Geology Reference
In-Depth Information
Soft ground, including severely weathered rock, may be excavated by
hand or by tunnel boring machine. For open-face excavation, beha-
viour can be predicted using classi
cation such as the Tunnelman
'
s
Classi
cation of Heuer (1974), which allows prediction of whether the
soil will stand
firmly whilst the liner is put in place or is likely to ravel,
run,
flow, squeeze or swell. Behaviour depends on the nature of soil,
water conditions and stress levels. For example, un-cemented sand
might be expected to
flow below the water table, especially at depth.
Such empirical predictions are also useful for weathered rocks where
the application of conventional soil mechanics principles is question-
able (Shirlaw
et al
., 2000). When tunnelling in soil or in mixed-face
conditions, it is the behaviour of the weakest or most mobile material
that generally governs the need for, and magnitude of, the support
pressure that is needed at the tunnel face.
If the soil is stiff and cohesive, then NATM methods can work
successfully, as has been achieved, for example, in the London Clay
(van der Berg
et al
., 2003) and in the Fort Canning Boulder Bed and the
Old Alluvium in Singapore (Shirlaw
et al
., 2000). Where soils are
unstable, then various options include grouting, dewatering, freezing
or the use of compressed air. All of these are costly, may have severe
health and safety implications and restrictions, and take time to install.
Nevertheless, such methods are often necessary to recover and restart a
tunnel that has encountered a major problem and perhaps collapsed.
Tunnel boring machines used in soft ground are of the closed-face
type, as illustrated in
Figure 6.14 a
and
b.
Guidance on machine selection
and use is given by the British Tunnelling Society (BTS, 2005).
Earth pressure balance (EPBM) and slurrymachines use pressurised soil
at the cutting face to hold up the ground as the tunnel advances. In an
EPBM machine, the broken down soil remains in the plenum chamber
behind the cutting head, balanced by pressure in the Archimedes screw,
which removes the spoil under the control of the operators. In a slurry
machine, which tends to be used in higher permeability soils, bentonite
slurry is introduced to the plenum chamber, mixes with excavated soil,
which is then removed for separation, disposal and re-use (bentonite) by
pipes rather than on a muck conveyor. Permanent concrete lining is
formed from precast segments, directly behind the machine, and this
liner is used as a reaction to push the TBM forward. TBMs often work
well for the speci
c conditions for which they are designed but also
commonly run into problems with the machine getting stuck or running
into rock that is either too hard or too soft or too wet for the type of
and collapse in Singapore, even using sophisticated EPBMs. Similarly, an
EPBMmachine was recently stopped by silt breaching the tunnel liner on a
contract in the UK. A further example is discussed in
Chapter 7.
Recovery
