Characteristic Values for Geotechnical Design of Offshore Monopiles in Sandy Soils â€“ Case Study-Paper

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The paper presents guidance for selecting geotechnical parameter values for conceptual design of offshore wind-turbine monopiles in the Hollandse Kust (noord) wind farm zone, where soils are predominantly sandy. The Dutch government made the guidance and supporting site data publicly available to help developers prepare tenders before turbine locations were fixed.<br /><br />The ground model combined seismic interpretation with data from 33 boreholes and 75 cone penetration tests (CPTs). CPT profiles were grouped into four categories and spatially allocated across the site, helping designers account for variations in ground conditions. Geological processes and post-depositional changes limited the correlation between many CPT groups and specific geological units.<br /><br />A central challenge was that practical tests often provide derived values rather than direct measurements of the parameters needed for design. The authors distinguish Eurocode 7 “derived values” from “characteristic values” used in limit-state design, and introduce “indicative values” as preliminary estimates of in-situ conditions. Indicative low, mean and high values provide a starting point for designers to select characteristic values.<br /><br />The guidance covered 17 parameters relevant to monopile design. For the coefficient of earth pressure at rest (K₀) and small-strain shear modulus (Gmax), the paper illustrates how CPT-based estimates were assessed alongside laboratory and in-situ methods. Selection considered method limitations, statistical variation, geological context and uncertainty in converting test results to representative in-situ values. For Gmax, for example, a relatively broad indicative range reflected substantial uncertainty across available methods.<br /><br />The study emphasizes that uncertainty—especially systematic bias between measured or correlated values and actual in-situ properties—must be addressed explicitly. Its integrated ground model and parameter guidance helped reduce project risk and support a smoother tender process, while leaving detailed-design teams responsible for deciding whether additional site investigation is needed.

Keywords

offshore wind turbine

monopile design

geotechnical parameters

Hollandse Kust Noord

ground model

cone penetration tests

sandy soils

Eurocode 7

characteristic values

small-strain shear modulus