How a Tunnel Is Dug Under Water.
Fig. 1. On the left is a cross section showing, in diagram, the back
view of a shield. The heavy black circle is the “tail” or “skin.”
The small circles within the tail are the hydraulic rams which at a
pressure of 5,000 pounds to the square inch force the shield forward.
The square compartments within the shield are the openings through
which the men pass to dig away the ground. In the middle of the shield
is shown the swinging “erector” which picks up the iron lining plates
and puts them in position.
The view on the right is a longitudinal section of the tunnel showing
the shield and the bulkhead wall across the tunnel with the air locks
built into it. The front of the shield ahead of the doors is made with
a sharp edge called the “cutting edge” and this makes it easier for the
shield to advance in case all the ground in front has not been removed.
This view shows how the tail overlaps the last portion of the iron
lining.
Some distance behind the shield comes the concrete bulkhead wall with
the air locks contained in it. There are two shown in the view. The
upper one is the emergency air lock, always kept ready so that in case
of an accident the men have a means of escape even though the lower
part of the tunnel is filled with rushing water or mud. The lower air
lock is for the passage of men and materials during ordinary working.
This view also shows that all the tunnel ahead of the bulkhead wall
is under compressed air while the finished tunnel behind the bulkhead
wall is under the ordinary or normal air pressure. When the tunnel is
finished the air locks and bulkhead walls are removed.
[Illustration: FRONT VIEW OF A DRIVING SHIELD
This shows the front of one of the shields used on the Pennsylvania
Railroad tunnels crossing the North River at New York. The cutting edge
is clearly seen and the various compartments, each with its door, which
divide up the front of the shield. These shields weighed about 200 tons
each.]