Offshore wind turbines are subjected to more significant wave and wind environmental loads at extreme weather conditions, making subsoil experience various loading stages with different amplitudes. To investigate the coupling effect of both cyclic shear stress ratio (CSR) and stage amplitude ratio (Ar) between normal and extreme weather conditions, a series of bi-directional simple shear tests with five different Ar and three CSR values were conducted on marine sand using the variable-direction dynamic cyclic simple shear (VDDCSS) apparatus. In the tests, soil samples were compacted under vertical stress and then sheared in undrained conditions by applying two shear stresses acting in different horizontal directions. Test results indicated that the cyclic strain, pore water pressure ratio, and cyclic strength were significantly determined by the value of stage amplitude ratios and the CSRs: at the same CSR, cyclic strains, and pore water pressure increased while cyclic strength decreased with the Ar. Comparing the test data between various cyclic stress ratios found that the CSRs can accelerate shear strains, pore pressure accumulation, and cyclic strength attenuation.

The cyclic loading of foundation structures in sand leads to an accumulation of plastic deformations in the structures. For shallow foundations of high and slender structures such as wind energy converters (WECs), an accumulation of the plastic rotations is expected under cyclic eccentric loading that is imposed by wind loads, which could be crucial for the proof of serviceability. A practical approach to predict the behavior of shallow foundations under high-cycle eccentric loading is under research. In this paper, a numerical approach, the cyclic strain accumulation method (CSAM), which has been validated for cyclically loaded monopiles, is adopted for shallow foundations under eccentric cyclic loading. Modifications to the CSAM are described, which are necessary to apply it to shallow foundations. The results that are gained with the modified method are compared with a medium-scale model test, in which the deformations of a footing with a diameter of 2.0 m under eccentric one-way cyclic loading were investigated. It can be concluded that the CSAM can make realistic predictions and shows satisfying agreement with the measured cyclic behavior. Although more experiments are needed to finally validate the method, the CSAM could be a promising numerical approach to account for the cyclic behavior of shallow foundations under eccentric cyclic loading in sand.