Reductions in Hydraulic Conductivity of Sands Caused by Microbially Induced Calcium Carbonate Precipitation

Microbially induced calcite precipitation (MICP) modifies soil behavior and properties through the precipitation of calcium carbonate (CaCO3) in the pore space. It has gained prominence as one strategy for biologically induced soil improvement. This study investigates the effect of MICP on hydraulic conductivity reduction and presents permeability reduction models for MICP-treated sands. Four column experiments, each with a different size of poorly graded sand, were subject to low-concentration equimolar MICP treatments while monitoring hydraulic conductivity reduction and precipitated CaCO3 distribution. Multiple MICP treatments produced homogeneous distributions of CaCO3 and caused a gradual reduction in hydraulic conductivity of 50%–90% until a CaCO3 content of ∼10%–15% was achieved. The high-resolution X-ray computed microtomography (CMT) and scanning electron microscopy (SEM) imaging reveals that the pore-scale precipitation behavior changes from a contact-cementing pattern in fine sands to a mixed pattern of contact-cementing and surface-coating precipitation in coarse sands as the grain size increases. The Kozeny–Carman type of permeability models appear to well capture the hydraulic conductivity reduction caused by MICP as a function of volumetric pore fraction of CaCO3. The experimental results presented in this study advance our understanding of the pore-scale CaCO3 precipitation patterns in different sizes of sands and their effect on hydraulic conductivity. Additionally, this study provides unique and reliable hydraulic conductivity data that can be used to develop hydraulic conductivity models for MICP-treated sands.