Study On Strength Properties Of Lime Fly-ash Improved Loess Under Freeze-thaw Cycles

Most of the northwest of China is belong to seasonal permafrost region,and the lime fly-ash improved loess roadbed in the seasonal permafrost region is deeply affected by freeze-thaw disasters,such as subgrade depression,subsidence,grouting,and so on,after cycles of freezingthawing,the strength features of roadbed mostly develop toward a harmful direction.The strength of soil determines the carrying capacity of road roadbed,so high attention must be paid to the influence of freezing and thawing on the strength of roadbed soil.For this purpose,this paper studies the strength features of lime fly-ash improved loess under the cycle action of freeze-thaw,and the change of soil micro structure is the essence of the change of macroscopic physical properties,so the micro structure change of lime fly-ash improved loess after freeze-thaw cycle have been studied.Also the relationship between micro structure and soil strength have been analyzed,this study provides a theoretical basis for disaster prevention of lime fly-ash improved loess roadbed in seasonal permafrost regions.The main achievements of this paper are as following:(1)This paper studied the strength characteristics of lime fly-ash improved loess under freeze-thaw cycle,the freeze-thaw cycle tests of lime fly ash improved loess under different initial conditions are carried out,and then the tri-axial shear tests are carried out on the soil samples after freeze-thaw cycle.the variation rules of stress-strain curve,failure strength and shear strength of soil with the number of freeze-thaw cycles are obtained.The test results show that the stress-strain curves of lime fly-ash improved loess are softening curves under different freezethaw cycles,moisture content,mix ratio and confining pressure,and the increase of soil failure strength is positively correlated with moisture content and consolidation confining pressure.it is negatively correlated with the number of freeze-thaw cycles and mix ratio,and the cohesion C of soil shear strength increases with the increase of moisture content and decreases with the increase of freeze-thaw cycle times and mix ratio.Under the influence of various factors,the variation range of internal friction angle is within 4°,and there is no obvious changes.(2)Based on the results of concretion and drainage shear test,the shear strength parameters are sorted out,the change rules of cohesion of lime fly-ash improved loess with the increase of freeze-thaw cycle times is summarized,and the freeze-thaw cycle strength deterioration model is established.(3)The micro structure change rules of lime fly-ash improved loess under freeze-thaw cycle was studied,and the micro structure pore characteristics of lime fly-ash improved loess were quantitatively analyzed by SEM scanning electron microscope and IPP image processing software.The test results show that with the increase of freeze-thaw cycles index,the void area ratio of lime fly-ash improved loess increases,the carrying capacity per unit area decreases,and the shear strength of soil per unit area decreases.(4)This paper studied the relationship between micro structure and macroscopic strength.Combined with the parameters of freeze-thaw cycle index N,pore area ratio K,damage variable D and cohesion C etc,the microscopic parameters of lime-fly ash improved loess are related to macroscopic physical properties.the shear strength is negatively correlated with the freeze-thaw cycles index,pore area ratio and damage variables.