Biocementation of soft soil by carbonate precipitate and polymeric saccharide secretion

Abstract

This study utilised native denitrifiers of cattle manure to oxidise its organic matter and induce calcium carbonate cementation in a synthetic soft soil. The soft soil is prepared by remoulding a 50% kaolinite + 50% sand mix with ultra-pure water at 21% water content. The prepared specimen is classified as soft soil based on its low unconfined compression strength of 16 kPa. Extraneous calcium nitrate provides nitrate ions (electron acceptors) for the metabolism of denitrifying bacteria and calcium (Ca2+) ions for calcium carbonate formation. The facultative anaerobe community peaks and declines between 7 and 14 days. A small addition of magnesium oxide balances the pH reduction caused by acetic acid formation during microbial degradation of cattle manure. Acetic acid serves as a carbon substrate for the metabolic activities of denitrifying bacteria. The CO2 (gas) released during the metabolic activities of bacteria dissolves in water and forms bicarbonate ions. The anion combines with calcium ions in the alkaline environment to form calcium carbonate cement. In addition, the native extracellular polysaccharide (EPS)-secreting bacteria in cattle manure facilitate EPS bonding of soil aggregates. Soil structure, interfacial frictional resistance mobilised by cattle manure fibres, EPS bonding and calcium carbonate cementation enhance the compression strength of the stabilised specimen by 720%.