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- Cyclic loading (4)
- Offshore pile foundation (4)
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- Assessment criteria (2)
- Bridge transition zone (2)
- Densification (2)
- Ratcheting (2)
- Train-track-bridge-interaction (2)
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- Anwachsen (1)

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This paper deals with the system identification of a mechanical structure supported by nonlinear springs subjected to an external load. If all mechanical parameters of the system were known, the displacement of the system subjected to this load could be easily calculated. However, the monitoring applications often deal with the inverse problem. The loads and displacements of the system are known and certain mechanical Parameters of the system are sought. The solution of such inverse problems can be difficult, especially when they have a nonlinear and multimodal character, which often makes them appear intractable at first sight. However, evolutionary computing can be applied to solve this inverse, nonlinear and multimodal problem. Sometimes a prior knowledge exists on certain system properties, which is difficult to implement into analytical or numerical solvers. This knowledge can play a decisive role in identifying the System properties and it can be easily included as a boundary condition when applying evolutionary algorithms.
This article discusses how and under what conditions the unknown spring resistances can be identified. The practical application of this procedure is exemplified here with the mechanical system of a pile foundation.

The stability and geometric nonlinearities of slender structures are a major topic in structural design. While this topic is most relevant in the field of Structural Engineering, e.g. for steel or concrete structures, only few applications take the role of soil-structure-interaction explicitly into account. The focus of this paper is placed on the impact of soil support and its modelling for the buckling analysis based on examples both for pile foundations and for railway track stability. The general interaction between steel design and the geotechnical input will be addressed. The paper discusses and summarizes a range of subtopics based on experience and current research at the author’s institute.

Relevant factors for the liquefaction susceptibility of cyclically loaded offshore monopiles in sand
(2013)

The offshore foundations may exhibit a relatively high liquefaction susceptibility due to the full saturation of the porous seabed and the cyclic nature of the typical offshore loads. Here, the particular relevance of some of the main factors that affect the liquefaction susceptibility of an offshore monopile will be addressed, focusing on the possibility of a progressive accumulation of residual pore water pressure within the saturated soil around a monopile under cyclic lateral loading. The discussion is based on numerical results obtained with a coupled FE model of the offshore foundation which includes the Biot-Zienkiewicz u-p model. A constitutive model of the Generalized Plasticity type has been used for the soil in order to reproduce important features of its behaviour under cyclic loading. This paper presents the findings derived from a parametric study of the problem and shows that the accumulation of residual pore pressure can produce significant changes of the pile's behaviour under external loading. The paper also investigates the effects caused by the loading from a realistic storm of moderate magnitude and the consequential transient degradation of the foundation's stiffness.

The surface erosion of soil samples caused by an impinging jet can be analyzed using the jet erosion test (JET), a standard experimental test to characterize the erosion resistance of soils. This paper specifically addresses the flow characteristics of a laminar impinging jet over the irregular surface of granular beds to discuss the pertinence and relevance of commonly used empirical estimations based on a selfsimilar model of a free jet. The JET is here investigated at the microscale with a coupled fluid-particle flow numerical odel featuring the lattice Boltzmann method (LBM) for the fluid phase combined with the discrete element method (DEM) for the mechanical behavior of the solid particles. The hydrodynamics of a laminar plane free jet are confronted with the results from a parametric study of jet impingement, both on solid smooth and fixed granular surfaces, that take into account variations in particle size, distance from jet origin, and jet Reynolds number. The flow characteristics at the bed surface are here quantified, including the maximal values in tangential velocity and wall shear stress, which can be regarded as the major cause of particle detachments under hydrodynamic solicitation. It is shown that the maximal velocity at the impinged surface can be described by the free jet self-similar model, provided that a simple empirical coefficient is introduced. Further, an expression is proposed for the maximal shear stress in laminar conditions, including a Blasius-like friction coefficient that is inversely proportional to the square root of the jet Reynolds number. To conclude, finally, the JET erosion of different cohesionless granular samples is analyzed, confirming that the threshold condition at the onset of granular motion is consistent with the Shields diagram and in close agreement with previous experimental results.

