Influence of clay tectonics in the North Sea on offshore wind foundation design and installation (Clay Tectonics)

The occurrence of Clay Tectonic Features (CTFs) in Paleogene clay layers in the Southern Bight of the North Sea was first reported in the 1980s (e.g. Henriet et al. 1983 and Henriet et al. 1988). These features include polygonal fault networks and other deformations such as diapiric structures. One of these clay layers is the Kortrijk Formation which is the main substrate for the installation of the planned offshore wind farms (OWF) in the Princess Elisabeth Zone in the Belgian part of the North Sea. Even though the presence of CTFs in this formation is well documented, the effect of these features on the stability and geotechnical properties of the clay, especially under significant external forcing (e.g. piling of a monopile), is unknown and requires in-depth study. Moreover, evidence exists that faults were reactivated after they were formed, underlining that these features may negatively affect the stability of the clay. Therefore, the Clay Tectonics project will apply an integrated geophysical / geological / geotechnical approach to investigate the occurrence and formation of clay tectonic deformation features in stiff clay layers, as well as their influence on offshore wind foundation design and installation.

The following scientific objectives are envisaged:

• The development of an innovative field measurement and processing strategy to generate ultra-high resolution (pseudo) 3D seismic data volumes that adequately visualize the different types of CTFs occurring in the Kortrijk Formation. A thorough characterization of the CTFs in the selected study areas is foreseen, requiring that both large-scale elements (e.g. properties of the fault network as a whole) and small-scale elements (e.g. features related to individual faults) are well represented in the seismic data. In this regard, various acoustic techniques with different (high to ultra-high) resolution will be combined and a pipeline will be developed to optimally process and integrate the different datasets.
• A detailed geological analysis and reconstruction of the CTFs in the Kortrijk Formation to further elucidate their genesis and (potential) reactivation, as well as the underlying processes and parameters affecting the distribution, style and intensity of the deformations. This objective includes a careful mapping and characterization of the prevailing CTFs in the 3D seismic volume, an advanced statistical analysis of the variation in the prevailing seismic attributes of the deformation features, as well as a detailed investigation of reactivation features.

The integration of geophysical and geotechnical parameters for a realistic geotechnical characterisation of the Kortrijk Clay Formation (ground model), to be used for numerical simulations in the context of offshore wind development. In order to reach this objective a dedicated onshore field sampling survey of fault sections in the Kortrijk Formation is foreseen, followed by a thorough mineralogical and geotechnical characterization of the sediment. Finally, a detailed comparison of a design or installation assessment with and without due consideration of the faults will be performed to quantify the foundation-related risks.