Study On The Influencing Factors And The Underwater Weatherability Resistance Of Microbial Induced Carbonate Precipitation For Sand Solidification

The requirements for the bearing capacity of the foundation soil are increasing in the construction of urban infrastructure,and the phenomenon that the foundation soil needs to be strengthened due to failure to meet the requirements.Microbial induced carbonate precipitation(MICP),as a new method for improving foundation soil,uses the mineralization of urease-producing bacteria to precipitate calcium carbonate with the function of cementing loose soil.Because of its simple mechanism,small disturbance to soil,low energy consumption,according with national sustainable development strategy,it has good development prospects.In this paper,Bacillus pasteurii is used as a test bacterium to carry out grouting and strengthening research on sand.Firstly,the effects of the speed of the shaker,the inoculation ratio,the amount of substrate added before the inoculation,and the ratio of urea and calcium chloride in the reaction solution on the growth characteristics,urease activity,and calcium carbonate production of the target strain were studied.Secondly,the effects of factors such as bacterial solution concentration,reaction solution concentration,and reaction time on the morphology and crystal form of the microbial mineralization reaction were studied.Then microbial grouting tests were performed on standard sand and sand sphericity alternative materials,respectively,and the effect of particle shape on the curing effect of MICP technology for grouting soil was studied.Finally,the sand samples solidified by microbial grouting were immersed in groundwater and artificial simulated seawater for a certain period of time,and the weather resistance of the MICP solidified sand samples under groundwater and seawater conditions was studied.The main conclusions of the paper are as follows:(1)When cultivating the Bacillus pasteurii bacterial solution,adding substrate before inoculation can improve the urease activity of the bacterial,but an excessively high concentration of the substrate will restrain the growth of the bacterial.In the range of 1%?3%inoculation ratio and 150rpm?210rpm shaker rotation speed,with the increase of inoculation volume or rotation speed,the peak of urease activity of the bacterial solution shows a tendency of increasing and then decreasing.As the theoretical production of calcium carbonate is controlled to be equal,with the concentration ratio of urea to calcium chloride in the reaction solution increases,the actual production of calcium carbonate gradually increases,but after the proportion of urea concentration increases to a certain extent,the increase of calcium carbonate's actual production is no longer significant.(2)Under concentrations of reaction solution with different gradients of 0.25 to 3 mol/L,the calcium carbonate crystallinity corresponding to the sediment is relatively high at the reaction solution concentration of 0.5mol/L and lmol/L.The crystallinity of the sediment crystals induced by the higher concentration reaction solution is lower.(3)The proportion of calcium carbonate crystal form induced by the pure bacterial solution changes little with the mineralization time,and the calcium carbonate crystal phase is dominated by vaterite throughout.The proportion of the proportion of vaterite crystals in the solution without bacterial and 50%concentration of bacterial solution changes relatively large,but it gradually becomes dominated by vaterite crystal phase with time going.In the process of microbial mineralization,the bacterial cells play a regulatory role on the direction of nucleation and growth of calcium carbonate crystals.(4)After microbial grouting,the standard sand sample is cemented into a whole by loose particles,and the strength of sample is improved.When the sample preparation conditions are consistent,the biosolidification effect of particles with low sphericity and certain angularity is better than that of pure round particles.(5)Under the same grouting and particle composition conditions,as the initial dry density of the sample increases,the content of calcium carbonate in the grouted sample gradually decreases,but the unconfined strength of the sample increases first and then decreases.Therefore,only calcium carbonate,which plays a role in cementing,contributes to the growth of sample's strength.(6)The standard sand samples solidified by microbial grouting,after being soaked in groundwater and artificial simulated seawater for 60 days,show no large area of dropout on the surface layer,and the decrease in the strength values of the samples is small,and the maximum decrease is 8.5%and 10.8%,respectively.