2414
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
The analysis predicts liquefaction of the soil near the skirt
tips and build-up of pore pressure inside the caisson during the
second load parcel. Stress redistribution towards the baseplate
will cause an additional increase in pore pressure inside the
caisson. The abrupt increase at t = 2000s is due to the nonlinear
dependency of generated pore pressure on the CSR, which
increases at the start of the second load parcel. Much of the pore
pressure is dissipated by the end of the last load parcel, even
though cyclic loading continues (at 60% of the second parcel).
As the pore pressure dissipates, settlements due to the cyclic
loading are expected.
The discretization of cyclic loading in load parcels and
subsequently in subdivisions affects the accuracy of the
analysis, but the results seem to converge as the number of
subdivisions is increased. Where short drainage paths or high
CSR values are involved, sufficiently short steps are required.
The rate of pore pressure dissipation is affected by the length of
the skirts. Longer skirts result in slower dissipation and higher
potential for pore pressure accumulation inside the caisson.
5.2
Monopod
5.2.1
Model
The monopod caisson (20x10m) is subjected to three degree of
freedom loading, including a horizontal and moment load. A 3D
FE model of half the caisson is sufficient, taking advantage of
the plane of symmetry formed by the vertical and the direction
of aligned wind and wave loading. A six hour design storm,
consisting of 2160 waves in five load parcels, was adopted.
5.2.2
Results
The five load parcels are distinguishable in the pore pressure
response plotted in Figure 7 and peak pore pressure occurs right
after the peak of the storm. The permanent horizontal load due
to wind and/or current causes an asymmetric cyclic shearing in
the example, so the observed peak pore pressure (4 kPa) does
not occur on the center line. The consequences, such as
potential differential settlements and tilting of the turbine,
should be examined in a more advanced analysis.
Figure 7: Example of excess pore pressure history, monopod caisson
6 CONCLUSIONS AND FURTHER DEVELOPMENTS
A pore-pressure generation and dissipation model has been
developed to study the effect of cyclic loading on suction
caissons in sand. Example analyses have shown that the
proposed model can be successfully applied to the study of
suction caissons, both in 2D and in 3D. However, the model
needs further improvement to allow prediction of the complete
liquefaction behaviour, including settlements, of a caisson.
The model can be used to predict which areas are prone to
pore pressure build-up, estimate the rate of pore pressure build-
up and to some extent how fast this pore pressure is dissipated.
Analysis of the type presented here may be useful to assess
the geotechnical and structural risks related to cyclic loading of
caissons in sand such as:
•
reduction in caisson bearing capacity due to generated
pore pressures;
•
caisson foundation stiffness reductions;
•
pore pressure induced total and differential
settlements for offshore wind turbine structures;
•
analysis of the effect of scour on pore pressure
gradients.
The model can be improved to reflect more realistic soil
behaviour. As some zones underneath the suction caisson
liquefy, the load is transferred to other parts of the foundation.
This leads to secondary pore pressure increases which are not
yet considered in the presented model.
If sufficient soil data are available, the cyclic shear strength
curves could include dependency on the relative density and
initial shear stresses in the soil.
Finally, a large part of the vertical load on suction caissons is
taken by friction between the caisson skirts and the soil. A
systematic study of the influence on the liquefaction potential
would be interesting.
7 ACKNOWLEDGEMENTS
The work described in this paper was performed as a part of the
author’s master thesis (Versteele 2012), supervised by professor
Charlier (Université de Liège), whose guidance is gratefully
acknowledged. Development of the model and calculations
were performed at, and with support of Cathie Associates
SA/NV.
8 REFERENCES
Andersen K.H. and Berre T. 1999. Behaviour of a dense sand under
monotonic and cyclic loading.
Proceedings of the 12
th
ECSMGE
,
Vol 2, Geotechnical Engineering for Transportation Infrastructure,
667-676
Byrne B.W. and Houlsby G.T. 2003. Foundations for offshore wind
turbines.
Phil. Trans. R. Soc. Lond.,
Vol 361, 2909-2930
DNV 1992. Classification notes No 30.4 – foundations.
Det Norske
Veritas,
Norway
DNV 2011. Design of offshore wind turbine structures, Offshore
Standard DNV-OS-J101,
Det Norske Veritas
, Norway
EWEA 2011. Wind in our sails – The coming of Europe’s offshore
wind energy industry.
Houlsby G.T., Ibsen L.B. and Byrne B.W. 2005. Suction caissons for
wind turbines.
Proc. International Symposium on Frontiers in
Offshore Geotechnics (ISFOG)
, Taylor & Francis Group, Perth,
Australia
Lee K.L. and Focht J.A. 1975. Liquefaction potential at Ekofisk tank in
North Sea.
Journal of Geotechnical Engineering Division, ASCE,
101(GT1), 1-18
Rahman M.S, Seed H.B. and Booker J.R. 1977. Pore pressure
development under offshore gravity structures.
Journal of
Geotechnical Engineering Division, ASCE,
103(GT12), 1419-1436
Seed H.B. and Idriss I.M. 1980. On the importance of dissipation effects
in evaluating pore pressure changes due to cyclic loading.
International Symposium on Soils under Cyclic and Transient
Loading, Swansea
, 569-570
Senders M. 2008. Suction caissons in sand as tripod foundations for
offshore wind turbines.
Ph.D,
The University of Western Australia,
Australia
Taiebat H.A. 1999. Three dimensional liquefaction analysis of offshore
foundations.
Ph.D. Thesis
, The University of Sydney, Australia
Verruijt A. and Song E.X. 1991. Finite element analysis of pore
pressure build-up due to cyclic loading.
Deformation of soils and
displacement of structures, Proc. 10
th
European Conference on Soil
Mechanics and Foundation Engineering
, 277-280
Versteele H. 2012. Cyclic loading of suction caisson foundations for
offshore wind turbines.
M.Sc. Thesis,
Université de Liège, Belgium
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
0
1
2
3
4
5
6
Pore pressure [kPa]
time [h]
under baseplate,
center line
skirt tip level,
center line
skirt tip level,
underneath skirt
