TUNNEL ENGINEERING

hoe is used to loosen the soil and scoop it onto the
conveyor. From there, it is discharged on a loading
conveyor mounted on a movable carriage behind
the shield. The loading conveyor dumps it into
mine cars, usually of about 4-yd 3 capacity in large
tunnels. The muck trains are rolled back through
the tunnels to an access shaft. The individual cars
are hoisted up the access shaft and dumped into
hoppers for discharging into trucks.
Tunnel Linings n Except in very stiff or
compact soils, segmental ring liners are used in
shield tunnels. These used to be of cast iron but
today steel or precast concrete is used. The
segments are brought in by mine cars, unloaded
by hoists mounted on the conveyor carriage, and
deposited within reach of the erector arm. This is a
telescoping, counterweighted arm pivoted on the
center line of the tunnel for full rotation by a
hydraulic motor (Fig. 20.17). A gripper at its
outer end engages lugs or bars in the segments
and places these, starting at the bottom. A
short, tapered segment forms the key. See also
Art. 20.17.
TUNNEL ENGINEERING
Tunnel Engineering n 20.33
Packing n Since the shield has a larger
diameter than the lining, a void exists around the
liner rings. This may permit a cave-in and cause
settlement. The usual practice when segmental
liners are used is to inject pea gravel into this void
through grout holes in the liners immediately after
the shield has been advanced (Fig. 20.16). Cement
grout is later injected into the gravel to solidify it. In
a section of the Victoria line of the London subway
in deep, very stiff clay, an articulated cast-iron
lining was installed and expanded against the clay
behind the shield. The adjacent rings were pressed
into contact by the jacking forces but were
not bolted. Expansion of steel ribs with wood
lagging has also been used to achieve tight fit
against the soil.
Semicircular or semielliptical shields have
been used as temporary supports for the roof or
arch of excavations, mostly in dry or dewatered
soils, for example, for tunnels at shallow depth
where open-cut operations are prohibited by
circumstances. They are advanced in a manner
similar to that for circular shields.
(J. O. Bickel and T. R. Kuesel, “Tunnel
Engineering Handbook,” Van Nostrand Reinhold
Company, New York.)
Fig. 20.17 Section through a conventional shield (used in 1930 for the Detroit, Mich.—Windsor, Ont.,
Tunnel) for tunneling with compressed air.
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