Biocementation of soft clay by Extra cellular Polysaccharides and Microbially Induced Carbonate Cementation Using Indigenous Microbes in Cattle Manure

Abstract

Soft soils are composed of large portions of silts and clay and are characterized by low strength and high compressibility owing to their high-water content. Biocementation techniques are emerging as a strong alternative to conventional methods owing to their eco- friendliness and sustainability. Biocementation techniques bonds soil particles through biologically precipitated cementing products, such as extracellular polymeric saccharides (EPS) and microbially induced calcite precipitation (MICP). Extracellular polymeric saccharides are constituents of protective biofilms that shield microorganisms from environmental stress and are responsible for cohesion of microorganisms and adhesion of biofilms to surfaces. They are bound to surfaces by growth of fibrous bridges, filling of voids and formation of van der Waal s, hydrogen, and ionic bonds. Existing studies have relied on extraneous polymer addition to overcome the need for microbial and nutrient injection, time for cultivation and excrement secretion and compatibility of the microbes with host mineral. A major drawback of extraneous addition is the inability of polysaccharides to penetrate the deeper layers of a porous solid owing to formation of surficial crust and/or resistance by the micro-porosity of the system. The problem of binding particles located in deeper layers can be addressed by in-situ secretion of polysaccharides in the connected voids network of a solid ensuring cementation through entire depth. Microbially induced calcite precipitation (MICP) is another preferred biocementation technique in which bacteria possessing urease enzyme hydrolyses solution of externally injected urea to produce bicarbonate ions. The anions react with calcium ions in soil to form calcium carbonate that bond soil particles and produce stabilized soil. Flushing of microbes during repeated injection resulted in uneven distribution of precipitated calcite. Further, competition with native bacteria for nutrients led to starvation and diminished population of injected...