ABSTRACT
The foundation stiffness under operational loads is a crucial point in the design of monopile foundations. As key design issues, structural eigenfrequencies as well as the structural fatigue loading are significantly affected. According to measurement results, the current calculation methods lead to an underestimation of the actual existing foundation stiffness. The reason is most likely that the stiffness is usually figured out from load-displacement curves for virgin monotonic loading, whereas actually the stiffness under un- and reloading conditions is relevant for the operational behavior. Concerning this matter, the proposed paper presents numerical simulations to derive the foundation stiffness under un- and reloading condition. Herein, the HSsmall soil model is applied, that accounts for the strain dependency of soil stiffness. By varying the load level and the cyclic load amplitude, it is demonstrated that the secant foundation stiffness for un- and reloading is only slightly affected by the load level and the magnitude of the un- and reloading amplitude. Hence, the application of constant foundation stiffness in the overall dynamic structural analysis appears reasonable.
