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- Micromechanical modelling (4)
- Offshore pile foundation (4)
- Offshore wind turbines (4)
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- Jet erosion (3)
- LBM-DEM (3)
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- Pore pressure accumulation (3)

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Driven steel piles are commonly used as deep foundations for a wide range of engineering structures, particularly in the offshore branch. They are also an interesting example among the broad spectrum of geotechnical applications where the fluid-solid interaction at the pore-scale can play a major role for the macromechanical behaviour of the whole system.
In the context of the geotechnical practice for offshore wind-farm structures, both the industrial design and the actual dimensions of the large piles used as foundations in the seabed are often driven by factors such as the soil resistance to driving (SRD), which are still not well understood and often estimated based on mere empirical correlations or overly simplified one-dimensional models. In particular, the role of the micromechanical effects during the installation process (e.g. local dilatancy or contractancy) and their consequences on the pore pressure levels at the pile-tip and on the effective resistance to driving, are generally either disregarded or at most assumed to be covered by the simplified engineering “black-box” solutions.
Here, we propose a general framework to address such local aspects of a geotechnical application involving fluid-saturated soils while retaining the focus on the micro-scale phenomena. We advocate for an approach that combines the relative simplicity of the Discrete Element Method (DEM) for the solid mechanics with the capabilities of the Lattice Boltzmann Method (LBM) for the fluid dynamics. In this sense, we aim to compile some useful techniques and practical recommendations for an efficient GPU-based implementation of a micromechanical LBM-DEM simulation tool.

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.

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.

The shaft bearing capacity often plays a dominant role for the overall structural behaviour of axially loaded piles in offshore deep foundations. Under cyclic loading, a narrow zone of soil at the pile-soil interface is subject to cyclic shearing solicitations. Thereby, the soil may densify and lead to a decrease of confining stress around the pile due to microphenomena such as particle crushing, migration and rearrangement. This reduction of radial stress has a direct impact on the shaft capacity, potentially leading in extreme cases to pile failure. An adequate interface model is needed in order to model this behaviour numerically. Different authors have proposed models that take typical Interface phenomena in account such as densification, grain breakage, normal pressure effect and roughness. However, as the models become more complex, a great number of material parameters need to be defined and calibrated. This paper proposes the adoption and transformation of an existing soil bulk model (Pastor- Zienkiewicz) into an interface model. To calibrate the new interface model, the results of an experimental campaign with the ring shear device under cyclic loading conditions are here presented. The constitutive model shows a good capability to reproduce typical features of sand behaviour such as cyclic compaction and dilatancy, which in saturated partially-drained conditions may lead to liquefaction and cyclic mobility phenomena.

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.

The aim of an ongoing research project is to develop a design approach for typical offshore driven piles (e.g. Jacket piles) based on the application of injections by compaction grouting directly at the pile shaft. The paper aims to present the results of laboratory and in-situ tests, which reveal the efficiency and the promising potential of the optimised foundation concept for a more economic dimensioning of pile foundations and to increase their bearing capacity in non-cohesive soil at any moment after installation.

In this lecture, both the physical behaviour and the general design procedures for deep foundations in the context of the offshore wind energy generation are presented. The first part of the lecture deals with the phenomenology and design of the pile foundations relative to the bearing of axial loads. The second part dwelves on the particularities of the lateral loads as well as on two research topics beyond the current design procedures, namely the hydromechanical coupling and the aging and fatigue phenomena as observed in the course of large-scale field tests.

This presentation deals with the phenomenology and design of pile foundations for offshore wind turbines, and is divided in two lectures.
The first lecture presents a brief introduction to the context and peculiarities of such foundations, and then focuses on the particular case of axially loaded piles. This part is most relevant for the relatively slender piles of the multi-pile substructures (i.e. jackets and tripods). A clear distinction between physical phenomenology and practical design is drawn here.
The second lecture continues with the case of lateraly loaded offshore piles, which bears most relevance for the case of the monopile foundations. Here again, a clear separation between physical reality and design methods is intended.
Finally, the last part of the second lecture introduces several advanced topics which lie outside the classical design approaches, namely the cyclic pile fatigue and the so-called pile setup (i.e. the time effects on the axial pile capacity). The relevance of these two topics is illustrated with experimental results from a field testing campaign on real large-scale piles.

