Microbiological, Environmental, and Compositional Factors in Efficacy of MICP Treatment in Clayey Soils
Authors: Asghari Somaye 
Published: December 2022

Expansive clayey soils can cause billions of dollars of damage to infrastructure such as roads and foundations annually. Researchers propose many techniques (e.g., pre-wetting, soil replacement, and chemical stabilization) to improve the mechanical properties of these soils; however, some of these methods are impractical in certain situations, and are unsustainable in others due to the economic and environmental impacts. One possible method for enhancing soil’s mechanical properties is Microbial Induced Calcium Carbonate Precipitation (MICP). This environmentally friendly technique is a biological process where microbes play a key role in precipitating calcium carbonate. This precipitating calcium carbonate can coat soil particles and cement the soil matrix, thereby reducing the swelling potential. MICP is a complicated process. Many environmental variables such as the soil type, composition, chemistry, and microbial communities present in the soil control the rates and amounts of carbonate precipitation. The application of MICP in clay soils is an active area of research, however due to the complex nature of MICP and the clayey soils, not all the parameters impacting MICP have been comprehensively or systematically described. Moreover, the MICP performance of the soils tested in other studies varied considerably depending on the soil types. This leads to a fundamental question: What geochemical and environmental factors influence MICP performance and how these factors can be used as predictors of the MICP effectiveness in expansive soils? Answering this question is essential in the development of optimization strategies capable of enhancing the competitive advantages of MICP over traditional soil improvement methods; Moreover, understanding these factors prior to applying MICP to the soils can be a promising key for saving time, energy, and money. To determine the factors controlling MICP effectiveness in expansive soils, we performed a series of physical, chemical, microbiological, and compositional experiments in clayey soils collected from different geographical locations.

To determine how soil’s clay content and gradation impacts calcium carbonate (CaCO3) precipitation, several artificial clay/sand mixes were prepared and examined for urease activity and calcite precipitation. The test results showed that clay has more urease activity and precipitation calcite than sand despite the two having similar relative populations of indigenous ureolytic bacteria.

To determine the role of microbial communities in CaCO3 precipitation, we measured CaCO3 precipitation using
Rapid Carbonate Analysis (RCA) and examined its correlation with soil ureolytic bacteria determined through 16SrRNA DNA sequencing. These
observations show MICP treatment can increase ureolytic strains in all soils. However, this increase is not correlated with calcium carbonate
precipitation in soils.

Additional testing on 6 soil samples from multiple geographical locations showed that compositional characteristics such as Cation Exchange Capacity (CAC) and Specific Surface Area (SSA) have a significant positive correlation with the efficiency of MICP.

The overall results suggest that the performance of MICP treatment is better in clayey soils compared to other non-clayey soils. Moreover,
the results suggest that compositional properties such as CEC and SSA of the soil could be the reasons for the observed differences in
CaCO3 precipitation in soils. Therefore, it is possible that CEC and SSA can be used as indicators of the MICP effectiveness
prior to any MICP treatment in soils.