| How to improve the ability of loess foundation to resist collapsible deformation has always been a hot topic in the research of soil property improvement.The current traditional loess improvement methods have some problems,such as unsatisfactory improvement effect and environmental pollution.Correspondingly,A new,green and sustainable microbially induced calcium carbonate precipitation(MICP)technology systematically solves these problems.In this paper,Bacillus pasteurii was used as the biological mineralizing strain,and the optimum culture experiment was carried out.On this basis,direct shear tests were carried out to explore the effects of calcium sources,cementing fluid concentration and curing days on the reinforcement of microbial loess.Through a series of laboratory tests and field disintegration tests,such as consolidation undrained triaxial shear test,permeability test,collapsibility test,etc.,the strength improvement of MICP on the fractured loess was studied,and the improvement effect of MICP on the water property of the loess was evaluated,which provided scientific basis for the field application of MICP in the reinforcement of loess foundation.Finally,combined with electron microscope scanning and X-ray diffraction test,etc.,The strengthening mechanism of microbiological reinforcement of loess was revealed.The following points of research were obtained:(1)Bacillus pasteurii was selected as the microorganism for the MICP mineralization reaction and incubated for 32-44h.The value-added of the bacteria was in the logarithmic phase,and their growth and metabolic capacity were vigorous and showed high activity for the subsequent experimental study of microbial reinforced loess.The number of bacteria(OD600)showed a positive correlation pattern with the urease activity,with the increase in the number of bacteria,the urease activity was significantly enhanced.The growth and reproduction of Bacillus sp.was most suitable under the conditions of bacterial inoculation ratio of 1%by volume,ambient temperature of 30℃,pH of 8 and yeast powder liquid medium.(2)Calcium chloride was determined as the optimal calcium source,and the optimal ratio was obtained when the concentration of cementing liquid was 1mol/L and the curing time was 14d.Compared with remolded loess,the cohesion and internal friction angle of MICP reinforced loess samples can be increased by 343%and 32%respectively.When the curing days were 14 days,the cohesion of MICP-solidified loess samples increased significantly,reaching 55.5kPa.The internal friction angle changed little.(3)MICP can effectively improve the shear strength of fissured loess.The fissure inclination angle has an important effect on the mechanical strength of the loess The cohesion and internal friction angle of MICP-reinforced fissured loess increased and then decreased with the increase of the fissure inclination angle,showing a fitting relationship of a cubic function.The effect of fracture inclination on the deterioration of remoulded loess is more pronounced than that of MICP-reinforced fractured loess specimens,and the microbially generated calcium carbonate precipitates have a cementing effect,which reduces the deterioration effect of fractures on the specimens.(4)The permeability coefficient of the cured loess specimens increased and then decreased gradually with increasing time.In the indoor disintegration tests,the disintegration of MICPreinforced loess was significantly reduced by 9.8～46%compared with that of plain loess,and the average disintegration rate of the specimens decreased gradually with the increase of the maintenance days.The results of the field disintegration tests showed that the disintegration of the unreinforced in-situ loess specimens was significantly greater than that of the MICPreinforced loess,and the disintegration rate of the unreinforced loess specimens was faster.(5)Mercury injection test showed that the pore size distribution of the microbe reinforced loess decreased significantly.XRD test confirmed that the generated product was calcium carbonate,and the crystalline type was aracharite and calcite.SEM test showed that the MICPsolidified loess encapsulated soil particles and filled the intergranular pores through the formation of calcium carbonate minerals.Thus,the cementation and bridging effects are achieved,and the overall strength of the sample is significantly improved. |
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