of the claims against consultants were directly related to the
geotechnical investigation.
What can be done to bring these numbers down? Conduct
more borehole tests? Or maybe it just makes more sense to
do an effective site investigation in the first place?
Geotechnical unknowns usually exist in reverse correlation
to the scale and quality of the investigation. We are always
asked by project managers, “What is the reduction in risk for
each dollar I spend on a geotechnical investigation?”
Unfortunately, the correlation between the scale of
geotechnical investigation (i.e. the number of boreholes) and
risk is not a universal one. Different geological regions have
different soil deposits/property variability. Even for a site or
region with natural variations of geotechnical conditions,
different construction methods have different sensitivities to
the geotechnical condition. Risk is the effect of the threats,
not the threat itself. Therefore, the project affects a difference
on the risk level and hence a difference on the required level
of geotechnical investigation. For example, an underground
tunnel excavation is more sensitive than an open trench
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installation. Subsequently, one conducts more boreholes for
a tunnel as compared to an open trench excavation.
No single geotechnical investigation can function universally,
but here is the good news: it is possible to arrive at an upper
bound for necessary geotechnical work!
This idea started by studying cost overruns in tunnelling
projects. The study shows an upper bound correlation between
the ratio of total length of boreholes over tunnel length as
compared to the cost overruns. The common median ratio
of Borehole:Tunnel Lengths is 0.42 and there are tunnel
projects with 50 per cent, or more, cost overruns when the
scale of geotechnical investigation is this low and at this
median BH:Tunnel Length ratio. The same data suggests
that if the number of the boreholes are increased to a ratio
of 1 (Borehole:Tunnel Length), the upper estimate of possible
cost overrun can be reduced to a more reasonable estimate
of 15 to 20 per cent. The study also suggests there is not much
benefit going beyond a Borehole:Tunnel Length ratio of 1.5.
Generally, we prefer to have this type of information in the
context of the investigation cost. For major infrastructure
projects, it is recommended to budget, for all phases of the
geotechnical investigation, costs ranging from 1.5 to 2.2 per
cent of the construction cost and have a contingency up to
three per cent of construction cost ( for complex conditions).
For low-rise buildings, that number is as low as 0.2 to 0.4 per
cent of construction costs. It is safe to say all construction
projects generally sit somewhere between a low-rise building
and a major infrastructure project.
The cost of an investigation is not always an accurate
indication of its effectiveness. Appropriate choice of
investigation methods, research on the project area (other
construction, existing geotechnical information), geological
modelling, quality of the investigation and a proper engineering
assessment all play a key role in conducting a truly effective
investigation. By employing these principles the geotechnical
risk can be greatly reduced while spending the same level of
money on the investigation. Don’t just spend money, do the
right investigation!
Masoud Manzari is a senior geotechnical and hydrogeological
engineer with over 23 years of experience and has been involved
in a wide range of civil engineering projects, specifically for
structures built on challenging soil sites.
Mark Tigchelaar is president and founder of GeoSolv Design/
Build Inc. Tigchelaar is a licensed engineer in Ontario and
Alberta, the chair of the industry Advisory Board of York
University and the past chair of the Canadian Geotechnical
Society-Southern Ontario section.
TECHNICAL
Risk is the effect of the threats, not the threat itself.
Therefore, the project affects a difference on the risk
level and hence a difference on the required level of
geotechnical investigation.
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