Geology Reference
In-Depth Information
The main options are drill and blast, a roadheader excavating machine
or to use a TBM that may be either open (without a protective shield) or
shielded.
Generally, drill and blast tunnels are more
flexible than TBMs and allow
dif
cult ground conditions to be understood and overcome, but theymay
be much more time consuming unless a number of access points can be
found to allow operations to proceed from several faces at the same time.
Holes are drilled in the face, and explosives placed in the holes. Issues
of tunnel blast design are addressed by Zare & Bruland (2006). The
holes are detonated sequentially to break to a free face over micro
seconds. The aim is to break the rock to manageable size so it can be
excavated (mucked out) readily with machines, without further blasting
or hammering. Other aims may be to keep blast vibrations to a mini-
mum and not cause damage or offence to nearby residents, and usually
to keep as closely as possible to the excavation shape prescribed by the
designers, i.e. minimising overbreak. Typical advances per round are 3
to 3.5m, sometimes up to 5m in very good rock conditions. Depending
on the size of tunnel and ground conditions, the full face may be blasted
in one round or may be taken out as a series of smaller headings
top, or
side, that may be supported by sprayed concrete with steel mesh or steel/
carbon
-
fibres, rock bolts, and/or steel arches or lattice girders, before the
tunnel is advanced.
Figure 6.15
shows a tunnel portal following the
rst
blast, with steel arches being erected to protect the tunnel access.
first blast and
mucking out,
construction of
temporary steel
arches to protect
tunnel portal,
Queens Valley
Reservoir, Jersey,
UK.
