Design Focused Ground Models for Offshore Rock-Socketed Pile Solutions: St Brieuc OWF

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The St. Brieuc Offshore Wind Farm, located 16 km off the coast of Brittany, uses jacket foundations supported by drilled and grouted rock-socketed piles. Its geology includes several bedrock formations, dolerite dykes, and variable superficial deposits and weathered rock, creating uncertainty for foundation design and installation.<br /><br />The project adopted a phased investigation strategy, combining four generations of geophysical surveys with two phases of borehole and in-situ geotechnical testing. Early findings informed turbine layout and concept design. An evolving 3D observational ground model integrated seismic reflection and refraction data with borehole and cone penetration test results. It predicted the elevations of key boundaries: the base of superficial deposits, the top of residual soil and weathered rock, and the top of fresh, high-strength bedrock. The model was updated as new investigation data became available and used to target later surveys and boreholes.<br /><br />Outputs included engineering terrain-unit maps, location-specific stratigraphic summaries, cross-sections, and mapping of potential constraints such as bedrock outcrops and dolerite dykes. These materials helped assess ground conditions at each turbine and substation location, guide further investigation, and support potential turbine micro-siting.<br /><br />For pile design, location-specific geology and rock properties were used to determine embedment requirements and account for geohazards. The model also provided confidence levels for critical geological boundaries; where confidence was low, additional pile-length allowance was included. An onshore pile load test helped validate the design methodology.<br /><br />The case study demonstrates how design-focused 3D geological modelling can improve the targeting and value of offshore ground investigations, clarify site variability, and reduce design and installation risks for rock-socketed foundations.

Keywords

St. Brieuc Offshore Wind Farm

offshore wind farm

3D geological modelling

jacket foundations

rock-socketed piles

geotechnical investigation

seismic survey

borehole testing

bedrock geology

pile design