The effect of preparation of biocemented sand sample on its 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 effects the content and strength of the precipitate. The objective of this study is to investigate the optimum dosage of urea-calcium chloride and urease enzyme from test tube experiments, on the calcium carbonate 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 improves 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. The shear behaviour of the precipitate seemed to be influenced by its morphology, which was different for different chemical concentrations and urease activities. Stress dilatancy relations were determined for EICP treated sands and appear to follow Taylor's work-hypothesis relationship.