Assessment of Pier Foundations for Onshore Wind Turbines in Non-cohesive Soil

Authors: Mauricio Terceros, Jann-Eike Saathoff, Martin Achmus

Abstract:

In non-cohesive soil, onshore wind turbines are often found on shallow foundations with a circular or octagonal shape. For the current generation of wind turbines, shallow foundations with very large breadths are required. The foundation support costs thus represent a considerable portion of the total construction costs. Therefore, an economic optimization of the type of foundation is highly desirable. A conceivable alternative foundation type would be a pier foundation, which combines the load transfer over the foundation area at the pier base with the transfer of horizontal loads over the shaft surface of the pier. The present study aims to evaluate the load-bearing behavior of a pier foundation based on comprehensive parametric studies. Thereby, three-dimensional numerical simulations of both pier and shallow foundations are developed. The evaluation of the results focuses on the rotational stiffnesses of the proposed soil-foundation systems. In the design, the initial rotational stiffness is decisive for consideration of natural frequencies, whereas the rotational secant stiffness for a maximum load is decisive for serviceability considerations. A systematic analysis of the results at different load levels shows that the application of the typical pier foundation is presumably limited to relatively small onshore wind turbines.

Keywords: Onshore wind foundation, pier foundation, rotational stiffness of soil-foundation system, shallow foundation.

Digital Object Identifier (DOI): doi.org/10.5281/zenodo.3566307

References:

[1] A. Quast, „Zur Baugrundsteifigkeit bei der gesamtdynamischen Berechnung von Windenergieanlagen“, Mitteilung des Instituts für Grundbau, Bodenmechanik und Energiewasserbau der Universität Hannover, Heft 69, 2010.
[2] C.J. Warren-Codrington, “Geotechnical Considerations for Onshore Wind Turbines“, University of Cape Town, November 2013, p. 205.
[3] DNV/Risø “Guidelines for the design of wind turbines”, Second Edition, 2002.
[4] Deutsches Institut für Bautechnik, „Richtlinie für Windenergieanlagen. Einwirkungen und Standsicherheitsnachweise für Turm und Gründung“, Schriften des DIBt, Heft 8, Fassung Oktober 2012.
[5] Deutsche Gesellschaft für Geotechnik (DGGT), „Empfehlungen des Arbeitskreises Baugrunddynamik“, 1.Auflage, Dezember 2002.
[6] Earth Systems Southwest, “Patrick and Henderson (P&H) Foundations for Wind Turbine Support”, Brochure, USA, 2009.
[7] Earth Systems Southwest, “Patrick & Henderson Tensionless pier”, Presentation, USA, 2008.
[8] E. Ntambakwa, Y. Hao, C. Guzman, M. Rogers, “Geotechnical Design Considerations for Onshore Wind Turbine Shallow”, Geotechnical and Structural Engineering Congress, Volume 2, 2016, pp. 1153-1165.
[9] J.A. Santos, A.G. Correia, “Reference threshold shear strain of soil and its application to obtain a unique strain-dependent shear modulus curve for soil”, Proceedings of 15th International Conference on Soil Mechanics and Geotechnical Engineering, 2001, pp. 267-270.
[10] J. Ohde, „Zur Theorie der Druckverteilung im Baugrund“, Der Bauingenieur 20, H. 33/34, pp. 451-459, 1939.
[11] K. Morgan, E. Ntambakwa, “Wind Turbine Foundation Behavior and Design Consideration”, American Wind Energy Association Annual Conference and Exhibition, Houston, Texas, Volume 2, 2008, pp. 585-598.
[12] L.C. Reese, “Statement of Company Experience in Wind-Turbine Structures”, report, 2008.
[13] M. Achmus, K. Thieken, J.E. Saathoff, M. Terceros, J. Albiker, “Un- and reloading stiffness of monopile foundations in sand”, Applied Ocean Research, Volume 84, pp. 62-73. March 2019.
[14] M. Achmus, “Soil Investigation and Foundation for Wind Turbines”, HdT-Conference, July 2013.
[15] Nordex, “Turbine and Tower Technical Data”, February 2012.
[16] R.B.J. Brinkgreve, S. Kumarswamy, and W.M. Swolfs, “PLAXIS 3D Manual”, 2016.
[17] T. Benz, “Small Strain Stiffness of Soils and its Numerical Consequences”, Ph.D. thesis, University of Stuttgart, Stuttgart, Germany, 2007, 193 pp.
[18] T. Schanz, „Zur Modellierung des mechanischen Verhaltens von Reibungsmaterialien“, University of Stuttgart, Habilitation, 1998.
[19] United State Patent “Tensionless pier foundation, patent number 5586417”, Inventors A. P. Henderson & M. B. Patrick, 1996.