Research On The Mechanical Properties And Constitutive Model Of Silty Sand After Freeze-thaw Cycles

The seasonal frozen region is widely distributed in China,accounting for about 54%of the land area.The subgrade soil freezes and melts periodically due to the seasonal variation of temperature in cold region engineering.In cold season,the water in the soil will freeze when temperature reaches the freezing temperature,leading to the volume expansion and uneven displacement of ground surface.While in warm season,the freezing soil will thaw,resulting in the volume decrease.Meanwhile,the pore water will discharge under the external loading.The physical and mechanical properties will be changed after freeze-thaw cycles,influencing the engineering properties of subgrade in seasonal frozen regions,which should be taken into consideration.Therefore,research on the influence factors and variety regularity of mechanical parameters under freeze-thaw cycling is meaningful to the engineering construction in cold regions.Based on the research status,the variety regularity of modulus,failure strength,shear strength parameter as well as the stress-strain relation were studied by applying the lab experiment,theoretical analysis and numerical analysis methods,considering the influence of cooling temperature,number of freeze-thaw cycles and confining pressure.The evolution law of yielding locus as well as the strength properties was analyzed based on the experiment results.A double-yield-surface model considering both shear yielding and volumetric yielding was established,which can provide evidence for the study on mechanical properties of subgrade soil in seasonal frozen region.The main research results can be summarized as follows:?1?The unconsolidated undrained tri-axial test was conducted under various confining pressures,cooling temperatures and numbers of freeze-thaw cycles,and the stress-strain relations and mechanical parameters,such as modulus,failure strength,shear strength parameters,were analyzed.Results showed that the mechanical indexes of silty sand changed obviously under different numbers of freeze-thaw cycles.The modulus and failure strength reached the minimum values at cycle 7-9.The influence of cooling temperature was weak and had no certain rules.Meanwhile,the method of significance was adopted to analyze the influence level of various factors and interactions between them.The confining pressure as well as number of freeze-thaw cycles had significant influence on the mechanical parameters,while the influence of cooling temperature was not considerable.And interactions between confining pressure and number of freeze-thaw cycles,cooling temperature and number of freeze-thaw cycles both had significant influence.?2?Based on the UU test results,when the confining pressure was relatively low,the unfrozen silty sand showed strain softening phenomenon and then exhibited strain hardening after a few numbers of freeze-thaw cycles.Both the unfrozen silty sand and the one after freeze-thaw cycles showed strain hardening when the confining pressure was high.The typical hyperbola expression can be adopted to describe the hardening stress-strain relations.The normalized stress-strain relationships considering various normalized factors were established and the normalized conditions were given.By comparing the fitting data with the experiment results,the established normalized relationships can describe the stress-strain properties well.?3?The consolidation drained tri-axial tests?CD tests?were conducted under different numbers of freeze-thaw cycles and confining pressures with the cooling temperature of-5? to study the yielding and strength properties of freeze-thaw silty sand.The volumetric yielding locus and shear yielding locus of unfrozen as well as freeze-thaw silty sand can be fitted by an elliptical function and a linear function over the origin,respectively.The shear strength increased with the normal stress and decreased firstly and then increased with the number of freeze-thaw cycles.The linear Mohr-Coulomb criterion can be used to present the strength properties of silty sand after freeze-thaw cycles.?4?A double-yield-surface model was established by introducing the residual ratio of modulus and the number of freeze-thaw cycles based on the CD tests results.The model can consider the influence of both confining pressure and freeze-thaw cycling,by applying the associated flow rule.The hardening parameters were expressed as the functions of the plastic strain as well as number of freeze-thaw cycles for both volumetric and shear hardening.The data calculated by the model agreed well with the experiment results,indicating that the proposed double-yield-surface model can predict the stress-strain relations well.?5?The finite difference scheme of double-yield-surface constitutive model of silty sand considering the influence of freeze-thaw cycles was derived based on the incremental theory of plasticity.The established model was realized for secondary development in VC++ environment offered by FLAC^3D.Comparison between the experimental results and FLAC^3D numerical simulation data under various numbers of freeze-thaw cycles and confining pressures indicated that the development of the established model is reasonable.The subgrade model was established by FLAC^3D to analyze the influence of freeze-thaw cycles on deformation,which indicated that the subsidence on the subgrade surface increased after freeze-thaw cycles.