Pile-soil interaction

Motivation and objectives

The capacity of offshore wind energy generated in Germany’s exclusive economic zone is to be steadily increased in the coming years. Despite this steady growth, the scenario for the end of life of the wind turbines - and especially their foundation structures - has not yet been fully determined. The German Federal Maritime and Hydrographic Agency (BSH) has set the operating period for all German offshore wind farms at a maximum of 25 years, unless otherwise approved. After this initial 25-year permit period, all structures above and below the waterline must be decommissioned in accordance with state-of-the-art technology. Possible options are a partial or complete decommissioning based on technical feasibility and the market readiness of a potential decommissioning method.

In light of the future reuse of land used for the construction of wind farms, the pursuit of a fully circular economy and environmental considerations, it is worth discussing whether the complete removal of foundation elements should be pursued. One possible method for the complete removal of the monopiles used as foundation elements is extraction using hydraulic overpressure. Large-scale tests using this extraction method were conducted at the IGG-TUBS as part of the DeCoMP research project. The GROW research consortium also conducted tests on this extraction method as part of the HyPE-ST research project. The investigations show that applying hydraulic pressure to the pile system influences the pile’s resistance to extraction. However, the mechanisms responsible for this have not yet been conclusively identified. Possible causes include stress degradation resulting from an expansion of the pile’s cross-section or hydrodynamic deformation of the grain structure, which alters the pile-soil contact area.

Small-scale experiments are currently being conducted at the IGG-TUBS, in which imaging techniques are used to investigate the movement of soil particles inside the pile, with the aim of better understanding the pile-soil interaction during the extraction process as well as the underlying failure mechanism. A half-model and a model for conducting experiments in a computed tomography (CT) scanner are available for the studies.