Sand densification around the pile has traditionally been regarded as an explanation for the grain migration and soil subsidence that often occur around cyclic laterally loaded piles embedded in sand. Supported by new empirical evidence, this paper proposes that, additionally to some soil densification around the pile, the main cause for the continuous "steady-state" grain migration is a convective cell flow of sand grains in the vicinities of the pile head. Such convective flow would be caused by a ratcheting mechanism triggered by the cyclic low-frequency lateral displacements of the pile. Furthermore, the experimental results suggest that the limit between the convective cell and the static soil is marked by a distinct direct shear surface. This might shed some light into the complex phenomena related to the pile-soil interaction in the upper layers of the bedding, which are normally the main contributor for the lateral load-bearing capacity of piles.

Porenwasserdruckaufbau und Bodenentfestigung um Pfahlgründungen von Offshore-Windenergieanlagen
(2012)

Die Gründung von Offshore-Windenergieanlagen erfordert oft die Installation von Stahl-Pfählen mit großem Durchmesser, entweder als Monopile- oder Mehrpfahl-Konfiguration (Jacket, Tripod, usw.). In dieser Veröffentlichung werden die Auswirkungen einer zyklischen lateralen Belastung auf Offshore-Pfahlgründungen untersucht. Hierbei wird besonderes Augenmerk auf die Entwicklung des Porenwasserdrucks im umgebenden gesättigten Boden gelegt. Es wird gezeigt, dass der progressive Aufbau von Porenwasserdruck zu erheblichen Veränderungen des Verhaltens der Gründung unter externer Belastung führen kann, die die Stabilität bzw. Gebrauchstauglichkeit der Anlage potenziell gefährden. Dazu werden auch einige Effekte, die während eines realistischen Sturms von moderater Stärke entstehen, untersucht. Die transiente Abminderung der Gründungssteifigkeit wird hierbei thematisiert. Die genannten Aspekte wurden von den Autoren mittels eines gekoppelten zweiphasigen numerischen Modells der Offshore-Gründung untersucht. Das verwendete Stoffgesetz für den Boden, im Rahmen der Theorie der verallgemeinerten Plastizität, kann wichtige Merkmale von zyklischem Bodenverhalten reproduzieren, wie z. B. die Tendenz zu einer fortschreitenden Verdichtung unter zyklischer Belastung, die für Verflüssigungsphänomene unter undränierten Bedingungen verantwortlich ist. Die Notwendigkeit der Verwendung eines solchen Ansatzes wird hier anhand von Vergleichsberechnungen mit einem einfacheren Stoffmodell bzw. mit einem ungekoppelten Modell erörtet. Abschließend werden einige Implikationen der untersuchten Fragenstellungen für die praktische Bemessung von Offshore-Monopiles diskutiert und spezifische Empfehlungen gegeben.---------------------------------------------------------------------------------------------------------------------------
For the installation of wind turbines in the offshore environment and their foundation in the seabed it is customary to employ large-diameter steel piles, either in monopile or multi-pile configurations (jacket, tripod, etc. ...). The effects of cyclic lateral loading on the offshore piles and particularly the possibility of a progressive accumulation of residual pore water pressure within the saturated embedding soil are discussed in this article. Here it is shown that this can lead to significant changes of their behaviour under external loading, which can potentially compromise the foundation's stability or serviceability. Furthermore, some of the singular effects arising during a realistic storm of moderate magnitude as well as their potential for transient damages to the foundation's stiffness are addressed in the paper. For the investigation of these phenomena the authors have employed a coupled bi-phasic analytical model of the offshore foundation. The constitutive model employed for the seabed, in the frame of the theory of Generalized Plasticity, can reproduce some complex features of cyclic soil behaviour such as the tendency for a progressive densification under cyclic loading, which is responsible for the soil liquefaction phenomena in undrained conditions. Finally, some implications and specific recommendations for the design of offshore monopiles in the frame of the limit states are provided.

