Multi Scale Experimental Research On Anti-liquefaction Performance Of Liquefiable Sands Strengthened By Microorganisms

Microbial induced calcium carbonate precipitation?MICP?is a new technique of in-situ grouting.Calcium carbonate is formed by the interactions of microorganisms and calcium salts that can improve the liquefaction of sand soil liquefaction resistant characteristics by cementation and filling.This technique has a potential value in liquefied sand layer,slope stability and subgrade reinforcement field.In this paper,we firstly determined the optimization scheme of microbial grouting treatment of liquefiable sands from material scale,and monitored shear wave velocities from different points on large scale samples by bender elements.Then,the liquefaction resistance of different reinforcement schemes corresponding to the samples was studied by dynamic traxial test.Through centrifuge shaking table tests,this study also systematically explored the seismic performance of sand liquefaction by microbial reinforcing using model simulation.It can provide the basis for the application and development of MICP technology.Firstly,we studied the effect of MICP cementation through comparative study of different kinds of nutrients and nutrient consumption.The results showed that Ca?CH₃COO?₂ had the best reinforcing effect.Secondly,we studied the effect of flow rate on sample stiffness,permeability and the uniformity of calcium carbonate after the sample size amplification.The experimental results showed that measuring pressure and the ammonium ion of the samples was a simple and non-destructive test method for studying the effect of microbial treatment of liquefiable sands.The balance between stiffness and uniformity should be taken into account in engineering application.Later,multi-channel monitoring of shear wave velocity of liquefiable sands was studied.After grouting,the shear wave velocity measured at each point was significantly increased,which indicated that the soil anti-liquefaction performance can be significantly improved by microbial reinforcement.Again,as is known to all,the effect of microbial formulation and grouting batches on MICP reinforcement is extremely important.We compare the liquefaction resistance of different cemented samples by dynamic triaxial tests through the adjustment of both the two mentioned before.In this way,the corresponding microbial reinforcement scheme can meet the needs of different engineering projects.Finally,Centrifuge shaking table tests were carried out to simulate at model scale.Three models,dry sand model?Model-1?,saturated sand model?Model-2?and microbial reinforcement model?Model-3?,were established.Comparing centrifuge experimental results of 3 models under 30g,we systematically analyzed acceleration variation,pore pressure variation,surface subsidence,the lateral displacement response in free field,and also sand stress-strain,shear stiffness-strain relationship at depth of 3m,5m and 8m.It was found that the resistance of sand liquefaction was significantly improved by microbial reinforcement.Sand liquefaction and plastic characteristics caused by calcium carbonate cementing were the fundamental factors of shear stress-strain hysteresis loop irregular changing under dynamic load.At the same time,spatial heterogeneity of microbial and calcium carbonate for large scale models in centrifuge test simulation was systematically discussed,which had been paid little attention before.Then,we had a preliminary understanding about microbial and calcium carbonate heterogeneity distribution along the height of the model box.