Study On The Mechanical Properties Test And Strength Prediction Model Of Unsaturated Expansive Soil Under Freeze-thaw Cycle

The stability of engineering of expansive soil in seasonal frozen area is seriously challenged due to the bad engineering properties of expansive soil and the special climatic conditions.The water migration under freeze-thaw cycle and the accompanying phenomena caused by the water change cause the swell-shrinkage deformation and mechanical properties deterioration of the shallow expansive soil,then seriously affect the safety of project operation in this area.Therefore,according to the engineering characteristics of expansive soil in seasonal frozen area,it is necessary to carry out relevant research on compaction expansive soil based on freeze-thaw cycle for prevention and treatment of freeze-thaw damage in engineering of expansive soil in seasonal frozen area.In order to study the deformation characteristics and the mechanical properties of shallow expansive soil in seasonal frozen area under freeze-thaw cycle,the self-made model box is used to test the one-dimensional soil column model of the compacted expansive soil under freeze-thaw cycle firstly,the variation of temperature,pore water pressure and vertical displacement inside the soil in freezing-thawing environment was studied.Then,through the automatic triaxial test system to conduct the unconsolidated-undrained triaxial shear test on the samples with different moisture contents subjected to 0?7 cycles of freezing-thawing to study the variation of mechanical parameters such as stress-strain relationship,elastic modulus and shear strength with the number of freeze-thaw cycles,moisture content and confining pressure of compacted expansive soil,and the prediction model of the failure strength of expansive soil is constructed based on the triaxial shear test data.The main conclusions of this study are as follows:(1)Under the condition of one-way cooling,the soil in the shallower part is closer to the cold end,the temperature gradient in the soil is larger,the cooling rate is faster,and the temperature decreases rapidly after freezing,which leads to the cooling curve has no obvious constant temperature stage compared with the typical cooling curve of wet soil.During the freezing process,the internal temperature of the soil is lowered form top to bottom.After a certain degree of freezing in the upper soil,the lower soil loses the free surface when it freezes.As the temperature further decreases,the unfrozen water further undergoes phase transformation,causing the lower soil to be squeezed by the ice expansion,resulting in a small increase in pore water pressure.During the melting process,the melting of the upper soil leads to the rapid reduction of ice expansion and extrusion pressure,and the pore water pressure in the lower soil rapid reduction after the rapid increase.(2)The change of pore water pressure and vertical displacement during freeze-thaw cycle is greatly affected by the number of freeze-thaw cycles and the depth.The shallower the depth of the soil layer,the more severe the fluctuation of the pore water pressure during the freezing and thawing process,and the greater the variation magnitude of the vertical displacement.During the first freezing and thawing process,the variation magnitude of pore water pressure and vertical displacement is larger.As the number of freeze-thaw cycles increases,the variation magnitude of the two parameters gradually decreases.During the first freezing and thawing process,the frost heaving amount of the soil is greater than the amount of thawing settlement.Under the late freeze-thaw action,the cohesive force and friction of the soil are weakened,and the ability of the soil to resist sedimentation is reduced,the frost heaving amount of the soil is less than the amount of thawing settlement,and the soil appears to be sinking compared with the initial state.(3)The freeze-thaw cycle,moisture content and confining pressure have significant effects on the stress-strain relationship of expansive soil in the seasonal frozen area.In the moisture content range of 18.2%?24.2%,when the moisture content of compacted expansive soil is not more than the optimum moisture content,the form of stress-strain curve is strain softening,and the failure modes of samples are all brittle failure.When the moisture content is greater than the optimum moisture content,the form of stress-strain curve changes from the strain softening to the strain hardening as the number of freeze-thaw cycles,the moisture content and the confining pressure increase,and the failure modes of sample changed from brittle failure to plastic failure.(4)The elastic modulus,shear strength and failure strength of compacted expansive soil decrease with the increase of the number of freeze-thaw cycles,and they all tend to be stable after multiple freeze-thaw cycles.With the increase of moisture content,the elastic modulus,shear strength and failure strength decrease sharply.With the increase of confining pressure,the relationship of elastic modulus and confining pressure of the same moisture content sample show no obvious regularity,and the shear strength increases with the increase of confining pressure,and the rate of increase of shear strength decreases with the increase of moisture content.(5)Considering the effects of freeze-thaw cycles and moisture content on the failure strength of expansive soil in the seasonal frozen area,a two-dimensional prediction model considering the influence of a single factor and a three-dimensional prediction model considering the influence of the interaction of two factors were constructed,and the reliability and rationality of the prediction model are verified by the comparison between the test results and the prediction results.