Modelling of excess pore pressure accumulation in sand around cyclically loaded foundations

Publikation: Qualifikations-/Studienabschlussarbeit › Dissertation

Autoren

  • Jann-Eike Sören Saathoff

Organisationseinheiten

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Details

OriginalspracheEnglisch
QualifikationDoktor der Ingenieurwissenschaften
Gradverleihende Hochschule
Betreut von
  • Martin Achmus, Betreuer*in
Datum der Verleihung des Grades15 Dez. 2022
ErscheinungsortHannover
PublikationsstatusVeröffentlicht - 2023

Abstract

Insbesondere bei Sturmereignissen kann es im Boden an zyklisch belasteten Offshore-Fundamenten zu einer Akkumulation von Porenwasserüberdrücken kommen. Der Porenwasserüberdruck reduziert die effektiven Spannungen im Boden und kann daher die strukturelle Integrität negativ beeinflussen, indem dieser die Boden-Bauwerk-Interaktion zusätzlich beeinträchtigt. Insbesondere für Offshore-Windenergieanlagen ist eine genaue Abschätzung von Verformungen von großer Bedeutung. Obwohl die Berücksichtigung dieses Degradationseffekts auf die Tragfähigkeit von den beteiligten Zertifizierungs- oder Genehmigungsstellen gefordert wird, existiert derzeit keine allgemein anwendbare und akzeptierte Methode für den rechnerischen Nachweis. In den vergangenen Jahrzehnten untersuchten mehrere Forschende die zyklische Porenwasserüberdruckakkumulation, die sich um Offshore-Windenergieanlagen aufgrund von zyklischen Belastungen aufbaut. Sie versuchten, den Verlust der Tragfähigkeit und die Akkumulation der plastischen Rotation zu quantifizieren. Auch wenn einige Konzepte existieren, so wird keines als allgemeingültige Methodik angesehen. In dieser Arbeit wird eine neue numerische Methode – die sogenannte Excess Pore Pressure Estimation Methode (EPPE) – vorgestellt, die es erlaubt, das in zyklischen Einfachscherversuchen ermittelte Bodenverhalten auf das Tragverhalten des gesamten Fundaments zu übertragen. Dabei berücksichtigt das numerische Modell die zyklische Porenwasserüberdruckakkumulation unter Verwendung der element-spezifischen mittleren Spannung und Spannungsamplitude sowie der äquivalenten Zyklenzahl. Die Simulation des Porenwasserüberdruckaufbaus infolge bestimmter zyklischer Beanspruchungen basiert auf undrainierten Bedingungen, d.h. der Porenwasserüberdruckaufbau infolge bestimmter zyklischer Beanspruchungen wird unter Vernachlässigung des gleichzeitigen Konsolidierungsprozesses abgeleitet. Die Übertragung von Laborergebnissen auf Elemente innerhalb des Finite-Elemente-Modells in Form von Konturdiagrammen ermöglicht die Berücksichtigung von standortspezifischen zyklischen Einfachscher- und Triaxialversuchsergebnissen. Jeder Integrationspunkt wird individuell auf der Grundlage von last- oder weggesteuerten zyklischen Laborversuchsergebnissen ausgewertet. Die gesamte Porenwasserüberdruckakkumulation während eines Sturmereignisses, wird dann für einen bestimmten Bemessungssturm ermittelt. Auf Grundlage des abgeleiteten Porenwasserüberdruckfeldes wird im zweiten Schritt eine Konsolidierungsanalyse durchgeführt. Als Ergebnis der Analyse werden elementbasierte Porenwasserdruckabbaukurven abgeleitet. Der Verlauf des akkumulierten Porenwasserüberdrucks bis hin zum Ende des Sturms (oder des zyklischen Belastungsereignisses) wird durch analytische Superposition ermittelt. Für ein tiefgehendes Verständnis des zyklischen Bodenverhaltens wird das zyklische Antwortverhalten in verschiedenen Laborgeräten bei unterschiedlichen Lagerungsdichten und unter verschiedenen Spannungszuständen untersucht. Ein Konturansatz, der auf last- und verschiebungsgesteuerten Versuchsergebnissen basiert, wird abgeleitet. Um die Elementantwort aus numerischer Sicht zu untersuchen, wurde auch ein implizites Modell kalibriert. Die Ergebnisse werden im Detail erläutert. Anschließend werden verschiedene explizite Ansätze vorgestellt und hinsichtlich ihres Abschätzungsverhaltens der zyklischen Porenwasserüberdruckerzeugung, ihrer prognostizierten Gründungskapazität und ihrer Modellannahmen verglichen. Damit ist beabsichtigt, bestehende Ansätze und deren Anwendbarkeit in einer umfassenden Gesamtstudie zu untersuchen. Es wird ein generisches und modulares, explizites Modell vorgestellt, das leicht mit fachspezifischem Sachverstand bewertet werden kann. Die verschiedenen Berechnungsschritte können nach Bedarf durch weitere Schritte ergänzt werden. Im Rahmen dieser Arbeit werden Ergebnisse aus zyklischen Laborversuchen für einen beispielhaften Nordseesand vorgestellt und auf eine Referenz-Monopile-Gründung innerhalb eines vordefinierten Sturmereignisses angewendet. Der EPPE-Ansatz hilft bei der Quantifizierung des Verflüssigungsrisikos und der Ermittlung eines angemessenen Sicherheitsniveaus. Mit der aktuellen Methodik ist es möglich, das Degradationspotenzial für verschiedene Standorte einfach und schnell zu bewerten.

