2320
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
1.1
Project Development Phase
During the project development phase, the developer is
usually focused on wind resource assessment, land agreements,
power purchase agreements, and identifying potential investors.
The geotechnical aspect is secondary and is typically limited to
site visits and reporting of surficial characteristics such as
terrain topography, accessibility, proximity to bodies of water,
etc. The initial environmental permitting effort presents an
opportunity to identify geotechnical conditions that carry cost
implications as most environmental permitting efforts include
an evaluation of geo-environmental conditions. However, the
development phase of a wind project rarely includes
geotechnical field investigation activities. However, the
development phase is the most opportune time to identify
significant geotechnical risks. The findings of a preliminary
geotechnical investigation conducted during the development
phase rarely render the project non-pursuable. However,
preliminary geotechnical investigations are critical to proper
planning and allocation of risks to the appropriate stakeholders.
The achievement of the benefits of this proposed shift can be
formalized through techniques such as geostatistics, Bayesian
updating, statistical inference and neural networks (Christian et
al. 2006 and Lin and Hung 2011). An immediate benefit of a
more holistic development phase exploration is to focus the
detailed exploration effort on the critical issues or portions of
the project area. In addition to desk studies, recommended
development phase exploration techniques include:
Geophysical surveys using seismic methods such as Multi-
Analysis of Surface Waves (MASW) at all proposed turbine
locations, possibly excluding locations where rock is at the
surface. An MASW survey provides depth profiles of shear and
compression wave velocities. The information is used to gain an
insight into site stratigraphy and to estimate elastic moduli
needed to verify foundation stiffness requirements. The MASW
survey, conducted at the project development phase, helps to
identify soft locations or locations with potential difficulties as
an aid to micrositing of turbines. This exercise lessens the
likelihood of needing very large foundations or performing
costly ground modifications at soft sites. MASW surveys are
also quick and relatively low cost, making them the ideal
qualitative tool that is suited for the development phase.
Preliminary geotechnical exploration borings using a limited
number of traditional SPT, SCPT, CPT, or DMT borings spread
over different portions of the project area. Exploration pits may
also be used along planned access road alignments. Information
obtained through site visits and review of available published
information and online maps can be used to decide on the
locations of the preliminary borings so that the captured range
of variability is as wide as possible. Information from the
preliminary exploration is used to assess the type and range of
variability of site geomaterials, to identify potential foundation
types and to plan the full investigation. For example, if a
gravity base (shallow) foundation is deemed feasible, an effort
should made at the project development stage to assess the
depth range of the stationary groundwater table in order to
decide if buoyancy will be a design consideration. If soft
materials are encountered requiring consideration of deep
foundations, the depth of borings during the full investigation
can be adjusted.
The preliminary geotechnical exploration should also include
electrical and thermal resistivity testing as this input is critical to
sizing the electrical collection system which is associated with a
significant share of project cost.
1.2
Engineering Design Phase
During this phase, a full geotechnical investigation must be
carried out to finalize the design. The full geotechnical study is
designed to complete the investigation and to fill the gaps
remaining after the preliminary exploration. The full
investigation should confirm and refine the assessment of the
risks identified during the preliminary investigation and should
assess any additional risks that may be uncovered. At a
minimum, standard practice includes at least one exploratory
boring at every turbine location extending to a depth of interest
not less than the largest base dimension of the structure
(DNV/Risø 2002, GL 2010). For a typical shallow foundation
used to support wind turbines, the explored depth is 1 to 1.5
times the foundation diameter. Common current practice is to
perform geophysical testing during the full investigation phase
at a limited subset (approximately 10%) of turbine locations. In
the proposed redistribution of effort, a more extensive
geophysical survey is recommended at the development phase.
A non-exhaustive list of risks that should be assessed as early as
possible during the development and preliminary design phases
(but prior to the final design phase) is shown in Table 1.
Geotechnical exploration activities help in identifying these
risks but are not the sole resource.
Table 1. Non-exhaustive list of potential wind farm geotechnical and
eo-environmental risks (in no particular order).
g
No.
Risk
Identification tools
1 High groundwater
Drilling, excavation pits,
monitoring wells and
permeability testing.
2 Flooding, storm surge, tsunami
Records, maps
3 Shallow bedrock / blasting
Visual, drilling, MASW
4 Slope stability & landslides
Visual, geologic study
5 Mine subsidence
Records, LiDAR, maps
6 Coal seams
Drilling, records
7 Karst subsidence, caves & voids
Records, drilling,
LiDAR, maps, type of
underlying rock,
groundwater regime
8 Shrink/swell (expansive) soils
Laboratory testing
9 Frost heave
Records, climatic data
10 Permafrost
Records, climatic data
11 Freeze-thaw
Climatic records
12 Collapsible soils
Laboratory testing
13 Excessive consolidation / tilt
Laboratory testing
14 Aggressive environments: high
sulfates, high salinity, corrosion
Laboratory testing
15 Alkali-Silica Reaction (ASR)
Testing, local
information
16 Peat bogs and soft grounds
Visual, drilling, MASW
17 High seismicity / liquefaction
Exploration, design
codes
18 Hurricanes
Records, design codes
19 Volcanic activity
Records, geologic study
20 Scarcity of gravel / road base
Visual, local information
21 Buried pipelines & infrastructure
Records
22 Forest logging roads
Drilling, excavation pits
23 Drifting sands
Visual, local information
24 High soil electrical resistivity
Field and lab testing
25 High soil thermal resistivity
Field and lab testing
