Geology Reference
In-Depth Information
Typically, the cost of a site investigation is only a small part of the
overall project cost (less than a few percent), yet clients often require
some persuasion that the money will be well spent and might
be especially reluctant to allow a staged approach because of the
impact on programme. He might be reluctant to allow thinking and
planning time as the GI data are received and especially unwilling to
pay for a revised design as the ground models are developed and
re
ned. Sometimes the engineer might adopt a fast-track approach
whereby GI, design and construction are carried out concurrently,
although this approach carries the risk that information gained later
might impact on earlier parts of the design and even on constructed
parts of the works. The programming can sometimes go awry, as on
a site in Algeria where the author was trying to set out locations for
drilling rigs in the same area as a contractor was preparing to construct
foundations which obviously did not make sense. It turned out
that design engineers had made assumptions about the ground condi-
tions without waiting for GI, thinking that surface footings would
be adequate. This proved incorrect and the design needed complete
revision. In a similar manner to fast tracking, an observational
approach is sometimes adopted, especially for tunnelling, whereby
ground conditions are predicted, often on rather sparse data, and
provisions made for change if and when ground conditions turn
out to be different from those anticipated (Powderham, 1994).
The observational method often relies on instrumentation of ground
movements, measured loads in structural members, or water levels,
whereby performance is checked against predictions. This can go
seriously wrong where the ground behaves outside predictions
-
perhaps because the geological model is fundamentally incorrect or
because instrument systems fail or are not reacted to quickly enough.
Examples where instruments were not reacted to early enough include
the Heathrow Express Tunnel (Muir Wood, 2000) and the Nicholl
Highway collapse in Singapore (Hight, 2009); these are described in
some detail in
Chapter 7.
An observational approach should also
generally be adopted for rock slope construction, although it is
seldom referred to as such. Basically, it is very dif
cult to characterise
the complete rock fracture network from a few boreholes and therefore
it is very important to check any design assumptions during construc-
tion and to be prepared to come up with different solutions for stabi-
lisation as the rock is exposed and structures identi
ed and mapped
(see
Box 1-1).
A large part of any site investigation budget will generally be taken up
in sub-surface investigation and characterisation of
the ground
