Experimental Study On Soil Consolidation By Improved Microbial-Induced Calcium Carbonate Precipitation

Microbial-induced calcium carbonate precipitation(MICP)is a new technology by microbial mineralization of reinforced soil.The mechanism is that microbial metabolism produces urease,which hydrolyzes urea into ammonium ions and carbonate ions.Using bacteria as nucleation site to combine carbonate ion with calcium ion to form calcium carbonate precipitation to consolidate soil is an economical,green and environment-friendly technology for improvement of foundation and slope.In this paper,based on this technology,by changing the composition and ratio of different substances in the culture medium,the culture method of Sporosarcina pasteurii was divided into two steps,and the culture method of producing high urease was studied.Under this culture mode,on the one hand,the solidification and cementation of sand were analyzed by adding four kinds of fibers in the sample: carbon fiber(CF),basalt fiber(BF),polypropylene fiber(PP)and polyester fiber(PET)combined with microbial induced calcium carbonate precipitation technology.On the other hand,MICP technology was combined with HPMC,a viscous material,to explore and analyze the surface cementation and solidification.The main contents and research conclusions of this paper are as follows:(1)In this study,a simple two-step culture method was established to improve urease activity of Sporosarcina pasteurii by using suitable culture medium.Urea plays an important role in the culture of S.pasteurii,especially in the first pre-culture stage,which was a necessary and insufficient condition for the pre-culture of S.pasteurii.In the second expanded culture stage,urea was no longer an essential component.The pre-culture stage will have an impact on urease in the expanded culture stage of S.pasteurii.The first pre-culture with type I medium and the expanded culture with type II medium could obtain higher urease activity and higher unit urease activity.In addition,by improving the initial urease activity of S.pasteurii,the content of calcium carbonate induced by MICP technology will also increase,and the mechanical properties of samples will be strengthened correspondingly.SEM images showed that the samples treated by MICP technology will form a firmer structure.XRD test showed that the crystal type of calcium carbonate produced by MICP technology was not changed by the initial urease activity of the bacteria liquid of aspergillus pasteurensis.SEM images showed that the samples treated by MICP technology will form a firmer structure.XRD test showed that the crystal type of calcium carbonate produced by MICP technology was not changed by the initial urease activity of the bacteria liquid of S.pasteurii.(2)The soil treated by MICP is brittle.Discrete fiber inclusion is an effective way to enhance soil ductility.In this study,we collaboratively utilized these two techniques for soil reinforcement,the Mechanical Properties of Carbon Fiber(CF),basalt fiber(BF),polypropylene fiber(PP)and polyester fiber(PET)with different content(0 ～ 1.8% of sand weight)to enhance the anti-permeability,unconfined compressive strength(UCS)and tensile strength(TS)of the biobonded sand were compared.The microstructure of the samples was showed by X-ray diffraction and SEM.Through research,it was found that fiber inclusions could promote the precipitation of calcium carbonate.With the increased of fiber content,the calcium carbonate content gradually increases,and the added of four kinds of fibers would not change the crystal type of calcium carbonate.When the volume fraction of the four fibers was about 1%,the fiber had the most significant influence on unconfined compressive strength(UCS).With the increased of fiber content,the impermeability and splitting tensile strength(TS)were improved.Generally speaking,the performance of four kinds of fibers was evaluated by improving the impermeability,strength and ductility of fiber reinforced bio-cemented sand,and the order from high to low was: CF > BF > PP> PET.Considering the cost,basalt fiber(BF)was the most cost-effective fiber material among the four fibers.(3)HPMC,a viscous material,was used to improve the surface layer by inducingcalcium carbonate precipitation with the high urease activity of S.pasteurii.The surface strength,calcium carbonate content in hard shell,water resistance and ammonia retention rate of samples were enhanced under different bacterial liquid and cementing liquid contents and MICP treatment times.The microstructure of the samples was showed by XRD and SEM.The research showed that by adding HPMC and increasing the number of MICP treatments,the calcium carbonate content in the surface crust could be effectively increased,but it had no effect on the total amount of calcium carbonate induced.The combination of HPMC and MICP technology could effectively reduce the loss of rain erosion and increase their water resistance.The addition of HPMC could also effectively improve the ammonia absorption rate and reduce the release of ammonia during MICP technology,which was of great significance to environmental protection.The microstructure showed that by adding HPMC and increasing the number of MICP treatments,the surface structure of samples could be denser,calcium carbonate could fill more pores and cement soil particles,and the crystal type of calcium carbonate could not be changed by adding HPMC.