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- Cyclic lateral load (3)
- Numerical model (3)
- Offshore pile foundation (3)
- Pore pressure accumulation (3)
- Assessment criteria (2)
- Bridge transition zone (2)
- Cyclic loading (2)
- Cyclic loads (2)
- Offshore foundations (2)
- Offshore wind turbines (2)

#### Organisationseinheit der BAM

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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.

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.

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.

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.

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.

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.

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.

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

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.

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.

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

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.

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.