Study On The Structure Evolution Of Deep Soil Under Freeze-Thaw Effect And Its Influence On Creep Deformation

With the increase of freezing depth and frozen wall thickness in recent years,more attention should be paid to the influence of frozen wall thawing settlement on structural stability.For the sake of prevention and control of thawing settlement,it's of great significance to explore further understanding of the mechanical properties of thawed soil.In this paper,we selected deep thawed clay as the research object to study microstructure evolution and creep deformation rule of deep soil under freeze-thaw effect,by means of indoor testing and numerical simulation.Research works in this article include:(1)obtain quantitative information and pore characteristics of deep soil microstructure by SEM,MIP and Nitrogen adsorption tests,and analyze its evolution rule under freeze-thaw effect systematically.(2)Based on the deviatoric stress obtained from the consolidated-undrained triaxial shear test,we carried out creep tests through hierarchical loading method,and analyzed the effect of freeze-thaw on the creep deformation of deep soil.(3)Combined with quantitative information of deep soil microstructure,we established granular flow models of deep soils under different freeze-thaw states,calibrated the parameters of deep soil,and analyzed the influence of freeze-thaw action on creep deformation from microcosmic perspective.The main achievements of this paper are as follows:(1)The proportion of angle zoning in the fifth area where angle between the long axis and the vertical direction is between 80~100 degrees increased under the freeze-thaw effect;The undisturbed soils increased by 0.8% and the remolded soils increased by 1.4%.In the meanwhile,the proportion of the ratio of long and short axis in first area where the ratio is between 1~2 increased under the freeze-thaw effect too;The undisturbed soil increased by 2.4% and the remolded soil increased by 1.0%.(2)Mesopore in remolded soil increased under the freeze-thaw effect.But for the mesopore in undisturbed soils,the influence of freeze-thaw effect is quite different under different heat flow directions.The mesopore in undisturbed soil increased when the heat flow direction is horizontal but decreased when direction is vertical.Under the effect of freezing and thawing,the pore shape of undisturbed soils changed from L3 type(the bottle type pore)to L2 type(the pore system is more complex.the shape of micropore is mainly closed at one end while the shape of macropore include both closed and opened).But for remolded soils,the pore shape changed from L2 to L3.(3)The deviatoric stress-strain curves of deep remolded clay transformed from post peak softening to gradual hardening due to the effect of freeze-thaw.Because of the effect,the shear strength of remolded soil decreased by 36.5% and secant modulus decreased by 30.2%.The significant effects of various factors on shear strength are: dry density,confining pressure,freezing temperature and thawing temperature.The significant effects of various factors on secant modulus are: confining pressure,thawing temperature,freezing temperature and dry density.(4)The maximum strain at different levels of creep increases with freezing-thawing effect.Under different experimental conditions,the axial strain-time curves appeared the same rules,mainly including three stages.The long-term strength of deep soil decreased by 48.2% after freezethaw effect and the significant effects of various factors on long-term strength are: confining pressure,freezing temperature,dry density and thawing temperature.(5)The force chain structure diagrams show that the evolution speed from "columnar oriented" to "mesh type" of force chain structure is accelerated and the crevice is more developed under the effect of freeze-thaw.From the displacement field,we can see that,under the freezethaw effect,the specimen shows a more obvious shearing property,and the movement trend of particle displacement field is becoming more obvious.The freeze-thaw effect intensifies the movement and rotation of particles at the failure zone.The maximum coordination number of the specimen is increased by 6.3% after freeze-thaw effect.