Cyclic shear strength for offshore wind gravity-based foundation analyses in glacial soils from an existing database

Abstract

The Gulf of Bothnia presents notable challenges for offshore wind development due to its metocean and geological conditions. The seabed substrate varies widely, from soft post-glacial sediments to dense glacial tills and mixed outwash deposits, with bedrock locally close to or outcropping at the seabed. These hard soils provide suitable conditions for gravity-based foundations (GBFs), but reliable design requires an understanding of cyclic soil behaviour under loading from wind, waves, and winter ice. This study evaluates the cyclic shear strength of glacial soils using Direct Simple Shear (DSS) contour diagrams derived from established databases and representative load histories from the Baltic and North Seas, with the aim of supporting preliminary GBF design in glacial environments. The influence of key soil properties, such as plasticity, density, and consolidation state, and the effects of partial drainage are assessed. The study also discusses key uncertainties associated with database representativeness, cyclic contour and stress-path selection, and load history generalisation. The results enhance the reliability of early-stage GBF assessments and support the sustainable development of offshore wind infrastructure in the Gulf of Bothnia and similar glaciated seabed conditions.