Impact Dynamic Experiment And Constitutive Model Of Unsaturated Frozen Soil Under Different Temperature And Moisture Content

Frozen soil is a multi-phase composite natural geological material composed of soil skeleton,pore ice,unfrozen water and gas.Its physical properties are significantly affected by the environment.Frozen soil in nature is mostly in an unsaturated state under the action of freeze-thaw cycle and water vapor migration.For the convenience of research,the influence of unfrozen water and gas is usually ignored to study its mechanical properties and establish relevant theoretical models.With the continuous development of cold region engineering,it is necessary to consider the influence of unfrozen water and gas to improve the theoretical research of unsaturated frozen soil.In tunnel excavation,blasting and other practical projects,unsaturated frozen soil is inevitably subjected to impact load,so it is necessary to study its dynamic mechanical properties under high strain rate load and establish relevant theoretical models.The dynamic mechanical properties of unsaturated frozen soil are mainly affected by moisture content,freezing temperature and loading strain rate.In order to study the influence of the above factors on the dynamic mechanical properties of unsaturated frozen soil,in this paper,the unsaturated frozen soil samples with different initial moisture content（20%,25%and 30%）at three freezing temperatures（-10℃,-15℃and-20℃）were investigated by split Hopkinson pressure bar（SHPB）equipment under three groups of different loading strain rates(470 s^-1,590 s^-1 and 710 s^-1).The results indicated that the peak stress of unsaturated frozen soil samples increased as the loading strain rate increased and the freezing temperature decreased,showing obvious strain rate effect and temperature effect,the peak stress and elastic modulus increased as the freezing temperature decreased and the initial moisture content increased.With the increase of loading strain rate,unsaturated frozen soil changes from ductile failure to brittle failure,with the decrease of freezing temperature and the increase of initial moisture content,the brittle characteristics of dynamic stress-strain curve of unsaturated frozen soil gradually become obvious.According to the characteristics of dynamic stress-strain curve of unsaturated frozen soil,the decrease of temperature and the increase of initial moisture content will lead to the enhancement of compressive strength of unsaturated frozen soil.Therefore,considering the action of unfrozen water and gas,and introducing matrix suction,a typical physical quantity of unsaturated frozen soil,to describe their influence on the compressive strength of unsaturated frozen soil.Based on the elastic-plastic theoretical framework,and introducing damage to describe the weakening effect of compressive strength with the increase of strain due to crack propagation during impact,a dynamic constitutive model of unsaturated frozen soil under uniaxial impact loading is established.The comparison between theoretical calculation results and experimental results indicated that the model can well describe the dynamic mechanical behavior of unsaturated frozen soil under impact loading.By analyzing the characteristics of dynamic stress-strain curve and crushing characteristics of unsaturated frozen soil,the crushing characteristics of unsaturated frozen soil is obviously different under different working conditions.In order to reveal the internal mechanism of different fracture forms,the relationship between the absorption energy of unsaturated frozen soil and the compressive strength and failure forms was analyzed,the results show that the greater the compressive strength of unsaturated frozen soil,the greater the energy needed to be absorbed for failure.Considering that unsaturated frozen soil will absorb some heat energy due to transient temperature rise under impact loading,the specific heat is introduced to describe the difference of energy absorption characteristics of unsaturated frozen soil at different initial moisture content and freezing temperature.In order to eliminate the influence of sample size effect,based on the damage described by micro-fracture theory,an energy-time history model of unsaturated frozen soil considering transient temperature rise during impact is established.The comparison between the calculated results of the model and the experimental results indicated that the model can well describe the relationship between the absorption energy density of unsaturated frozen soil under impact loading and time,and reveal the strength characteristics and crushing process of unsaturated frozen soil from the energy point of view.