| Expansive soil is a kind of special disastrous clay which has obvious swelling and softening after water is met,but which has obvious shrinking and cracking after water is lost.Therefore,the engineering constructions are often badly damaged in the distribution area of expansive soils.Traditional expansive soil improvement methods have many disadvantages,such as unstable improvement effect,high construction intensity,high cost and environmental pollution.For these reasons,scholars at home and abroad have been constantly looking for new ways to improve expansive soils.With the continuous development of science and technology,many new methods and technologies for the improvement of expansive soils have been emerged in the recent years.Among these new technologies,the technology of microbial induced calcite precipitation(MICP)is applied to the improvement and reinforcement of soft soils,which has been widely concerned by the academic and engineering circles.The technology of MICP has the advantages of obvious improvement effect,environmental protection and low energy consumption.It is the leading technology for the improving of soft soil or for the reinforcing of foundation in China at present.Therefore,it has not only great theoretical significance,but also has broad engineering application prospects to carry out relevant research in this field.Based on the item "nonlinear rheological properties of expansive soils" supported by the National Natural Science Foundation of China(Grant No.50978097),the experimental study on the engineering characteristics of expansive soil improved by the technology of MICP has been carried out in this thesis.The main research works involved in this thesis and the main research conclusions have been obtained are as follows.1.The main research works(1)Through comparative analysis of the ability of different bacterial species in MICP technology,such as the ability of include inducing calcite precipitation and its environmental adaptability,the most suitable bacterial species for improving expansive soil has been selected when both the physical properties and the mineral composition of expansive soil are considered.Through a series of experimental study on the growth process and reproduction process as well as mineralization process of the bacilli,the influence laws of these processes on the engineering characteristics of improved expansive soils have been explored.(2)By analyzing the physical properties and chemical compositions of expansive soils,the technical scheme of improving expansive soil based on MICP technology has been determined.According to the biochemical reaction process of MICP technology,the method of adding calcium source in the improvement process is determined,and the technical problem of low calcium content in natural expansive soil has been solved.At the same time,the optimum moisture content when expansive soil is compressed is determined as the standard to control the amount of bacterial liquid and cementation liquid.Thus,the technical problem that expansive soil cannot be soaked with bacterial liquid and cementation liquid in the improvement process has been solved.(3)A series of experimental studies on the swelling and contraction characteristics and physical characteristics as well as strength characteristics of expansive soils before and after improvement are carried out.The influence laws of different amount of cementation liquid on the swelling and contraction characteristics,physical properties and strength characteristics of expansive soils have been explored,and relative influence mechanisms have also been revealed.(4)Based on the theory of Duncan-Chang model and modified Cambridge model,the stress-strain relations of expansive soils improved by MICP technology are discussed.The constitutive models of expansive soils improved by MICP technology have been established,and the relevant model parameters have been determined.According to these established constitutive models,the stress-strain analysis theory of expansive soil improved by MICP technology has been proposed.2.Main research advances and innovative conclusions achieved(1)Through the analysis on the adaptability of a variety of bacteria in the improvement of expansive soil by MICP technology,it is found that Bacillus Pasteurii is the most suitable strain for improving expansive soils.The growth curve of Bacillus Pasteurii is fitted by a function,and a fitting analysis model which is in the form of exponential function between OD600 value and the time in the growth period of Bacillus Pasteurii,has been established.(2)Through the analysis on the chemical composition of expansive soil,it is found that the content of calcium in expansive soil is low,and the precipitation of calcium carbonate induced by microorganism is less within the process of MICP.Therefore,in the process of improving expansive soil based on MICP technology,cementation fluid is prepared and calcium chloride is added to supplement calcium source.Through the research on the biochemical reaction process of microorganisms in expansive soil and the process of inducing calcium carbonate precipitation,it has been determined that the ratio of urea and calcium chloride in cementation fluid is 1:1,and that is,both contents are 1mol/L.(3)Through the experimental study on the swelling characteristics of expansive soil before and after improvement,it is found that the swelling characteristics of expansive soil can be obviously improved by using MICP technology.Compared with the unimproved expansive soils,the free swelling rate,no-load swelling rate and 100kPa loaded swelling rate of the improved expansive soil are significantly reduced.At the same time,it is found that the liquid limit water content,plastic limit water content and plastic index of expansive soils are all decreased significantly with the increasing of the amount of cementation liquid.According to the swelling criterion(swelling potential)of expansive soil,the expansive soil before improvement is medium expansive soil,and after improvement by MICP technology,it has become non expansive soil.(4)Based on the direct shear test,isotropic consolidation test and consolidated drained triaxial compression test of expansive soils improved by MICP technology,it is found that the compressibility and shear strength of expansive soils can be improved obviously by using MICP technology.Similarly,the compressibility of expansive soils improved by MICP technology can also be reduced.Under the same consolidation pressure,with the increasing of the amount of cementation liquid,the volume change amount of the improved expansive soil is decreased gradually.Compared with the unimproved expansive soil,the indices of shear strength of improved expansive soils(include unsaturated soils and saturated soils)all have been improved obviously.When the amount of cementation liquid is different,the shear strength indices of the improved expansive soils are all increased with the increasing of the amount of cementation liquid.(5)Based on the theory of Duncan-Chang model and the theory of modified Cambridge model,the results of isotropic consolidation rebound test and consolidated drained triaxial compression test have been analyzed theoretically.A Duncan-Chang constitutive model and a modified Cambridge constitutive model of expansive soils improved by microorganism have been established respectively,and the relevant model parameters have been determined.Through the research,it is found that the changes of constitutive model parameters of expansive soils improved by MICP technology are related to the amount of cementation liquid.Based on the established constitutive models as are said above,the stress-strain analysis theory of expansive soils improved by MICP technology has been proposed.To sum up,the experimental study on swelling characteristics and physical characteristics as well as strength characteristics of expansive soils improved by MICP technology have been carried out in this thesis mainly.Through theoretical analysis,the species of microbe used to improve expansive soils have been determined,and its growth law has been explored.Through the experimental study and theoretical analysis of the swelling characteristics and physical characteristics as well as strength characteristics,the influence law of the addition amount of cementation liquid on the improvement effect of expansive soil has been explored,and the related influence mechanism is revealed.Based on the constitutive model theory of soils,the constitutive models of expansive soils improved by MICP technology have been established,and the stress-strain analysis theory of improved expansive soils has been put forward.The above research results provide a reliable theoretical basis for the application of MICP technology to improve expansive soils,and they also laid a technical foundation for the engineering application of MICP technology to improve expansive soils. |
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