Effects of Biological Stabilization on the Water Retention Properties of Unsaturated Soils

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.