Geology Reference
In-Depth Information
the ground ahead of the machine without stopping and drilling in front of
the face, which disrupts operations and is therefore to be avoided.
The design and use of modern hard rock TBMs is covered comprehen-
sively byMaidl
et al
. (2008). In good rockwith highRQD, open TBMs are
sometimes used, but generally only for relatively small diameter tunnels.
The tunnel advances by jacking forward against grippers that are
extended laterally against the tunnel walls. Clearly, if the rock becomes
poor quality then there may be problems with the grippers. There is also
no way of preventing groundwater ingress other than by grouting, pre-
ferably in advance of the machine.
In Chapter 7,
a case (SSDS) is presented
where open-rock TBMs were selected, anticipating good rock conditions
with low water in
ows, and the operations were halted when in
ows
became too great and grouting in advance was extremely dif
cult.
In poorer-quality rock, generally, shielded TBMs are used. A single-
shield machine pushes against the liner, as for soil TBMs
(Figure 6.14c)
.
In other set-ups there are two shields; the rear shield has grippers and
provides the reaction against which the front shield can push forward.
The cutter head has discs that rotate as the cutter head itself rotates. The
thrust of the machine causes the rock to fail, mainly in tension. A major
consideration is the lifetime of the cutting discs before they need to be
replaced, as addressed by Maidl
et al
.(2008).Acaseexamplein
Chapter 7
describes considerable wear in an EPBM used to tunnel
through abrasive sandstone.
Rock tunnelling, in general, relies largely on the rock mass locking up as
joints and interlocking blocks of rock interact and dilate during the
process of convergence towards the excavation. Good-quality rock
often forms a natural arch and no or little support is needed. However,
in weaker ground, such as in fault zones, the rock mass cannot support
itself, even with reinforcement, and requires arti
cial support in the form
of steel arch ribs, typically encased in shotcrete. Optimising support
requirements in weaker ground requires prediction of likely convergence
rates, making observations as excavation is undertaken, i.e. observational
methods, and then applying support such as rock bolts and/or shotcrete
and/or steel arch ribs to control the movement and prevent excessive
loosening (Powderham, 1994). In stronger, blocky rock masses, rock
movement will be much less, and the purpose of the support is then to
prevent loss of loose blocks and wedges, which would destabilise the arch
and maybe lead to ravelling failure.
