Pile foundations for offshore wind turbines: Numerical and experimental investigations on the behaviour under short-term and long-term cyclic loading [Elektronische Ressource] / Pablo Cuéllar. Betreuer: Stavros Savidis

Pcuellar

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• Publié par: technische_universitat_berlin
• Publié le: 01 janvier 2011
• Nombre de lectures: 164
• Langue: English
• Poids de l'ouvrage: 16 Mo

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Pile Foundations for Offshore Wind Turbines: Numerical and
Experimental Investigations on the Behaviour under Short-
Term and Long-Term Cyclic Loading


von Pablo Cuéllar
*)Ingeniero de Caminos, Canales y Puertos
geboren am 16. Januar 1978 in Madrid


von der Fakultät VI - Planen Bauen Umwelt
der Technischen Universität Berlin
zur Erlangung des Akademischen Grades
Doktor der Ingenieurwissenschaften (Dr.-Ing.)
genehmigte Dissertation

Promotionsausschuß:
Vorsitzender: Prof. Dr.-Ing. Karsten Geißler
Gutachter: Prof. Dr.-Ing. Stavros Savidis Prof. Dr.-Ing. Werner Rücker
Gutachter: Prof. Dr.-Ing. Manuel Pastor

Tag der Einreichung: 15. Dezember 2010
Tag der Prüfung: 11. Februar 2011

Berlin 2011
D 83

*) Bauingenieur / Civil engineer
ABSTRACT

The behaviour of pile foundations for offshore wind turbines deviates from classical assumptions
and accumulated experience mainly due to their large diameter, reduced slenderness and elevated
ratio of lateral to vertical loads. The offshore environment poses the additional challenge of large
numbers of load cycles from wind and waves and the possible influence of transient changes of
pore water pressure around the pile, both of them issues that are still not well understood and
also not being contemplated in current design guidelines.
In saturated soils, short-term cyclic loading within the extreme regime often involves a pore-
pressure build up that eventually can lead to liquefaction phenomena and foundation failure. On
the other hand, the effects of the long-term cyclic loading on the foundation's stability and espe-
cially on its serviceability must also be studied.
The purpose of this work was to gain an insight into both aspects, while developing a practicable
numerical tool for a short-term prognosis and deriving useful criteria for design. To achieve such
goals, the investigations have been broadly divided into a first part with a theoretical orientation
for the analysis of short-term transient effects in the frame of the Finite Element Method, and a
second empirical block for the study of long-term phenomena by means of model tests in a re-
duced scale.
The general picture that arises from these investigations is that of a foundation subject to harden-
ing and soil densification in the long term, but also affected by transient episodes of significant
softening during the storms as a consequence of pore pressure accumulation.
Hence the importance of considering the coupling effects between soil stress and pore water
pressure in design, even for those cases where a soil liquefaction could in principle be disregarded
on the grounds of a high relative density of the soil.
For practical design purposes, it seems sensible to distinguish between piles in fully drained con-
ditions, in partially drained conditions and in undrained conditions, and analyse them correspon-
dingly employing different strategies. Some considerations in this respect have been given in the
final sections of this thesis.
i
KURZFASSUNG

