| Expansive soil is a special disastrous clay that is widely distributed in the world.In engineering construction,traditional methods such as soil replacement method,soil improvement method and pile foundation reinforcement are usually used to treat expansive soil foundation.However,using traditional methods to improve expansive soil has the disadvantages of high cost,long construction period and unfriendly to the environment.Therefore,the theoretical and engineering circles have been looking forward to exploring a more economical,convenient,ecological and environmentally friendly method for improving expansive soil.In recent years,Microbial induced Calcite Precipitation(MICP)method has been widely used to improve soft soil.Because the calcium carbonate precipitation formed by microorganisms in the process of mineralization has the function of cementation of soil particles and filling of soil pores,thus achieving the purpose of improving the engineering characteristics of soil.MICP method is a simple,lowcost,fast,efficient and environmentally friendly biotechnology.After expansive soil is improved by MICP method,the study on its creep characteristics and longterm strength characteristics is the premise of analyzing the deformation,strength and stability of improved expansive soil.Relying on the National Natural Science Foundation project "Nonlinear rheological characteristics of expansive soil"(project No.:50978097)and the Hunan graduate scientific research innovation project "Study on creep characteristics and long-term strength of expansive soil improved with microorganisms"(project No.:CX20220747),Experimental study and theoretical analysis on the creep and long-term strength characteristics of expansive soil improved with MICP method have been carried out in this thesis.Main research works and innovative research conclusions in this thesis are as follows.1.Main research works carried out(1)Through a series of indoor compression creep tests and triaxial shear creep tests,compression creep and shear creep characteristics of expansive soil improved with MICP method have been studied.The influence laws of MICP method on compression creep characteristics and shear creep characteristics of expansive soil have been explored clearly.(2)Through the test of scanning electron microscope imaging,the scanning electron microscope images have been processed by Image pro plus software.The evolution law of MICP method on pore characteristic parameters of expansive soil is explored clearly,and the influence mechanism of MICP method on macroscopic creep characteristics of improved expansive soil is revealed from microscopic level.(3)According to the theory of rock and soil rheology,a fractional order shear creep model of expansive soil improved with MICP method is established through applying fractional calculus theory and damage theory.Using numerical analysis method,the parameters of creep model are determined,and the creep equation of expansive soil improved with MICP method has been established.(4)Using isochronous curve method and transition curve method,a calculation equation of long-term strength of expansive soil improved with MICP method has been established.On this basis,the influence law of MICP method on long-term strength of expansive soil has been clearly explored.2.Main innovative conclusions obtained(1)The creep of expansive soil improved with MICP method is related to creep time and stress level loaded,and its compression creep has been experienced three stages these are instantaneous deformation,decay creep and stable creep.There is a significant impact of cementing liquid amount used in MICP method on compression creep characteristics of expansive soil.Under same stress level,creep deformation of expansive soil improved with MICP method is significantly decreased than that of unimproved expansive soil.When the ratio of bacterial liquid to cementing liquid is 50ml/100ml,creep deformation of soil sample is the smallest.so that too much cementing liquid will not help to reduce creep deformation of expansive soil.(2)After expansive soil is improved,its pores number is significantly reduced,soil structure is become more compact,and a larger particle aggregation structure has been formed on the surface of soil particles.After compression creep for soil is carried out,its structure is denser,and total soil pore number is reduced,and the pores with medium pore size and small pore size are the main ones.With the progressing of creep process,total area of small pores in soil is increased and total area of large pores is decreased.(3)In the process of improving expansive soil with MICP method,there are four functions of calcium carbonate precipitation formed by microbial mineralization.By these functions,particles have been wrapped,soil particle structure has been bridge connected,soil pores have been filled and soil particles have been cemented.Therefore,the roughness of soil particles surface is improved,the connection between soil particles is strengthened,and the ability of soil to resist deformation is increased.(4)The characteristics of shear creep for expansive soil are similar before and after improvement.Both unimproved expansive soil and improved expansive soil have obvious nonlinear creep characteristics,and their nonlinear degree is all related to stress level and creep continued time.During creep continued time,when the deviator stress level is low,the creep of soil samples only includes two parts these are instantaneous deformation and decay creep.When deviatoric stress level is high,four complete creep stages these are instantaneous deformation,decay creep,steady creep and accelerated creep have been undergone for creep process of soil samples.(5)In order to describe the characteristics of accelerated creep of expansive soil improved with MICP method,a fractional-order viscoplastic damage body is proposed by applying fractional calculus and statistical damage theory and considering damage deterioration of viscous-like coefficient of soft element.A fractional-order damage creep model,which can describe the shear creep evolution law of expansive soil improved with MICP method,is established by using fractional-order viscoplastic damage body.(6)Using isochronous curve method and transition creep method,a calculation equation of long-term strength of expansive soil improved with MICP method is established,and the analysis method to determine its long-term shear strength τf∞and its long-term shear strength indices c∞ and φ∞ is proposed.After the improvement by MICP method,long-term strength of expansive soil has been greatly improved.Compared with shear strength obtained from conventional triaxial shear test,long-term strength of expansive soil improved with MICP method is about 68%～70%of its short-term strength.Long-term shear strength indexes c∞ and φ∞ of expansive soil improved with MICP method are 19.9%and 26.1%lower than their instantaneous strength indexes c0 and φ0,respectively.In conclusion,by studying the creep and long-term strength characteristics of expansive soil improved with MICP method,influence laws of MICP method on compression creep,shear creep and long-term strength characteristics of expansive soil have been explored clearly.From microscopic level,influence mechanisms of MICP method on macroscopic mechanical characteristics of improved expansive soil,such as creep and long-term strength,have been revealed respectively.Based on theoretical analysis of tested results,calculation and analysis methods of creep,long-term shear strength and long-term shear strength indexes of expansive soil improved with MICP method have been proposed.The above research conclusions laid a theoretical foundation for analyzing deformation,strength and stability of expansive soil improved with MICP method,also provided a test basis for improving and expanding engineering technology of expansive soil improved with MICP method. |
PDF Full Text Request