Nasser Hamdan

Carbonate Mineral Precipitation for Soil Improvement Through Microbial Denitrification

ABSTRACT Microbially induced carbonate precipitation (MICP) and associated biogas production may provide sustainable means of mitigating a number of geotechnical challenges associated with granular soils. MICP can induce interparticle soil cementation, mineral precipitation in soil pore space and/or biogas production to address geotechnical problems such as slope instability, soil erosion and scour, seepage of levees and cutoff walls, low bearing capacity of shallow foundations, and earthquake-induced liquefaction and settlement. Microbial denitrification has potential for improving the mechanical and hydraulic properties of soils because it promotes precipitation of calcium carbonate (CaCO3) and produces nitrogen (N2) gas without generating toxic by-products. We evaluated the potential for inducing carbonate precipitation in soil via bacterial denitrification using bench-scale experiments with the facultative anaerobe Pseudomonas denitrificans. Bench-scale experiments were conducted (1) without calcium in an N-rich bacterial growth medium in 2.0 L glass batch reactors and (2) with a source of calcium in sand-filled acrylic columns. Changes of pH, alkalinity, NO3− and NO2− in the batch reactors and columns, quantification of biogas production and observations of calcium-carbonate precipitation in the sand-filled columns indicate that denitrification led to carbonate precipitation and particle cementation in the pore water as well as a substantial amount of biogas production in both systems. These results document that bacterial denitrification has potential as a soil improvement mechanism.

Enzyme induced carbonate precipitation (eicp) columns for ground improvement

Columns of improved soil created by Enzyme Induced Carbonate Precipitation (EICP) offer the potential for non-disruptive, cost effective ground improvement for a variety of geotechnical purposes. EICP employs urease enzyme to precipitate CaCO3 from an aqueous solution of calcium chloride and urea to fill the soil pores (increasing dilatancy and reducing compressibility) and cement soil particles (increasing shear strength). EICP is similar to Microbially Induced Carbonate Precipitation (MICP) except that, instead of employing microbes to generate the urease enzyme, the enzyme is obtained from agricultural sources. A major advantage of agriculturally-derived urease compared to microbial urease is its small size and water solubility, which allows penetration through the pore throat of finer grained soils such as silts, whereas ureolytic MICP is essentially restricted to soils of fine to medium sized sand or larger. The small size of the enzyme can also mitigate the potential for bio-clogging due to carbonate precipitation and biofilm formation, both of which my limit the applicability of MICP. Bench top tests in the laboratory show that cemented columns of soil can be created by infusing a cementation solution through a perforated tube or pipe or by mix and compact methods. EICP columns can be installed in patterns similar to root piles (pali radicii) for slope stability, micro piles for foundation support, and stone columns or soil cement columns to support embankments and restrict lateral spreading in liquefiable soils. Furthermore, EICP piles could be installed under existing structures without causing heave or settlement, making them ideal for remediation of poor (e.g. liquefiable) foundation soils.