Geological, Geophysical and Geotechnical Aspects of Boulders and their Influence on Offshore Foundations

1. Home
2. Publications
3. Online Library
4. Geological, Geophysical and Geotechnical Aspects of Boulders and their Influence on Offshore Foundations

The presence and size of boulders are important drivers for the selection of offshore wind turbine foundations and cable routes. Inadequate understanding of the boulder issue can jeopardise the installation and performance of penetrating foundations and can involve significant delays and associated cost increases. For geotechnical design, this requires sufficient insight into where boulders come from, where they can be expected, how they can be detected, avoided and/or mitigated, and which boulders still can have an acceptable effect on the performance and integrity of the foundation. The paper briefly presents the geological, geophysical and geotechnical aspects of boulders from the perspective of foundation engineering. The first part explains that boulders can be expected in areas and stratigraphic levels affected during the Pleistocene glaciations. Deposits with potential boulders include glacial till, towards the bottom and along the slopes of tunnel valleys and its infill, or drop stones in glacial clays. The second part of the article discusses various geophysical techniques that can be used for boulder detection and the feasibility of selected methods, such as 3D UHR seismics, and acoustic coring. It shows that the costs of boulder detection increase significantly as the smaller boulders need to be detected. This raises the question of what boulder size should be detected. The third part of the article presents some simplified methods to assess the behaviour of a boulder during installation of intrusive foundations including downward/lateral pushing of the boulder and crushing boulders, and some installation experiences.

5th International Symposium on Frontiers in Offshore Geotechnics (ISFOG2025)

7 - Pile design and installation in challenging soil conditions: glauconite, carbonated soils, cemented soils, gravels and rocks