Influence of diatom microfossils on sediment shear strength and slope stability

Abstract

Diatom microfossils have been detected in many natural marine sediment deposits around the globe and are held responsible for the disobedience to well-established geotechnical relationships between index-properties and shear strength. We revisit the static shear strength and present the first cyclic undrained shear strength experiments on diatom microfossil—clayey-silt mixtures to study the role of diatoms on submarine slope stability. It is attested that the angle of internal friction (Φ) increases with diatom content, however, we provide evidence for a significant overestimation of Φ in previous studies. Based on direct shear tests at varying normal stresses ≤ 600 kPa we find Φ = 32° in contrast to 43° in pure diatom. Similarly, to static shear strength, cyclic shear strength increases with diatom content, however, in contrast to static shear strength the most drastic increase does not occur from 0% to 25% diatoms but from 75% to 100%. Interestingly, diatomaceous sediments tend to fail by liquefaction although well-established relations between index properties and liquefaction susceptibility predict the opposite. Liquefaction failure is observed solely in samples containing ≥ 50% diatoms whereas samples with lower diatom content fail by cyclic softening. We conclude diatom microfossils in marine sediments significantly contribute to an increased slope stability under static and cyclic loading conditions since diatoms lead to higher resistance independently of the loading mode. The strength increase is interpreted as a result of particle interlocking and surface roughness, which is very efficient given the highly variable habitus of diatom species.