Abstract: Diatomaceous soil is a kind of natural sedimentary soil that formed in lacustrine or marine environment, mainly composed of clay minerals and diatom remains. The light weight and high porosity of diatom results in a significant difference of physical properties between diatomaceous soil and common clayey soil without diatom, such as low density, high porosity and high water content. These physical properties cannot be predicted by empirical equations of conventional soil mechanics and still need to be explored. This study measured typical physical indices of particle size composition, specific gravity, specific surface area and Atterberg limits of diatom-kaolin mixtures at different diatom contents to enhance systematical comprehension of physical properties of diatomaceous soil. Also, the scanning electron microscope tests were performed to reveal the microscopic mechanism of these physical properties of diatomaceous soil. The results indicate that increasing diatom content causes increases in silt fraction, specific surface area and cation exchange capacity, and a decrease in specific gravity. As the diatom content increases, the Atterberg limits show ascending tendency with pore fluid being NaCl solutions of different concentrations. Despite both the liquid and plastic limits increase, the plasticity index remains almost unchanged. The microscopic investigation suggests that the abovementioned physical properties of diatomaceous soil mainly depend on the large inner cells and high water-retention capacity of diatoms. The tests also show that diatom-kaolin mixtures with diatom content higher than 80% possess low plasticity or nearly non-plastic during the test despite of the high Atterberg limits of pure diatom. This phenomenon means that the Atterberg limits cannot reflect the plasticity of diatomaceous soil, and the current classification methods for fine-grained soils according to Atterberg limits are inappropriate for diatomaceous soil. This study can provide data reference and theoretical support for research on engineering behaviors of diatomaceous soil.

Key words: diatom, diatomaceous soil, physical properties, microstructure, Atterberg limits, plasticity