Microbial Carbonate Precipitation in Soils

Ahmed Al Qabany, Cambridge University
Geotechnical Engineering Group


In this study the use of an innovative biological technology, namely microbially induced carbonate precipitation (MICP) is discussed. The precipitation of calcium carbonate (CaCO3) in sand using ureolytic bacteria shows a lot of promise and could be utilized in numerous geotechnical and environmental applications such as liquefaction control, slope stability, facilitating excavation and tunnelling, soil improvement against soil liquefaction and CO2 sequestration.

This study seeks to optimize the use and efficient control of Bacillus pasteurii to raise the pH of the system and induce the precipitation of CaCO3 in sands. Laboratory tests were conducted to investigate the effect of changing treatment factors such as chemical concentrations, retention times and effective input rates (mole/litre/hour) on chemical efficiency. Chemical efficiency was measured based on weight measurements of CaCO3 precipitation compared to the amount of chemical reactants injected to samples. Based on the experimental results, the optimum time required for the precipitation process to take place in porous media for a specific range of bacterial optical density was determined. Results show that, below a certain urea and CaCl2 input rate of (0.042 mole/litre/hr) and for a bacterial optical density (OD600) between 0.8-1.2, the reaction efficiency remained high (over 90 %) and that the amount of precipitation was not affected by the liquid medium concentration (for input concentrations up to 1 mole/litre). However, the precipitation pattern at the pore scale was found to be affected by the injected concentration. Scanning electron microscopy (SEM) images taken of different samples at different levels of cementation showed that, for the same amount of precipitation, the use of lower chemical concentrations in injections resulted in better distribution of calcite precipitation especially at lower cementation levels. The use of the bio-cementation process in three main applications was also investigated. These applications were modification of soil hydraulic properties, soil improvement, and carbon dioxide mineral sequestration. Unconfined compressive strength (UCS) and permeability experiments were conducted for samples during treatment, where the increase in strength and decrease in permeability were correlated with the amount of CaCO3 precipitation. It was found that for strength improvement, MICP was highly promising (an average UCS of 1 MPa at 6 % cementation) but also highly dependent on the chemical concentration of the treatment solution. The reduction in permeability was also affected by Microbial Carbonate Precipitation in Soils Ahmed Al Qabany the chemical concentration, but generally was not as high as initially thought (an average decrease of approximately 75 % at 2.5 % cementation). Nevertheless, it was found that the use of MICP in these areas would depend on the specific application and the surrounding conditions. For many applications, it is useful to assess the degree of CaCO3 precipitation by nondestructive testing. Thus, the feasibility of S-wave velocity measurements to evaluate the amount of calcite precipitation by laboratory testing was investigated. A correlation between shear wave velocity and the amount of CaCO3 precipitation in soil was developed in order to enable the use of S-waves as an indicator of the amount of precipitation during an MICP treatment, where conductivity was also assessed as a monitoring tool for the MICP process. If chemical reaction efficiency was assumed to be constant throughout each test, the relationship between S-wave velocity (Vs) and the amount of CaCO3 precipitation was found to be approximately linear. This correlation between S-wave velocity and calcium carbonate precipitation validates its use as an indicator of the amount of calcite precipitation, where conductivity was found to be more suitable to indicate chemical changes in the treatment solution and not the amount of precipitation. Furthermore, radial flow tests were conducted on a relatively larger scale in order to further validate the control parameters obtained in the study in a larger setup (chemical concentrations and retention time) and to investigate how those parameters may be applied in the presence of a velocity gradient in radial flow conditions. It was found that the presence of preferential flow paths or irregularities in the treatment area affects the retention time of chemicals in the targeted cementation area, and resulted in significant heterogeneities in the precipitation distribution after treatment. These heterogeneities were found to lead to a decrease in chemical and overall efficiency of the MICP process if not properly taken into account in the design of the process. Results also showed that the pattern of decrease in permeability with cementation obtained from smaller tests were still applicable on this larger setup and that the increase in S-wave velocity with cementation was different from previous tests as it was highly affected by the confinement of the sand model. This research and the obtained results constitute a step towards better understanding, and control of MICP and its use in different field applications.