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th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
entire soil mass. Thus, in designing a shallow foundation for a
wind turbine, for example, traditional practice assumes an
infinite horizontal correlation length where a single value is
assumed for the soil in each layer. Furthermore, while focus is
on variation in the vertical direction, geotechnical exploration
rarely goes beyond one boring at the center of the foundation
unless there is strong reason to believe conditions are non-
uniform in the lateral directions, such as in cavitose terrain.
Thus, knowledge of the vertical spatial variation is often limited
to the line of the boring. On the other hand, knowledge in the
horizontal direction is limited to the observation and verification
of the exposed foundation bearing surface. This is very limited
information but standard practice. This is also why at least two
forms of exploration should be carried out at each turbine
location: a traditional boring and a seismic survey (MASW).
2.2
Uncertainty Caused by Measurement Error
Measurement uncertainty is related to the equipment being
used, in-situ or laboratory test procedures, and random data
scatter. Naturally, measurement error is different for different
test procedures. Reported measurement error data have been
summarized for various laboratory and field tests by various
investigators (e.g. Phoon and Kulhawy 1999). It is worthwhile
to note that the highest variability attributed to in-situ test
measurement error is that corresponding to the Standard
Penetration Test (SPT). The error introduced by sample size is
sometimes considered as a measurement error. Normally, the
greater the number of data points or sample size, the smaller the
error. However, beyond a rather low number of samples, it is
more important to capture the full range of variability than to
obtain more data points. There are numerous simplified rules to
estimate standard deviation and variability based on the range
and number of samples (Tippett 1925, Withiam et al. 1997,
Whitman 2000 and Foye et al. 2006). For this reason, the effort
to capture the full range of variability as early as possible is
very important to the early assessment of risks.
2.3
Uncertainty Caused by Transformation Error
Transformation or model errors are introduced when test
measurements are used to calculate the desired design properties
using empirical or theoretical relationships. The sources of the
error include the fitting errors in the case of empirical equations
and the simplification/idealization errors in the case of
theoretical relationships. The transformation errors for several
design properties (undrained shear strength, effective stress
friction angle, Young’s modulus, horizontal stress coefficient,
etc.) have been compiled (e.g. Phoon and Kulhawy 1999) for
various laboratory and in-situ test methods. Noteworthy remarks
from these compilations include:
Higher variability (as expressed in higher coefficients of
variation) result for sand properties obtained though
correlations with SPT blow counts, especially if
“universal” empirical relationships are used; i.e.,
relationship not calibrated to a specific geology. Hence,
“local knowledge” seems to be important for interpretation
of SPT results.
Higher variability is typically obtained for sand properties
than for clay properties.
3 CONCLUSIONS
Wind energy projects are almost always developed and built
under compressed schedules where project realization phases
overlap. They also cover large terrains that involve wide
variability of geotechnical and geo-environmental conditions.
For these reasons, geotechnical risks must be addressed as early
as possible during the development phase to avoid overlooking
fatal hazards that can shelve or financially devastate the project.
This paper proposes to conduct extensive, low cost and quick
geophysical surveys during the development phase to help with
turbine micrositing and to gain an insight into the variability of
the entire project area. The paper lists potential hazards that
should be assessed and discusses sources of geotechnical
uncertainty and how they relate to wind energy projects.
4 ACKNOWLEDGEMENTS
The author would like to acknowledge his employer,
Renewable Energy Systems Americas Inc, for support in the
preparation and presentation of this paper.
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