Implications of glauconite sands on US offshore wind development

Glauconite is a high specific gravity, greenish black iron-potassium pellet found in shallow marine depositional environments along the US Atlantic Continental Shelf in areas associated with offshore wind developments. Glauconite affects the geotechnical properties of sediments in which it forms and poses risks to foundation installation and performance due to its transformation from a coarse-grained to a fine-grained material due to crushing. Dense, authigenic deposits exhibit less compressibility than other crushable soils, but degrade rapidly under loading due their high internal porosity, linked to their maturity. In situ cone penetrometer testing results exhibit high tip resistance, high sleeve friction, and positive or negative excess pore pressures depending on microstructure, hindering site characterisation using standard soil classification charts. Examples of high glauconite content (> 90% by weight) sand behaviour from onshore deposits along the Atlantic coast are presented, as part of the Joint Industry Project: Piling in Glauconitic Sands (PIGS). Soil properties are shown to be highly sensitive to initial specimen conditions and preparation method, affecting particle size distribution, plasticity and thixotropy, and shear strength. Its implications on offshore geotechnical site investigation and pile foundation design are discussed.