Keywords: wind turbines, pile foundations, soil-structure interaction, Winkler springs, numerical modeling.

Wind turbine towers founded on soil formations are subjected to dynamic excitations (wind or earthquake), thus, their overall response is directly affected by the phenomenon of soil-foundation-structure interaction [1,2,3,4,5]. However, the role of this interaction cannot be regarded a-priori as beneficial or detrimental for the dynamic response of the towers. In the present work the basic parameters of the problem are investigated, and a thorough examination of the circumstances under which the dynamic interaction may determine the response of the whole system is performed. For this purpose, elaborate numerical simulations are used to assess the dynamic response of a typical wind turbine tower, which can be founded (by a rigid mat foundation or by a monopile) either on the elastic half-space, or on a soft soil layer overlying stiff rock.

The results presented illustrate that three-dimensional finite element modeling is capable of predicting the correct soil-pile stiffness, given that the mechanical properties of the soil have been accurately assessed. It is also stressed that extra attention is needed when simulating the interface between soil and pile, since a sophisticated simulation of the interface is needed to take into account phenomena like soil-pile separation, drag forces, gap closure, etc. Moreover, an appropriate constitutive law should be used for modeling soil behavior, since the existing ones such as Mohr-Coulomb, are proven to be unable to accurately predict the ultimate soil resistance due to the significant hardening they exhibit. The numerical results also demonstrate that the response of the tower may be sensitive to the mechanical and geometrical properties of the underlying soil layers, and that the interaction may be even detrimental for the distress of the superstructure. Therefore, soil-foundation-structure interaction, which is usually neglected in engineering practice, should be taken realistically into account to achieve an optimum balance between cost, effectiveness, reliability and safety of wind turbines.

1

API, "American Petroleum Institute: Recommended practice for planning, designing, and constructing fixed offshore platforms", Report RP 2A, 17th Edition, 1987.

2

R.W. Boulanger, C.J. Curras, B.L. Kutter, D.W. Wilson, A. Abghari, "Seismic soil-pilestructure interaction experiments and analyses", ASCE Journal of Geotechnical and Geoenvironmental Engineering, 125(9), 750-759, 1999. doi:10.1061/(ASCE)1090-0241(1999)125:9(750)

3

B.W. Byrne, G.T. Houlsby, "Assessing novel foundation options for offshore wind turbines", World Maritime Technology Conference, London, UK, 2006.

4

DNV and RISØ, "Guidelines for design of wind turbines", 2nd Edition, Denmark, 2002.

5

R.T. Klinkvort, "Laterally loaded piles - centrifuge and numerical modeling", MSc Thesis, Department of Civil Engineering, Technical University of Denmark, 2009.

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