# Diatomaceous Soils and Advances in Geotechnical Engineering--Part I
## 3 Review of Progress in Geotechnical Lines
### Shear Strength of Diatomaceous Soils
Therefore, measurements conducted with this technique may present erroneous values for Su [44]. Ref. [6] investigated a type of DS which has been found in road projects in the Tengchong region of southwest China's Yunnan province. This soil was characterized by various methods such as sieve analysis, chemical analysis, XRD and electron microscopy analysis, shrinkage testing, uniaxial unconfined compression testing, triaxial shear testing, and direct shearing. The results revealed that \"Tengchong diatomaceous clay\" is an unusual deposit with both expansive clay soil and soft clay rock characteristics. Ref. [6] explains that diatoms improve the connection between microstructures and transform the mechanical properties of soil. Due to its significant number of expansive materials, Tengchong diatomaceous soil is an expansive soft rock prone to engineering problems and geological hazards. According to [2], the presence of diatoms increases the Atterberg limits and compressibility and allows high pore and low effective confining pressures during shear tests. Ref. [2] explains that all the organic soils used in their research presented high values of \(\phi^{\prime}\) under triaxial tests in CU conditions. They also mention that the strain mechanisms of the microstructural elements can help to explain the unusual behavior of this type of soil. The theory proposes the contributions of subhorizontal planes and non-organic microstructural elements (microfossil skeletons) to the high values of friction angles. However, this study does not discriminate based on the state of preservation of the frustules nor based on their concentration, shape, or species. With the above, Ref. [2] explains the reason behind the results obtained for Dutch organic soils when diatoms are present (extremely high effective resistance). The authors point out that these values are not as expected and that this phenomenon has yet to be fully explained. Even though the values obtained are high, they are not applied in practice. Figure 14 exposes the results presented by [25] regarding the \(\phi^{\prime}\) of multiple DSs of varied origin as a function of concentration; the parameter increase for any species or origin is highlighted. Table 3 presents references regarding values of effective friction angle \(\phi^{\prime}\) obtained from triaxial tests. These data are complemented by other characteristics of the deposits, the artificial soils with the presence of fossils, or the tests themselves, highlighting aspects such as the type of fill, the depth of sampling, the type of triaxial test, the shear condition evaluated, and the range of applied stresses, among others. The values of \(\phi^{\prime}\) recorded in the literature fluctuate between 22\({}^{\circ}\) and 65\({}^{\circ}\), highlighting values higher than 60\({}^{\circ}\) obtained in deposits with a high organic content. Other authors [37] evaluated the shear strength of DS of two origins (Mexican and Colombian), in peak and residual conditions, by using the \"Annular Shear test\" or \"Ring Shear test.\" Including both species in fine kaolin matrices increased the peak and residual strength, with these behaving similarly to dense sand. Regarding the friction angle, as the diatom content increased, the peak and residual conditions also increased. The Mexican sample at a 100% concentration reported friction angles greater than 35\({}^{\circ}\) in the peak condition and greater than 19\({}^{\circ}\) in the residual condition.