Zitieren

Modelling of excess pore pressure accumulation in sand around cyclically loaded foundations. / Saathoff, Jann-Eike Sören.
Hannover, 2023. 259 S.

Publikation: Qualifikations-/Studienabschlussarbeit › Dissertation

Saathoff, J-ES 2023, 'Modelling of excess pore pressure accumulation in sand around cyclically loaded foundations', Doktor der Ingenieurwissenschaften, Gottfried Wilhelm Leibniz Universität Hannover, Hannover. https://doi.org/10.15488/13232
Saathoff, J.-E. S. (2023). Modelling of excess pore pressure accumulation in sand around cyclically loaded foundations. [Dissertation, Gottfried Wilhelm Leibniz Universität Hannover]. https://doi.org/10.15488/13232
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@phdthesis{e02ea296851942649debd4abcba115e7,
title = "Modelling of excess pore pressure accumulation in sand around cyclically loaded foundations",
abstract = "In particular during storm events an accumulation of excess pore pressures may occur in the soil around cyclically loaded offshore foundations. The excess pore pressure build-up reduces the effective stresses in the soil and, hence, may negatively affect the structural integrity by influencing the soil-structure interaction. Besides a loss in bearing capacity, large plastic deformations may occur to the structure. Especially for offshore wind turbines an accurate estimation of such deformations is of great importance. Even though the consideration of this degradation effect on the bearing capacity is commonly demanded by the involved certification or approval bodies, no general applicable and accepted method for the calculative verification currently exists. Over the past decades several researchers investigated the excess pore pressure build-up around offshore foundations due to environmental cyclic loads. They tried to capture the loss of bearing capacity, the accumulation of plastic rotation and the essential influence on the serviceability limit state and fatigue design. However, even if there are some sophisticated concepts, none of them is seen as the simple general applicable choice. Within this thesis a new numerical method – termed Excess Pore Pressure Estimation method (EPPE) – is presented in great detail. This method allows for the transfer of the soil behaviour obtained in cyclic simple shear tests to the bearing behaviour of the entire foundation. Herein, the numerical model accounts for the cyclic excess pore pressure accumulation by respecting the element-based mean stress and stress amplitude as well as an equivalent number of load cycles. The simulation of the excess pore pressure build-up due to certain cyclic loading is based on undrained conditions, i.e. the excess pore pressure build-up due to cyclic loading is derived by disregarding the simultaneous consolidation process. The respected transfer method, in the form of contour plots, enables the consideration of site-specific cyclic direct simple shear and triaxial test results from laboratory devices to elements within the finite element model. Each integration point is evaluated individually. Based on the derived excess pore pressure field, a consolidation analysis takes place in the second step. The actual accumulated excess pore pressure in each element at the end of the storm (or cyclic loading event) is then found by analytically superposing the excess pore pressure decay curves from the consolidation analysis. For a deeper understanding of cyclic soil behaviour, the cyclic response in different laboratory devices with different densities and under varying stress states was investigated by the author. A contour approach based on cyclic load- and displacement-controlled test results is derived to study the element response from the numerical point of view and use these for the calibration of an implicit model. Moreover, different explicit approaches are presented and compared in terms of their estimation behaviour of cyclic excess pore pressure generation, their predicted foundation capacity and their model assumptions. The intention is hence to examine existing approaches and their applicability by means of an elaborate comprehensive study. A simple modular explicit model is presented which can be easily assessed with engineering judgment. If needed, the different individual calculation steps can be exchanged with more sophisticated ones. For a reference sandy soil, results of cyclic laboratory tests are presented and used on a reference monopile foundation for a predefined storm event. The EPPE approach helps to quantify the risk of capacity degradation as well as to evaluate an appropriate safety margin. It is possible with the current methodology to evaluate the degradation potential for different sites quite easily and fast.",
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year = "2023",
doi = "10.15488/13232",
language = "English",
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Download

