Precipitation of calcite by indigenous microorganisms to strengthen soils

Enrichments for indigenous microorganisms capable of hydrolyzing urea in the presence of CaCl2 were performed on potentially liquefiable saturated soils in both the laboratory and in situ during a field experiment. Following enrichment, treatment of soils with nutrients, CaCl2 and urea resulted in significant in situ precipitation of calcite, even at depth, by indigenous microorganisms. The biomineralized soils showed properties that indicate calcite precipitation increased resistance to seismic-induced liquefaction. Cone penetrometer testing of soil treated in the field indicated a two to three-fold increase in cone penetration resistance in treated soil compared to untreated soil.;We found that the precipitation of calcite by indigenous microorganisms was possible in a wide range of soils. In laboratory experiments on six distinct soil types, a range from 2.3 to 19.1% of calcite (w/w) was precipitated within the soil pore space over the course of 3--10 treatments.;Additional large-scale tests were performed in a soil microcosm. Enrichment and biomineralization solutions were delivered into the microcosm and not allowed to drain so that the soil within the microcosm remained partially saturated. The experiment resulted in a 33-fold (7.14 MPa) increase in cone tip resistance compared to untreated soil.;Cultivable microoganisms that were able to preferentially use urea as a nitrogen source were isolated from six types of soil and identified by 16S rDNA sequence analysis. Phylogenetic relationships were mapped based on 16S rDNA and ureC sequence homology. Urease activity was measured in the ten candidate isolates and it was found that all of the microorganisms studied constitutively regulated urease and urease activity remained relatively high, even in the presence of 200 mM ammonia. These experiments show that ureolytic bacteria capable of constitutive regulation of urease may be more prevalent than previously thought.