# Diatomaceous Soils and Advances in Geotechnical Engineering--Part I
The mechanical behavior and index properties of soils that record considerable fossil contents cannot be explained by applying the typical patterns associated with organic or inorganic soils. Some of the systems proposed in geotechnical engineering for soil classification--for example, those based on plasticity (Casagrande methods)--are inadequate for correctly categorizing this type of sediments [8; 14; 15; 16]. Multiple authors [17; 18; 19; 20] have warned that the results obtained from DSs are unexpected, inconsistent with classical soil mechanics theories, or present problematic or uncertain engineering properties [19; 21; 22; 23]. The abundance of fossils in a soil matrix can modify its physical, chemical, and geotechnical properties to such an extent that it differs from conventional results [8]. Ref. [24] reported large diatomic sedimentary deposits in Oita (Japan), where natural moisture contents are high (>141%) but simultaneously present high stiffness. The water-holding capacity is proposed to be associated with the fossil internal volumes and the pores that characterize the frustules. The authors of [11] associate the unusual behavior of these soils to the empty skeletons of the frustules, their irregular patterns, and their abrasive surfaces. However, there is no clear evidence to relate or quantify this aspect, either from the number of pores per unit area, size, or the irregularity of their surface [17; 25]. In the Colombian scenario, the lack of information regarding DS is such that the analysis of \" soils with special characteristics\" is restricted to the study of phenomena such as expansivity, collapsibility, dispersivity, the presence of vegetation, and the presence of water bodies [26], ignoring the effects of secondary consolidation, potential absorption, and high shear strength angles linked to the high liquid limits, usual in DS. Therefore, the characterization of these deposits should at least consider analysis by microscopy [13; 27] in order to recognize the presence of fossils, their concentration, and their state of preservation and, thus, to understand the incidence of these variables on the projects' performance. Few studies have attempted to investigate the relationship between mechanical properties and the microstructure of microfossil deposits [24]. Tests such as SEM, X-ray diffraction (XRD), or Porosimetry allow one to obtain a better understanding of the soil, which cannot be conceived by regular mechanical tests [13; 28]. In order to characterize and quantify the volumetric proportion of diatomaceous particles and relate them to the physical properties of soils, Ref. [15] extracted samplesfrom central and southern Oregon, based on which they made SEM observations and applied quantitative stereology procedures using manual particle selection. The accuracy of the method was validated by applying it to artificial samples dosed by weight (75% diatomaceous earth--25% kaolin). Some sources that have tried to quantify the fossil content in soil samples have applied indirect methods, for example by assessing the amorphous silica content through dissolution or X-ray diffraction. However, these methods do not provide information on the shape or porosity of the particles, factors that are quickly released by SEM observation [15] (see Appendix A). There are reports of places where considerable DS deposits have been identified [7; 29]--for example, in the Equatorial Pacific [30], Mexico [31; 32], Japan [33; 34], the USA [4; 35], Chile [18], China [23], Colombia [12; 19; 36; 37], Poland [38], as well as places with the presence of living diatoms, which leads us to infer that the underlying strata present fossil sedimentation. In Colombia, more than 73 places with microscopic algae are recorded [39]. It is essential to increase characterization and mechanical evaluation studies in all places that have a history of volcanic activity, such as the Pacific fire belt, since the presence of DS is enhanced there [18; 31; 32; 40; 41].