Pore-pressure accumulation and soil softening around pile foundations for offshore wind turbines
(2012)

The saturated sand surrounding an offshore pile
foundation under quasi-static cyclic lateral load can show the
physical phenomena of macromechanical densification and
convective granular flow. Based on the results from physical
model tests at different geometrical scales, this paper
provides a certain quantification of such phenomena and discusses
their causes and consequences. The progressive sand
densification leads to subsidence of the soil surface and a significant
stiffening of the pile behaviour. Conversely, the ratcheting
convective motion of two closed cells of soil beneath
the pile-head is responsible for an endless grain migration at
the soil surface, the inverse grading of the convected material
and a direct shear of the sand at the distinct boundary of the
revolving soil domain. In this respect, and from a macromechanical
perspective considering the soil as a continuum, it
appears that the convecting material tends to follow gradient
lines of shear stress during its ratcheting motion. Concluding
the paper, the practical relevance of these phenomena and
their extrapolation to other conditions are briefly discussed.

The authors are currently investigating the possibility to apply compaction grouting for offshore pile foundations (Jacket piles as well as monopiles) as a possible retrofitting technique for an optimised foundation concept. In this research project, we are developing a design approach aiming to predict the ideal amount and properties of a grout for a specific soil situation and desired improvement of pile bearing capacity after Installation and during service time. Both numerical and experimental tests have been carried out to investigate the injection process during which a highly viscous grout is injected into the soil under high pressure to displace and compact the surrounding soil without fracturing it. The implicit Material Point Method (MPM) based on a mixed formulation is the numerical technique chosen to deal with the expected large deformations and the arbitrary shape of the developing grout bulb. The usage of MPM prevents both the need of remeshing and the numerical instability induced by extensive mesh distortion. For validation with experimental results, we have constructed a testing chamber with one transparent sidewall. This chamber enables us to observe the injection process directly at the transparent vertical window and to measure the in-plane soil displacements and strains by means of the Digital Image Correlation (DIC) technique.
The results already reveal the interrelation of soil and grout properties for a successful usage of this common ground improvement technique.

Offshore-Pfähle in Mehrpfahlgründungsstrukturen wie Jacket-Gründungen müssen nach der Rammung überwiegend zyklisch axialen Belastungen widerstehen. Die Abmessungen der Pfähle werden sowohl von der inneren Tragfähigkeit und dem Ermüdungsverhalten des Stahlquerschnittes als auch von der äußeren Tragfähigkeit und der Pfahl-Boden-Interaktion bestimmt. Aufgrund der großen Zahl von Pfahlgründungen, die für aktuelle und zukünftige Windparkprojekte benötigt werden, besteht ein dringender Bedarf, die Dimensionierung und die damit verbundenen Kosten des Einzelpfahles zu optimieren. Bezüglich der Pfahltragfähigkeit sind wichtige aktuelle Forschungsthemen eine mögliche Traglaststeigerung durch Anwachsen und die geeignete Abschätzung der zyklischen Degradation. Zur Untersuchung beider Effekte wurde eine großmaßstäbliche Versuchsanlage auf dem Testgelände BAM TTS in Horstwalde nahe Berlin errichtet. Auf diesem Versuchsfeld können große Stahlrohrammpfähle zyklisch druck- und zugbelastet werden. Zudem kann die Untersuchung von Alterungseffekten durch zeitlichen Versatz zwischen den Messkampagnen untersucht werden. Erste Ergebnisse zeigen einen Anstieg der Pfahltragfähigkeit bis zu ca. 60 % nach einer Standzeit von ca. 18 Monaten. Für die Degradation der Tragfähigkeit der zyklisch belasteten Pfähle zeigen die ersten vorläufigen Ergebnisse ein zum Teil unerwartetes Verhalten. Zusätzliche Untersuchungen werden derzeit zur weiteren Abklärung durchgeführt.

