Causes of the high friction angle of diatomaceous soil: microscale and nanoscale insights

Abstract

Diatomaceous soil has geotechnical properties that differ fundamentally from those of common non-diatomaceous soils due to the presence of diatom microfossils with biological origins. Despite its dominant fines content, diatomaceous soil usually has high frictional shear resistance (approaching that of sand). Currently, the exact role of diatoms in controlling soil strength and underlying mechanisms remain obscure. Here, the frictional strength of diatomaceous soil is evaluated by way of angle of repose and direct simple shear tests on diatom–kaolin mixtures with differing diatom content. The microscale and nanoscale structures are characterised in detail using scanning electron and atomic force microscopy to establish how soil structure evolves with diatom content and shear. For the studied diatom–kaolin mixtures, the angle of repose and internal frictional angle are high and increase with diatom content, especially when the diatom content exceeds 20%. Diatom content controls the frictional strength through its intricate morphology (cylindrical, saucer and disc shapes), very rough surface (hundreds of times rougher than flaky minerals) and stiff frustules with high Young's modulus. These features increase the particle coordination number and produce interparticle interlockings, both of which prevent particle rearrangement during shear and improve the frictional strength. This paper provides new insights into the multiscale structure of diatoms and improves understanding of the shear strength of diatomaceous soils.