Primary Study on Ureolysis-Based Microbially-Induced Calcium Carbonate Precipitation Technique for Geotechnical Applications

Environmental concerns and application limitations of the conventional techniques for geotechnical engineering problems persuaded the geotechnical engineers to look for alternative solutions. In this regard, biogeotechnics which deals with bio-mediated and bio-inspired solutions to the challenges that vex geotechnical systems has been recently introduced. Within only around one decade of emerging this topic, it has received many attentions by researchers as it offers the promise of cost-effective, sustainable and non-disruptive solutions for a diversity of geotechnical applications. Microbially induced CaCO3 precipitation (MICP) is one the biological solutions, through which calcium carbonate is precipitated as binding agent between grains and/or filling materials within soil pores, by mediation of microorganisms. Successful development and implementation of the MICP technique for ground improvement would have wide application to many important geotechnical problems, such as increasing stiffness and shear strength to mitigate liquefaction potential; to enhance bearing capacity of soil beneath foundation and reduce associated settlements; to stabilize slopes; to facilitate excavation, boreholing and tunneling; to control erosion; and reducing permeability to reduce seepage of dikes and cut-off walls. Application of this technique may be especially useful underneath or near existing structures, where the application of conventional soil improvement techniques is restricted due to high cost or ground deformation associated with available technologies. There are several mechanisms for MICP. Ureolysis mechanism is the most concentrated one in the literature, in which ureolytic microorganisms are employed as biological mediators. Since introducing the ureolysis-based MICP into geotechnical engineering, many researches has been carried out on development and application of this technique into soil at laboratory scale. Although much success and progress have been attained by these studies, it was found that there is still lack of some geotechnical insights in biology-oriented side and some biological insights in geotechnics-oriented sides; for instance, no cheap, fast and geotechnical engineer-friendly method for estimation of precipitation progress was presented; and few attention has been also paid to the mineralogy of the precipitated calcium carbonate as it can influence mechanical properties of the treated soil and durability. In the present study, a hybrid of conductometry and precipitation mass measurement method in treatment solution (Biogrout) has been developed for monitoring the precipitation progress, which facilitates controlling over precipitation pattern, i.e. estimation on type, time, amount and place of precipitation within soil. It was found that the precipitation pattern in every treatment solution follows a logistic function, which was herein designated as microbial CaCO3 precipitation characteristics curve (MCPCC) since it represents three important characteristics (i.e. precipitation rate, precipitation ratio and lag duration). Effect of some significant influencing factors on these characteristics was investigated. Statistical models for precipitation rate and ratio were also presented. Potential type and morphology of the precipitates at different environmental conditions have been also assessed. It was observed that calcite as the most stable type of calcium carbonate does not always occur. Formation of non-stable phases can cause change in mechanical properties of treated soil as change in type and morphology of the precipitates during time was demonstrated in this study. Treatment of soil samples by using soaking and injection methods at high and low rate of precipitation were also separately evaluated. Different soil treatment methods were found to influence the mineralogy, shear wave velocity and unconfined compressive strength.