Experimental Study On Physical And Mechanical Properties And Microstructure Of Microbial Solidified Sand In Salt Solution

The MICP technology is a process in which carbonate ions generated by microbial decomposition of urea react with calcium ions to form calcium carbonate precipitates.Calcium carbonate is used to fill the gap of soil particles and cement the adjacent soil particles,so that the loose soil can be bound into a solid body with certain strength,thereby improving the mechanical properties of the soil.As a new way of foundation reinforcement,MICP technology has been widely concerned.At present,most of the studies on MICP do not start from the actual environment of soil,only consider in fresh water environment.However,in engineering practice,the groundwater environment of many soils is not a pure water environment,but a solution environment with various ions and certain salinity.Therefore,in order to study the effect of salt solution on the performance of soil strengthened by MICP.In this paper,salt solutions with concentrations of 0,20,35,60,80 and100g/L were prepared with Na Cl respectively.The bacterial activity test and in-solution mineralization test of MICP were carried out by using Sporosarcina pasteurii in different concentrations of salt solutions.The unconfined compression test,calcium carbonate content test,dry density test,water absorption test and dynamic triaxial test were carried out on MICP solidified sand samples in different concentrations of salt solution to study the changes of microbial and urease activities in different concentrations of salt solution.The solidification effect and difference of calcareous sand reinforced by MICP in different concentration salt solution environment were compared.Combined with optical microscope,XRD phase analysis and SEM scanning electron microscope test,the reinforcement mechanism of samples was analyzed.The main conclusions and achievements of this paper are as follows.（1）The existence of salt solution can inhibit the growth and reproduction of bacteria,and the inhibition increases with the increase of salt solution concentration.Although the growth of bacteria and the activity of urease were inhibited in the simulated seawater environment,the inhibition effect was not too great,which did not make the bacteria lose their activity completely,which proved the feasibility of MICP strengthening sand in the seawater environment.（2）In the solution mineralization experiment,the change trend of calcium carbonate production was consistent with that of bacterial activity.The decline of bacterial activity is the main reason for the decrease of Ca CO₃ production in salt solution environment,and it is also the fundamental reason for the decrease of sand column reinforcement strength.The decrease of calcium carbonate productivity is the most direct reason for the decrease of reinforcement strength of sand column.（3）The unconfined compressive strength of sand sample decreases with the increase of salt solution concentration,which indicates that salt solution will reduce the reinforcement effect of MICP.The experimental results are consistent with the results of bacterial activity test and in solution mineralization test.From the test results of strength,dry density and water absorption,the reinforcement effect in simulated seawater environment is still considerable.（4）Under different concentrations of salt solution,the dynamic strength of the specimens strengthened by MICP decreases with the increase of salt solution concentration.However,the dynamic strength of the specimens strengthened with MICP is higher than that of the specimens without MICP in any salt solution.After MICP treatment,the liquefaction resistance of sand is greatly improved.（5）From the microstructure test,it can be seen that the amount of calcium carbonate gradually decreases with the increase of salt solution concentration,and the filling density of sand particles decreases continuously.The existing form of calcium carbonate changes from wrapping to adhesion,which reduces the effective connection between sand particles and reduces the strength of sand column.（6）In the salt solution environment,because the cell membrane of bacteria is similar to the semi permeable membrane,and the concentration difference between the inside and outside of the bacteria,the water inside the bacteria seeps out,causing the bacteria to lose water,affecting the normal growth and reproduction of bacteria.As a result,the bacteria have not been completely wrapped by the calcium carbonate crystals produced by MICP,but die ahead of time.