| Expansive soil is a special disastrous clay,when water is met it is swollen and when water is lost it is shrunk.Engineering construction in expansive soil distribution area,great potential hazards are often brought to engineering due to its poor engineering characteristics.So that,in the process of engineering construction,it is usually necessary to mix lime,cement or other inorganic materials in expansive soil to improve and treat it.It is shown that good improvement results have been achieved using these traditional improvement methods by a lot of engineering practice.However,these methods will not only a lot of resources are consumed but also be uneconomical.They will not only construction period prolonged,but also serious environmental pollution is caused,which is not in line with the concept of green ecological construction.Therefore,it is very necessary to find a kind of expansive soil improvement method which is met the ecological and environmental requirements of our country.With the development of modern microbial technology,improvement method of Microbially Induced Carbonate Precipitation(MICP)has been used in the field of geotechnical engineering.By this method to reinforce and improve soft soil,good treatment effect can be produced.Compared with using lime,cement,sand gravel and fly ash to improve soil,MICP method is environmentally friendly,and improvement effect is more obvious.Consequently,study on the improvement of expansive soil by MICP method has not only important theoretical significance,but also broad engineering prospects.Taking expansive soil from Nanning,Guangxi as research object and relying on the National Natural Science Foundation project "Nonlinear rheological characteristics of expansive soil"(project No.:50978097),Experimental study on consolidation and strength characteristics of expanded soil improved by MICP method has been carried out in this thesis.Main research works carried out and innovative research conclusions obtained in this thesis are as follows.1.Main research works carried out(1)For expansive soil before and after improvement by MICP method,onedimensional consolidation test,direct shear test and scanning electron microscope imaging test are carried out.Through experimental study,influence laws of MICP method on consolidation and strength characteristics of improved expansive soil have been explored clearly.Through quantitative analysis,evolution mechanisms of microstructure of improved expansive soil have been revealed.(2)Using IPP software,scanning electron microscope imaging results are quantitatively analyzed,and evolution law of pore characteristic of improved expansive soil during one-dimensional consolidation compression deformation have been explored clearly.Influence mechanism of cementation and filling effects on microstructure of expanded soil,these are formed by calcium carbonate precipitation has been revealed.(3)According to fractal theory,chaos theory,damage theory and critical state theory,expansive soil improved by MICP method is theoretically analyzed by using IPP software and MATLAB software.Influence law of cementing liquid content in the process of MICP method on fractal characteristics and chaotic characteristics of improved expansive soil has been clearly explored.Fractal models of porosity,void ratio and shear strength of MICP method improved expansive soil considering microdamage have been established.(4)Using PFC2D software,a discrete element analytical model for expansive soil improved by MICP method has been established.Influence laws of cementing liquid content on force chain structure,micro-fabric and local porosity of improved expansive soil have been clearly explored.Evolution mechanism of the microstructure of MICP method improved expansive soil has been revealed.2.Main innovative conclusions obtained(1)Through one-dimensional consolidation test and direct shear test,it is found that when cementing liquid content is not more than 125mL,with the increasing of cementing liquid content,compression deformation characteristics of expansive soil improved by MICP method has been significantly ameliorated and its shear strength has been significantly increased.By analyzing scanning electron microscope imaging,it is found that under the action of consolidation pressure,the contact between particles in expansive soil have been gradually changed from point-point contact or point-surface contact to surface-surface contact.In improved expansive soil,there are obvious calcium carbonate precipitations.In the process of compression deformation,calcium carbonate precipitations in improved expansive soil are gradually changed from spherical to flat,and are interpenetrated in the pores between soil particles.(2)Through quantitative analysis of micro structure of expansive soil,it is found that the total pores number and total pores area of improved expansive soil are all lower than those of unimproved expansive soil during compression deformation.With the increasing of consolidation pressure,the change trend of pore number proportion and pore area proportion of improved expansive soil are all gentler than that of unimproved expansive soil.With the increasing of cementing liquid content,this phenomenon is more obvious.So that,the changing of micro-pore characteristics is an important factor affecting the improvement of strength characteristic and deformation characteristic of expansive soil improved by MICP method.(3)Fractal theory and chaos theory are applied to quantitatively analyze microscopic pore characteristics of expansive soil.Analysis results show that the chaos of expansive soil is improved obviously the ability of expansive soil to resist external loads is increased effectively by using MICP method.In the process of compression deformation,the fractal dimension of improved expansive soil is higher than that of unimproved expansive soil.The larger of cementing liquid content,the larger of fractal dimension of improved expansive soil,and the better of its overall structure properties.Based on fractal theory,damage theory and critical state theory,the porosity model,void ratio model and shear strength model of expansive soil improved by MICP method have been established,and the analysis method of shear strength indices have been proposed.The rationality of all models has been verified by comparing and analyzing model calculated results and tested results.(4)Based on discrete element analysis theory,direct shear process of expansive soil improved by MICP method is numerically simulated.Through numerical simulation for direct shear process,evolution law of the microstructure of improved expansive soil has been analyzed.It is found that with the increasing of cementing liquid content,the cohesion between particles of improved expansive soil by MICP method is increased,the strength of force chain between particles is enhanced,and the change of local porosity is more gradual.These phenomena show that the shear strength of soil is higher and the improved expansive soil is more isotropic.In conclusion,through experimental study on consolidation and strength characteristics of microbial improved expansive soil,influence laws of MICP method on consolidation compression and strength characteristics of expansive soil have been clearly explored.Influence mechanisms of the change of microstructure and pore characteristics on macroscopic mechanical characteristics of improved expansive soil have been revealed.Through quantitative analysis,pore characteristic model and shear strength model of improved expansive soil have been established.The rationality of the model and analysis theory are verified by comparing and analyzing model calculated results with numerical simulation results and tested results.The above research conclusions lay a theoretical foundation for quantitative analysis the impact of microstructure evolution of MICP method improved expansive soil on its macroscopic mechanical characteristics,and they also provide a theoretical basis for perfecting application technology of MICP method in expansive soil engineering. |
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