A method is presented to accurately capture the 3D interaction phenomena of a foundation pile embedded in soil, in a computationally efficient non-local 1D model. It is shown how to extract the global stiffness kernels from simulations with the 3D inhomogeneous continuum, and implement them in a 1D non-local Winkler-type model that can subsequently serve as a stand-alone, condensed substructure. The presented method for obtaining the kernels removes the need to assume certain distributions for these functions. We show that the method is very versatile, and yields accurate results for a wide range of pile geometries and soil stiffness profiles, over the full depth of the embedded pile. For the dynamic case, the discretized global stiffness kernels (matrices) become complex-valued (they include soil stiffness, inertia and damping), and the 1D model proves to be capable of mimicking also the out-of-phase part of the response. The method being straightforward and fast, the engineering community is served the benefits of accuracy (3D model) and speed (1D model), without the need of empiric tuning.

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In an attempt to decrease the modelling uncertainty associated with the soil-structure interaction of large-diameter monopile foundations, a hydraulic shaker was used to excite a real-sized, in-situ monopile foundation in stiff, sandy soil in a near-shore wind farm. The response in terms of natural frequency and damping of a pile-only system is significantly more influenced by the soil than a full offshore wind turbine structure, and therefore ensures a higher degree of certainty regarding the assessment of the soil reaction. Steady-state vibration amplitudes with frequencies between 1 and 9 Hz were retrieved from strain gauges vertically spaced along the embedded pile, and accelerometers attached to the top of the pile and to the shaker. The measured response is used to validate an effective 1D stiffness method, which is applied as a smart initial guess for a model-based identification of the effective soil-structure interaction properties in terms of stiffness, damping and soil inertia. The performance of the stiffness method is compared to the currently employed p-y stiffness design method. While the effective stiffness method seems to overestimate the actual low-frequency stiffness with about 20%, the p-y method appears to underestimate this stiffness with 140%. The assumption of linear soil behaviour for most of the occurring pile displacements is shown to be acceptable. A damping ratio of 20% (critical) is identified as effective soil damping for the monopile, which is estimated to correspond to a 0.14% damping ratio contribution from the soil for the full structure. The unique measurement setup yielded a ‘first-off’ opportunity to validate a soil-structure interaction model for a rigidly behaving pile. We have shown that indeed such a pile reacts stiffer than predicted by the p-y curve method, and that its response can be modeled more accurately with our recently developed effective stiffness method.

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It is widely accepted that the initial stiffness derived with the p-y methodology as prescribed by the American Petroleum Institute does not capture the true small-strain stiffness for rigidly behaving piles. We present an alternative method, capturing the 3D effects in the soil-pile interaction, in which the soil is characterized with in-situ seismic measurements. As the design of the foundations of offshore wind turbines often involves many expensive load simulations (load cases), the method also includes finding an effective 1D or Winkler stiffness yielding a similar pile response as in the 3D model. The method is exemplified for a real 5 m-diameter monopile embedded in stiff dense sand, in a near-shore wind farm in The Netherlands. The dynamic properties of this pile have been tested (prior to installation of the super-structure) with a unique measurement setup using a hydraulic shaker, exciting between 1 and 9 Hz. The response of this stand-alone pile is highly sensitive to the soil and allowed us to verify the effective 1D stiffness with much higher certainty as opposed to the usual situation including dynamic disturbances and uncertainties related to the interaction with the super-structure. The effective soil damping of the pile is estimated, and the performance of both the standard design (p-y) stiffness method and the proposed effective stiffness method is assessed in view of the measured strains and accelerations.@en

A procedure is presented for the derivation of an effective small-strain soil stiffness governing the soil– structure interaction of large-diameter monopiles. As a first step, geophysical measurements are used to estimate the depth-dependent shear modulus G of the soil stratum. The second step is to use this modulus and an estimated Poisson’s ratio and density in a 3D model, which captures the deformation of both the monopile and the soil. As a final step, a new method is proposed to use the computed 3D response for identification of a depth dependent stiffness of an effective Winkler foundation. This stiffness can be used in a 1D model, which is more fit for design purposes. The presented procedure is deemed more appropriate than the often used ‘‘p–y curve” method, which was once calibrated for slender flexible piles and for which the input is based on the large-strain cone penetration test. The three steps are demonstrated for a particular design location. It is also shown that the displacements of the 3D model are smaller and the resulting fundamental natural frequency is higher than calculated with the p–y method. @en