Geological and geotechnical characterisation for offshore wind turbine foundations: A case study of the Sheringham Shoal wind farm

• Soil conditions of a realistic offshore wind farm are characterised.
• Characterisation focuses on important soil parameters for wind turbine foundations.
• Soils have high stiffness and strength due to overconsolidation or dense state.
• Brittleness and variability of chalk can be an issue during pile driving.
• Stiffness degradation and soil variability is a challenge for foundations design.

This paper characterises the soil conditions at the Sheringham Shoal wind farm in the North Sea, in the context of designing foundations for offshore wind turbines. The purpose is to provide a realistic reference case study for research and practice in the emerging field of offshore wind energy. The soil data was obtained between 2005 and 2008 in an investigation program involving field measurements and laboratory experiments. The characterisation focuses on soil properties which are relevant for evaluating the performance of foundations for offshore wind turbines. The soil profile at the Sheringham Shoal site consists of four main units: two clay strata interbedded with a sand layer and underlain by a chalk bed. The heavily overconsolidated state of the clays and the densely compacted state of the sand dictate the behaviour of these soils. The chalk bed is characterised by varying degrees of weathering leading to significant variation in properties ranging from those typical for stiff soil to soft rock. These four main units, though differ considerably in soil composition, share several common characteristics including high shear strength and high stiffness but their shear stiffnesses reduce significantly both at high mobilised static shear strength and at medium to high mobilised strength under cyclic loading. The soil stiffness is an important input parameter for the design of offshore wind turbine foundations and hence the degradation of stiffness is an important issue at this wind farm. A number of soil properties at the site show considerable variability which requires a specific foundation design tailored for each location in order to be both safe and cost-effective.