This presentation deals with the phenomenology and design of pile foundations for offshore wind turbines, and is divided into two lectures.
The first lecture presents a brief introduction to the context and peculiarities of such foundations, and then focuses on the particular case of axially loaded piles. This part is most relevant for the relatively slender piles of the multi-pile substructures (i.e. jackets and tripods). A clear distinction between physical phenomenology and practical design is drawn here.
The second lecture continues with the case of lateraly loaded offshore piles, which bears most relevance for the case of the monopile foundations. Here again, a clear separation between physical reality and design methods is intended.
Finally, the last part of the second lecture introduces several advanced topics which lie outside the classical design approaches, namely the hydromechanical coupling effects (i.e. the excess pore-pressure generation around the monopiles), the cyclic pile fatigue and the so-called pile Setup (i.e. the time effects on the axial pile capacity). The relevance of the latter two topics is illustrated with experimental results from a field testing campaign on real large-scale piles.

Design challenges for offshore wind-farms. From foundation mechanics to wind-farm aerodynamics
(2018)

This talk provides a brief introduction on general engineering aspects of offshore wind energy production. Then some geomechanical issues for the foundation of OWTs into the seabed are introduced, while the results from experimental investigations and coupled computational analysis are discussed.
In the second part of the seminar, the hydromechanical Wave–Tower interaction is firstly discussed. Then, some general aspects of the windfarm aerodynamics are introduced. On the one hand, some modelling possibilities for the wake analysis of single turbines and turbine groups are discussed. And on the other hand the relevance of such analyses for a proper windfarm layout optimization is pointed out.
Concerning the geomechanical issues the talk shows that: i) The pile’s bearing capacity can degrade under cyclic loading (waves, wind, …). ii) The time effects can be relevant: Capacity improvement can be substantial, but also fragile. iii) There are cyclic PWP effects: Cyclic interaction with pore water may lead to soil softening and an uncoupled analysis (current design practice) is potentially unsafe.
And concerning the hydromechanical and aerodynamical design considerations, this seminar shows that: i) Numerical analysis of turbine’s interaction with wind/waves is useful and affordable. ii) Simplified models can provide insight into windfarm aerodynamics. iii) Turbulent wake analysis is very relevant for the windfarm layout.

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.

Erosive phenomena at the mesoscale – Perspectives and challenges using coupled LBM-DEM models
(2017)

The physical phenomena related to the erosion of granular materials by a fluid flow are ubiquitous and often present major challenges and threats to a wide range of civil engineering constructions and infrastructures. Catastrophic earth-dam failures and large sinkholes are just some of the possible outcomes of the different forms of erosion (a.o. surface erosion, suffusion, piping, backwards erosion, etc…). However, little is known about the actual mechanical origins of erosion, while the assessment of erodibility is generally performed by means of experimental tests and empirical correlations.
Here we provide a general overview of some current research models aiming to clarify the micromechanical phenomena and their macromechanical consequences taking place in different erosion scenarios. The employed numerical techniques rely on the coupling of two well-stablished particle methods for the fluid and solid phases, namely the Lattice Boltzmann Method (LBM) and the Discrete Element Method (DEM) respectively. Further ingredients of our numerical models include an elastoplastic cohesion model for intergranular solid bridges and a subcritical debonding model for the simulation of transient damage processes within the soil matrix.

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.

Erosion of soils affects both natural landscapes and engineering constructions as embankment dams or levees. Improving the safety of such earthen structures requires in particular finding out more about the elementary mechanisms involved in soil erosion. Towards this end, an experimental work was undertaken in three steps. First, several model materials were developed, made of grains (mostly glass beads) with solid bridges at particle contacts whose mechanical yield strength can be continuously varied. Furthermore, for most of them, we succeeded in obtaining a translucent system for the purpose of direct visualization. Second, these materials were tested against surface erosion by an impinging jet to determine a critical shear stress and a kinetic coefficient. Note that an adapted device based on optical techniques (combination of Refractive Index Matching and Planar Laser Induced Fluorescence) was used specifically for the transparent media. Third, some specifically developed mechanical tests, and particularly traction tests, were implemented to estimate the mechanical strength of the solid bridges both at micro-scale (single contact) and at macro-scale (sample) and to investigate a supposed relationship with soil resistance to erosion.

