Research On The Relationship Of Subgrade Soil Macroscopic Mechanical Behavior And Microstructural Characteristics In Seasonally Frozen Soil Region

In seasonal frozen soil region,under the repeated cycles of winter frozen spring thaw,the macroscopic mechanical behavior of compacted subgrade soil will change in varying degrees with the increase of freezing and thawing times.The change of roadbed performance often causes serious road structure disease.Exploring the mechanism of many freeze-thaw effects on the physical and mechanical properties of compacted subgrade soil can provide reliable theoretical basis and important reference materials for engineers to prevent or control road diseases.Because the change of macroscopic mechanical behavior of compacted subgrade soil is essentially the change of microstructure of roadbed soil,it has become an urgent task for many researchers to discuss the relationship between the microstructure of roadbed soil and the macroscopic mechanical properties of roadbed soil.In order to analyze the relationship between macroscopic mechanical behavior and microstructure of compacted subgrade soil after many freeze-thaw processes,three different plastic indices were obtained based on static triaxial compression test.The change law of macroscopic mechanical behavior of typical compacted subgrade soil in different moisture content is obtained by scanning electron microscope(SEM),and the change law of microstructure parameters of compacted soil sample under the same condition as static triaxial test is obtained by means of scanning electron microscope(SEM).The changes of macroscopic mechanical behavior and microstructure characteristics of compacted subgrade soil with the times of freeze-thaw action are summarized,and the relationship between them is sought by means of mathematical techniques.The influence mechanism of microstructure change on the macroscopic mechanical properties of compacted subgrade soil is obtained,which reveals the causes of roadbed diseases in seasonal frozen soil regions,controls the quality of subgrade engineering,and reduces the occurrence of subgrade diseases.The reliable basic data and theoretical basis are provided.Specific findings indicate that: After freezing and thawing,the macroscopic mechanical properties(cohesion,angle of internal friction,shear strength and static elastic modulus)of silty sand and silty soil decreased,while the cohesive force and static elastic modulus of clay decreased,and the internal friction angle and shear strength increased.With the increase of moisture content,the macroscopic mechanical properties(cohesion,angle of internal friction,shear strength and static modulus of elasticity)of soil samples decrease.With the increase of plastic exponent,the cohesion,shear strength and static elastic modulus of soil samples decrease,while the internal friction angle increases.The agglomeration of silty sand is enhanced by freezing and thawing cycle,the splitting property of silt and clay particles is enhanced,and the agglomeration of soil particles decreases with the increase of plasticity index.The cleavage increases with the increase of plasticity index.With the increase of plasticity index,the average pore diameter and pore size fractal dimension of the three soil samples fluctuate less and less,and for silt and clay,freeze-thaw and moisture content have little effect on the pore size of soil sample.The grain abundance of typical compacted subgrade soil with different moisture content and freeze-thawing action has no obvious regularity,but it develops to round shape in general,and the surface fluctuation degree of three kinds of soil samples is small,and the surface undulating degree of clay is greatly affected by freezing and thawing.The particle orientation of the three soil samples is poor.And the degree of correlation between the orientation of particles and moisture content is not significant.Among the microstructure parameters of subgrade soil,the microstructural parameters related to the macroscopic mechanical properties(cohesion,angle of internal friction,shear strength and static modulus of elasticity)are mean particle size,average pore size,mean abundance and fractal dimension of surface fluctuation.