Impact Experiments And Related Mechanical Properties Of Frozen Soil Under Uniaxial Condition And Passive Confined Pressure

Frozen soil is the soil in which rock and soil particles are cemented by ice at negative temperatures.The recent extension of engineering construction in cold regions and the wide application of artificial ground-freezing methods require a better understanding of the dynamic mechanical properties of frozen soil under impact loading.Thus far,research on the static and quasi-static mechanical properties of frozen soil has been quite extensive and in-depth,whereas that on the dynamic mechanical properties of frozen soil under impact loading remains poor.Therefore,it is crucial to study the dynamic behavior and constitutive model of frozen soil in the one-dimensional and three-dimensional stress states.In this study,uniaxial impact compression experiments at different strain rates were carried out on frozen soil with different freezing temperatures using a split-Hopkinson pressure bar(SHPB),and the dynamic stress–strain curves under the corresponding conditions were obtained.The experimental results show that the dynamic mechanical behavior of frozen soil exhibited obvious strain-rate and temperature effects,as well as obvious elasticviscoplastic deformation characteristics.Furthermore,the dynamic stress–strain curves of the frozen soil showed instability with the evolution of microscopic damage.The strength and failure strain increased with increasing strain rate;the elastic modulus at the same temperature had no obvious correlation with the strain rate;as the temperature decreased,the elastic modulus increased,the strain hardening effect was enhanced,the strength increased under the same strain rate,and the strain rate sensitivity of strength increased.According to the deformation characteristics of frozen soil under uniaxial impact loading,the dynamic mechanical behavior of frozen soil is described by introducing an advanced ratedependent,continuous,damage-evolution model and an elastic-viscoplastic constitutive model based on the Chaboche constitutive theory and using the Drucker–Prager yield criterion.To obtain a numerical solution of the elastic-viscoplastic constitutive equation,the constitutive equation was discretized into an incremental form using Euler's method,and the closest-point projection algorithm was used as the numerical integral to update the stress state during deformation of the frozen soil.Finally,the uniaxial dynamic constitutive model was solved numerically.A comparison between the calculated and experimental results shows that the model reasonably describes the strain-rate and temperature effects of frozen soil.Based on the uniaxial experimental results,the expression of the equivalent elastic constants of frozen soil was derived using homogenization theory.Meanwhile,a plastic mesomechanical model of frozen soil was established using the isotropic tangent modulus method.Furthermore,a dynamic constitutive model of frozen soil was constructed by introducing a continuous damage-evolution model related to the freezing temperature and loading strain rate.The calculated results of the constitutive model agree well with the measured stress–strain curves,which verify the rationality and validity of the model.Impact compression experiments of three different loading strain rates under passive confined pressure were carried out on frozen soils with three different freezing temperatures,using SHPB equipment.The experimental results show that during the impact compression process,the frozen soil first underwent elastic deformation,and the average stress and generalized shear stress increased linearly with increasing axial strain.The value of confining pressure divided by axial pressure was constant.Then,when the stress was loaded to the elastic limit,the frozen soil suffered from shear failure;the pore ice lost its bearing capacity;and the thawed soil became the main bearing body.Subsequently,the average stress was hardening,and the deformation characteristics are similar to those of conventional soil in a corresponding state.The volume deformation is shown as shrinkage.Moreover,the dynamic mechanical properties are closely related to the loading strain rate and freezing temperature;the secant modulus,elastic modulus,and strength,including the volume yield strength and shear strength,increase with an increasing loading strain rate.By contrast,with a decrease in the freezing temperature,the Poisson's ratio and lateral pressure coefficient decrease,and the secant modulus,elastic modulus,and shear strength increase.