Laboratory-prepared iron oxide coatings on sands: Submicron-scale small-strain stiffness

Chemical weathering of iron-bearing rock minerals results in the formation of iron oxides, which can chemically adsorb onto soil surfaces, and/or absorb into their molecular structure. The objective of this paper is to use laboratory-prepared, iron oxide-coated sand to study the role of iron oxide coatings on the small-strain stiffness of coarse-grained soil. Ottawa sands were geochemically coated with iron oxides of goethite and hematite via a heterogeneous suspension reaction in the laboratory. Specimen preparation techniques were chosen to ensure that the soils were not cemented during preparation. SEM images, ICP analysis, and geotechnical index tests were used to characterize the submicron-scaled iron oxide coatings, and the bender element method was used to measure the shear wave velocities of uncoated and the two iron oxide coated sands. Contact mechanics and submicron scale mechanistic approaches were explored to interpret the experimental data, and results indicated that small-strain stiffness of iron oxide coated sands was higher than that of uncoated sands. Results also suggested that a small-strain stiffness behavioral hierarchy associated with iron oxide thermodynamic stability may exist. This study demonstrated that iron oxide coatings significantly influence sand grain-to-grain behavior, even in the absence of cementation or augmented contact area effects, due to an increase in the number of particle contacts present in the iron oxide coated sands.

► VH stiffness under K0 loading is higher for Fe oxide-coated than for uncoated sands.
► Fe oxide coatings boost the vertical-stress stiffness effect over that of void ratio.
► Fe oxide coatings enhance sand grain-to-grain number of contacts and inter-locking.
► Adsorbed coatings of goethite and hematite onto quartz are discrete, sub-micron sized.