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Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
geotechnical design information is available and project specific
geotechnical risks can be identified, as shown in Table 2. At this
point the design parameters have been selected based on
geological memoirs and nearby site investigations. The
undrained shear strength of the main clay unit has been used as
an example to show how the design inputs change as more
information is collected.
Table 2. Design undrained shear strength of main clay unit and primary
eotechnical risks following desk study
g
Design parameter
Geotechnical risks
125 to 350 kPa
No wind farm specific ground information
Presence of regional geohazards
Several foundation types possible
The design information and project risks are reviewed at this
stage. Referring to Figure 2, this level of design information and
risk may be acceptable to some clients, in which case the design
and construction risks will be managed with a conservative and
uncertain design, or passed onto a third party with large
financial consequences. However in this case, to confirm the
ground conditions and add confidence to the design, site
investigation works were specified.
4.3
Geophysical survey
A site wide sub-bottom geophysical survey was carried out.
The primary objectives were to: identify the geological units
present; identify any unexpected geological features; and,
collect information to allow preliminary geotechnical boreholes
to be positioned.
4.4
Preliminary geotechnical survey
A preliminary geotechnical survey consisting of seabed CPT
and composite CPT and sampling boreholes was carried out.
The primary objectives were to: validate the desk study and
geophysical survey; collect information for ground model
development including in situ testing and sampling in the
primary units; and, identify unexpected ground conditions and
risks.
Following completion of the preliminary geotechnical site
investigation, the results were interpreted and a ground model
produced.
4.5
Ground model development
The ground model was developed using a collaborative
geophysical and geotechnical approach. The engineering units
were identified and geotechnical parameters selected using the
CPT and laboratory testing results. Wind farm specific ground
information is now known, and the regional geohazards
understood, allowing preliminary design to commence. The
design parameters and risks (Table 3) have been refined,
resulting in a more efficient foundation.
Table 3. Design undrained shear strength of main clay unit and primary
geotechnical risks following preliminary site investigation and ground
odel development
m
Design parameter
Geotechnical risks
Low: 145 kPa
Local variation in site geology
Mean: 240 kPa
Advanced design parameters unknown
High: 325 kPa
4.6
Design team and stakeholder review
The confidence and expected accuracy of the geotechnical
interpretation and risks were reviewed by the design team and
stakeholders. It was decided that there was sufficient knowledge
of the site to progress the design and construction planning and
manage the geotechnical risks.
Some of the project infrastructure will be designed and
developed by others. There is now sufficient knowledge of the
site to be able to define a contract ground model baseline, and
enable the risk to third parties of unknown ground conditions to
be appropriately handled.
4.7
Future geotechnical survey
The residual project risks are not acceptable for the finalisation
of detailed design. To obtain this information, a small number
of sampling boreholes (to obtain samples for advanced
laboratory testing) complemented by one CPT to the expected
foundation depth (to identify local variability and confirm the
units present) will be carried out. Subsequent investigation may
be required depending on the results of this investigation and
the developer’s acceptable risk.
5 CONCLUSIONS
Offshore wind projects in their early stages present considerable
challenges to engineers managing the geotechnical hazards
given the large complexity, cost and time scale of the projects.
A review of the site investigations undertaken for various
projects highlights repeated mistakes leading to inefficient site
investigations; namely the site investigations not being planned
to mitigate project specific geotechnical hazards and surveys not
being specified by the foundation designers.
Lessons must be learnt from the wider construction industry,
including: the need for formal approaches to the management of
geotechnical risk; acceptance that a staged site investigation will
deliver the most cost effective results; and, considering a
contracting structure that brings the detailed designer onto the
project from a very early stage.
The authors have proposed an engineer led approach for
managing geotechnical risks, where the design team and
stakeholders are actively involved in assessing and mitigating
the geotechnical risks, allowing the geotechnical foundation
design and site investigation to develop with the project and
ensuring that survey work is specified to directly control project
risks.
These principles are being applied to the first project of the
Hornsea Round 3 development area, where design and
construction risks are being mitigated early with a relatively
small amount of site investigation works.
6 REFERENCES
BS EN 1997-2:2007.
Eurocode 7 Geotechnical design – Part 2: Ground
Investigation and testing
Clayton C.R.I. 2001.
Managing geotechnical risk: improving
productivity in UK building and construction.
Institution of Civil
Engineers and Thomas Telford, London.
Det Norske Veritas AS. 2011.
Offshore Standard DNV-OS-J101 Design
of Offshore Wind Turbine Structures.
Det Norske Veritas, Oslo.
Liingaard M.A. Mygind M. Thomas S. Clare M. and Pickles A. 2012
Evidence of tertiary intrusive rock at the West of Duddon sands
offshore wind farm.
Society for Underwater Technology
Proceedings of the 7th International Conference, London 2012,
145-152.
Muir Wood A.M. 2001.
Tunnelling: management by design.
E&FN
Spon, London.
