Up to now the IBF has dealt with gravity foundations as well as with pile foundations (namely monopiles) for offshore wind turbines. For both foundation types, small-scale physical model tests as well as numerical calculations have been carried out. Both foundation technologies are also used onshore. In the area of gravity foundations we are engaged in the scientific attendance of a large-scale physical model test of Ed. Züblin AG.
In regard to spread foundations, IBF offers consulting and advice related to questions like geometry optimization (plate, ring, single feet), installation, additional measures (skirts, grouting, score protection) and monitoring, in regard to pile foundations also to installation (driving or drilling of shafts).
Physical Model Tests
The IBF has performed and will carry out further small-scale physical model tests on complete wind turbine foundations. In the framework of a research project, such experiments have at first helped to a general understandig of soil-structure interaction.
A suitable model soil is filled into a test container with controlled density. A foundation structure (spread foundation or pile) scaled 1:50 to 1:100 is placed or driven. The loading is realized load-controlled in two orthogonal directions and displacements are recorded with extremely high accuracy. The influence of different loading sequences and foundation geometries on the accumulated deformations of the system can thus be investigated.
Finite Element Models
Numerical models are in many cases finite element models of a complete wind turbine foundation including the affected subsoil. The soil behaviour under cyclic loading is described using high-class constitutive relations like hypoplasticity with intergranular strain combined with the high-cycle accumulation model. The numerical models allow for class A predictions of settlements and rotations due to given loading sequences. They have to be validated against the results of physical models and field tests. For example, the calculations were able to simulate a back rotation which is typical for cyclic loading with decreasing amplitudes and could be of large economic relevance, but is not taken into account in common design methods.
Engineering Design Procedures
IBF is involved in the development of design procedures and rules and contributes from its background with cyclic laboratory tests, small-scale physical and numerical modelling. Engineering-like design procedures have to take into account the ultimate limit state as well as a limitation of deformations during the lifetime of turbines. Present research projects are focused on integrative design concepts for wind turbines involving the whole proofing chain from subsoil investigation and laboratory testing over load characterization to recommended calculation models.