How do construction engineers dig tunnels underwater?

There are two fundamental methods of tunneling underwater that have enabled engineers to overcome the problem of floods and collapsing walls.

The Thames tunnel was built in the mid-nineteenth century by a system that ensured that the air pressure within the tunnel exceeded the water pressure without, thus preventing the inflow of water.

First, the face of the tunnel is sealed with heavy airtight gates. Necessary equipment, shields, conveyors, dump cars, and segment erectors, are placed within the tunnel, and workers pass back and forth through air locks and a decompression chamber used for acclimatization. Air is pumped into the tunnel until the pressure inside is higher than that of the water pressure outside.

In addition, pressure grouting with cement, bituminous emulsions, or gelling salt solution is usually necessary. When loose gravel presents a problem in the tunnel, temporary jetties may be built above from which are lowered tubes that inject a bentonitesilicate cement mixture, forming an impervious strip on either side of the digging. Any water leaking into the tunnel is pumped out, and eventually a waterproof lining is constructed.

The second, and newer, method of tunneling underwater involves first constructing the tunnel, then lowering the prefabricated sections into the water. The section of the Paris Metro that runs under the Seine, built in 1910, was constructed in this way, as were the Chesapeake Bay Tunnel and the Ij Road Tunnel in Amsterdam.

Concrete or steel sections, generally nearly 200 feet in length, are floated out and sunk into dredged trenches. Temporary end walls are then removed, and the joints are sealed. The trench holding the tunnel is filled in to secure its position.

Pile foundations help support the tunnel, and it is essential that their tops be made level so that weight is distributed evenly across the long sections. This was accomplished in Holland’s Rotterdam Metro by first sinking each tunnel section onto four piles.

Other piles to support the section had been driven in slightly farther, but were topped with false heads, each containing a nylon sleeved cavity. When cement grout was pumped into the sleeves, the false heads rose to the level of the tunnel.