# Diatomaceous Soils and Advances in Geotechnical Engineering--Part I
The grain size distribution of DSs generally classifies them as fine soils, mainly within the silt range (0.002 mm \(<\) SD \(<\) 0.075 mm). The presence of diatoms (frustules) leads to an increase in the liquid limit (LL) [32; 40] and plasticity index (PI) [12], simultaneously with an increase in the effective friction angle (\(\phi^{\prime}\)) [18; 42]. This contradictory behavior has not been fully understood, nor has it been associated with the diatom species present in the studied samples. Some researchers have attempted to explain the high values of shear strength in diatomaceous deposits, in terms of \(\phi^{\prime}\), through an assumption of interlocking between soil particles, of which some are fragments of frustules [2]. However, in this scenario, the correspondence between increases in resistance, the concentration of fossils, the state of deterioration (fracturing) of the pieces, the representative forms of the species, and the compositional condition (single or multispecies) is not precisely defined. The relationship between diatoms and soil particles has not been wholly characterized [32]. Ref. [8] suggests that a way to evaluate the concentration of diatoms in a given soil should be found so that it can be related to the geotechnical behavior of the latter. It is concluded that the state-of-the-art regarding the variation in the physical properties and mechanical response of soil deposits with the presence of diatom fossils is poorly developed [15; 18]. Published results are generally limited to the report of friction angle (\(\phi\)) values and consistency limit records [12; 32; 37]. Ref. [8] state that many Japanese soils do not follow the patterns that regularly describe soil behavior because they contain significant amounts of diatoms, while [15] posit that diatomaceous soils significantly affect the engineering behavior of the medium, even when they are found in tiny proportions (20%). Although the presence of fossils in soils has been widely assessed from a geological point of view and their geotechnical performance has been observed, explanations for unexpected or non-typical responses have not yet been given. Considering the presence of DS in different parts of the world and its unique behaviors, its influence should be recognized and it should be considered as a distinct soil type in the classification of marine sediments [43]. A new classification system for \"fossiliferous soils\" is considered for inclusion in addition to the existing classification of organic and inorganic soils [8; 44]. Few documentary sources elucidate the influence of species on DS behavior. When they do, are limited to the description of immediate details such as particle size variation and frustule content [14], but do not delve into subjects such as mechanical response or dynamic effects. The authors of [44] compared the geotechnical behavior between samples with the presence of centric and pennate particles. The latter, contained in a diatomaceous ooze, is associated with an open and less dense fabric, high void and water content, and high porosity. This fabric collapses rapidly with increasing vertical stresses. Diatomaceous oozes with mostly pennate particles have a more remarkable ability to reproduce in situ stress conditions when subjected to one-dimensional consolidation tests. This type of diatomaceous ooze is prone to generate high pore pressures under seismic stresses and,therefore, they are cataloged as factors that generate submarine landslides [44]. On the other hand, the disk shape of the centric particles is related to a denser, naturally oriented fabric, which results in a more resistant structure. The effect of diatom species on geotechnical responses is a relevant topic for future research [44]. Diatom-rich sediments show different behaviors when evaluated in the laboratory and analyzed in their natural condition. It has also been concluded that these soils somehow support the lithostatic and tectonic forces that are slowly transmitted to them.