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Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
surveys; early CPT refusal and lack of boreholes on location
meant that insufficient information was collected at many
locations; and, the advanced cyclic testing undertaken was not
optimised for the design load being applied to the soil.
The above issues meant that stratigraphy was not certain for
the full foundation depth leading to very conservative design
and uncertain driveability.
2.1.2
Project 2 – value ~€0.3bn
The site investigation comprised a site wide sub-bottom
geophysical survey with several rounds of geotechnical works
comprising CPTs and boreholes.
The primary issue was the detailed designer being appointed
after the site investigation was specified. The boreholes were
not deep enough and insufficient information was gathered
about the bedrock.
The developer had to commission another round of
geotechnical investigation to gather further information for
design and risk mitigation, which resulted in project delays and
additional costs.
2.1.3
Project 3 – value ~€1.5bn
The site investigation comprised a site wide sub-bottom
geophysical survey with staged geotechnical works comprising
CPTs and boreholes. Extensive advanced lab testing and down-
hole geophysical methods and pressuremeter tests were
undertaken.
Again, the detailed designer was appointed after site
investigation had been undertaken, resulting in an insufficient
number of boreholes to allow an efficient design.
2.1.4
Project 4 – value ~€0.1bn
The site investigation comprised a site wide sub-bottom
geophysical survey with staged geotechnical works comprising
CPTs and boreholes.
The specification for the surveys was directly copied by the
client from one of their previous projects in very different
geology, and the detailed designer was appointed after the site
investigations had been undertaken. This resulted in the
geophysical techniques not being suitable for the geology and
the developer had to repeat the geophysical survey to an
enhanced specification.
2.2
Mediocre practice
2.2.1
Project 5 – value ~€1bn
The site investigation comprised sub-bottom geophysical survey
with one round of geotechnical investigation comprising full
depth CPTs and boreholes.
The geophysics was incorrectly scoped, so when planning
the geotechnical campaign, the detailed designer could not
utilise the geophysics. The project programme was also
compressed, meaning the geotechnical campaign was carried
out in one stage.
More value, cost savings and mitigation of geotechnical risks
could have been achieved with a collaborative geophysical and
geotechnical interpretation and a staged geotechnical campaign
with ground model development.
2.2.2
Project 6 – value ~€1bn
The site investigation comprised sub-bottom geophysical survey
with one round of geotechnical investigation comprising full
depth CPTs and boreholes with geophysical borehole logging.
Again, the geotechnical campaign was carried out in one
stage with the scope being developed whilst offshore. A more
cost effective survey could have been achieved with a staged
geotechnical campaign with ground model development.
2.3
Good practice
2.3.1
Project 7 – value ~€2bn
The site investigation comprised sub-bottom geophysical survey
with multi-stage geotechnical investigation comprising CPTs
and boreholes. Extensive advanced laboratory testing was
undertaken.
The foundation designer was involved in the specification of
the surveys, and a ground model was developed based on
collaborative geophysical and geotechnical interpretations. The
geotechnical investigations were specified considering the
confidence in the ground model and the foundation design.
2.3.2
Project 8 – value €1.5bn
The site investigation comprised sub-bottom geophysical survey
with multi-stage geotechnical investigation comprising CPTs
and boreholes.
The early geophysics identified a considerable geological
hazard which was successfully mitigated (Liingaard et al. 2012)
through ground model development based on collaborative
geophysical and geotechnical interpretations. A multi-stage
geotechnical investigation allowed the detailed designer to
develop a specific design method and scope the investigations
to verify the variations from design codes.
3 SUGGESTED APPROACH
The above projects highlight the issues regarding quality and
more importantly the effectiveness of the geotechnical
investigations. There are common problems with scoping and
management of site investigations. The offshore wind industry
should look to the experience gained on projects in other fields
of engineering, especially the lessons learnt by large onshore
infrastructure projects. In particular they should: consider more
formal approaches to the management of geotechnical risk;
accept that a staged site investigation will deliver the most cost
effective results; and, consider a contracting structure that
brings the detailed designer onto the project from a very early
stage.
3.1
Management of geotechnical risk
Managing ground risks solely through the traditional practice of
thorough site investigation will not lead to the most cost
effective project. Simply undertaking the most detailed site
investigation that the budget will pay for leads to investigations
being undertaken that are not specifically targeting the
unknowns that are truly affecting the project, and also does not
allow for the fact that the acceptable level of risk at construction
is very different depending on the project developer and their
attitude towards risk.
Developers should therefore seek to apply a more formal
approach of managing geotechnical risk so that the site
investigation specifically targets the unique project hazards.
Clayton (Clayton, 2001) suggests formal processes for
managing geotechnical risk, these have been adapted to form
the recommended process detailed in Figure 1.
Figure 1 shows a design team led approach to planning,
undertaking and reviewing effective site investigations. The
design team identifies all hazards affecting the project (design,
installation, operation and decommissioning). An assessment is
made of the significance of those hazards in consultation with
all project stakeholders. The site studies/investigations are
planned to specifically target the hazards. The results are
studied and the residual hazards reviewed. The consultation is
repeated with the stakeholders and the need for further
investigation evaluated.
