Engineering Properties of Biocementation Coarse- and Fine-Grained Sand Catalyzed By Bacterial Cells and Bacterial Enzyme

Biological induced calcite precipitation is a potential method being investigated for improved soil stabilization. In terms of the associated urea hydrolysis concept, three main strategies have been developed over the last 2 decades: (1) using live urease-producing bacteria, (2) using plant-extracted urease, and (3) using bacterial-extracted urease. This paper focused on evaluating the comparative benefits of two of these methods (i.e., live bacterial cell or extracted bacterial urease methods for induced calcium precipitation) in terms of their biocementation performance. Cell-based induced carbonate precipitation (ICP) (i.e., MICP) testing was completed on standard Ottawa coarse-grained sand (#20/30), and bacterial-enzyme-based (i.e., BEICP) testing was conducted individually on both coarse-grained and fine-grained (#50/70) sands. Distinctly higher unconfined compressive strength (UCS) was achieved with the BEICP method when evaluated at similar levels of calcium precipitation. Residual permeability levels remained markedly higher after BEICP testing versus MICP. The UCS of BEICP coarse-grained treated sand was approximately 450–1,500 kPa, whereas that of fine-grained treated sand had a notably lower range (i.e., 250–900 kPa) when evaluated at similar levels of CaCO3 production. These results indicate that calcium carbonate content is not the sole factor which impacts the strength of biocemented sand. Additional test-tube investigation of ICP-derived CaCO3 precipitation was used to evaluate the chemical conversion efficiency for each method, i.e., live cells (i.e., Sporosarcina pasteurii) or bacterial-extracted urease. The calcite precipitation ratio declined at higher substrate chemical concentrations. However, this ratio increased with higher rates of enzymatic activity.