# Diatomaceous Soils and Advances in Geotechnical Engineering--Part I
## 3 Review of Progress in Geotechnical Lines
### Compressibility in Diatomaceous Soils  
Significant magnitudes varying between 80 and 90% have been reported regarding the void content present in DS or diatomites [65,80]. Diatomaceous deposits are composed of silica frustules with rough surfaces and harbor pores, which confer a low soil density and high sorption potential [18]. Some compressibility studies on DS have been carried out in undisturbed samples recovered from deposits [18] or artificial mixtures [5,23] that generally feature a fine clayey matrix of kaolin or bentonite type [41], in which the content of frustules can be controlled [12,32]. DSs exhibit a higher compressibility range than fine soils with a similar geotechnical classification [18]. DSs have large intraskeletal porosities and perfect skeletal pore networks. As consolidation develops, the clay matrix accompanying the fossils reorganizes around the fossils, almost as if an arching phenomenon is applied [8]. The presence of fossils within a saturated, fine-grained matrix leads to a higher soil compressibility. However, the strain mechanisms involved still need to be better understood [29]. Oedometric tests conducted in DS have shown values of the Cc/Cs ratio (normal compression index/recovery index) greater than 30, a considerable magnitude, as fine soils record values ranging from 3 to 7 [18]. The Cc values obtained in DS in undisturbed conditions (Cc: 4-8) or even remolded conditions (Cc: 2.1) are higher than those of fine clayey soil with an acceptable configuration and fabric (Cc: 0.44-1.78 Louisville Clay--Canada); this is due to the breakdown of the diatomaceous skeletons and rearrangement [40]. With the ample capacity of microfossils to harbor water within their rigid structures, it is not unexpected that these soils might report high compression index values, even at considerable depths. Diatomaceous oozes have been studied in the North Sea and found to have Cc values as high as 5.0 [8]. In artificial samples made up of 100% diatomaceous earth mined in Luneburg Heath (Germany), it was observed that the compression index (Cc) and the secondary compression index (Ca) increase along with the diatom content and vertical stress. This soil reports a constant increase in Cc, reaching values of 2.9 for vertical stresses of 1600 kPa [44]. Differential behaviors have been recorded for submarine sediment samples classified as diatomaceous ooze extracted from the South Sandwich Trench (British Territory located at the southern end of the Atlantic Ocean bordering Argentina to the west), in which peak values of Cc >6 have been reported for vertical stresses near 20 kPa, followed by a reduction to 3.5, a magnitude associated with vertical stresses of 400 kPa. Ca values of approximately 0.065 have been recorded in 100% diatomaceous earth samples for vertical stresses of 1600 kPa. For diatomaceous oozes, the Ca value has been found to be close to 0.1 for vertical stresses of 200 kPa [44]. For soil samples obtained at a depth of 250 m from around Kansai Airport (Japan), abundant in diatoms, Cc values of 4.7 have been obtained. In this case, an alternation between normally consolidated (NC) and overconsolidated (OC) layers was identified. The observed behavior found by the authors [8] indicated that the OC layers presented a higher or stronger content of microfossils. In contrast, the NC layers reported either a low concentration or a high deterioration of frustules, which was generated once the yield stress was overcome due to the effect of deposition. Therefore, the Cc values expected for soils in the OC condition are higher than those that might be expected for the NC condition. Marine sediments with diatom microfossils are systematically associated with a decrease in consolidation state, going from \"highly overconsolidated\" in the upper 100 m below sea level to the \"normally consolidated\" condition below that depth.