MS7.6 - Dynamic Soil-Structure Interaction and Wave Propagation

Presently, over 80% of the offshore wind turbines in Europe are founded on monopiles. These large tubular substructures are commonly installed via impact hammer. During impact piling, a hammer is mounted on the pile top and the applied pulses progressively drive the pile into the seabed. Notwithstanding the simplicity and robustness of this method, impact piling poses alarming problems related to pile structural damage and significant underwater noise emissions, which are harmful to aquatic species. With a view to improve monopile installation in terms of performance and environmental aspects, alternative techniques are investigated. Vibratory pile driving is used onshore for decades and possesses advantageous features such as high installation speed and low axial pile stresses. In offshore monopiles, vibratory installation is hindered by the incompleteness of available field observations and knowledge gaps related to drivability, energy efficiency and lateral pile response. To further boost the performance of vibratory methods, the Gentle Driving of Piles (GDP) has been proposed and tested successfully by TU Delft. The GDP method is based on simultaneous application of low-frequency axial and high-frequency torsional vibrations, with a view to enhanced installation performance and reduced underwater noise emissions. In this paper, the focus lies in the study of the GDP method with the aid of field data and numerical modelling. Medium-scale field tests have been executed at Maasvlakte II site, at the port of Rotterdam, in which the proof of concept of the method was achieved. In these tests, piles were installed by means of GDP and conventional installation techniques, in order to facilitate their comparison. Furthermore, a 3-D axisymmetric model is presented for the analysis of vibratory and GDP installations. In particular, the model is comprised by a thin cylindrical shell (pile), a linear elastic layered half-space (soil) and a history-dependent frictional interface. The coupled non-linear pile-soil problem is solved by a hybrid frequency-time approach based on sequential application of the Harmonic Balance Method (HBM). Comparison of numerical results against field data testify the model validity for the study of pile installation and showcase its predictive potential. Both axial vibratory driving and GDP are analyzed in terms of installation performance, induced ground motion and energy efficiency, with the aid of the benchmarked model. Conclusively, the effect of the high-frequency torsional excitation is realized, with friction force redirection being the major mechanism that leads to superb installation performance.

The tests of resistance to dynamic impact with shaped charge (ŁK) were carried out on a homogeneous armor plate made of ARMOX 370T Class 1 steel, used as a standard in the production of ballistic shields. shock loads. The chemical composition of the steel was determined in spectrometric tests using a spark discharge spectrometer S1 MiniLab 150 by GNR INDIA and confirmed by a GDS-750-QDP glow discharge spectrometer by LECO. Composition tests confirmed the compliance of the chemical composition of the board with the EN 10029 standard for the ARMOX 370T grade. Numerical calculations of the considered penetration with a shaped charge were carried out in the Abaqus program. For this purpose, geometric models were made in the Inventor program and then the geometric models were exported in the .step format to the computing environment. The Johnson-Cook (J-C) mathematical model was used for elastic-plastic materials, including plastic parts. The calculations were based on numerical models separately established for individual elements of the load and cover, assuming material constants appropriate for the materials used for the transition from hybrid methods using the SPH and FEM methods.

Pile foundations are commonly used to support the superstructures of bridges, buildings and heavy industrial plants. The determination of pile stiffness and damping is an important step in the analysis of the soil-structure-interaction of pile-supported structures subjected to dynamic loading due to earthquakes, machinery, seawaves, etc. To date, impedance functions for different types of soil-foundation systems are typically available from analytical, semi-analytical and numerical approaches. The finite element method (FEM) with viscous boundaries consisting of dashpots has gained the most popularity among the numerical methods. However, this numerical approach generally requires large bounded domains for satisfactory accuracy, since viscous boundaries absorb only plane waves propagating perpendicularly to the artificial boundary. Moreover, very few numerical studies in which impedance functions for pile foundations are computed, are validated with experimental data, since field validation studies for deep foundations are very limited. In the present paper the three-dimensional dynamic interaction of pile foundation in layered soil and the accompanied response of the pile foundation on the free field excitation are investigated by means of numerical and experimental studies. To this end, a three-dimensional FE numerical scheme with efficient absorbing layers is developed to simulate the interaction of piles with layered soils in frequency domain. In the FE solver the Perfectly Matched Layer (PML) technique is successfully implemented and the waves propagating outward from the bounded region are attenuated and absorbed. Results of the new developed FE-PML numerical scheme are verified against numerical and experimental results. The numerical results for the verification of the FE-PML model are obtained by means of the SC-SASSI Software based on exact analytical solutions (Green’s functions) for dynamic loads in layered structures, while the experimental validation is conducted by means of vibration measurements on pile foundations. First, a single pile in a homogeneous viscoelastic half-space is modelled and the computed impedance functions are verified with analytical solutions. Next, the influence of the wave propagation through the pile foundation is modelled and validated through vibration measurements. Both the numerical simulation and the in situ measurement results exhibit a reduction of the vibration amplitude due to the piles and this reduction strongly depends on their stiffness. All simulation results demonstrate the ability of the developed FE/PML numerical tool to study the dynamic interaction of piles in layerd soils and to provide better insights into wave propagation patterns within highly heterogeneous soil media.