Iron-and calcium-based biogrouts for porous soils

Chemical grouts are often used to reduce the hydraulic conductivity of soil for seepage control purposes. However, chemical grouts can be expensive and environmentally unfriendly. Therefore, two new biogrouts or microbial-based grouting materials were developed. The first was a calcium-based biogrout, which contained urease-producing bacteria, calcium chloride and urea for the crystallisation of calcite due to enzymatic hydrolysis of urea. The second was an iron-based biogrout, which consisted of urease-producing bacteria, ferric chelate, and urea for the precipitation of ferric hydroxide due to enzymatic hydrolysis of urea. Both biogrouts reduced the hydraulic conductivity of loose clean sand from 5 × 10^-5 to 1·4 × 10^-7 m/s when the precipitated metal content was at 1·5% w/w or lower. The unconfined compressive strength of the treated specimens also increased. However, at a low metal content, the amount of improvement was still much less than that obtained by chemical grouting. Therefore, suitable geotechnical applications of the calcium-based and iron-based biogrouts at a precipitated metal content of 1·5% w/w or less would be mainly the bioclogging of porous soils for the sealing of aquaculture or wastewater treatment ponds and landfill sites, and for plugging of the piping in dams and retaining walls in sandy soil.