The bridge design for railway bridges is far more dependent on the interaction with the traffic and the carriageway than for road bridges. This is especially true for the specific demands of the track in highspeed railways. Both maintenance and safety of the track have to be considered. The most relevant sections for the design criteria can be located at the bridge transition zones. Based on experimental investigations it is shown that bridge joint displacements, changes in stiffness and uplifting of the sleeper are causes for an increased degradation and loss in strength of the ballasted track. With respect to high speed vibrations of the bridge deck can have an even more decisive impact. Bridge deck vibrations can lead to destabilization of the bailast. In a numerical study the behavior of the track at the bridge is illustrated.

Among different devices developed quite recently to quantify the resistance to erosion of natural soil within the broader context of dyke safety, the most commonly used is probably the jet erosion test in which a scouring crater is induced by impingement of an immersed water jet. A comprehensive experimental investigation on the jet erosion in the specific situation of a cohesionless granular material is presented here. The tests were performed by combining special optical techniques allowing for an accurate measurement of the scouring onset and evolution inside an artificially translucent granular sample. The impinging jet hydrodynamics are also analyzed, empirically validating the use of a self-similar theoretical framework for the laminar round jet. The critical conditions at the onset of erosion appear to be best described by a dimensionless Shields number based on the inertial drag force created by the fluid flow on the eroded particles rather than on the pressure gradients around them. To conclude, a tentative empirical model for the maximal flow velocity initiating erosion at the bottom of the scoured crater is put forward and discussed in the light of some preliminary results.

The main degradation process at bridge transition zones due to traffic loads is the appearance of differential settlements. Abrupt stiffness changes, repeating traffic loads and relative displacements of the superstructure ends on bridges often aggravate this problem. In this contribution, a 3D finite element (FE) model extended with a boundary formulation in the frame of the scaled-boundary finite element method (SBFEM) for a transient analysis of train-track-bridge interaction is presented. This numerical model permits an assessment of bridge transition zone with respect to permanent deformations of the track. The main focus lies on the modeling strategies for the vehicle and their impact on suitable assessment criteria for bridge transition zones. For this purpose, two different modeling strategies for the vehicle, a moving load model and a multibody model, have been compared and discussed on the basis of the assessment criteria. The results indicate that the model of the vehicle has a minor effect for an assessment on the embankment, but that the assessment on the bridge may show significant differences depending on whether the inertial components of the vehicle (multibody model) are considered.

A comprehensive numerical model for the analysis of offshore foundations under a general transient loading is presented here. The theoretical basis of the model lies on the Swansea formulation of Biot's equations of dynamic poroelasticity combined with a constitutive model that reproduces key aspects of cyclic soil behaviour in the frame of the theory of generalised plasticity. On the practical side, the adoption of appropriate finite element formulations may prevent the appearance of spurious numerical instabilities of the pore pressure field. In this respect, the use of a coupled enhanced-strain element is here proposed. On the other hand, the practicality of the presented model depends ultimately on its computational efficiency. Some practical recommendations concerning the solution strategies, the matrix storage/handling procedures and the parallel multi-processor computation are here provided. Finally, the performance of the model with a benchmark study case and its practical application to analyse the soil–structure interaction of an offshore monopile under a realistic transient storm loading are discussed.

A mechanical structure supported by nonlinear springs subjected to an external load is considered. If all mechanical parameters of the system were known, the displacement of the system subjected to this load could be easily calculated. If not all of the parameters are known, but the load and the displacement are measured at one location, an inverse problem exists. In the presented problem the nonlinear springs are unknown and have to be determined. At first glance a problem needs to be solved, which is underdetermined due to the number of unknown variables. However, evolutionary computing can be applied to solve this inverse, nonlinear and multimodal problem. Sometimes a prior knowledge exists on certain system properties, which is difficult to implement into analytical or numerical solver. This knowledge can play a decisive role in identifying the system properties and it can be easily included as boundary condition when applying evolutionary algorithm. This article examines how and under what conditions the spring resistances can be identified. The procedure is exemplified at a mechanical system of a pile foundation.