Experimental Study On Improving Of Expansive Soil By MICP Method Using Centrifugal Concentration Microbe Solution

The MICP method is developed in the past ten years is a new technology in microbial geotechnical engineering,which has been widely used in the fields of geotechnical engineering,such as soft soil improvement,reinforcement and seepage prevention and so on.However,at present,relevant research on the application of MICP method to improve expansive soil,is not seen many reporting in relevant literatures.In practical engineering,the use of MICP method to improve expansive soil needs to strictly control water used amount in microbe solution and cementation solution to ensure the maximum compaction of expansive soil achieved.So that,in combination with actual situation of expansive soil,control water used amount in microbe solution under the premise of ensuring improvement effect of expansive soil.it is necessary to carry out experimental study on improving of expansive soil by MICP method using centrifuging concentrated microbe solution.To carry out relevant research in this field,is the premise for forming related engineering technology to the improvement of soft soil by MICP method.Relying on the National Natural Science Foundation project "Nonlinear rheological characteristics of expansive soil"（project No.:50978097）,Experimental study on improving of expansive soil by MICP method using centrifuging concentrated microbe solution has been carried out in this thesis.Expansive soil in Nanning,Guangxi has been improved by MICP method of using centrifuging concentrated microbe solution.Through experimental study on swelling characteristics,physical characteristics and microstructure,the influence laws of concentrate ratio of microbe solution on macroscopic characteristics of improved expansive soil have been clearly explored.At micro level,influence mechanisms of concentrate ratio of microbe solution on engineering characteristics of improved expansive soil have been revealed.Through triaxial consolidation drainage shear test,the influence rules of microbe solution concentrate ratio on the stress-strain relations of improved expansive soil have been clearly explored.And the influence mechanisms of microbe solution concentrate ratio on shear strength of expansive soil have been revealed.Through the analysis of strength characteristics of improved expansive soil,the model parameters of Duncan-Chang model of improved expansive soil have been modified.Relation models between parameters and the concentrate ratio of microbe solution have been established,and analysis method of shear strength of improved expansive soil has been proposed.The main research works and innovative conclusions of this thesis are as follows.1.Main research works carried out（1）The concentration of microbe solution in different time periods is determined by spectrophotometer,and the growth law of Sporosarcina Pasteurii is explored clearly.Using centrifugal concentration technology microbe solution has been concentrated proportionally,and the influence of microbe solution concentrate ratio on the absorbance of microbe solution is explored.After centrifugal concentration for microbe solution,MICP method using centrifuging concentrated Microbe Solution has been proposed to improve expansive soil for the first time by analyzing growth law of microbes.（2）Experimental study on swelling and physical characteristics of improved expansive soil have been carried out.Influence laws of microbe solution concentrate ratio on macroscopic characteristics of MICP method improved expanded soil have been explored clearly,and influence mechanisms of microbe solution concentrate ratio on engineering characteristics of improved expansive soil have been revealed.（3）Through scanning electron microscope imaging and X-ray diffraction test,the evolution of mineral composition and micromorphology of expansive soil before and after improvement have been compared and analyzed.The influence law of microbe solution concentrate ratio on the microstructure of improved expansive soil has been explored clearly.Through determination of calcium carbonate production,effect mechanism of microbe solution concentrate ratio on the benefit of microbial mineralization has been revealed.（4）Through triaxial consolidation drainage shear test,influence law of microbe solution concentrate ratio on the relation between deviatoric stress and axial strain of MICP method improved expansive soil has been explored clearly,and the Duncan-Chang model parameters applicable to improved expansive soil have been modified.Relations between microbe solution concentrate ratio and the parameters of constitutive model of expansive soil have been established.Furthermore,calculation models of tangent modulus of improved expansive soil with different microbe solution concentrate ratio have been established.2.Main innovative conclusions obtained（1）Through the analysis on growth law of Sporosarcina Pasteurii,it is found that the optimal incubation time of microbe solution is 36～48h,and the maximum absorbance corresponding to microbe solution concentration is OD600=1.6.By measuring the concentration and absorbance of microbe solution after it is concentrated,it was found that the concentration and absorbance of microbe solution all are gradually increased with the increasing of microbe solution concentrate ratio under different conditions of microbe solution concentrate ratio.