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.

The mechanical behavior of sands treated using microbially induced carbonate precipitation (MICP) has been investigated at the macroscale and the microscale. Triaxial and confined compression tests with embedded shear and compression wave (S-wave and P-wave) sensors were conducted on two MICP-treated sands, Ottawa 50/70 and 20/30 silica sands. Triaxial compression tests were conducted at three different confining pressures (25, 50, and 100 kPa). Tests were also performed at calcium chloride (CaCl2) concentrations of 0.1 and 0.3 M, resulting in specimens with average calcium carbonate (CaCO3) content ranging from 1.5 to 2.5% for the 50/70 sand and from 1 to 1.6% for the 20/30 sand. In contrast to previous research, the results of triaxial tests presented in this paper show an increase of the soil strength even at 1% calcium carbonate content. After the tests, samples taken from the specimens were utilized to measure the CaCO3 content and to perform analysis using scanning electron microscopy (SEM) ...

A baseline study was conducted to evaluate the influence of the composition of the enzyme-induced carbonate precipitation (EICP) treatment solution on the efficiency of carbonate precipitation. EICP is an emerging biogeotechnical ground-improvement technique in which calcium carbonate is precipitated from an aqueous solution within the soil pores. A solution consisting of 1 M urea, 0.67 M calcium chloride (${\mathrm{CaCl}}_2$ ), and $3\mathrm{g}/\mathrm{L}$ urease enzyme (reported activity $\mathrm{3,500}\mathrm{U}/\mathrm{g}$ ) was identified as the preferred composition of the solution for EICP treatment of soil, because that concentration of ingredients results in high precipitation mass, high precipitation efficiency, and high enzyme efficiency. It was also shown that effluent from EICP treatment may be used as a source of ${\mathrm{CaCl}}_2$ and urea but cannot be used as a source of urease enzyme. Adding calcite seeds facilitates densely agglomerated calcite crystal formation. Unconfined compression tests on EICP-treated specimens rinsed with deionized water to flush organic matter and ammonium chloride from the specimens suggest that there may be a threshold carbonate content above which the strength dramatically increases. The strength of EICP-treated soil is influenced not only by the amount of carbonate precipitation and but also by the method of preparation and by particle packing. Rinsing of the EICP-treated specimens with deionized water was found to cause dissolution and flushing of organic matter and ammonium chloride salt precipitates.

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.