Study On Mechanical Properties Of Cement Modified Subgrade Silty Soil In Seasonally Frozen Area

The northeast belongs to the typical seasonally frozen area.Silty soil is widely distributed which is inevitable encountered in the construction of road engineering.The mechanical properties of silty soil would change significantly under repeated freeze-thaw cycles and lead to serious road diseases.Therefore,the effective improvement of silty soil is of great significance for the construction of road engineering,the prevention and control of subgrade diseases,and environmental protection in seasonally frozen area.At present,the engineering community has used cement to improve silty soil according to the engineering construction experience and the effect is obvious.In order to provide a theoretical and experimental reference for cement modified subgrade silty soil engineering practice,the combination of laboratory test,electron microscopic scanning test and theoretical analysis are used to study the change law of the physical properties,static characteristics and dynamic characteristics of cement modified silty soil under freeze-thaw cycles,at the same time the mechanism analysis is carried out from the microstructural changes.The static triaxial test and the dynamic triaxial test are used to systematic study the variation of the cohesion,internal friction angle,shear strength,static elastic modulus and dynamic elastic modulus of the compacted silty soil before and after freezing and thawing,which experienc different number of freeze-thaw cycles,different cement content and different moisture content.The electron microscopic scanning test is used to qualitatively and quantitatively analyze the changes of particles and pores of cement modified silty soil under freeze-thaw cycles.The gray correlation analysis method is used to find the microscopic parameters effect on the cohesion,internal friction angle and shear strength of cement modified silty soil,and the influence of freeze-thaw cycles on its microstructure parameters is further explored.Results showed that:(1)the cohesion,shear strength and static modulus of cement modified silty soil increase with the increase of the number of freeze-thaw cycles,the change of the internal friction angle is not obvious,but the mechanical properties of improved soil are significantly better than unmodified silty soil.(2)The cohesion,internal friction angle,shear strength and static elastic modulus of the modified silty soil have tendency of increasing with the increase of cement content.(3)The law of macroscopic mechanical properties of silty soil before and after improvement is not uniform with the change of moisture content.(4)The plastic deformation of unmodified silty soil under cyclic loading is significantly greater than that of cement modified silty soil at the same test conditions.The dynamic elastic modulus of unmodified silty soil decreases with the number of freeze-thaw cycles.The value of dynamic elastic modulus of cement modified silty soil changes in the range of 10.86MPa~13.98 MPa,which is significantly higher than that of unmodified silty soil and preferentially stabilized in small fluctuation.(5)The unmodified silty soil is in honeycomb structure after freezing and thawing,the particles are in the “fragmented” state and tend to be equiaxed,the shape of the particles is more irregular,the mesopores increase,and the orientation of particles is weakened.The cement modified silty soil is in flocculated structure under freeze-thaw cycles,the particles are in the “aggregation” state.The whole is composed of large particles,with small particles attached.The particles tend to be equiaxed,the circularity of particles is reduced,the macropores are increased,and the orientation of particles is remarkable.