Effect of biocementation on sand sample strength measured using direct shear tests

Abstract

Urea, calcium and urease are the primary components of enzyme-induced calcium carbonate precipitation (EICP) method. The composition of these ingredients significantly affects the content and strength of the precipitate. The objective of this study is to investigate the optimum dosage of urea–calcium chloride (CaCl2) and urease enzyme from test-tube experiments, on the calcium carbonate (CaCO3) precipitate when used with sand. The results of samples prepared using two different methods are compared using direct shear tests to note the effect of biocementation on the strength of the cemented sand. Bench-scale conductivity tests show that large urea–calcium chloride concentrations in combination with adequate urease activity improve the precipitate efficiency. The method of treatment also affects the strength of the treated sand, with the injection method being more efficient in cementing the sand particles compared to the mix-and-compact approach. While both the sample preparation methods resulted in the increase in peak friction angle and cohesion intercept, concentration of urea–calcium chloride higher than 0.75 mol/l and urease activity more than 30 kU/l reduced the overall shear strength. Precipitate strength was governed by its morphology, which varied depending on chemical concentrations and urease activity. Stress–dilatancy relationships for EICP-treated sands appear to follow Taylor's work-hypothesis.