Application of Enzyme-Induced Carbonate Precipitation (EICP) to Improve the Shear Strength of Different Type of Soils

Urease enzyme derived from the agricultural source precipitates calcium carbonate (CaCO₃) from an aqueous solution of urea and calcium chloride via urea hydrolysis, which strengthens the soil by cementing and bridging soil particles. Different combinations of urea, calcium chloride and urease enzyme will have varying influence on the mechanical properties of the treated soil. This work aims to analyse the efficiency of enzyme-induced carbonate precipitation (EICP) on three different soil types (silty sands, clayey sand and silt). The optimum combination of urea, calcium chloride and urease enzyme is studied by mixing different concentrations, and the combination of these reagents in beakers and optimum precipitation is evaluated by gravimetrically measuring the amount of CaCO₃ precipitated. The observed optimum combination is applied to three different soil types, and the improvement in the compressive strength of the soils specimens due to carbonate precipitation is observed by conducting unconfined compressive strength (UCS) tests. The precipitation experiments in the beakers reveal that the increase of the urea-CaCl₂ concentration may inhibit the activity of urease, thereby precipitating lower amount of CaCO₃. The results of the UCS tests show that the technique of enzymatic calcium carbonate precipitation improves the shear strength of the all the three types of soils, however, more strength gain was obtained in the case of clayey sand. The microstructural observations with the help of scanning electron microscopy (SEM) and X-ray powder diffraction (XRD) tests verify the existence of calcite in the pores of compacted soil specimens.