Simplified biogeochemical numerical model to predict pore fluid chemistry and calcite precipitation during biocementation of soil

Abstract

Microbially induced calcite precipitation (MICP) technique has gained attention recently as a novel method to enhance the engineering properties of soils, especially sandy soils. However, the applicability of this method to field scale is challenging and requires understanding of the factors affecting MICP process under variable subsurface conditions. This study presents a laboratory investigation and numerical predictive model to assess pore-water chemistry and calcite precipitation during the biocementation process. Laboratory experiments were conducted to assess the microbial treatment of Narmada sand in plastic tubes using three bacterial strains and two cementation media concentrations. Calcite precipitation via ureolysis as a result of biogeochemical reactions was measured. The effects of pH and electrical conductivity (EC) on the rate of urea hydrolysis and calcite precipitation were assessed. The presence of calcite crystals was analyzed using scanning electron microscopy (SEM). The SEM images confirmed the formation of calcite at the surface and between the sand particles. A simplified numerical model was developed to estimate the rate of urea hydrolysis and their effects on the biocementation of sand. Three stages of MICP process were identified: bacterial ureolysis, dynamic equilibrium between liquid-gas interface and oversaturation of ions, and calcite precipitation. The variations of pH and EC at these three stages were modeled. The predicted pH, EC, and calcite precipitation based on the simplified model were found to be in close agreement with the experimental results. The numerical model can be used to assess and optimize the system variables for effective MICP field applications.