| With the cross-fertilization of microbiology,chemistry,geotechnical engineering and other multidisciplines,microbial deposition of calcium carbonate technology has received extensive attention and research.This technology has potential value in improving cemented loose sand in situ and it is gradually becoming a new type of impermeable technology.Compared with traditional methods,this technology is an environmentally friendly technology with simple mechanism,rapid and efficient reaction,and high environmental tolerance.This paper focuses on the application and research of MICP technology in the field of sandy soil permeability improvement.Firstly,the activation and culture of microorganisms were carried out to analyze the changes of bacterial solution concentration and urease activity during the growth of microorganisms.Then,the orthogonal test and response surface design test were conducted to explore the priority and level of different influencing factors in the process of sand improvement by MICP,focusing on the effects of bacterial solution concentration,colloid concentration,colloid time and number of colloid injections on the improvement effect,and the changes of permeability,strength and calcium carbonate content of the specimens during the improvement process were analyzed.Finally,we analyzed the changes of microscopic pores in the specimens during the improvement process based on SEM images,and investigated the relationship between microscopic pore ratio and macroscopic permeability coefficient.The following conclusions were obtained in the test under the condition of five cycles of cementation process:(1)MICP technology can effectively improve the permeability of sandy soil,and the permeability coefficient can be reduced by two orders of magnitude compared with the permeability coefficient of the original sandy soil.The optimal ratio of influencing factors was determined to improve the permeability coefficient of sandy soils as an indicator,and the optimal ratio with unconfined compressive strength as the index influence factor.The quadratic polynomial regression model of the permeability coefficient constructed and unconfined compressive strength by the response surface design test was validated to have a trial relative error of 4.756% and 3.93%,and the model was highly accurate and reliable.(2)The permeability coefficient of the sandy soil specimens improved by MICP technology is inversely proportional to their own calcium carbonate content.The improvement of the permeability of the sandy soil is minimal when the calcium carbonate content inside the specimens is less than 5%,and the permeability coefficient only decreases significantly when the calcium carbonate content exceeds 10%.The strength of the improved sandy soil shows an increasing trend with the increase of the calcium carbonate content.(3)Microscopic mechanism of sand permeability improvement by MICP technology: the number of large pores and pore area ratio inside the specimen are reduced by calcium carbonate deposition;calcium carbonate divides the large pores into several small pores,which increases the directional probability entropy of pores inside the specimen and makes the pores more uniformly distributed inside the specimen,which in turn reduces the seepage channels inside the specimen and leads to the reduction of its macroscopic permeability.(4)Compared with the macroscopic measured pore ratios,the values of microscopic two-dimensional pore ratios are small due to the influence of threshold values,and the values of microscopic three-dimensional pore ratios are large due to the influence of the preparation method of electron microscope scanned specimens,but the relative errors are less than 10%,which prove that the calculated micro-porosity ratios have certain degree of reliability.The double logarithmic form of lgk-lge eliminates the difference in order of magnitude between microscopic pore ratios and permeability coefficients,and well describes the linear regression between the two relationship. |
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