Geology Reference
In-Depth Information
4.3.3.4 Discussion
It is evident that site investigation cannot provide a fully detailed
picture of the ground conditions to be faced. This is particularly true
for tunnelling, because of the length of ground to be traversed, the
volume of rock to be excavated and often the nature of the terrain,
which prevents boreholes being put down to tunnel level or makes
their cost unjusti
able. Instead, reliance must be placed on engineering
geological interpretation of available information, prediction on the
basis of known geological relationships and careful interpolation and
extrapolation of data by experienced practitioners. Factors crucial to
the success of the operation, need to be judged and consideration given
to the question: what if? It is generally too late to introduce major
changes to the methods of working, support measures, etc. at the
construction stage, without serious cost implications.
Site investigation must be targeted at establishing those factors that
are important to the project and not to waste money and time inves-
tigating and testing aspects that can be readily estimated to an accep-
table level or aspects that are simply irrelevant. This requires a careful
review of geotechnical hazards, as advocated above. Even then, one
must remain wary of the unknowns and consider ways in which
residual risks can be investigated further and mitigated, perhaps
during construction, as addressed in
Chapter 6.
There is a somewhat unhealthy belief that standardisation (for
example, using British Standards, Eurocodes, Geoguides and ISRM
Standard Methods) will provide protection against ground condition
hazards. Whilst most standards certainly encompass and encourage
good practice, they often do so in a generic way that may not always be
appropriate to the project at hand and they may not provide speci
c
advice for coping with a particular situation. Ground investigations are
often designed on the basis of some kind of norm
ts-all
approach to ground investigation. It is imagined that a certain number
of boreholes and tests will suf
-
a one-size-
ce for a particular project, essentially
irrespective of the actual ground conditions at the site. This ignores the
fact that ground investigations of average scope are probably unneces-
sary for many sites but will fail to identify the actual ground condition
hazards at rare, but less forgiving sites. Similarly, an averaging-type
approach will mean that many irrelevant and unnecessary samples are
taken and tested whilst the most important aspects of a site are perhaps
missed or poorly appreciated. This is, unfortunately, commonplace.
If the hazards are considered in a systematic way, as discussed
earlier, then the risks can be thought through fully and this will
help the ground investigation to be better focused. The process is
illustrated in
Box 4-5
for a hydroelectric scheme involving the
construction of a dam, reservoir, power station and associated
infrastructure.
