Biological Stabilization of a Swelling Fine-Grained Soil: The Role of Microstructural Changes in the Shear Behavior

In many engineering projects, improving soil shear strength is one of the goals. There are different methods for soil improvement among which biological soil stabilization techniques have emerged in recent decades. These techniques have been introduced as environmentally friendly techniques and have been shown to give promising results. However, different aspects of their effect on the soil mechanical properties have not yet been comprehensively understood. One of these aspects is the effect of different ingredients such as the effect of culture media as well as bacterial concentration on the improvement in the soil shear strength. In this study, the effect of one of the most commonly used biological stabilization techniques, namely microbial induced calcite precipitation on the shear strength of a swelling fine-grained soil has been studied by means of laboratory-scale tests and the influence of bacterial concentration as well as the chemicals used in the treatment (culture media) have been looked into. The results have been carefully analyzed and presented in terms of suitable formulas for practical purposes. Furthermore, the mechanisms contributing to the shear strength increase have been discussed based on the observed changes in the soil micro-fabric disclosed by X-ray diffraction analysis. The results reveal that by increasing bacterial concentration, the soil cohesion and the soil friction angle increase for which empirical relationships are presented. The increase in bacterial concentration thus increases the soil shear strength. Furthermore, the effect of culture media on the soil mechanical properties is examined. While the sole use of culture media slightly increases the soil shear strength, its impact on the soil shear strength is much less than that of the complete biological solution which includes bacteria. This proves the hypothesis that the improvement is indeed induced by the bio-geo-chemical processes which bacteria bring about rather than the chemical compounds present in the biological solution.