Study On Effect Of Freeze-thaw Cycle On Dynamic Performance Of Improved Aeolian Sand And Fractal Law Of Damage

The climatic environment of the four seasons in western Liaoning makes the base of the road easily affected by periodic temperature fluctuations,it will form freeze-thaw phenomenon in the effective freezing depth,and seriously affect the service life of subgrade engineering.Aeolian sand is widely distributed in western Liaoning Province.The application of aeolian sand to subgrade fill is economical and environmentally friendly.However,it is difficult to directly improve the dynamic performance and freeze thaw damage ability of unimproved aeolian sand for subgrade filling.Therefore,studying the dynamic properties of the aeolian sand and the pore fractal characteristics after freezing and thawing cycles are of great significance for preventing the dynamic disaster and freeze-thaw cycle damage of the improved aeolian sand roadbed.To solve the above problems,based on the National Natural Science Foundation(51774166)under the support of GDS,through the frozen three axis instrument,freeze-thaw cycle test machine and scanning electron microscope,carried out three triaxial test,freeze-thaw cycle test,SEM test,creep test,the main research work can be summarized as the following 5 aspects:(1)According to the actual stress state of high speed railway subgrade in seasonal frozen area,cyclic loading test under the condition of confining pressure and water content of frozen soil was carried out by using GDS three axis instrument;the dynamic stress-strain hysteresis curve was obtained under different experimental conditions,through the analysis of hysteretic curve,the influence of the three factors on the initial dynamic elastic modulus and the plastic deformation;and the variation of the dynamic parameters with the number of cyclic loading under the three experimental factors are determined;the influence of negative temperature on shear strength is determined by using the non recoverable deformation as the criterion of failure.(2)The freeze-thaw cycle tests under the conditions of different temperatures,confining pressure and water content have been carried out to determine the effect of the freezing thawing cycle on the shear strength,dynamic parameters,dynamic strength and critical stress ratio;a new test method is sought to determine the critical dynamic stress ratio with the highest accuracy by the least number of tests;according to the irregular characteristics of the hysteretic curve at the initial stage of cyclic loading,a joint calculation model is proposed which is closer to the characteristic of the actual hysteresis curve.(3)The pore distribution of improved aeolian sand after freezing and thawing cycle is observed by SEM observation test;the spatial distribution of cement,particles and pores is obtained by MATLAB image processing technology;fractal dimension of pore space is determined by "Box cover method",fractal law of pore surface pore evolution of the whole specimen is built up by fractal theory and macro mesoscopic measure mapping relation.(4)The fractal law of the attenuation of shear strength parameters is established by the linear relationship between the number of freezing and thawing cycles and the fractal dimension,the influence mechanism of the shear strength parameter attenuation on the Mohr-Cloumb boundary and the Drucker-Prager boundary degradation is determined;a statistical damage constitutive model based on Weibull distribution and the law of damage evolution are established.(5)The hysteretic curve under cyclic loading shows that the dynamic strain is lagging behind the dynamic stress.The creep test of the specimens after the freezing and thawing cycle is carried out to study the influence of the freezing thawing cycle on the rheological parameters E1?E2??1 and ?2 of the Nishihara model;the rheological behavior of cement modified sand under dynamic loading is analyzed and the corresponding relationship between dynamic stress and dynamic strain is established through the loading time function,reconstruction of hysteresis curve is realized;the relation between the unclosed strain of hysteresis curve and the degradation of yield boundary under the action of freezing and thawing cycle is established.The paper contained 76 figures,15 tables and 136 references.