Study On The Strength And Water Stability Of Two Strains Of Bacteria Solidified Purple Soil

Purple soil widely distributes in the Three Gorges Reservoir Area of Chongqing and plays an important role in geotechnical engineering and agricultural production.It suffered shorter weathering procedure and expresses lower cohesive strength.Under the influence of ground force,climate factors and the water fluctuation,purple soil contains imperceptible dangers of landslides,debris flow and soil erosion,as well as other geological disasters,which brings troubles to the human production and living in these areas.MICP has a wide application prospect.It takes the bacteria in the soil as the medium,and when the environmental and material conditions is appropriate,it generates calcite among adjacent soil particles,which achieves the objectives of enhancing soil strength,reducing soil erosion and containing heavy metal pollution.Bacillus megaterium（B.m）and Sporosarcina pasteurii（S.p）are widely used in microbial reinforcement technology because of their stable and higher urease activity（U_a）.However,the control and detection of the reaction process are still the difficult,as well as the challenge in achiving the higher productivity of MICP.Based on above,this paper carried out experiments to improve the strength and slaking property of purple soil in the Three Gorges Reservoir Area through MICP.Research contents and conclusions are as follows:（1）Firstly,revealing strains’micromorphology,U_a,specific urease activity（SU_a）and growth characteristics.The morphology（length and width）of S.p is about 3～5 times bigger of B.m,resulting in a large difference in the size of their mineralized crystals.The size of calcite crystals produced by B.m is relatively uniform,while the size of the counterpart produced by S.p is more uneven.Under the same incubation conditions for48h,the peak biomass of B.m was about 2.06～2.18×10⁸cfu/m L（OD≈2.3）,the peak U_awas about 0.54m M/min,the p H value increased from 5 to 8.24,and the SU_a displyed 1.3m M U/（min·OD）at about 8h.The peak biomass and U_a of S.p were 1.68～2.12×10⁷cfu/m L（OD≈1.5）and 3.6m M/min,respectively.The p H went from 5 to 7.6.The highest value of SU_a was 6.8 m M U/（min·OD）at about 16h.The effects of passage,temperature,initial p H value,centrifugal acceleration and nickel ion content on the two strains were studied.The results of Logistic function fitting and parameter analysis showed that the catalytic effect of nickel chloride on B.m was better than that on S.p.These two strains have inapparent antagonism or synergistic effect when they were mixed up.（2）A set of combined devices was designed indepently and applied in doing orthogonal test with four factors in three levels.The SEM image,strength improvement,change of permeability coefficient and micro-strengthening characteristics of the purple soil cemented by B.m under pressure were analyzed.The results show that the optimal grouting condition is 150k Pa（grouting pressure）,28ml/h（flow rate）,1mol/L（concentration of cement solution）,and the utilization rate of the cementing solution were67%,48%and 36%after 1,3,and 5 cycles of treatment respectively.The permeability coefficient of the sample decreases by 1-2 orders of magnitude from the initial 2.5×10^-4cm/s.After 5 cycles of treatment,sample’s peak unconfined compressive strength（UCS）increased 2 times,calcium carbonate production and the increment of peak unconfined compressive strength（ΔUCS）shows the approximate linear relationship,more than 1.1%of calcium carbonate production can make theΔUCS improved.The E₅₀ is negatively correlated with the dispersion of calcite and fluctuated with the increase of calcite.The E₅₀ of cured sample obtained by pressure grouting is up to 301MPa,which is 3.2 times that of uncured sample.More than or equal to 5 cycles of grouting and appropriate increase of grouting pressure and flow rate can obtain a better soil solidification effect.（3）For S.p strain,the single factor test explored the influence rule of grouting pressure and flow rate on curing effect.As the grouting pressure increases to 200k Pa,the permeability coefficient of the sample decreased by 2～3 orders of magnitude.UCS of sample linearly increased at first,and then decreased with the increase of grouting pressure,the peak UCS was 2.84MPa.Compared with samples uncured and normal cured,the UCS strength of pressure MICP cured sample increased by 97%and 88%at the highest.At 150k Pa grouting pressure,the yield of calcite（7.6%）was the highest,which is 2.6 times of that normal cured（2.9%）.The permeability coefficient of solidified samples with a grouting flow rate of 270ml/h is 11～15 times higher than that of samples with a low grouting flow rate of 10ml/h.Keep the grouting pressure in constent,UCS of the sample increased first and then decreased linearly with the increase of the flow rate,the optimum flow rate under pressure grouting is about 30ml/h.The Kjeldahl test for effluent shows that the urea hydrolysis efficiency still changed with time even if the grouting pressure and flow rate are always constant.（4）The slaking characteristics of the remolded purple soil and MICP cured purple soil samples were studied.The result shows that slaking process of purple soil samples shows four stages:water absorption period,equilibrium period,accelerated slaking period and stable period.There is a critical dry density K_d in the range of 1.39～1.45g/cm³.The sample can completely disintegrate if it’s dry density less than K_d.The residual amount（W_r）and peak soaking amount（W_p）getting more when the dry density is higher,whereas the slaking rate（P）getting smaller.There is K_m in the range of 18～21%.When the initial moisture content is higher than K_m,the sample will remain stable in a certain period of time with extremely low P and W_r.Inversely,the sample will completely disintegrate in a short time.The P increasd linearly about 5 flod with the increase of water temperature from 15 to 30℃.The samples with more coarse grain soil content and good grading properties show better slaking property.After 1～3 times MICP treatment,the W_p of the cured samples increase about 4～5g per cycle.The W_r of the samples also increased from0g to 36.13～51.41g,and the P decreased to 1/6～1/14 of that of the uncured samples.The mophorlogy of produced MICP on samples with different treatment times are quite different,but MICP treatment improve the wslaking property of purple soil is feasible.