Tunnelling risk - Practical Engineering Geology - page 395

Table AE-1.1

LIKELIHOOD CONSEQUENCE

TITLE DESCRIPTION SCALE TITLE DESCRIPTION SCALE

Catastrophic Total loss of a section

of tunnel

5

Likely to occur during
the construction of the
tunnel, possibly on
more than one
occasion

Major damage or
delay to tunnel or
major
environmental
impact affecting
programme

Probable 3

Critical 4

Some damage or
delay to tunnel or
some
environmental
impact affecting
programme

Serious 3

Likely to occur at least
once during
construction
of the tunnel

Occasional 2

A routine
maintenance repair
to tunnel or minor
hindrance

Marginal 2

Unlikely to occur
during construction of
the tunnel

Remote 1

Of little consequence
to programme

Negligible

1

Risk assessment
The assessment of risks associated with the use of a shielded
TBM was carried out separately from that for drill and blast
excavation.
Table AE-1.3 provides a brief summary of the hazards identi ed for

a drill and blast option with level of probability of occurrence and
likely consequence and hence risk. Possible ways of mitigating the risk
are identi

ed. The original reports
go into far more detail of the nature of hazards, ways to mitigate
them and the potential ways those mitigation measures might prove
unsuccessful leaving residual risks. An example of a summary hazard
and risk register for this project (TBMoption) is given at Appendix E-2.

ed and the residual likely risk identi

Conclusions
It was found that the number of hazards and residual risks associated
with a shielded TBM construction would be greater than for the drill
and blast method. The principal reasons were:

the relative in exibility of mechanised excavation and lining sys-

tems to deal with conditions for which they may not have been
speci

-

cally designed.