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 piles have to withstand predominantly cyclic axial loads when they are installed in multi-pile configurations, as in jacket foundations. The dimensions of the pile are governed by both the internal capacity and the fatigue behaviour of the steel cross-section as well as by its external capacity in the pile-soil interaction. Owing to the large numbers of piled foundations required for current and future offshore wind farms, there is an urgent need to optimize the dimensions and related costs of single piles. With regard to the pile capacity, two major topics of research are the determination of possible capacity gains due to pile ageing effects and proper consideration of cyclic degradation. In order to investigate both effects, a large-scale testing facility has been constructed at the BAM TTS site in Horstwalde near Berlin. This open-air facility allows large tubular driven piles to be loaded cyclically in both tension and compression while studying the ageing effects by introducing delays between the testing campaigns. First results already show a moderate increase in pile capacity over time. Concerning the anticipated capacity degradation of cyclically loaded piles, preliminary results show an unexpected behaviour. Additional tests are currently being conducted for further clarification.

This talk provides a brief introduction on general engineering aspects of offshore wind energy production. Some geomechanical issues for the foundation of OWTs into the seabed are introduced, while the results from experimental investigations and coupled computational analysis are discussed.
In the second part of the seminar, the hydromechanical Wave–Tower interaction is firstly discussed. Then, some general aspects of the windfarm aerodynamics are introduced. On the one hand, some modelling possibilities for the wake analysis of single turbines and turbine groups are discussed. And on the other hand the relevance of such analyses for a proper windfarm layout optimization is pointed out.
Concerning the geomechanical issues the talk shows that: i) The pile’s bearing capacity can degrade under cyclic loading (waves, wind, …). ii) The time effects can be relevant: Capacity improvement can be substantial, but also fragile. iii) There are cyclic PWP effects: Cyclic interaction with pore water may lead to soil softening and an uncoupled analysis (current design practice) is potentially unsafe.
And concerning the hydromechanical and aerodynamical design considerations, this seminar shows that: i) Numerical analysis of turbine’s interaction with wind/waves is useful and affordable. ii) Simplified models can provide insight into windfarm aerodynamics. iii) Turbulent wake analysis is very relevant for the windfarm layout.

Here we investigate the physical mechanisms behind the surface erosion of a cohesive granular soil induced by an impinging jet by means of numerical simulations coupling fluid and grains at the microscale. The 2D numerical model combines the Discrete Element and Lattice Boltzmann methods (DEM-LBM) and accounts for the granular cohesion with a contact model featuring a paraboloidal yield surface. Here we review first the hydrodynamical conditions imposed by the fluid jet on a solid granular packing, turning then the attention to the impact of cohesion on the erosion kinetics. Finally, the use of an additional subcritical debonding damage model based on the work of Silvani and co-workers provides a novel insight into the internal solicitation of the cohesive granular sample by the impinging jet.

Numerical analysis can be useful for the investigation of important aspects of offshore foundation
prototypes that otherwise could hardly be studied experimentally, like the evolution of pore-water pressure around
the monopile foundation of an offshore wind turbine under extreme loading. A combination of mixed pressuredisplacement
formulations along with a constitutive model for sands based on the Generalized Plasticity Theory
can replicate accurately the soil behaviour in saturated conditions. However, additional issues must be taken into
account in order to perform numerical simulations of offshore piles. Some implications of the Babuska-Brezzi
restriction, as well as considerations about the pile-soil interface and suitable solution strategies are discussed
here. Due to the high cost of the transient analysis, the parallel computation offers a promising perspective, but
can be complex and needs to be implemented carefully in order to avoid a performance deterioration. A brief
overview on current trends and functional software is given here.

In this paper a shear test, which helps to study local behavior of the soil-pile interaction, is modelled numerically with the Finite Element Method as a 2D plane strain problem. A normal pressure on top and shear displacement on side were applied. So far, the material behavior was considered elastic for the sake of simplicity.
The effect of thickness on contact elements and the presence of in plane stress has been highlighted. The purpose of the paper is to find a suitable contact element which represents more close to reality a soil-pile interaction problem under cyclic axial loading. Moreover, an insight on the presence of in-plane stress shows that it needs to be considered cautiously.

We focus here on the major and still relevant issue of soil erosion by fluid flows, and more specifically on the determination of both a critical threshold for erosion occurrence and a kinetics that specifies the rate of eroded matter entrainment. A state-of-the-art is first proposed with a critical view on the most commonly used methods and erosion models. It is then discussed an alternative strategy, promoting the use of model materials that allow systematic parametric investigations with the purpose of identifying more precisely the local mechanisms responsible for soil particle erosion and ultimately quantifying both critical onsets and kinetics, possibly through existing or novel empirical erosion laws. Finally, we present and discuss several examples following this methodology, implemented either by means of experiments or numerical simulations, and coupling erosion tests in several particular hydrodynamical configurations with wisely selected mechanical tests.

