Experiments and predictions of physical properties of sand cemented by enzymatically-induced carbonate precipitation

Abstract

A grouting technique that utilizes precipitated calcium carbonate as a cementing material is presented. The enzyme urease is used to enhance the rate and the magnitude of the calcium carbonate precipitation. Evolutions in the mechanical and the hydraulic properties of treated sand samples are examined through unconfined compression and permeability tests, respectively. The grout is mainly composed of urease, which bio-catalyzes the hydrolysis of urea into carbon dioxide and ammonia, urea, and calcium chloride solutions. This method employs chemical reactions catalyzed by the enzyme, and ultimately acquires precipitated calcium carbonate within soils. The mechanical test results show that even a small percentage of calcium carbonate, precipitated within soils of interest, brings about a drastic improvement in the strength of the soils compared to that of untreated soils—the unconfined compressive strength of the samples treated with <10 vol% calcium carbonate precipitation against the initial pore volume ranges from ∼400 kPa to 1.6 MPa. Likewise, the hydraulic test results indicate the significant impervious effects of the grouting technique—the permeability of the improved samples shows more than one order of magnitude smaller than that of the untreated soils. Evolutions in the measured hydraulic conductivity and porosity are followed by a flow simulator that accounts for the solute transport process of the injected solutions and the chemical reaction of the calcite precipitation. Predictions of the changes in permeability with time overestimate the test measurements, but those of the changes in porosity show a good agreement with the actual measurements, indicating that such simulations should become a significant supplementary tool when considering real site applications.