Experimental Study On Soil Improvement Using Microbial Induced Calcite Precipitation (MICP)

With increase of global population and development of society, demand forconstruction land continues to expand. Traditional chemical grouting materials andchemical stabilization agents often contain toxic chemicals, which have adverseimpact on the environment. It is very urgent to study a new environmental friendlysoil improvement method. The microbial induced calcite precipitation (MICP) is acommon biomineralization process widespread in natural environment. Themechanism is simple and the process is fast and efficient, as well as goodenvironmental tolerance. This technology could be used for soil improvement, whichwill bring enormous environmental and economic benefits.This paper studied on MICP improving soil using urea hydrolysis catalyzed bySporosarcina pasteurii, as well as the effect of soil improvement and influence factors.It can provide basic data and scientific basis for application of MICP on soilimprovement. The study results were listed as follows:(1) The bacterial culture and its urease activity were studied at first. Theoptimum conditions for Sporosarcina pasteurii growth are30℃, pH8~9,38~40h,when culture at200rpm and1%(V/V) inoculums. It can be storage at4℃for at least30d.(2) The influencing factors of urea hydrolysis MICP process were discussed. Theincrease of bacteria (or urease) concentration can speed up the process of ureahydrolysis. When urea concentration was less than1.6M, the bacterial urease activityrose with the growth of urea concentration. However, the high concentrations ofCaCl2inhibited the urease activity significantly. When both of CaCl2and ureaconcentrations (less than1.5M) rose, the amount of CaCO3generated through MICPprocess increased. The pH has little effect on urease activity when ranging from6to9.The high temperature (>30°C) promoted urease activity obviously.(3) A full contact flexible mold made by geotextile and completely stirred tankreactor was used for research of MICP treated soil, which can great enhance soil improvement effect and uniformity.(4) The soil improvement effect of MICP catalyzed by bacteria and manualextracted urease was studied. The raise of bacterial (or urease) concentration,cementation media concentration, and reaction time can increase the unconfinedcompressive strength of MICP treated samples and the amount of produced CaCO3.Sand particle size has a great influence on MICP treated samples, the MICPimprovement effect on standard sand (D50=0.46mm) is better than that ofMississippi sand (D50=0.33mm). The MICP treated samples had good water stabilityand thermal stability. Comparing the result of MICP treated sample catalyzed bybacteria and manual extracted urease, the improvement effect of bacteria was alwaysbatter than urease when their initial urease activity were similar. Besides, the soilimprovement of MICP by urease was also low efficiency and high cost, so it is notsuitable for practical application compared with MICP by bacteria.