A great deal is possible, even on the seabed. Together, wind and waves shake the foundations of the offshore wind turbines. "ere is also the “normal” current and ebb and #ow of the tides that are so distinctive in the North Sea. Pore water pressure can also occur on the seabed, loosening it. Where monopiles are involved, the worst case is that the stability of the whole turbine can be altered – even if such a monopile rammed into the seabed has a diameter of up to eight metres. Over half the planned offshore wind turbines in the North and Baltic Seas are to have monopile foundations, and over 40 % are to have multi-pile foundations, designed for example as a tripod. "is is why pile foundations and their loads deserve special attention. For one thing above all is to be avoided in plant operations, and that is the risk of turbine tilt.

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

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.

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.

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 track on bridges is affected by cyclic and dynamic action arising from the train passage and the reaction of the track supporting structure. Concerning the track, the current bridge design procedures include two major engineering tasks: On the one hand the stability of the track supported on the bridge deck has to be satisfied. In order to reduce the deterioration of the track the elasticity of the track has to be adapted for the stiff bridge deck. Furthermore, lateral stability has also to be satisfied. The vibration of the bridge structure has to be taken into account because it can affect the stability of the ballasted track. On the other hand the transition zone between bridge structure, abutment and the track on the subgrade is a matter of permanent concern. The changes in stiffness, in the dynamic behavior arising from the rolling stock and the relative deflection between the bridge structure and the abutment induce a nearly unavoidable problem for the long term behaviour on the track.
BAM has conducted intensive experimental work to study the behaviour of the ballasted track on dynamically excited bridges. The state of the art is a design value for the deck acceleration of 0.35 g as established in the ENV 1991-1. In our experimental investigations the design value was verified. The influence of a frequency content higher than 30 Hz and of single vibration amplitudes on the track deterioration was evaluated. Both vertical and lateral stability were investigated. The influence of vibrations on the lateral stability can be more severe since the lateral stability could involve a sudden failure of the track. Furthermore, a simulation tool was developed to calculate the lateral stability on a vibrating bridge deck. For this simulation tool a parameter study was undertaken [1] where combinations of the curvature of the bridge-track system, the design temperature and the bridge deck vibrations were investigated. Parts of this study are incorporated in this paper.
In a current research project the investigations are extended to the interaction of the train-track-dynamics and the long term behaviour of the transition zone. The main emphasis is on the integration of the possible deterioration of the structural components in the numerical modelling of the relevant structural elements of the vehicle, the bridge and bridge abutment structure. An outlook is given to a complete model for the assessment of the track behaviour at railway bridges.

Most railway embankments in the UK were built in the Victorian era and are of end-tipped construction using materials (usually cohesive) excavated from adjacent cuttings, resulting in a clod-and-matrix structure. Historically, there has been a lack in understanding of the mechanical behaviour of such railway embankments. In the next decade railway traffic in the UK, particularly freight, is forecast to grow considerably. Consequently, there is a need to improve the understanding of how increases in rail traffic loading may influence the mechanical behaviour of railway embankments and thus track performance. The Rail Safety and Standards Board in conjunction with Network Rail is currently undertaking a programme of applied research into this topic. As part of these studies a programme of physical model tests has been carried out. Physical model tests can provide high quality data on system performance under a large range of loading conditions and geometrical configurations. The data can be generated rapidly, with test periods of weeks or months rather than years or decades. Additionally, boundary conditions are well-defined and controlled, compared with the complex situations encountered in full scale embankments in the field. This paper summarises the physical model test programme, the development of the test set-up, the tests themselves and the conclusions drawn.

Offshore wind turbines enter unknown territory, especially where the foundations are concerned. This is because offshore wind power can only make use of the experience from the common offshore constructions used by the oil and gas industry to a limited extent. The offshore wind industry has tried to reduce foundation dimensions, especially the pile lengths, as much as possible compared with those of the oil and gas industry. This is because with the large number of wind turbines involved it can provide considerable economic advantages. On the other hand, the stability of the foundations is additionally at risk because due to the much larger number of cyclic loads they are subjected to it is very difficult to predict how they will behave. Since offshore wind farms are manufactured in series, every systematic fault in the foundation acts as a series fault for a large number of turbines. This calls for monitoring – and the right dimensions of pile foundation, the most common type of foundations used for wind turbines