2410
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
highest
in situ
estimates of shear modulus. The seismic cone
provides estimates of G
ovh
and these values are lower than those
of the cross-hole testing. The Long and Donohue (2010) CPT
correlation (from equation 6) shows comparable variations in G
o
with depth and falls between the two other
in situ
test datasets.
However the Hardin and Black (1969) method based on OCR
and overburden (equation 7) shows much higher estimates of G
o
(despite using the lower values of OCR from Figure 3).
Figure 4. Small-strain shear modulus with depth.
Similar variations in the elastic shear moduli (G
ovh
and G
ohv
)
have been observed with
in situ
tests previously (Pennington et
al. 1997). These would be expected to be equal for a perfectly
cross-anisotropic material. However, the different travel times
may result from the averaging of the shear wave velocity
through layered strata (for vertical travel), compared to lateral
wave velocity through the stiffest layers. This leads to cross-
hole measurements tending to measure the stiffest layers, rather
than the average stiffness for SCPT measurements.
3 DISCUSSION
The design of gravity base foundations for onshore wind
turbines requires accurate estimates of strength parameters for
bearing capacity and stiffness parameters for displacements,
within at least 1B of the founding level. The adoption of the
most appropriate methods for site investigation to determine
these parameters is debated in the industry and various
published correlations are commonly used. Unfortunately many
of these correlations have been previously developed for
geologically young and relatively simple materials, and their
applicability beyond their original databases can be uncertain.
The different methods of determining the undrained shear
strengths show the crustal materials are quite strong,
particularly near the founding depth, with undrained shear
strength of up to 300 kPa reducing to 100 kPa at the crust base.
Given the relatively high c
v
(and permeability) and field vane
values, there is a possibility that the CPT and vane estimates
may be artefacts due to partial drainage. The crustal zone also
has fissuring related to drying/wetting and frost action, and field
shearbox tests on similar materials have indicated that bulk
strengths can reduce considerably, and therefore representative
values may be closer to 60-80 kPa (Lo, 1970). However,
whether crustal fissures and associated strength changes are
significant for such large shallow foundations is questionable.
Since overconsolidation ratio is often used as a component of
correlations to determine geotechnical parameters, accurate
estimation is important. Overconsolidation in tills is often
attributed to loads from the overlying ice, however if drainage is
inhibited, then only a small degree of consolidation will occur.
The measurement of preconsolidation pressure in tills using
laboratory testing has been found to be quite difficult due to the
high pressures often required to fully define compression curves
and the effects of sample disturbance (which lead to under-
estimation of
vp
’
). The difficulties with this process are evident
in the wide range of estimates for OCR shown in Figure 3.
Stiffness anisotropy is often evident in soils from
in situ
and
laboratory measurements. The data in Figure 4 shows the
general difficulties in choosing appropriate estimates of the
small-strain stiffness (G
o
). Indeed cross-anisotropy in till may
be difficult to justify, since sub-glacial shear and consolidation
could have effects on the anisotropy of the
in situ
stress and
fabric. Rocking stiffness (k) for circular surface loads (radius,
R) is estimated using equation 8, (DNV/Risø, 2002):
) 1(3
GR8 k
3
(8)
where
is Poisson
’
s ratio and G is the shear modulus
determined from the shear modulus ratio G/G
o
that corrects the
stiffness for degradation due to strain level (this is typically 0.25
for the presumed strain levels of 10
-3
for wind turbines).
Manufacturers recommend criteria for rocking stiffness to
ensure the natural frequency of the turbine remains above the
main excitation frequencies. The range of small-strain moduli in
Figure 4 indicate rocking stiffnesses from 50 to 170 GNm/rad,
which is in excess of typical requirements of 40 GNm/rad, but
still represents quite a significant range of stiffness.
4 CONCLUSIONS
There is currently little guidance for choosing cost effective site
investigation methods and interpreting the results for this type
of geotechnical structure on glacial tills in Ontario. It is
anticipated that the completion of this project will provide some
of the missing knowledge and insight required in this area.
5 ACKNOWLEDGEMENTS
We wish to acknowledge the support of NSERC, Golder
Associates, Michael Cookson, JJ Davis, and Paul Dawson.
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