Geotechnical Properties of Subglacial Till at Baltic Sea Offshore Wind Farm Sites

Abstract Detailed geotechnical soil investigation surveys have been performed with onshore laboratory testing and interpretation to support the development of two wind farm sites, herein denoted Sites 1 and 2, in the Baltic Sea, offshore Poland. The two wind farms are planned to have about 50 wind turbines each with a total combined capacity of about 1.4 GW. Large diameter monopiles have been selected as the foundation concept with a target embedment length ranging from 25 m to 40 m. The presence of an overconsolidated, high to ultra-high strength subglacial till layer, located at a depth of about 20 m to 40 m below seabed, is critical for the monopile foundation design. This layer presents significant challenges in terms of pile drivability, while also providing support to the monopile foundation design, both for the foundation capacity as well as the overall structural stability and integrity. The onshore laboratory testing included classification testing, static and cyclic strength testing, and testing to determine in-situ stress history and deformation properties. The interpretation included the integration of the geophysical data, geological, in-situ testing, and geotechnical laboratory testing to derive representative soil parameters for the subglacial till. X-ray scans showed signs of a complex geological process having taken place. The subglacial till is heterogeneous, with sandy, gravelly clay alternating with homogenized deformed clay layers. It exhibits poor sorting, including intermittent sand layers and erosive contacts at various depths. Seismic reflections display varying amplitudes and facies characteristics, reflecting its intricate geological history linked to the last glaciation event. The presence of inclined layers, deformed laminations, and fractures help to confirm glaciotectonic origins. Key parameters for strength assessment are presented. The cyclic strength properties are compared to experience from literature offering insights into its cyclic loading behavior. The thorough investigation provides essential insights into the geotechnical properties of subglacial till at the two offshore wind farm sites, crucial for designing reliable monopile foundations in challenging marine environments.