During the last few years Ed. Zueblin AG has been designing a new type of gravity base foundation for multi-megawatt offshore wind turbines. As there is no proven experience available up to now, which allows the verification of calculation models for describing the mechanical behavior of cyclically loaded gravity base foundations, Zueblin decided to carry out full-scale model tests with their novel foundation type. The cyclic loading with more than 1 million load cycles is applied by hydraulic jacks and steel cables to the foundation, which is located inside a flooded construction pit close to the North Sea coast. For the measurements of pore pressures, total stresses and displacements in the subsoil, as well as of the foundation itself, approximately 150 measuring sensors in total have been installed. The tests focus on the possible accumulation of differential settlements and pore pressures in the subsoil due to the cyclic loading.
The validation and calibration of soil-mechanical models and calculation tools for the dimensioning of gravity base foundations for offshore wind turbines is carried out generally on the basis of monotonic and cyclic laboratory tests and small-scale model tests. Small-scale model tests are useful to get an idea of the principal mechanical soil behavior of this foundation type. Furthermore smallscale tests are an economic way to investigate the influence of different design parameters on the mechanical behavior of the foundation. These tests can be repeated easily after design modifications under identical starting conditions. However, the scale effect of small-scale model tests may lead to impreciseness or even failures, because it is impossible to scale down the grains of sand without changing their mechanical behavior. This scale-effect has to be evaluated and interpreted. The remaining uncertainties then may be eliminated by a monitoring of the behavior of real structures.