（2）Under the conditions of different microbe solution concentrate ratio,through the tests on swelling characteristics and physical characteristics of improved expansive soil,it is found that with the increasing of microbe solution concentrate ratio,the indices of expansive soil such as expansion rate indices and plasticity index and standard hygroscopic moisture content are all gradually decreased and are all gradually tended to be stable,plastic limit water content of expansive soil is gradually increased,and its grain grading curve of soil is become more gentle.（3）Comparative analysis of microstructure and mineral composition of expansive soil before and after the improvement,it is found that the surface morphology of soil particles changed from smooth surface before improvement to rough surface after improvement.The contact forms between soil particles are edge-to-surface contact and granular aggregated,and warping phenomenon of its edges is obviously weakened,and soil pores are obviously filled with calcium carbonate precipitation.With the increasing of microbe solution concentrate ratio,the number of aggregates,particle size and calcite content in soil are all gradually increased and gradually tend to be stable,and the proportion of clay minerals in soil is gradually decreased and tends to be stable.（4）Through the study on the relation between deviatoric stress and axial strain of improved expansive soil,it is found that with the increasing of microbe solution concentrate ratio,the maximum deviatoric stress（shear strength）,cohesion and internal friction angle in the soil are gradually increased,and finally tend to be stable.Based on Duncan-Chang model theory,the relation between compliance J and axial strain ε₁ of expansive soil is comparative analyzed.The study found that with the increasing of microbe solution concentrate ratio,test parameters a,b,and Janbu test constant n are all gradually decreased,while Janbu test constant k is gradually increased,and all parameters finally tended to be stable.（5）Through the analysis on Duncan-Chang model of improved expansive soil,the calculation models of model parameters a and b,Janbu test constants k and n,initial modulus Ei,tangent modulus Et,ultimate deviator stress（σ1-σ3）ult and failure ratio Rf of improved expansive soil have been established.Calculation methods for shear strength indices and shear strength of expansive soil improved by MICP method have been proposed.（6）After centrifuging concentrated microbe solution,experimental study and theoretical analysis have been carried out on microbial growth law,mineralization benefit of microbes,microstructure evolution of soil,swelling characteristics of soil and strength characteristics of soil.The research results show that it is completely feasible and reasonable to improve expansive soil by MICP method using centrifuging concentrated microbe solution.After centrifuging concentrating of microbe solution,not only solved the problem of water used amount is too much in improvement process of expansive soil,but also achieved better improvement effects on various characteristics of soil.In conclusion,Through the experimental study on the improvement of expansive soil by MICP method,which using centrifuging concentrated microbe solution,influence laws of microbe solution concentrate ratio on the growth,reproduction and mineralization benefits of microorganisms have been explored clearly.The feasibility and rationality of adopting the technology of centrifuging concentrated microbe solution in the improvement process are verified.The influence laws of microbe solution concentrate ratio on the microstructure,swelling characteristics and strength characteristics of improved expansive soil have been explored clearly.The influence mechanisms of microbe solution concentrate ratio on microstructure evolution,swelling characteristics and strength characteristics of expansive soil have been revealed.Through quantitative analysis,the parameters of Duncan-Chang model have been modified.Calculation methods of shear strength indices and shear strength of expansive soil improved by MICP method have been proposed.The rationalities of model and analysis theory have been verified by comparing the calculated results of the model with tested results.The adoption of centrifuging concentrated microbe solution technology not only solves the problem of water used amount is too much in the process of improving expansive soil by MICP method,but also improvement effect of expansive soil has been increased.The above research conclusions provide a new idea for improvement effect of MICP method on expansive soil,also provide a theoretical basis for expanding and improving engineering application technology of MICP method to improve expansive soil.