Effect Of Freeze-thaw Cycles On Mechanical Properties Of Silt Under Constant Pressure

Seasonal permafrost covers a wide area in China,accounting for about 53.5%of the country's total land area.With the periodic change of temperature in a year,the permafrost will constantly experience a cycle of freezing and thawing.Therefore,when constructing in seasonally frozen soil areas,it is necessary to consider the changes in the mechanical properties of the soil after freezing and thawing cycles.In practice,the soil body undergoes freezing and thawing cycles under external loads,so when studying the changes in the mechanical properties of seasonal frozen soils,it is particularly important to restore the actual stress state of the soil body during the freezing and thawing cycles.Silt sand is selected as the test material in this paper,and a grouped control test is carried out.The silt samples with relative densities of D_r 0.35,0.6,0.9 were subjected to a freeze-thaw cycle.According to the actual environment,the freezing temperature was-5?,-10?and-15?.The freezing process lasts for 8 hours,and the melting temperature is at 20°C,the melting process lasts for 8 hours.During the freeze-thaw cycle,in order to simulate the actual force of the silt soil foundation in nature,keep the silt sample under the effective confining pressure(100k Pa,200k Pa,300k Pa)during the continuous process of this cycle.After the freeze-thaw cycle is over,the unconsolidated and undrained triaxial shear test is carried out under the corresponding confining pressure,and the final analysis obtains the change law of the mechanical properties of the silt samples in different states after undergoing the freeze-thaw cycle under different conditions.The main research results are as follows:(1)The increase in the number of freeze-thaw cycles changes the deviatoric stress-strain curve of silt,and becomes strain-hardening after the seventh freeze-thaw cycle;the increase in the number of freeze-thaw cycles destroys the structure of silt and causes powder The breaking strength of sand decreases,but it increases with the increase of confining pressure;when the confining pressure is 100k Pa and 200k Pa,as the number of freeze-thaw cycles increases,the elastic modulus of silt sand decreases.In the case of 300 k Pa,the elastic modulus first decreases with the increase in the number of freeze-thaw cycles,and increases at the seventh freeze-thaw cycle.When the number of freeze-thaw cycles is the same,the elastic modulus of silt sand increases with the increase of the confining pressure;the internal friction angle of the silt sample increases with the number of freeze-thaw cycles,but the cohesion Is reduced.(2)In the case of 7 freeze-thaw cycles,the change of freezing temperature cannot change the type of deviatoric stress-strain curve of silt;the breaking strength of silt increases with the decrease of freezing temperature and the increase of confining pressure.But the change of freezing temperature has little effect on the damage strength of silt sand samples;when the freezing temperature decreases,the elastic modulus of silt sand first decreases and then increases.As the confining pressure increases,the modulus of elasticity increases,but its increase gradually decreases;the freezing temperature decreases,the internal friction angle of silt sand decreases,and when the temperature decreases to a certain degree,the internal friction angle of silt sand becoming steady.The change law of cohesion is the opposite.(3)For silt with relatively small density,its deviatoric stress-strain relationship curve is strain softening,while the deviatoric stress-strain relationship curve of relatively dense silt becomes a stress hardening type;the relative density and confining pressure Increase,increase the destructive strength of silt.When the relative density of silt sand increases from 0.35 to 0.6,the breaking strength increases greatly;the elastic modulus of silt sand increases with the increase of relative density and confining pressure,and the internal friction angle and cohesive force also follow.improve.When the relative density increases from 0.6 to 0.9,the increase in internal friction angle and cohesion is small,indicating that when the silt reaches a certain density,its shear strength gradually stabilizes.