How are underwater bridges made?

Question

I've always wondered that..

Answer 1

I believe they dam the river or lake to dry it out then build the tunnel and once the tunnel is ready to open they release the water.

Answer 2

If you mean how are the supports for a bridge that are underwater are made...

Some bridges will have the supports that are pre-cast concrete.  These pieces are made on land, and then using a pile-driver are driven into the sea/lake/river bed until they reach a depth determined by an engineer.

Other bridges the construction company will build a coffer-dam around the area.  This is a temporary barrier that will keep out most of the water.  The water that does get in will be pumped out.  The construction company then has a mostly dry area to build the support structures.

For a tunnel...

Like the "chunnel" between Britan and France, you get a large machine to core through the stone.  You then come in and pour concrete to make it structurally sound.

You can also use the coffer-dam construction mentioned above.  You section off the area.  Build the tunnel.  Let the water back in over the tunnel.

You can also build the tunnel in sections with pre-cast.  You then trench out where the tunnel will go and drop it in the water.  You will need some underwater construction to seal the piece together.  Pump out the water on the inside an finish construction.

Answer 3

Do you mean tunnels?