UNDERGROUND ENGINEERING - Introduction to Soft Soil Geotechnique: Content, Context and Application - page 251

buffer of slurry or air, or by a solid frame, referred to as earth-pressure balance
shield (EPB). The tunnel-boring machines (TBM) are large, complex and
impressive semi-automatic robots. Beside the shield and the cutting wheel at the
front, the size of which may reach more than 15 m, cut soil has to be removed,
tunnel lining elements have to be transported and installed, and the machine has to
be maintained (replacing chisels, removing obstacles). The design and operation of
a tunnelling project is multi-disciplinary, since the involvement of technical,
legislative, social, communicational and even political skills are required.
Soil behaviour
Two main aspects with regard to soil behaviour are considered in a TBM
operation: the front stability and the soil response on the lining. The front stability
is complex and a different behaviour is to be expected in sandy soil and in clayey
soil. Internal friction in sand may cause arching, which may induce more
favourable stress redistribution (see Fig 15.4). The slurry pressure at the front must
not be too low to disturb the cutting process and not too high to cause a blow-out
towards the surface. Careful monitoring is constantly required. Large obstacles
(boulders, buried trees, remnants of old foundations) may hamper the production
seriously, and they must be removed.
(a) frictional soil (b) cohesive soil
Figure 15.4 Schematic soil deformations during tunnel boring
Along the lining, for directional drilling, the soil may be compressed, while for
large diameter drilling over-cutting (the cutting wheel is usually slightly larger than
the external tunnel-lining diameter) will let soil locally expand, even if high
pressure grouting at the tail of the shield fills this space. Furthermore, the hole
which the TBM creates in the soil, will, according to Archimedes law, result in a
tendency of lifting, and this has led to instability during tunnelling through very
soft layers (peat) and dilatant layers (sand). All these aspects change the original
stress state of the soil and invoke pressures on the TBM and the tunnel lining.
Induced deformation and friction must be within limits in every stage to make the
operation a success.
A fundamental soil problem related to tunnelling, boring or in-situ wall
installation is what happens to a hole in the ground. Two aspects will be discussed,