Das Verhalten von Pfahlgründungen für Offshore-Windenergieanlagen weicht von den klassi-
schen Annahmen und gesammelten Erfahrungen vor allem wegen der großen Pfahldurchmesser,
geringeren Schlankheit und hohen Verhältnisse von lateralen zu vertikalen Lasten ab. Zudem
entsteht im Offshore-Bereich die zusätzliche Herausforderung einer großen Anzahl von Last-
wechseln aus Wind und Wellen und des möglichen Einflusses von vorübergehenden Verände-
rungen des Porenwasserdrucks um den Pfahl. Da in diesem Hinsicht noch viel Unkenntniss
herrscht, wurden beide Aspekte in den Bemessungsrichtlinien bisher nicht ausreichend in Be-
tracht gezogen.
Es ist aber bekannt, dass wassergesättigte Böden unter zyklischer Belastung oft einen Porenwas-
serdruckaufbau zeigen, der schließlich bis zur Bodenverflüssigung und einem Gründungsversa-
gen führen kann. Andererseits können grosse Lastspielzahlen erhebliche bleibende Verformun-
gen zur Folge haben, die die Gebrauchstauglichkeit der Gründung progressiv gefährden.
Wesentliche Ziele dieser Arbeit waren, einen Einblick in beide Aspekte zu gewinnen, ihre Folgen
aufzuzeigen und sinnvolle Kriterien für die Pfahlbemessung herzuleiten. Dabei wurde aber auch
ein praktikables numerisches Werkzeug entwickelt, um den Einfluss von Kurzzeit-
Sturmereignissen auf die Offshore-Pfähle prognostizieren zu können. Die Untersuchungen wur-
den deswegen in zwei Hauptrichtungen gegliedert: ein erster Teil, mit einer theoretischen Orien-
tierung, der der Analyse von kurzzeitigen Ereignisse anhand der Finite-Elemente-Methode ge-
widmet wurde, und ein zweiter empirischer Teil für die Betrachtung von Langzeitphänomene
anhand Modellversuchen in kleinem Maßstab.
In diesen Untersuchungen konnte gezeigt werden, wie sich der Boden an der Pfahlgründung
langfristig progressiv verfestigt (long-time hardening). Vorübergehend kann es aber auch zu einer
Entfestigung während der Sturmereignisse kommen (short-time transient softening). Sowohl die
abgeminderte Bettungssteifigkeit während der Entfestigung, als auch ihre kummulativen Effekte
im Sinne von bleibenden Verformungen, zeigen die Relevanz und Notwendigkeit der Berücksich-
tigung der Wechselwirkung zwischen Porenwasserdruck und Bodenverformung in der Pfahlbe-
messung. In der Summe werden die Phänomene und praktischen Folgen für die Bemessung der
Pfähle in dieser Dissertation hergeleitet.
ii
PROJECT FUNDING

BAM's doctoral program and the German Federal Ministry for the Environment, Nature Con-
servation and Nuclear Safety (BMU) have kindly provided the funding for these investigations,
which were carried out in the frame of the RAVE (research at Alpha Ventus) research program.


PERSONAL ACKNOWLEDGEMENTS

For the completion of this work I have enjoyed the assistance and support of many people, to all
of whom I am greatly indebted. First of all, my deepest gratitude is due to my academic advisor,
Prof. Manuel Pastor, for his enthusiasm, help and constant encouragement and for always mak-
ing everything so easy. With him there was never a problem that could not be solved.
For the past years I have also had the great privilege of working at BAM's department for Build-
ings and Structures under the direction of Prof. Werner Rücker who watched over my work and
granted me permanent support, assistance and understanding, especially during the final stages of
the doctoral endeavour. I am also very grateful to Prof. Stavros Savidis from the Technical Uni-
versity of Berlin for his interest on my work, his advice on academic matters and for supervising
this thesis. Finally, I am grateful to Prof. Karsten Geißler for assuming the chairmanship of the
doctoral committee and to Prof. Yuri Petryna for his disposition and gentle substitution at the
scientific defense.
Among my direct colleagues, I am particularly indebted to Pablo Mira and José Antonio
Fernández Merodo for their selfless support, advice and help. Together with Manolo Pastor, they
provided the sources of the GeHoMadrid Finite Element code and were a decisive help for deal-
ing with the intricacies of computational geomechanics. Fernando Pardo and the research group
within the Geotechnical Laboratory of CEDEX in Madrid have kindly hosted my "research-and-
training" stays at CEDEX.
For the experimental investigations I was able to rely on the experience and wisdom of Fred
Ziegler, Wilfried Wuttke and Hans-Joachim Peschke, who, along with many other individuals at
BAM (a.o. Margrit Kayser, Steven Georgi, Krassimire Karabeliov, Christian Knaust and all the
staff at BAM's division VII.2) made my work in Berlin much easier and particularly rewarding.
iii
Finally, my special appreciation is due to Matthias Baeßler, the leader of my work-group, who has
coordinated all of my German projects and has gradually guided me towards becoming an inde-
pendent researcher. His inquisitive style, sharp insight and critical judgement have been a perma-
nent stimulus and never precluded his unconditional assistance and friendship.
Last but not least, I want to acknowledge the marvellous support provided by my family. I cannot
thank them enough for their love and care. This dissertation, which is the product of their con-
stant encouragement and support, is dedicated to them.

Pablo Cuéllar. February 2011, Berlin.
iv
STUDENT: MEPHISTO: The threads flow on, unseen and subtle,
I have arrived quite recently Before we get into my views – Each blow effects a thousands ties.
And come, full of humility, What Department do you choose? The philosopher comes with analysis
To meet the giant intelect STUDENT: And proves it had to be like this:
Whom all refer to with respect I should like to be erudite, The first was so, the second so,
MEPHISTO: And from the earth to heaven’s height And hence the third and fourth was so,
This is a charming pleasantry. Know every law and every action: And were not the fir