.

For the Channel tunnel between England & France, they simply dug through solid rock deep beneath the seafloor and set concrete bracing in place.

As far as earthquakes are concerned, I suppose that the Engineers thought of that when they designed the bracing for the tunnel. Hopefully it will not leak.

Answer 3

They are building the tunnel by parts... then, piece to piece they are assembling it in the water. they have a verry good protection for earthquakes and special devices so that the water can't go through the walls of the tunnel

Answer 4

I'm sure there are some good engineers out there that can answer this better, but my understanding is that many of these underwater tunnels are composed of pre-fabricated sections that are sunk, attached together underwater on the bed of the bay or river, and then the water is emptied.