Cyclic loading could lead to progressive degradation of soil in terms of plastic strain accumulation, pore pressure build up and changes in soil strength and stiffness. The present paper outlines an explicit method to predict the accumulated foundation displacement under cyclic loading. Cyclic contour diagrams, derived from cyclic laboratory tests, are linked to the finite element software PLAXIS by means of a Python interface. The effects of cyclic loading on the accumulation of strain is taken into account by the modification of the elastic shear modulus of the soil in a cluster-wise division in the finite element mesh.
The design of wind turbines relies on knowledge of different engineering disciplines with multiple interfaces. For the design of the foundation structure, iterative loops between the wind turbine manufacturer and the foundation designer (incl. the geotechnical design team) are needed to update the load calculations at the interface level (bottom of tower). Wind turbine manufacturers provide the irregular variation of fatigue and extreme loads by means of aeroelastic and hydrodynamic analyses, which in turn provide the basis for the foundation design.
In each loop the geotechnical designers have to ensure that the foundation obeys a series of criteria such as foundation capacity, when experiencing the maximum load (ULS), and the prediction of the foundation response during the lifetime of the structure (SLS). The analysis of the foundation capacity can be done by applying simplified approaches such as a Winkler model approach using p-y curves or by use of finite element modeling, cf. DNV GL AS (2016). The prediction of the foundation response over the lifetime of the structure remains challenging.
During a lifetime of 25 years, offshore structures undergo millions of irregular loading cycles distributed randomly over time and direction (Andersen et al. 2013). These cyclic loads are likely to cause relevant changes in the behavior of the soil-structure interaction with time. In this regard, different requirements should be addressed, such as: ensuring bearing capacity following cyclic loading, which may differ from the capacity under monotonic loading; assuring that the cyclic displacement of the structure obeys the requirements provided by the wind turbines manufacturer; assessing the change in the foundation stiffness and damping; and ensuring stability against the pore pressure build up during a cyclic storm event.