Dynamic Creep Characteristics And Dynamic Constitutive Model Of Freezing-thawing Saline Soil

Since the 20th century,with the proposal of the‘The Belt and Road Initiative’and other plans,China has paid more and more attention to the construction of the western region.A large number of traffic engineering projects have been or will be located in the western cold regions.However,some soil masses in the western region（especially in Xinjiang and Qinghai）belong to sulfuric acid saline soil.When the temperature decreases,the solubility of sulfate also decreases,and partial salt precipitation to form crystals,resulting in increased porosity and reduced strength of the soil.At the same time,transportation vehicles such as high-speed rail are equivalent to applying a long-term cyclic load to the foundation.Under the action of this load,uneven settlement of saline soil foundations with different salt content will occur,which has a negative impact on the safety and comfort of high-speed rail traffic.In this thesis,the dynamic properties of remolded frozen thawed saline soil were studied through dynamic triaxial cyclic loading.The effects of temperature,salt content,and dynamic stress amplitude on the dynamic properties and cumulative plastic deformation of the soil were analyzed;In addition,this article also studied the microstructure of soil samples after loading,a fractional order Nishihara model also was established to fit the creep results.The relevant conclusions studied in this article can serve as a reference for the construction and maintenance of ballasted railway embankments in saline soil areas.The conclusions are as follows:（1）Under short-term loading,the hysteresis curve area of soil increases with the increase of dynamic stress amplitude and salt content,and decreases with the increase of vibration frequency and freezing temperature.The area of hysteresis curve of unfrozen and thawed soil is smaller than that of frozen and thawed soil.During long-term loading,when the amplitude of dynamic stress is 10k Pa,the area of the soil hysteretic curve first decreases rapidly with the increase of the vibration frequency,then rises rapidly after falling to the lowest point,and then remains almost unchanged.When the dynamic stress amplitude is 30 k Pa,the area of the soil hysteretic curve will first decrease rapidly with the increase of vibration times,then slowly increase after falling to the lowest point.At about 10000 vibration times,the growth rate of the area of the hysteretic curve will decrease and almost remain unchanged.When the dynamic stress amplitude is 50 k Pa,the hysteretic curve areas of all soils except unfrozen and thawed remolded soils decrease with the increase of vibration frequency.The dynamic elastic modulus increases at the initial stage of loading when the amplitude of dynamic stress is small;when the dynamic stress amplitude is large,the dynamic elastic modulus decreases at the initial stage of loading.The dynamic elastic modulus of soil is inversely proportional to the salt content and directly proportional to the freezing temperature.The dynamic elastic modulus of unfrozen and thawed soil is larger than that of soil after undergoing freezing and thawing cycles.（2）The cumulative plastic deformation of soil mass is directly proportional to the amplitude of dynamic stress and salt content,and inversely proportional to freezing temperature.The cumulative plastic deformation of unfrozen and thawed soil is smaller than that of soil after undergoing freezing and thawing cycles.However,the effect of freezing temperature on the cumulative plastic deformation of soil is far less than the salt content.The creep of most specimens in this test belongs to stable and critical types,while only a few belong to destructive types.（3）The larger the pore diameter of the soil,the smaller the roundness value,and the deviation of the pores from the roundness.The higher the salt content of the soil,the higher the value of pore circularity under the same dynamic stress amplitude.The larger the dynamic stress amplitude of soil with the same salt content,the greater the roundness of soil pores.The porosity of soil increases after freeze-thaw.As the salt content increases,the proportion of small diameter pores increases after loading.The larger the dynamic stress amplitude of soil with the same salt content,the more proportion of small diameter pores.The higher the salt content of the soil,the more regular the pores of the soil after loading,and the smaller the fractal dimension;The larger the amplitude of dynamic stress on the soil,the more slender pores close after loading to form regular small pores,the smaller the fractal dimension of soil pores.（4）The use of fractional order Nishihara model to describe the creep of remolded saline soil can achieve good results.In the fractional order Nishihara model,the parameters of unfrozen and thawed soil mass E₂、η₁ are generally larger than those of soil mass undergoing freezing and thawing cycles.For the soil mass after freezing and thawing,it decreases with the increase of salt content,this indicates that the freeze-thaw cycle leads to the deterioration of soil mechanical properties.The UMAT subroutine can be used to introduce the fractional order Nishihara model into ABAQUS for numerical simulation,and the numerical simulation results are very close to the experimental results,which means a good fitting effect.