Glauconite Sand Challenges for US Offshore Wind Development

Abstract

Abstract Glauconite sand, otherwise known as ‘greensand’, is a challenging sediment type that can pose significant risks to foundation installation and performance. This is due to its tendency to transform from a stiff, high permeability coarse-grained material to a weak, low permeability fine-grained material due to particle crushing. Glauconite is an iron potassium mica, characteristically green and often found in peloidal form. It forms under reducing conditions within shallow marine depositional environments and has been found in coastal regions of the USA including locations along the Atlantic Continental Shelf associated with offshore wind farm developments. Due to its friable nature, glauconite affects the geotechnical properties of the sediments in which it forms. Geotechnical laboratory tests performed on glauconite sand samples reveal a wide range in particle size gradation and high specific gravity. Intact glauconite sand exhibits high strength, but under moderate disturbance it readily degrades into a clay-like material, increasing plasticity, reducing shear strength, and exhibiting strong thixotropic behavior. Given its tendency to crush, in situ testing with cone penetrometers produces high tip resistance and high sleeve friction, limiting the usefulness of standard soil classification charts. This paper presents an overview of the geological basis for glauconite sand formation, describes its depositional environment and maturation process, and presents results from recent tests performed on glauconite sands from the USA. Implications on pile installation and performance are discussed.