Geology Reference
In-Depth Information
The scope of site investigation is set out in
Box 4-1.
This should include
everything relevant to use of the site, including site history and long-term
environmental hazards and not just geology. All authorities (e.g. AGS,
2006) agree that site investigation should, ideally, be carried out in
stages, each building on the information gained at the previous stage,
as outlined in
Box 4-2.
A preliminary engineering geological model
should be developed for the site fromdesk study and
eld reconnaissance,
as outlined in
Chapter 3.
That model should then be used to consider
the project constraints and optimisation (e.g. the likely need for deep
foundations or the best location for a dam) and for designing the
rst
phase of GI. For a large project, this
first phase is usually carried
out during the conceptual phase. Further GI campaigns might be carried
out for basic design, for detailed design and often additional works
during construction. Engineering geologists should readily appreciate
that all sites do not require the same level of ground investigation. Some
have simple ground conditions, others more complex. At some locations,
existing exposures will allow the broad geology to be assessed and reduce
the need for GI. Projects may be situated in areas where the geology and
ground conditions are already well understood. For example, if designing
piles in London Clay, because of the wealth of published data and
little
should be needed in the way of testing to determine parameters for design.
Taking this further, experience shows that the majority of sites world-
wide do not have any particularly inherently hazardous conditions and
might be categorised as forgiving. Even with no, or no competent
investigation, the project is often completed without geotechnical diffi-
-
-
-
culty. Such sites need little investigation
enough to establish that there
are no particularly adverse hazards. In a review of the scope of ground
investigations for foundation projects in the UK, Egan (2008) found
that GI was either not conducted or was lacking borehole plans for
30% out of 221 projects, but he reported no adverse consequences. In
other words, the engineers took a risk, perhaps on the basis of previous
experience in an area, and apparently got away with it, although, as
Egan points out, a ground investigation might have allowed more
cost-effective solutions. Unfortunately, the world also has relatively
rare unforgiving sites with inherently dif
cult geotechnical conditions
that need careful and insightful investigation if problems are to be
1
It does not follow that London Clay is without hazards for construction projects, for
example, the Heathrow Express Tunnel collapsed during construction, as discussed in
Chapter 7.
De Freitas (2009) also provides a warning over geological variation
through the London Clay stratum and argues that data banks of geotechnical properties
need to be used with care from one area to another.
