Reactive transport model development and injection strategy optimization for soil improvement based on microbially induced calcite precipitation (MICP)

Abstract

An alternative method to cement-based ground improvement techniques is necessary due to the contribution of cement production to increasing carbon dioxide emissions. Microbially induced calcite precipitation (MICP) has garnered huge interest for its potential as an alternative but sustainable ground improvement method as it exploits natural microbiological processes. To this date, the MICP method is still limited to laboratory-scale experimentation or meter-scale pilot demonstration. Moreover, there is little or no guidance on the injection strategy to achieve a uniform calcite distribution at a field scale. To extend further this technique to a field scale, this dissertation aims to develop a reactive transport model to describe MICP process in subsurface, and to suggest guides for design of MICP treatment strategy for ground improvement. While the field-scale physical demonstration is costly and has limitation in conducting parametric studies, the reactive transport modeling enables parametric study to test various biological treatment plans. This study first develops a reactive transport model to describe the MICP process, in which the key model parameters are determined by back-calibration against the half-meter column experiment results. The parametric case study primarily examines the effect of bacterial density in biological solutions and the effect of cementation solution concentration on spatial distribution of precipitated calcite. The simulation results reveal that the density and distribution of the attached bacteria have the most pronounced effect on the spatial distribution of precipitated calcite, attributable to the sufficiently fast ureolysis rate of attached bacteria. Bio-augmentation with a low bacterial density causes a slow ureolysis rate and a precipitation reaction rate, and this allows the urea and calcium to be transported further and distributed homogeneously because only a small portion of the chemicals reacts to form calcite during the injection of cementation solution. Therefore, use of bacterial inoculum with a low bacterial density or control of the ureolysis rate facilitates in achieving a homogeneous calcite precipitation as long as a sufficient retention time is allowed in-between cementation treatments so as to complete the precipitation reaction. The findings in the parametric case study leads to introducing two efficiency factors: the spacing efficiency factor (E$_s$) and the calcite content efficiency factor (E$_{CC}$). These efficiency factors as well as the overlap factor (E$_{OL}$), a preliminary design guide of MICP injection strategy is proposed. The injection strategy includes the design of the injection parameters, such as flow rate, fluid volume, solution concentration, retention time, and number of treatment, and the design of injection wells, such as number of wells, vertical hole spacing and horizontal well spacing. This study provides insight into the major factors affecting spatial distribution of the precipitated calcite in MICP and proposes novel methodology and design guide to design MICP injection strategy for field-scale implementation.