Study On Mechanical Properties Changes Of Fill And Long-term Deformation For Permafrost Embankment

Subgrade will experience once freeze-thaw cycle within one year in permafrost regions,with the increase of freeze-thaw cycles,the freezing and thawing of soil-water induce the change of particle structure connection and position,which can seriously affect mechanical properties of fill and even cause subgrade failure possibly;Meanwhile,road construction in permafrost regions changes original heat balance of permafrost,which lead temperature rising and thawing of permafrost within certain depth and affect long-term stability of subgrade.For change of fill’s mechanical properties in permafrost regions and prediction of subgrade’s long-term deformation,the following contents would be completed in this article:(1)Unconsolidated and undrained triaxial shear test was conducted under different freezing temperatures,numbers of freeze-thaw cycles and confining pressures.The stress-strain behavior,failure strength,elastic modulus,cohesion and angle of internal friction were measured in unfrozen soil as well as in thawed soil in order to find regular change of those targets,meanwhile,the relations of failure strength and elastic modulus related to freezing temperatures,freeze-thaw cycles and confining pressures separately were got.Results show that:Freeze-thaw cycles can’t change the form of stress-strain curve.With the increase of freeze-thaw cycles,failure strength and elastic modulus,which are measured in 300kPa confining pressures and-5℃ freezing temperatures,increase firstly and then decline,finally attain a stable status.(2)The method of orthogonal test and significance was used to analyze the influence level of freezing temperatures,compaction,freeze-thaw cycles,moisture content and confining pressures as well as interactions between five factors.Results show that:All factors have significant influence on failure strength and elastic modulus except freezing temperature.Interactions between degree of compaction and moisture content,confining pressure and number of freeze-thaw cycles both affect failure strength of sandy silt significantly(3)Three-Direction freezing and thawing test under different moisture content,compaction,load and numbers of freeze-thaw cycles was carried on in order to obtained stable thawing settlement coefficient and frozen-heave rate in different conditions as well as prediction model.Results show that:There was a linear positive correlation between the stable thawing settlement coefficient(stable frozen-heave rate)and moisture content,but there is a negative linear correlation between the stable thawing settlement coefficient(stable frozen-heave rate)and compaction as well as load.(4)Distribution law of temperature field in roadbed was analyzed combining field ground temperature date in the test section of subgrade.The numerical simulation on moisture-temperature coupling was realized for permafrost subgrade to predict permafrost table in different position within 15 years.Test section of subgrade’s long-term deformation within 15 years was predicted by combining stable thawing settlement coefficient and frozen-heave rate with the change of permafrost table.Results show that:Test section of subgrade’s accumulative settlement will be 101mm 15 years later;Sunny-shady slope effect will lead differential settlement of subgrade.