Study On Microstrcture And Mechanical Properties Of Expansive Soil Improved By MICP Technology

Based on the National Natural Science Foundation of China project "Study on Nonlinear Rheological Properties of Expansive Soils"(Project Number:50978097),the study on the microstructure and mechanical properties of expansive soils improved by MICP technology are carried out.Through the microstructure test and a series of physical and mechanical properties of expansive soils in Nanning,Guangxi,the mineralization of microorganisms and the influence of the changes in the mineral composition and microstructure characteristics of the improved expansive soil have been explored and revealed its influence mechanism.The influence law of the treatment solution dosage and curing age on the hydrophilicity,swelling and mechanical properties of expansive soil are investigated,and the corresponding influence mechanism is revealed.Based on the K-G model,the influence of the dosage of the treatment solution on the parameters of the K-G model is explored.On this basis,applying the disturbed state theory,a modified K-G model for microbial improvement of expansive soil based on the disturbed state theory is established.Through experimental research and theoretical analysis,the following research progress and innovation results are mainly obtained.(1)Through X-ray diffraction test and scanning electron microscope test research,the changes in the mineral composition and microstructure characteristics of the improved and unimproved expansive soil are analyzed.The mineral composition of the improved expansive soil has changed significantly,and many calcite crystals have been added.There is obvious calcium carbonate precipitation on the surface of the improved expansive soil particles.The soil particles are cemented by the precipitate of calcium carbonate,meanwhile the pores between soils particles are filled by the precipitate of calcium carbonate.Consequently,the connection ability between soil particles is enhanced and the stability of soil particles is improved.It is further verified that the use of microbial technology to improve the expansive soil is feasible and more effective.(2)Through experimental research on the limit moisture content,standard moisture content and swelling characteristics,it is found that the liquid limit moisture content,plasticity index,and standard moisture content of the expansive soil are improved by microorganisms with the increase of the treatment solution content or the extension of the curing age.And the swelling rate is gradually reduced.It shows that after microbial improvement,the hydrophilic and swelling properties of expansive soil have been significantly improved.(3)Through direct shear test and consolidation compression test,it is found that the shear strength of the modified expansive soil has been significantly improved,and its compression properties have also been significantly improved.With the extension of the curing age or the increase of the treatment solution content,the shear strength indices of improved expansive soil gradually increases,its compression coefficient gradually decreases,as well as the compression modulus gradually increases.It is found that the modified expansive soil with a treatment solution content of 60%after 28 days of curing has changed from the unimproved high compressibility soil to the improved medium compressibility soil.(4)Through the study of isotropic consolidation drainage and iso-p triaxial consolidation drainage shear test,it is found that with the increase of the treatment solution content,the volume strain of the microbial modified expansive soil gradually decreases,and the deviator stress gradually increases.The research results show that microbial improvement of expansive soil can not only increase its strength,but also improve its deformation characteristics.(5)Through experimental research and comparative analysis of model parameters,it is found that with the increase of the treatment solution content,the tangent bulk modulus and tangent shear modulus of the microbial modified expansive soil gradually increase.Applying the theory of disturbed state combined with the analysis of the K-G model,analytical models such as the tangent bulk modulus and tangent shear modulus of the improved expansive soil are obtained.On this basis,a modified K-G model for microbial improvement of expansive soil based on the disturbed state theory is established.The above research results provide a reliable experimental basis for further research on expansive soil improved by MICP technology,and also lay a theoretical foundation for related research.At the same time,it provides a theoretical basis for the practical engineering application of expansive soil improved by MICP technology.It also provides a new idea for the engineering application of using modern biological-geotechnical technology to improve expansive soil.