In-Situ Precipitation of Calcium Carbonate

OTT Case 09-025 | US Patent Application 20110027850

Existing technology for stabilizing soils include cementation (e.g. permeation grouting), densification (e.g. vibro-replacement, deep dynamic compaction, compaction grouting), and thermal stabilization. While many of these approaches have proven successful, they are largely limited to undeveloped sites.

Biologically induced precipitation of calcium carbonate in soils is of particular interest to engineers and microbiologists as an alternative method for stabilizing soils. It is well know that urease positive microorganisms initiate the formation of calcium carbonate. Soil stabilization methods are being investigated whereby urease positive microorganisms, along with calcium chloride and urea are injected into soil.
Common obstacles to injecting microorganisms into soils are clogging of the sample and uneven distribution of bacteria and forming calcite, both of which tend to be more concentrated near the injection point, most likely a result of difficulties with bacteria transport and attachment in soil. Factors affecting the transport of bacteria through and attachment to soil grains include the properties of the cell surface, ionic strength of the carrier solution, flow rate, van der Waals forces and pore space geometry within the soil matrix. Once in the soil, bacteria also face challenges such as reduction in population from predation and competition as well as aboitic factors such as pH, osmotic pressure, temperature and availability of suitable nutrients.

Researchers at the University of Idaho have developed a soil stabilization process that uses indigenous bacteria. Microorganisms that are able to hydrolyze urea to ammonia are widely distributed in soils, comprising from between 17 percent to 30 percent of the total number of aerophilic, micro-aerophilic, and anaerobic microorganisms. Results of bench top testing and field trials have shown that such indigenous microorganisms are able to precipitate calcite in saturated, liquefiable soil and that the calcite can be precipitated in a quantity that is thought to be sufficient to improve the strength and stiffness of the soil. Advantages of this stabilization process over microorganism injection methods include a reduction in costs because bacteria are not separately grown and that these indigenous bacteria tend to be evenly distributed in soils down to depths that engineers typically treat.