TUNNEL ENGINEERING

ground are directly related to the percentage loss of
material outside the tunnel. Typical loss of material
lies between 0.5% and 2.5%. Factors affecting the
loss include the properties of the material
traversed, the face pressure used, the design of the
shield, and the rate of advance. Tunnels exist where
the loss has been zero. Soft ground TBMs are
generally launched from a relatively small shaft,
with subsequent parts of the machine being added
as progress is made.
The sealed front compartment of a slurry TBM is
usually filled with a bentonite slurry held in
TUNNEL ENGINEERING
Fig. 20.21 Preparing the Cairo Metro Shield for Assembly.
Tunnel Engineering n 20.43
equilibrium with the soil and groundwater
pressures acting at the face. The equilibrium is
often balanced by a compressed air reservoir and
flow controls. The slurry also acts as a lubricant and
holds loosened soil in suspension. The main
disadvantage of a slurry TBM is that the slurry
must be continuously circulated through a separator,
often located on the surface, to remove the
excavated material before returning the reconditioned
slurry to the face. One main advantage is
that the underground operators never come into
contact with the excavated material. The slurry
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