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AU - Saathoff, Jann-Eike Sören

N1 - Doctoral thesis

PY - 2023

Y1 - 2023

N2 - In particular during storm events an accumulation of excess pore pressures may occur in the soil around cyclically loaded offshore foundations. The excess pore pressure build-up reduces the effective stresses in the soil and, hence, may negatively affect the structural integrity by influencing the soil-structure interaction. Besides a loss in bearing capacity, large plastic deformations may occur to the structure. Especially for offshore wind turbines an accurate estimation of such deformations is of great importance. Even though the consideration of this degradation effect on the bearing capacity is commonly demanded by the involved certification or approval bodies, no general applicable and accepted method for the calculative verification currently exists. Over the past decades several researchers investigated the excess pore pressure build-up around offshore foundations due to environmental cyclic loads. They tried to capture the loss of bearing capacity, the accumulation of plastic rotation and the essential influence on the serviceability limit state and fatigue design. However, even if there are some sophisticated concepts, none of them is seen as the simple general applicable choice. Within this thesis a new numerical method – termed Excess Pore Pressure Estimation method (EPPE) – is presented in great detail. This method allows for the transfer of the soil behaviour obtained in cyclic simple shear tests to the bearing behaviour of the entire foundation. Herein, the numerical model accounts for the cyclic excess pore pressure accumulation by respecting the element-based mean stress and stress amplitude as well as an equivalent number of load cycles. The simulation of the excess pore pressure build-up due to certain cyclic loading is based on undrained conditions, i.e. the excess pore pressure build-up due to cyclic loading is derived by disregarding the simultaneous consolidation process. The respected transfer method, in the form of contour plots, enables the consideration of site-specific cyclic direct simple shear and triaxial test results from laboratory devices to elements within the finite element model. Each integration point is evaluated individually. Based on the derived excess pore pressure field, a consolidation analysis takes place in the second step. The actual accumulated excess pore pressure in each element at the end of the storm (or cyclic loading event) is then found by analytically superposing the excess pore pressure decay curves from the consolidation analysis. For a deeper understanding of cyclic soil behaviour, the cyclic response in different laboratory devices with different densities and under varying stress states was investigated by the author. A contour approach based on cyclic load- and displacement-controlled test results is derived to study the element response from the numerical point of view and use these for the calibration of an implicit model. Moreover, different explicit approaches are presented and compared in terms of their estimation behaviour of cyclic excess pore pressure generation, their predicted foundation capacity and their model assumptions. The intention is hence to examine existing approaches and their applicability by means of an elaborate comprehensive study. A simple modular explicit model is presented which can be easily assessed with engineering judgment. If needed, the different individual calculation steps can be exchanged with more sophisticated ones. For a reference sandy soil, results of cyclic laboratory tests are presented and used on a reference monopile foundation for a predefined storm event. The EPPE approach helps to quantify the risk of capacity degradation as well as to evaluate an appropriate safety margin. It is possible with the current methodology to evaluate the degradation potential for different sites quite easily and fast.

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