| The improvement of soft soil,such as saturated sand,silty and soft clay,is one of the primarily problems for infrastructure construction and safe operation.In the goal of carbon peaking and carbon neutrality,the development of green and energy-saving technology is the realistic demand of the infrastructure industry.As a novel green and efficient technology,biomineralization holds broad application prospects in the field of foundation improvement,disaster prevention and reduction.However,the poor uniformity of cementation distribution has become one of the major challenges in the application of bio-grouting technology to practical engineering.However,there is still a lack of research on the distribution control of cementation and its mechanism as well as the cementation effectiveness of calcium carbonate in soil.Due to the spatial compatibility,with the decrease of soil particle size,the difficulty of biomineralization technology changes from the uniformity of grouting reinforcement to the grout ability and even the effectiveness of MICP.Thus,the bio-grouting technology is mainly used for sand improvement,but less in fine-grained soil.Based on this,the paper systematically studied the biomineralization technology to improve soft soil.Firstly,the preparation method of biological solution was compared and selected,the effect of S.pasteurii solution component on the MICP process was investigated,and the bio-chemical solution formula was optimized.Then,based on the self-developed multifunctional bio-grouting experiment system,the effects of grouting technology and bio-chemical solution formula on mechanical properties,the spatial distribution of cementation,and mechanism of MICP were deeply studied using a series of sand column experiments.Furthermore,a novel M-EICP technology based on bacteria cell-enzyme mixed microbial solution was developed to strengthen silty-sand mixed soil and improve the uniformity of pure sand.Finally,the MICP and electro-osmosis combined method was proposed for clay reinforcement.The main research contents and results are as follows:(1)For the technical and economic feasibility of bacterial solution preparation and the selection of bio-chemical solution formula,comparing the bacterial solution preparation methods from various aspects,systematically optimizing the formula of bio-chemical solution and clarifying the weakening effect of MICP reaction on the ability of bacteria to induce calcium carbonate by single reaction,single bacterial-multi chemical solution and multi bacterial-chemical solution cycle MICP reaction.The results showed that it was more reasonable to take the original bacterial solution as the target biological solution in biological grouting technology.Resuspension of cell with fresh medium failure to promote the total output of calcium carbonate,but nearly doubled the preparation cost;the calcium carbonate induced by original bacterial solution hold the strongest cementing property,and the presence of bacterial cells agglomerate the carbonate precipitation;The weakening effect of MICP reaction on microbially induce calcium carbonate lead to reductio of the optimal chemical solution concentration from 2.0 M of single to 0.75 M of multi MICP reaction cycle,and the optimized formula was 10 U/m L microbial solution + 0.75 M chemical solution.The results provide support and reference for the selection of biochemical solution in biomineralization.(2)Independently developing a set of multifunctional bio-grouting experimental system,which can realize: 1)based on a set of equipment,completing the different scales bio-grouting experiments by just one apparatus;2)adjusting the bio-chemical solution grouting strategy in real time in the process of bio-grouting test,which solves the disturbance of the sample caused by the transformation of grouting process during the experiment;3)monitoring MICP reaction process in real time;systematically investigating the effects of bacterial solution amount,soil particle size and grouting technology on the improvement of mechanical properties and spatial distribution of calcium carbonate,revealing its control mechanism by a series of the micp reaction process monitoring in sand column.It was found that there was an optimal injection volume of 1.2 PV,and the excessive biological solution has little effect on the further improvement of soil strength;the upward of biological solution,revised bio-chemical solution injection direction and increasing the grouting rate of biological solution can improve the distribution uniformity of calcium carbonate,but has little effect on the horizontal distribution uniformity improvement of carbonate precipitation.The variation coefficients of calcium carbonate horizontal distribution with all grouting strategies are between 0.1-0.3;The significant influence of MICP reaction during chemical solution grouting on the uneven distribution of calcium carbonate was analyzed,it is believed that the distribution of calcium carbonate is the result of MICP reaction time difference at different positions and the uneven distribution of bacteria;The finite element simulation further revealed the important role of MICP reaction during chemical solution grouting on the calcium carbonate distribution,and the results showed that the uneven distribution of calcium carbonate can be reduced by increasing the concentration of chemical solution and grouting rate and reducing the concentration of bacterial solution.Furthermore,the distribution of ion concentration in pore solution corresponded to the distribution of calcium carbonate and permeability.(3)For the poor grout ability of bacterial solution in silt-sand mixed soil and the uneven cementation distribution in sand,developing an M-EICP technology based on bacteria-enzyme mixed biological solution.Studying the feasibility of this technology to improve silt-sand mixed soil,with the maximum silty particle of 50%;The effect of bacteria-enzyme amount ratio on the effective area of M-EICP technology was evaluated;exploring the feasibility of M-EICP technology to realize the uniform distribution of calcium carbonate in pure sand.The results showed that for the mixed soil with silt content less than 50%,the effective area of MICP technology increased with the decrease of bacteria enzyme ratio;for biological solution with bacteria cell/enzyme ratio of 0,the effective area in 50% silt sand mixed soil was 15 cm;For the less than 30% of silt content,the reinforcement range can be at least 30 cm.However,the intact bacterial solution can only be used for the improvement of silt sand mixed soil with silt content less than 10% or the construction of thin shell layer less than 5 cm on the soil surface;By controlling the bacteria cell / enzyme ratio,the uniform reinforcement of pure sand can be realized.The uniformity of sand reinforcement can be realized by properly matching the bacterial enzyme ratio with the particle size of soil.(4)For the improvement of soft clay,developing a MICP-EO combined reinforcement method which can realize the uniform improvement of soil.By a series of electro-osmosis reinforcement experiments,discussing the mechanical properties and surface settlement of clay treated by the combined reinforcement method,and expounding the mechanism of MICP in performance improve of clay.The results showed that the combined reinforcement method solve the problem of the low strength near the cathode in pure electroosmosis consolidation,improved the strength and uniformity of clay,and significantly improved the uneven settlement of soil surface.Compared with pure electroosmosis,the settlement variation coefficient decreases from 0.26 to 0.12,and the settlement difference decreases by 3.86 times;The observation of physical and chemical properties and microstructure further proved that the improvement of strength and uniformity by MICP depends on the formation of solid substances such as calcium carbonate and calcium hydroxide in soil pores.In summary,for the main problems in biomineralization technology for soil improvement,based on the optimization of MICP reaction,this paper systematically studied the uniformity of calcium carbonate distribution in pure sand,the ability of biological solution pouring in silt sand mixed soil and the effectiveness of MICP in clay,especially developed the new methods for silt sand mixed soil and soft clay,providing technical support and reference basis for biomineralization to improve soil,and provides a new idea for the development of new lowcarbon and energy-saving infrastructure technology. |
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