Consideration of soil as a living ecosystem offers the potential for innovative and sustainable solutions to geotechnical problems. This is a new paradigm for many in geotechnical engineering. Realising the potential of this paradigm requires a multidisciplinary approach that embraces biology and geochemistry to develop techniques for beneficial ground modification. This paper assesses the progress, opportunities, and challenges in this emerging field. Biomediated geochemical processes, which consist of a geochemical reaction regulated by subsurface microbiology, currently being explored include mineral precipitation, gas generation, biofilm formation and biopolymer generation. For each of these processes, subsurface microbial processes are employed to create an environment conducive to the desired geochemical reactions among the minerals, organic matter, pore fluids, and gases that constitute soil. Geotechnical applications currently being explored include cementation of sands to enhance bearing capacity and liquefaction resistance, sequestration of carbon, soil erosion control, groundwater flow control, and remediation of soil and groundwater impacted by metals and radionuclides. Challenges in biomediated ground modification include upscaling processes from the laboratory to the field, in situ monitoring of reactions, reaction products and properties, developing integrated biogeochemical and geotechnical models, management of treatment by-products, establishing the durability and longevity/reversibility of the process, and education of engineers and researchers.

Wind tunnel tests show that enzyme-induced carbonate precipitation (EICP) holds promise as a method for mitigation of fugitive dust emissions. Fugitive dust (wind-blown fine-grained soil) is a significant environmental problem in semi-arid and arid environments. Conventional methods for fugitive dust control, including the application of water, salt, or synthetic polymers, are either ineffective in arid climates, limited to short-term stabilisation, or very expensive. EICP employs hydrolysis of urea (ureolysis), a process catalysed by the urease enzyme, to precipitate calcium carbonate (CaCO₃) in the presence of calcium ions. Urease is a widely occurring enzyme found in many plants and microorganisms. Wind tunnel experiments were conducted to evaluate the use of a topically applied EICP solution containing plant-derived urease to stabilise soil against fugitive dust emission. Three different soils were tested: a native Arizona silty sand, a uniform medium-grained silica sand and fine sand-sized mine tailings from southern Arizona. The wind tunnel tests established the treatment concentrations at which EICP was more effective in suppressing fugitive dust than specimens prepared by either thoroughly wetting the soil or treatment with a salt–urea solution. These tests demonstrate the potential of EICP treatment as a means of mitigating fugitive dust emissions.

Specimens of silica sand treated via enzyme induced carbonate precipitation (EICP) showed surprisingly high strength at a relatively low carbonate content when non-fat powdered milk was included in the treatment solution. EICP is a biologically-based soil improvement technique that uses free urease enzyme to catalyze the hydrolysis of urea in an aqueous solution, producing carbonate ions and alkalinity that in the presence of calcium cations leads to precipitation of calcium carbonate. The strength achieved at less than 1.4% carbonate content via a single cycle of treatment was unprecedented compared to results reported in the literature from both EICP and microbially induced carbonate precipitation (MICP). Scanning electron microscope images show that in the specimens treated with the solution containing powdered milk the carbonate precipitate was concentrated at interparticle contacts. The impact of these results include reductions in the concentration of substrate and enzyme required to achieve a target compressive strength, reduction in the undesirable ammonium chloride by-product, and, depending on the desired strength, reduction in the number of cycles of EICP treatment. These advantages enhance the potential for development of a sustainable method of soil improvement via hydrolysis of urea.

A crude extract from jack beans (Canavalia gladiata) is demonstrated to be an effective source of urease enzyme for biocementation via enzyme-induced carbonate precipitation (EICP). Test tube tests of crude and purified extracts from jack beans, jack bean meal, soybeans, and watermelon seeds show that the crude jack bean extract results in the highest unit yield, defined as urease content per initial mass of source material, among these four plant sources. The efficacy of EICP using crude jack bean extract for biocementation was compared with the efficacy of three commercially available enzymes by biocementation of a granular soil. Unconfined compression tests on the granular soil specimens subject to biocementation via EICP demonstrated that the crude extract and the less purified commercially available enzyme were actually more effective than commercially available highly purified urease enzymes at enhancing soil strength, an effect attributed to the presence of complementary proteins in the less purified enzyme sources. The simple technique used to produce the crude extract from jack beans significantly lowers the cost of EICP, eliminating a major barrier to practical applications, including infrastructure construction and environmental protection.

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.

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.