Experimental Study On The Solidification Of Coarse-grained Saline Soil By Microbial-induced Calcium Carbonate Precipitation (MICP)

Saline soil is widely distributed in China,especially in the northwest inland areas such as Gansu,Qinghai,Ningxia,Xinjiang,etc.There are a large number of coarse-grained saline soils.Because of the poor stability of coarse-grained saline soil,it is very important for roads,railways and buildings.The hazards of soils are more serious;coupled with the special local climate and topographical conditions,the area of??saline soil in the northwest is increasing,and the problem of land salinization is becoming more and more serious.Therefore,it is extremely urgent to strengthen the coarse-grained saline soil in the northwest inland area and improve its engineering performance.The paper uses the microbial reinforcement（MICP）method to solidify and improve coarse-grained saline soil,and through a combination of macro-mechanical tests and micro-mechanism analysis,explores the effect of the MCP method in solidifying coarse-grained saline soil under different salt content conditions.Feasibility and reinforcement effect.The main research contents and results are:（1）Aiming at the current situation of coarse-grained saline soil in the northwest inland area,the microbial induced calcium carbonate precipitation（MICP）method is used to improve the solidified coarse-grained saline soil;through dynamic monitoring of bacterial growth,the bacterial parameter conditions are determined:the bacterial culture time is For 24 hours,the bacterial concentration is OD₆₀₀=1.7～1.8,and the urease activity is 0.4581mmol/min.The optimal reaction conditions for the MCP reinforcement test of urea hydrolysis are determined as follows:the ratio of urea concentration and calcium ion concentration is 1:0.5,and the urea concentration is1.0mol/L,calcium chloride concentration 0.5mol/L,nutrient broth 3g/L;the optimum temperature is room temperature（25°C）,p H=7.（2）According to the above test parameters,the MCP solidified saline soil test was carried out.The macro-mechanical test was carried out on the coarse-grained saline soil after 14 times of MCP low-speed seepage reinforcement.The results of the unconfined compressive strength test showed that the four groups contained The strength of coarse-grained saline soil with salt content has been greatly improved,and its strength value is between 2.88～4.55MPa;comparing the coarse-grained saline soil before and after reinforcement,the permeability coefficient is reduced by two orders of magnitude;when the positive pressure is 300k Pa,4 The shear strength of the experimental group is 558～654k Pa;that is,the coarse-grained saline soil strengthened by MICP has good compressive strength and strong resistance to shear failure,which effectively reduces the porosity of the coarse-grained saline soil,Proved that microbial solidification significantly improved the engineering mechanical properties of salt-containing sand.（3）Comparing the changes in the amount of calcium carbonate produced in the sample within the range of salt content（0%～9%）,the results show that salt has an inhibitory effect on the rate of bacteria catalyzing the hydrolysis of urea.The higher the salt content,the slower the rate of hydrolysis of urea,and the less calcium carbonate is produced,which leads to a decrease in the mechanical properties of coarse-grained saline soil with high salt content,indicating that the presence of salt weakens the effect of MCP reinforcement,but Pasteurella The salt environment has a certain tolerance and strong adaptability.Compared with the coarse-grained saline soil before reinforcement,the existing reinforcement effect has met the actual engineering needs.（4）Microscopic tests were carried out on the coarse-grained saline soil after MCP reinforcement.The results of mercury intrusion test showed that the pore size distribution of the soil particles decreased after reinforcement;XRD（X-ray diffraction test）determined the interior of the coarse-grained saline soil reinforced by MCP.The products produced are calcium carbonate,and there are two crystal forms:calcite and vaterite.It is the calcium carbonate formed by the reaction that plays a decisive role in the reinforcement of the soil;SEM（Scanning Electron Microscope Test）can clearly show that each group of samples is in the soil particles.There are calcium carbonate deposits on the surface and the pores of the soil particles.When the calcium carbonate deposits completely cover the soil particles and are widely distributed in the pores of the soil particles,and the adjacent soil particles are cemented into a polymerized state,the best reinforcement effect of the sample is achieved.Improve the compressive strength,impermeability and shear damage resistance of saline soil.