Experimental Study On The Mechanism And Physical And Mechanical Properties Of Calcium Carbonate Precipitation Induced By Bacillus Licheniformis To Improve Swelling Soil

Swelling soil is a kind of soil with special properties,and most of its clay minerals are composed of hydrophilic minerals such as montmorillonite and illite,so it has engineering characteristics such as high water absorption,high plasticity,rapid disintegration and violent expansion and contraction,and it is widely distributed in China,which brings great harm to engineering construction.Most of the existing improvement methods are based on mixing lime,cement and ionic curing agent in the swelling soil,etc.Although the improvement effect is significant,there are disadvantages such as high construction intensity,high cost and environmental pollution.Biologically induced calcium carbonate precipitation（MICP）is a kind of biomineralization process that exists widely in nature,with simple mechanism,fast and efficient,and good environmental tolerance.The use of this technology for swelling soil improvement will bring great environmental and economic benefits.In this paper,microbial improvement of swollen soil in Xiaomiao Town,Hefei City,Anhui Province was carried out by using Bacillus licheniformis isolated independently.The target microorganisms were obtained by enriching,isolating,identifying and preserving the indigenous microorganisms in the soil,and their growth characteristics and optimal growth conditions were studied;the response surface method was used to optimize the optimal process parameters of three conditions,namely,the concentration of cementing solution,the maintenance time and the concentration of bacterial solution,for the rate improvement of swelling soil;the four methods of swelling soil improvement,namely,microorganisms,fibers,fly ash and lime,were compared by a series of physico-mechanical tests to The effect of the four methods of swelling soil improvement,namely microbial,fiber,fly ash and lime,was compared through a series of physical and mechanical tests to verify their improvement effects.Based on the experimental research and theoretical analysis,the following conclusions can be obtained:（1）The strain that can induce the production of calcium carbonate precipitation and can use calcium propionate as carbon and calcium source was screened out from the soil by selecting the culture medium and identified as Bacillus licheniformis by DNA sequencing and database comparison.（2）The optimal environmental conditions for the growth of Bacillus licheniformis were determined by single-factor test as 2%inoculum（v/v）,p H 8,shaker speed 160 rpm and incubation temperature 35°C.（3）Using the Box-Behnken design in the response surface method,the three conditions of colloidal solution concentration,maintenance time and bacterial solution concentration in Bacillus licheniformis modified swelling soil were further optimized using the free swelling rate as the response value,and the optimal process parameters were derived as the colloidal solution concentration of 0.85 mol/L,maintenance time of 5 d and bacterial solution concentration of 1.3OD₆₀₀.（4）The swelling soil improved by Bacillus licheniformis was compared with the swelling soil improved by fly ash,basalt fiber and lime under the optimal process conditions,and the combined liquid-plastic limit determination test,standard moisture absorption and moisture content test,lateral limitless compressive strength test and triaxial compression test were conducted respectively.The physical and mechanical properties are greatly improved,and it has the advantages of environmental friendliness,low cost and easy construction.（5）Scanning electron microscopy tests were conducted on swelling soils before and after modification by Bacillus licheniformis.The connection between soil particles of swelling soils was obviously enhanced after modification,and the pores between soil particles were filled by the induced calcium carbonate,and the connection between soil particles became tighter,thus revealing the mechanism of swelling soil modification by Bacillus licheniformis.