Impact of diatoms on the load-deformation response of marine sediments during drained shear: Small-to-large strain stiffness

A comprehensive characterization of the seafloor is difficult, as natural marine sediments comprise granular materials. In particular, the presence of diatoms in granular mixtures significantly alters the engineering properties and behavior. Herein, we investigate the critical role of diatoms in the load-deformation response, shear strength, and small-to-large strain stiffness in sand-silt-diatom mixtures during the drained shear process. Mixtures with different fines contents (FCs) of 0%, 10%, 20%, and 30% were prepared in a triaxial cell incorporated with bender and piezo disk elements for elastic wave measurements. Triaxial tests were conducted under four different confining effective stresses, and elastic wave signals were monitored at every 0.5% of the axial strain. Test results show that a distinct transition exists in the stress-strain response and shear strength characteristics at FC = 20%. Using volumetric gravimetric analysis, we could clearly anticipate percolation threshold FC, where the specimen with 20% FC appeared to be on the boundary between the sand-controlled and transition zones in the textural triangular chart. For coarse-dominant mixtures (FC < 20%), silts enhanced particle interlocking and contributed to an increase in the contact area. In contrast, the internal friction of transitional mixtures (FC = 20%, 30%) decreased when a diatom with an excessively low shear modulus participated in load-carrying mechanisms. Using large-strain stiffness, we assessed fabric changes that reflect friction and the coordination number. Using small-strain stiffness from elastic waves, we estimated the state for a constant fabric. The characteristics of the moduli at large strain and those at small strain were distinctly different.