Effects of ferric oxyhydroxide coatings on sand shear response: a laboratory approach to chemical weathering

摘要

Geomaterials can exhibit dramatic changes in mechanical behavior upon chemical and physical weathering. Specifically, chemical weathering of iron Fe(II) minerals yields products, including iron Fe(III) oxides, hydroxides and oxyhydroxides, which precipitate as coatings on rock and soil surfaces due to their low aqueous solubility. The general objective of this research is to readily isolate, for controlled study in the laboratory, the effect of such chemical weathering on the stress-strain relationships and strength of coarse-grained soils. Particles of fine silica sand were coated by chemical adsorption of the iron Fe(III) oxyhydroxide akaganéite onto the sand surface under controlled pH and ionic strength conditions, and at standard pressure and room temperature. Akaganéite is a chemical weathering product formed by precipitation after oxidation of iron Fe(II) minerals, and is a precursor of the chemical weathering products goethite and hematite. To identify the effect of the oxyhydroxide coating on soil behavior, both the coated sand and uncoated control samples were tested in direct shear. Chemical-weathering effects were thus evaluated independently from stress history, parent rock discontinuities, and biological influence. The results presented herein show that the oxyhydroxide coating increased the residual strength, dilatancy and friction angle of the sand, with no strain delay exhibited in the peak shear strength.