Experimental Study On The Improvement Ofexpansive Soil Fillers Based On MICP

Expansive soil is a kind of special disastrous clay when it meets water expansion and water loss shrinkage.In the construction of railway and highway in the expansive soil area,it is usually necessary to improve the expansive soil excavated in the road cutting construction and then use it as embankment filler.However,the existing methods to improve expansive soils are mainly physical and chemical improvement.In order to meliorate the swelling and shrinkage,and to restrain the hydrophilicity of expansive soils,these methods of adding cement,lime,fly ash,ionic curing agent,and biological enzyme and rubber particles are used.These traditional improvement methods not only cost time and labor,but also cause serious pollution to the construction environment,and the improvement effect is not necessarily ideal too.Therefore,for engineering it is looking forward to find a more environmentally friendly and economical new method to improve the expansive soil filler combined with the embankment filling construction process.In recent years,with the development of biogeotechnic,the researches on improving soft soil by microbial technology have made gratifying progress.Based on the National Natural Science Fund project "Experimental study on nonlinear rheological properties of expansive soil"(Project No.50978097),the experimental research on improving the physical and mechanical properties of expansive soil filler by MICP technology is carried out in this thesis.Through comparative experimental study and theoretical analysis,the influence laws of microbial mineralization on the hydrophilic properties,swelling-shrinkage properties,strength and deformation properties of expansive soil are ascertained,and the corresponding mechanisms are revealed too.At the same time,the mineralization mechanism and improvement effect of microorganism in expansive soil are conducted in-depth discussion and analysis.Finally,based on Duncan-Chang constitutive model,the strength and deformation characteristics of the improved expansive soil are further studied,and the relevant model parameters are determined.The main research work and conclusions are as follows.1.Main research works carried out1)The basic physical properties of the expansive soil samples for the embankment filler of Nanning Road Project in Guangxi are tested,and the cultivation,rapid propagation and mineralization environment optimization of microorganisms in the laboratory are studied according to the microbial experimental specifications.2)A series of tests on hydrophilicity and swelling-shrinkage characteristics of expansive soils improved by microbes are carried out,including compaction test,limit moisture content test,standard moisture absorption water content test,etc.The mechanism and effect of expansive soil improved by MICP technology are explored and analyzed.The treatment method of multiple spraying and air drying of microbial solution is used to improve the expansive soil,the optimal microbes amount and the water consumption in expansive soil have been determined in the experimental study by considering the factors of microbial activity,concentration of microbial solution,mineralization efficiency and the best water content for expansive soil to be compacted.3)In order to study the improvement effect of different microbial content(MICP treatment times)on the expansion and contraction characteristics of expansive soil and its influence mechanism,the free expansion rate test,no-load expansion rate test,loaded expansion rate test,high-pressure consolidation test under the dry-wet cycle and so on,have all been carried out.4)The direct shear tests and conventional triaxial compression tests have been carried out,to study the strength and deformation characteristics of expansive soils improved by microbes,and the influence of microbial content on the shear strength index of the improved expansive soil and on the improvement effect for the strength of the expansive soil are discussed.5)Based on Duncan-Chang constitutive model,the influences of microbial content on the parameters of Duncan-Chang model,initial deformation modulus and the ultimate deviator stress of expansive soil filler samples have been analyzed.Furthermore,the research on the characteristics of strength and deformation of expansive soils improved by MICP technology has been further carried out.2.Main research conclusions acquired1)In order to solve the contradiction between the best water content required for the compaction of expansive soil with some factors such as microbial activity,bacterial solution concentration and mineralization efficiency,an improved treatment method of multiple spraying and air drying of microbial solution is proposed.By adding calcium salt and controlling the concentration and dosage of bacterial solution,the mineralization environment of microorganism in expansive soil is well improved.2)Thought the experimental study based on the compaction test,limit moisture content test and standard moisture absorption water content test,it is shows that with the increasing of microbial content,the liquid limit,plastic limit,plasticity index and maximum moisture absorption water content of the improved expansive soils are decreased obviously.The research results also indicate that the hydrophilicity of the improved expansive soil is effectively inhibited and the soil sample has been changed from the high liquid limit soil to the low liquid limit soil.3)Through the experimental study on the swell-shrink characteristic by a series of tests,such as free expansion rate test,no-load expansion rate test,loaded expansion rate test and high-pressure consolidation test under the change of dry-wet cycle and so on,it is found that with the increasing of MICP treatment times(microbial content),the expansion rate and contraction rate of the improved expansive soils are significantly reduced.The soil sample has been changed from medium expansive soil before improved to non-expansive soil after improved.4)Through direct shear test and conventional triaxial consolidation and drainage shear test,it is found that the strength indexes(cohesion c and internal friction angle φ)of the improved expansive soil are significantly improved,and with the increasing of the microbial content,the deviatoric stress of the sample is increased when it is destroyed,namely,the shear strength is increased.Furthermore,the stress-strain curve of the improved soil shows strain hardening type.5)Based on Duncan-Chang constitutive model,the fittings for the model parameters of expansive soils improved by MICP are carried out,and the relevant model parameters have been determined through analysis.It is found that under the same confining pressure,with the increasing of microbial content,the values of model parameters a and b are all decrease gradually,and the values of parameters K and n are all increased gradually,that is,the initial deformation modulus Ei and ultimate deviatoric stress(σ1-σ3)ult are all increased gradually,which indicates that the failure strength of improved expansive soil has been improved.6)Both experimental study and model analysis show that there is an optimal dosage of microbial solution(treatment times)existed in the process of improving expansive soil by MICP technology.In this study,after six times treatment by microbial solution,the physical and mechanical indexes of expansive soils can be all achieved the best improvement effect.As is mentioned above,a comparative study on the physical and mechanical properties of expansive soils before and after the improvement based on the MICP technology is carried out in this thesis.Through the research,the influence rules of microbial content on the improvement effect of expansive soil fillers have been explored,and the influence mechanisms of various factors on the improvement effect of physical and mechanical properties of expansive soil fillers have been revealed too.Based on the theory of Duncan-Chang model,a Modified model of constitutive relation of improved expansive soil has been established.At the same time,the relevant model parameters have also been determined.These research results lay an experimental study foundation for applying microbial technology to improve expansive soil fillers.