Study On Strength Characteristics And Microstructure Of Artificial Frozen Cement-soil

In recent years,the union reinforcement technology of "Artificial Ground Freezing method + Cement Soil" has been applied in the reinforcement of subway shield tunnel entrance and exit tunnel and subway repair project in weak and water-rich stratum.To explore the union reinforcement technology used in the process of artificial frozen cenment soil strength characteristics and microstructure changes,reveals the the influence law of cement content and initial moisture content,this paper conducts determination of physical parameters,freeze-thaw test,triaxial compression test and scanning electron microscopy test(SEM).The influences of these two factors on physical parameters,frost heaving and thawing settlement,triaxial strength and microstructure of artificially frozen cement soil during the application are analyzed.The main conclusions are as follows:(1)After this union reinforcement technology of silty clay and silt clay,the water content stabilized at 85% and 70% respectively after 14 days of curing;The relationship between permeability coefficient and cement content is logarithmic;When the cement content is more than 6%,the compression index of the two kinds of soil decreases and becomes stable around 0.015 and 0.010 after the cement improvement,that is,the cement content of 6% can better control the soil consolidation deformation.(2)In the-10℃ freeze-thaw test,the silty clay with 25% initial water content showed weak frost heave.When the cement content was 9%,the freeze-thaw amount of silty soil could be reduced by more than 90%.The addition of cement weakens the degree of soil moisture migration,and the difference of overall moisture content decreases from 5% to less than 1%.(3)Under normal temperature and freeze-thaw conditions,the triaxial compressive strength of artificial frozen cement soil increases with the increase of cement content,and the relationship between them can be fitted by the first-order decay exponential function.With the increase of initial water content,the compressive strength of silty soil decreases continuously,while that of silty clay increases first and then decreases.The artificially frozen cement soil failure strain is between 1% and 2%.Freeze-thaw phenomenon has a certain attenuation effect on the compressive strength of silty clay,which is enhanced by the increase of initial water content,and weakened by the increase of cement content(4)Under the condition of low temperature freezing,the plastic deformation range of soil stress and strain curve increases obviously.The strength of artificial frozen cement improved soil increases with the increase of initial moisture content.The silty soil first increases and then decreases with the increase of cement content,and reaches the maximum value at 3%.The compressive strength of silty clay increases slowly with the increase of cement content(6)In terms of microstructure,with the increase of cement content,the degree of bonding between the artificial frozen soil-cement skeleton is strengthened,the porosity of the soil decreases obviously,and the formation of freeze-thaw cracks in silty clay can be inhibited.With the increase of initial water content,the number of pores in artificially frozen cemented soil decreases,but freeze-thaw cracks in silty clay increase and increase.(7)For silty clay,the increase of soil strength is mainly due to the aggregate action of cement and the enhanced strength of soil skeleton particles due to water absorption.For silty sand,the specific surface area of the soil particles is low and the strength of the soil particles is high.The increase of the strength of the soil particles by cement is mainly to improve the adhesion between the soil skeleton particles.