The siliceous structure that protects diatoms, called frustule, is the main component of diatom sedimentary soils. These particles' physical and mechanical characteristics are challenging, given their geometric conditions of only a few microns. For this evaluation, specialized tools must be used, such as the Scanning Electron Microscope (SEM), the Atomic Force Microscope (AFM) and X-ray dispersion (XRD), among others. The bibliographic references show significant variability in the load-deformation behavior in frustules, diatoms or their organic components. Technical background information usually presents information on a single type of species. This research demonstrated the characterization and micromechanical evaluation of frustules of three morphologically distinguishable species of diatoms (Colombian, Mexican and Peruvian origin). The results showed similarities in the chemical composition of the three samples. The displacement records are variable depending on the species for the same load range. The location of the load application points by AFM on the different types of frustules is presented. The most significant deformation in the Mexican species and the regularity in the results of the Peruvian species stand out. Young's moduli were also calculated by applying the Hertz Model, which had the highest values in the Colombian sample.

Diatomaceous soils, composed of diatom microfossils with biological origins, have geotechnical properties that are fundamentally different from those of conventional non-diatomaceous fine-grained soils. Despite their high fines content, diatomaceous soils typically exhibit remarkably high shear resistance, approaching that of sandy soils. However, the exact role that diatoms play in controlling the mechanical properties of fine-grained soils and the underlying mechanisms remain unclear. In light of this, the shear strength response of diatomaceous soils was systematically investigated using consolidated undrained triaxial compression tests on diatom-kaolin mixtures (DKMs) with various diatom contents and overconsolidation ratios. The micro- and nano-scale structures of the soil samples were characterized in detail using scanning electron microscope (SEM) and atomic force microscope (AFM) to interpret the abnormal shear strength parameters of diatomaceous soils. The results indicated that the presence of diatoms could contribute to significantly higher strength, e.g. the friction angle of DKMs was improved by 72.7% to 37 degrees and the value of undrained shear strength tripled with diatom content increasing from 20% to 100%. Such significant improvement in soil strength with diatom inclusion could be attribute to the hard siliceous skeleton of diatoms and the interlocking between particles with rough surfaces, which were quantitatively analyzed by the surface roughness parameters with AFM. Furthermore, a conceptual model established based on the macro-mechanical tests and microscopic observations portrays a microstructural evolution of soils with increasing diatoms. The microstructure of soils was gradually transformed from the matrix-type to the skeletal one, resulting in a continual augmentation in shear strength through mutual interactions between diatom microfossils. This paper provides new insights into the multi-scale structural properties of diatoms and significantly advances our understanding of the mechanical behavior of diatomaceous soils. (c) 2024 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

Diatomaceous soils as a special soil containing diatom frustules are widely distributed in marine deposits. Although their soil properties have been partially tested and reported which exhibit high void ratio and compressibility, a thorough investigation is in lack. Moreover, the interpretation of the cone penetration test (CPTu), a widely-used in-situ test, to the properties of such soil is still unknown, hindering a proper estimation of the diatomaceous soils in engineering geology and geotechnical practice. This study reports the soil properties and mechanical behavior of the undisturbed diatomaceous soil obtained from Walvis Bay in Namibia, which shows that the soils had an extremely high void ratio (similar to 4.39) and low specific gravity (similar to 2.16), and consequently low density and high compressibility. Due to the porous property of diatom frustules, the diatomaceous soils also exhibited a high liquid limit and plastic limit. Although these properties provide an identification to super soft soils, the diatomaceous soils had discrepant mechanical properties, i.e., the values of strength parameters in diatomaceous soils were higher than those in typical soft soils. At the end, by comparing the results of strength parameters obtained from the laboratory tests on undisturbed soil samples and the in-situ CPTu records, this paper proposes the interpretation methods of CPTu results to the strength parameters (effective friction angle and undrained shear strength) in the diatomaceous soils.