Geology Reference
In-Depth Information
Figure 6.16 (a)
Convergence in
rock tunnel to
stable condition.
(b) Local failure
and ravelling to
ground surface.
a)
b)
Rock mass
dilates -
strengthening
Self stablisation of interlocking
rock mass with no specific
failure mechanism
Specific failure mechansim.
In worse case rock mass may
'unravel' back to the surface -
called 'chimneying'
If no self stablisation then may
need to reinforce with bolts,
shotcrete, steel arches or lattice
girders
Local support to prevent
failure - bolts and anchors
After blasting, and dust and gases have dissipated and safety checks
made (e.g. for methane or radon), the broken rock is mucked out and it is
the engineering geologist
s task to examine and map the geological
conditions exposed. The freshly blasted rock may well be unstable, and
the geologist should not approach the face until the contractor has
carried out all necessary scaling and/or rock support work to make the
tunnel safe. The contractor has overall responsibility for site safety and
his instructions should be followed at all times in this respect. A decision
will then be taken on whether the ground is as expected, if the ground
is changing (and probing ahead is required), and the support
requirements. Any potential for deteriorating conditions or, for example,
a major potential wedge failure, need to be identi
'
ed quickly so that
support measures can be taken. As illustrated in Figure 6.16, often the
rock mass is self-supporting. As the tunnel is excavated, the tunnel walls
move inwards, the rock mass dilates and generally locks up. If there is an
inherent weakness, such as a free wedge of rock or a fault zone, then local
collapse can be followed by ravelling failure, which could chimney to the
ground surface. In two of the examples discussed in Chapter 7, the
situation deteriorated quickly. If conditions are poor and getting worse,
then the ground might be supported in advance of the tunnel by an
umbrella of spiles or canopy tubes, and/or by pressure grouting.
In suitable rock, other mining approaches may be used, including the
use of large roadheaders that cut their way into the rock but do not
excavate the full face pro
le in one operation, unlike a TBM. In a tunnel
formed by drill and blast or roadheader, it is possible to examine and
record the ground conditions throughout construction and make deci-
sions as to the support required. In a TBM tunnel, little can be told about
 
 
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