Small-Scale Testing Rig for Long-Term Cyclically Loaded Monopiles in Cohesionless Soil

Monopiles used as foundations for offshore wind turbines are exposed to strong cyclic loading due to the wind and wave forces. This cyclic loading may induce an accumulated rotation and a change in stiffness with time depending on the mean level (i.e. the static loading), the amplitude, and the period of the cyclic loading. However, the design guidance on these issues is limited. Thus, in order to investigate the pile behaviour for cyclically long-term loaded monopiles, a test setup for small-scale tests in saturated dense cohesionless soil is constructed and presented in here. The cyclic loading is applied mechanically by means of a testing rig, where the important input parameters: mean level, amplitude, number of cycles, and period of the loading can be varied. The results from a monotonic and a cyclic loading test on an open-ended aluminium pile with diameter = 100 mm and embedded length = 600 mm proves that the test setup is capable of applying the cyclic long-term loading. The plastic deformations during loading depend not only on the loading applied but also of the relative density of the soil and, thus, the tests are carried out with relative densities of 77-88%, i.e. similar to those for an offshore condition. The force-rotation relationship obtained from the long-term cyclic loading test indicates that the rotation does not accumulate after a certain number of cycles. This, however, must, for validation, be further investigated against many more cyclic tests with varying magnitude of the cyclic loading.