Effect of Glauconite Content on the Compression and Shear Behavior of Sand

Abstract Glauconite sand presence at several offshore wind lease areas in the United States and Belgium has raised geotechnical concerns due to their high crushability and potential for clay-like behavior, introducing uncertainty in pile installation resistance and axial response. Field experience suggests that even relatively low glauconite content can affect glauconite sand behavior, yet no systematic study has isolated this factor as a governing variable. The compression, direct shear, and interface shear response of idealized silica–glauconite sand mixtures containing 0%, 10%, 25%, 50%, 75%, and 100% glauconite particles by weight was investigated under controlled density, moisture, confining stress, and interface roughness conditions. Direct and interface shear tests were conducted primarily under wet (near-saturated, deionized water) conditions at 150 kPa normal stress, with additional direct shear and one-dimensional compression testing at 1,500 kPa normal stress under both wet and dry conditions. Steel interface roughness spanned smooth ground steel to very rough three-dimensional–printed steel surfaces. Particle image velocimetry (PIV) was used to quantify shear-zone development and local strain fields. Results show markedly increased compressibility with glauconite content, especially under wet conditions. Under wet shearing, mixtures progressively transition from dilative, strain-softening behavior (silica-dominated) to contractive, strain-hardening behavior (glauconite-dominated), with multiple indicators supporting a transitional glauconite content between ∼ 25 % and 50% by weight. At higher normal stress under wet conditions, peak and residual direct shear strengths converge across mixtures while contraction increases significantly with glauconite content. Interface friction decreases with increasing glauconite content on smooth steel but increases with roughness for pure glauconite as the mechanism shifts from sliding to soil–soil shearing; PIV measurements confirm associated changes in shear-zone thickness, strain localization, and volumetric response. The findings provide a mechanistic framework for interpreting pile–soil interaction in glauconite deposits while highlighting dependence on the glauconite content, stress level, moisture, and interface condition.