Effects of clay content increasement by microbial decomposition of silicate minerals on the strength characteristics of sand
Sand foundation is prone to engineering distresses such as seismic liquefaction and seepage erosion for its co-hesionless characteristics.The clay content in soil is an important factor to affect the cohesion,which contributes to prevent seismic liquefaction and seepage erosion of the soil.The development of microbial mineralization and decom-position technology provides new possibilities to improve the cohesion of sand.The Bacillus mucilaginosus that can decompose silicate minerals was adopted to accelerate the biological weathering of sand containing silicate minerals such as potassium feldspar and black mica under artificial nutrient solution stimulating condition.The changing rules of the strength properties,clay content,mineral component,and intergranular microstructure of the sand before and after the microbial treatment were investigated.The results showed that the clay content in the sand treated by B.mucilagi-nosus was increased by 1.6％when compared with that before treatment.There was a positive linear correlation be-tween cohesion and processing time,as well as the effective cohesion was increased by 23 kPa within 120 d.How-ever,the effective internal friction angle was decreased slightly after the microbial treatment.The stress-strain curves of sand samples treated by B.mucilaginosus showed strain softening characteristics under different confining pressures,and the peak strain of partial stress was gradually decreased with the increase in treatment age,and the strain softening characteristics of soil were obviously enhanced.Silicate minerals such as potassium feldspar and biotite in sandy soil are decomposed and destroyed in different degrees.The surface of the massive potassium feldspar in the sand was broken,and the thin layer fell off,and the overall structure did not change significantly.The surface of the sheet biotite was uneven and cracked,as well as the decomposition effect of B.mucilaginosus was remarkable.The X-ray diffraction spectra of sample treated for 120 d showed new diffraction peaks of kaolinite which presented the clay mineral product in the sand.This indicated that B.mucilaginosus decomposed silicate minerals such as biotite and po-tassium feldspar to produce secondary clay mineral kaolinite,which increased the clay content in sand.The production of clay minerals increased the viscosity of sand,and the adhesion of extracellular polymers(EPS)produced by micro-bial metabolism was also an important factor for the improvement of soil cohesion.Therefore,the physical and me-chanical properties of sandy soil can be improved by the introduction of artificially cultured microorganisms to decom-pose silicate minerals.As an environmentally friendly technology,it has application potential in seismic liquefaction,seepage erosion and other engineering diseases of sandy soil foundation.
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