Predictive capabilities of nonlinear load-transfer curves
Predictive capabilities of nonlinear load-transfer curves for
axially loaded piles
Aligi Foglia, Martin Kohlmeier
Fraunhofer Institute for Wind Energy and Energy System Technology IWES, Department Support Structures
In water depths larger than 40 m offshore wind converters are likely to be supported by jacket structures founded on
pile foundations. To gain new knowledge on both ultimate capacity and load displacement behaviour of piles in
tension a number of experimental campaign are being conducted at the Test Centre for Support Structures of the
Leibniz University Hannover. Load transfer curves, as straightforward tool to estimate the load-displacement of axially
loaded piles, are crucial to the design of jacket sub-structures. This poster reports on the research objectives of
Fraunhofer IWES, Section Support Structures, within the project GIGAWIND in regards to load transfer curves for
axially loaded piles.
Implementation of load transfer curves
In most geotechnical problems the prediction of ultimate capacity and
initial stiffness is of great importance. While bearing capacity drives the
pile dimensions, the initial stiffness plays a role in the entire sub-structure
system for instance in the so-called superelement analysis (Thieken et
al., 2016). A straightforward method for the estimation of the initial
stiffness of axially loaded piles are the load transfer curves also called t-z
curves (API 2011; Fleming, 1992; Bohn et al., 2016).
The formulations of load transfer curves are rather heterogeneous. A
specific t-z curve may involve deformation parameters as well as ultimate
skin friction values. The data interpretation conducted so far showed
promising results. In this contribution, only a selection of three
representative t-z curves is reported. The best fit of the initial stiffness is
displayed by the load transfer curves of Fleming (1992) which are able to
retrospectively simulate the load displacement experimental curves until
at least 50% of the ultimate load. Further in the development of loaddisplacement curve the predictions present overestimations of the
experimental data. The curve of Bohn et al. (2016) captures the overall
shape of the experimental data acceptably well. However, the initial
stiffness is not well reproduced. The prediction of API (2011) cannot
match the experimental curve and shows a conservative initial stiffness
and a stiff response thereafter.
The Test Center for Support Structures of the Leibniz Universität
Hannover offers ideal conditions for high quality pile testing. Through
large-scale physical modelling design methods can be validated or new
design approaches can be tested and calibrated. The Fraunhofer IWES
has been carrying out numerous experimental campaigns concerning
instrumented and non-instrumented axially loaded piles. In Figure 1 the
installation and the test phase of a piled foundation is shown. The
database assembled through public funded projects by the Fraunhofer
IWES and additional existing well-documented databases (Yang et al.
2015) will be used within the GIGAWIND project in order to validate and
possibly put forward load transfer curves.
Experimental data of tensile load test
Prediction: ICP-05, t-z API (2011)
Prediction: ICP-05, t-z Fleming (1992)
Prediction: ICP-05, t-z Hyperbolic Bohn et al. (2016)
Vertical displacement (mm)
Fig. 2: Load-displacement curve of an experimental test of a pile loaded in tension together
with the retrospective simulation of three load transfer formulations. Inner plot depicts a
magnified view of the initial stiffness section of the load displacement curves.
Fig. 1: Photos of the large-scale experimental tests carried out at the Test Centre for Support
Structures. a) detail of a pile during the installation test; b) detail of Pile 4 during the tensile
load test. Campaign conducted within the European project IRPWind.
Conclusions and outlook
Load transfer curves for axially loaded piles are crucial to the
determination of the overall stiffness of jacket sub-structures for
offshore wind turbines (e.g. the so-called superelement simulations).
Large-scale geotechnical physical modelling gives irreplaceable
chances for testing or proposing new load transfer curves. Within the
project GIGAWIND the following objectives are being pursued:
American Petroleum Institute (API) (2011). Geotechnical and Foundation Design Consideration.
ANSI/API RECOMMENDED PRACTICE 2GEO.
Bohn C, dos Santos AL and Frank R (2016). Deve- lopment of Axial Pile Load Transfer Curves
Based on Instrumented Load Tests. J. Geotech. Geoenviron. Eng. 04016081.
Fleming, W. G. K. (1992). A new method for single pile settlement prediction and analysis.
Geotechnique 42(3), 411–425.
Thieken K, Achmus M, Terceros M, Dubois J and Gerlach T (2016). Describing Six-Degree-of-Freedom Response of Foundations Supporting OWEC Jacket Structures. Proceedings of the Twenty
-sixth (2016) International Ocean and Polar Engineering Conference, Rhodes, Greece, pp. 745–
Yang Z, Jardine R, Guo W and Chow F (2015). A Comprehensive Database of Tests on Axially
Loaded Piles Driven in Sand. Zhejiang University Press, Elsevier
• to examine existing well-documented database such as Yang et al.
(2015) and Fraunhofer IWES’ in relation to general load bearing
• to understand whether the stiffness of the piled foundations system is
affected by the presence of sensors on the pile shaft and if so
estimate how significantly and how alternative instrumentation
techniques could be adopted;
Good estimation of the load transfer curves are also possible with finite
element methods. Though the lack of well-established contact models
defining the interface between pile shaft and soil medium limits their
usage. Advancements in this direction would also be desired.
7. GIGAWIND-Symposium, 2 March 2017