Cyclic loading on shallow foundations - experimental investigations on the accumulation of deformations

Offshore wind turbines are exposed to cyclic loading due to wind and water waves. Transferred to the foundation, this repeated loading leads to an accumulation of soil deformations. These deformations may result in an inclination of the wind power plant structure which may exceed at a certain degree the serviceability limit of the wind turbine. Such an inclination can for example be caused by a strong storm event inducing cyclic loads on the structure or by a huge single wave. The theoretical, experimental and numerical approaches for the description of the cyclic behaviour are especially for shallow foundations still very insufficient. Therefore this contribution focuses on experimental model tests on shallow foundations on dense sand. The test setup and the applied loads are described in detail. The tests reveal that cyclic loads following a single huge load lead to a backward rotation of the foundation. This phenomena was also observed in-situ and for monopiles and can be explained on the basis of different settlement rates due to different void ratios in the subsoil on the leeward and the windward side. In order to quantify the amount of backward rotation, a so called Backward Rotation Index (BRI) is introduced. It is a useful quantity to generalise the experimental findings and is used to clarify the results of a sensitivity analysis where load amplitudes and average loads were varied. These tests reveal that already small symmetric load amplitudes lead to a significant backward rotation after a few hundred cycles. Higher load amplitudes result in faster backward rotation, while unsymmetric amplitudes with average loads decrease the rate of the backward rotation. The also presented settlements follow more or less straight lines in a semi-logarithmic diagram (settlements vs. logarithmic number of cycles) with an inclination depending on the load amplitude.