The composition of freshwater and saltwater lacustrine soft soil deposits in Mexico City

摘要

Clays from the Valley of Mexico show an extraordinary water content and a high plasticity index and deformability. These characteristics are generated by three main factors: microstructure, composition and load history. Peralta & Fabi, 1989, and Rodriguez, 1999, proposed that these atypical moisture values are mainly due to a high content of diatoms, and Carreon-Freyre et al. (2006) suggested that these values are due to the predominant presence of allophane; however, these studies are inconclusive. This paper presents the results of a study on the composition and microstructure of clay deposits from the Mexico City Valley carried out with a variable pressure Scanning Electron Microscope (SEM), X-ray diffraction analysis (X-ray) and conventional soil mechanics laboratory tests. The results from this study show that these deposits have an irregular, dispersed and flocculated microstructure and contain mainly montmorillonite and illite minerals. The carbonate content up to 7%, the moisture ranges between 40% and 412%, the plasticity index values vary between 10 ＜ PI ＜ 290%, and these clays contain varying percentages of diatoms and other components, such as pyrite framboids and ostracods, in very small percentages. Many types of diatoms were observed, including the following main species: campylodiscus clypeus, anomoeoneis sphaerophora, cyclotella meneghinianay cocconeis scutellum and placentula, which are species mainly from brackish water. The results suggest that the main factors that generate high humidity in these soft soil deposits are the microstructure (irregular, dispersed and flocculated, which is influenced by the sedimentation rate) and composition (carbonate content, smectite content and diatom type and concentration, which are influenced by volcanic activity and salinity). The diatom type and content are not principal factors in the generation of the high water content as mentioned previously.