Experimental Study On Strength And Permeability Characteristics Of Microbial Solidified Granite Residual Soil

Granite residual soil is mainly distributed in the humid and rainy southern region of China,because it has strong hydrophilicity,loose soil quality,softening in contact with water and poor mechanical properties,etc.,in the summer rainfall conditions are easy to cause a series of natural disasters such as collapse erosion,slope landslide,so it is necessary to improve the granite residual soil.In this paper,MICP technology was used to study granite residual soil in Gannan area,and Bacillus pasteurii was used to carry out microbial reinforcement tests on the soil.Firstly,the culture and growth law analysis of Bacillus pasteurii were carried out,and secondly,the bacterial growth conditions were optimized based on the influencing factors such as ambient temperature,ambient p H and inoculation number,and the optimal preparation ratio of cementation solution was determined by the optimization test of cementation solution.Through a series of tests such as triaxial consolidation drainage test,unconfined compressive resistance test,variable head infiltration,soil column infiltration and calcium carbonate content test,the mechanical strength and permeability characteristics of microbial reinforced granite residual soil were studied,and the microstructure composition and reinforcement mechanism of cemented remodeling granite residual soil samples were analyzed by scanning electron microscopy（SEM）observation and EDS analysis,and the following conclusions were mainly drawn:（1）Firstly,through microbial culture and activity tests,it was found that the growth of Bacillus pasteurii was mainly divided into four different stages（growth retardation stage,logarithmic growth stage,stable stage and decay stage）;The concentration of bacteria had a certain correlation with the strength of urease activity,and the optimal curing time of Bacillus pasteurii was 22h,when the bacterial concentration was the largest and urease activity was the strongest.The study of Bacillus pasteurii growth factors found that when the culture temperature,ambient p H value and bacterial inoculum were 30°C,9 and 3%,respectively,the OD₆₀₀ value corresponding to the culture 22h was the largest,and urease activity was the strongest.Through the composition ratio test of cementation solution,it was found that when the molar ratio of calcium chloride to urea in cementation solution was 1:1,the production of calcium carbonate was the largest.（2）Through the triaxial consolidation drainage test（CD test）,it was found that the shear strength of reshaped granite residual soil could be significantly improved by microbial curing,and the stress-strain curves were all strain hardened.When the concentration of cementation solution is in the range of 0.25～1mol/L,the peak strength of the sample is positively correlated with the concentration of cementation solution,and when the concentration of cementation solution is greater than 1mol/L,the test results are negatively correlated,so the optimal concentration of MICP reinforced cementation solution is 1mol/L;With the improvement of curing time,the peak strength,cohesion and internal friction angle of the improved specimen were increased to varying degrees,and the shear strength of the specimen was the largest when it was cured for 9 days.When the bacterial concentration OD₆₀₀ value is 1.5,the shear strength is the largest,but the increase is small,the optimal bacterial concentration OD₆₀₀ value of MICP reaction is 1.2;p H value is also one of the important factors affecting MICP reaction,p H values are found to be 5,7,9 and 11,respectively,the reinforcement effect is the best when the p H value is 9,that is,too acidic and too alkaline are not conducive to MICP reaction.（3）Using different bacterial concentration and cementation solution concentration to strengthen granite residual soil,through the reinforcement sample without lateral limit compressive test and calcium carbonate total test test,it is found that the stress-strain relationship curve is mainly divided into three stages（initial stage,nonlinear rising stage and nonlinear falling stage）,when the concentration of bacteria liquid is the same,the relationship curve between the unconfined compressive strength of the sample and the concentration of cementation solution shows a trend of first increasing and then decreasing,and the unconfined compressive strength value of the specimen with a concentration of 1mol/L is the largest.Moreover,the production of calcium carbonate is the largest.When the concentration of cementation solution is the same,the measured unconfined compressive strength of the sample is positively correlated with the concentration of the bacterial solution,the larger the concentration of the bacterial solution,the greater the non-lateral compressive strength of the sample,but when the concentration of the bacterial solution exceeds 1.2mol/L,the non-lateral compressive strength of the sample decreases slightly.Through scanning electron microscopy test,it was found that the pore size of granite residual soil reinforced by MICP technology was significantly reduced,and the surface of soil particles was very rough,when the concentration of bacteria liquid was large,the soil particles were arranged very closely,calcium carbonate crystals were generated at the pores of the soil body,and the adjacent soil particles were cemented into a unified whole,and the calcium carbonate crystals were hexahedral shapes,and the reinforcement effect was improved.（4）The concentration of bacterial solution,environmental p H value,cementation concentration and curing time were selected as research factors,and the single-factor test and orthogonal experimental scheme were used to reinforce the granite residual soil respectively,and the permeability test of the variable head showed that when other conditions were controlled,the higher the concentration of bacterial solution,the smaller the permeability coefficient of the reinforced sample,and the minimum value was 1.76×10-6cm/s.Both acidity and alkalinity are not conducive to MICP reaction,and the permeability coefficient of the reinforced sample is the smallest when the ambient p H is 9;When the concentration of cementation solution is in the range of0.25mol/L～1.0mol/L,the permeability coefficient of the sample decreases with the increase of the concentration of cementation liquid,and when the concentration of cementation liquid exceeds 1mol/L,the permeability coefficient becomes larger and the reinforcement effect is poor;The curing time is 9 days,and the permeability coefficient is minimal;Through orthogonal test,it is concluded that the factors affecting the permeability coefficient of the reinforcement sample are cementation concentration,bacterial concentration,curing time,p H value in order of importance,and the best reinforcement scheme for the test is bacterial concentration OD₆₀₀=1.5,environmental p H value is 9,cementation solution concentration is 1.0mol/L,and the curing time is 5 days.（5）Through the one-dimensional soil column infiltration test,it is found that the wetting front migration distance of the soil column will be reduced to different degrees after microbial reinforcement,the higher the concentration of bacteria at the same infiltration time,the smaller the wetting front migration distance and water cumulative infiltration,and the relationship curve between soil wetting front migration distance and infiltration time is fitted,and it is found that it conforms to the power function relationship,and the fitting degree R² is greater than 0.996;through experimental analysis of samples reinforced with different cementation concentrations,it is found that the concentration of cementation solution is 1mol/At L,the wetting front movement distance under the same infiltration time is the smallest,and the reinforcement effect of soil column infiltration is the best.Moreover,the relationship between wet front migration distance and soil cumulative infiltration can be expressed by the linear function function y=ax+b,and the fitting degree R²is above 0.99.