TUNNEL ENGINEERING

should be restricted. Personnel accessing shielded
areas should be protected against cave-ins. Personnel
should not be permitted in shields when they
are being installed, removed, or moved vertically.
Excavations may extend up to 2 ft below a shield
bottom, if the shield is designed to resist the forces
from the full depth of the trench and if soil will not
be lost from behind or below the bottom of the
shield. (See also Arts. 20.6 and 20.8.)
The shield is a cylinder made of welded steel
plate (Fig. 20.16). It has a diameter slightly larger
than the outside of the tunnel lining. The plate is
stiffened by two interior ring girders, the first one
installed a short distance behind the cutting edge.
TUNNEL ENGINEERING
Tunnel Engineering n 20.31
Depending on the diameter and loads, the
girders are braced by horizontal and vertical steel
struts. The cutting edge is beveled and reinforced
by welded steel plates to a thickness of up to 3 or
4 in. For loose ground, the upper half of the
shield is extended forward 12 to 18 in to form a
protective hood.
The tail of the shield overlaps slightly the end of
the finished lining and provides space for at least
one liner ring, and for underwater tunnels is
usually long enough to accommodate two rings.
The inside of the tail clears the lining by about 1 in
all around. For working in soft clay, the front of the
shield may be closed by a steel bulkhead with door-
Fig. 20.16 Longitudinal section through a shield used for tunneling through soft ground in free air.
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