The soil water retention curve (SWRC) is one of the most fundamental characteristics of unsaturated soils. Because unsaturated soils are subjected to a range of natural processes and engineered treatments, a thorough understanding is needed of how their retention properties change when exposed to each separate treatment. In recent years, several biological treatment methods, such as microbial-induced calcite precipitation (MICP), have been introduced as environmentally friendly techniques. This study investigates the effect of biological treatment by means of Bacillus sphaericus on the SWRC of both coarse-grained and fine-grained soil samples. Pore fluid pH, X-ray diffraction, and scanning electron microscopy tests are used to better delineate the mechanisms affecting biologically induced alterations of SWRCs. Results showed that higher bacterial concentrations produced higher air-entry values in coarse-grained soils. However, the fine-grained samples showed an initial increase followed by a decrease in the air-entry value when the bacterial concentration increased. Changes in the soil porous structure and double-layer thickness are thought to be main reasons causing the different behavior of fine-grained soils.

In this study, a grain-scale modelling technique has been developed to generate the capillary pressure–saturation curves for swelling granular materials. This model employs only basic granular properties such as particles size distribution, porosity, and the amount of absorbed water for swelling materials. Using this model, both drainage and imbibition curves are directly obtained by pore-scale simulations of fluid invasion. This allows us to produce capillary pressure–saturation curves for a large number of different packings of granular materials with varying porosity and/or amount of absorbed water. The algorithm is based on combining the Discrete Element Method for generating different particle packings with a pore-unit assembly approach. The pore space is extracted using a regular triangulation, with the centres of four neighbouring particles forming a tetrahedron. The pore space within each tetrahedron is referred to as a pore unit. Thus, the pore space of a particle packing is represented by an assembly of pore units for which we construct drainage and imbibition capillary pressure–saturation curves. A case study on Hostun sand is conducted to test the model against experimental data from literature and to investigate the required minimum number of particles to have a Representative Elementary Volume. Then, the capillary pressure–saturation curves are constructed for Absorbent Gelling Material particles, for different combinations of porosity values and amounts of absorbed water. Each combination yields a different configuration of pore units, and thus distinctly different capillary pressure–saturation curves. All these curves are shown to collapse into one curve for drainage and one curve for imbibition when we normalize capillary pressure and saturation values. We have developed a formula for the Van Genuchten parameter \(\alpha \) (which is related to the inverse of the entry pressure) as a function of porosity and the amount of absorbed water.

The detrimental effects of sand storms on agriculture, human health, transportation network, and infrastructures pose serious threats in many countries worldwide. Hence, wind erosion is considered a global challenge. An environmental-friendly method to suppress wind erosion is to employ microbially induced carbonate precipitation (MICP). However, the by-products of ureolysis-based MICP, such as ammonia, are not favorable when produced in large volumes. This study introduces two calcium formate-bacteria compositions for non-ureolytic MICP and comprehensively compares their performance with two calcium acetate-bacteria compositions, all of which do not produce ammonia. The considered bacteria are Bacillus subtilis and Bacillus amyloliquefaciens. First, the optimized values of factors controlling CaCO₃ production were determined. Then, wind tunnel tests were performed on sand dune samples treated with the optimized compositions, where wind erosion resistance, threshold detachment velocity, and sand bombardment resistance were measured. An optical microscope, scanning electron microscope (SEM), and X-ray diffraction analysis were employed to evaluate the CaCO₃ polymorph. Calcium formate-based compositions performed much better than the acetate-based compositions in producing CaCO₃. Moreover, B. subtilis produced more CaCO₃ than B. amyloliquefaciens. SEM micrographs clearly illustrated precipitation-induced active and inactive bounds and imprints of bacteria on CaCO₃. All compositions considerably reduced wind erosion.