Enzyme-Induced Carbonate Precipitation as a Novel Remedy for Expansive Soils: Assessing Microfabric and Swelling Characteristics

Highly susceptible to moisture changes, expansive soils are widely recognized as problematic soils. Swelling-induced damages to overlying engineering structures result in financial loss and billions of dollars in repair costs annually worldwide. Therefore, to mitigate swelling potential, various soil amendment approaches have been investigated to date. Sustainable soil treatment techniques such as microbially induced carbonate precipitation, although enjoying a low CO2 footprint, suffer from challenges pertinent to the use of living microorganisms at field scale treatments. Hence, the current study aims to explore the feasibility and efficiency of using enzyme-induced calcium carbonate precipitation (EICP) as an alternative eco-friendly method, yet to be examined for improving swelling characteristics of clayey soil. For this purpose, a series of laboratory experiments from macro- to microstructural scales involving free swell tests, swell pressure, pH measurement, X-ray powder diffraction, Bernard calcimeter, confocal Raman spectroscopy, and scanning electron microscopy have been conducted on the EICP-treated expansive samples. The experimental program focuses on the influence of constituent concentrations used in the EICP treatment on the swelling potential to obtain the optimal EICP solution. The study reveals that EICP can result in more than 60% reduction in the free swell index compared to untreated soil conditions. The results indicate that the EICP technique can double calcium carbonate content within expansive soil, increasing it by as much as 220%. Finally, it is demonstrated that there is a strong correlation between the amount of precipitated calcite and enhancement in the free swell index.