The Clair 1 platform was the first fixed offshore structure installed on the UK's Atlantic Margin. The seabed soils comprise thick layers of boulder-rich glacial clays whose unit weights are comparable to those of concrete (24 kN/m³) and undrained shear strengths up to 2000 kPa. This paper summarises the site investigations undertaken, ground conditions encountered, soil properties explored, foundation design and field monitoring measures.