Study On Mechanical Properties And Constitutive Relationship Of Saline Soil Under Freeze-thaw Cycles And Stress Paths

Saline soil is diffusely exists in the west of Jilin,and is a typical carbonate saline soil with a total area of 15,000 square kilometers.The western part of Jilin belongs to the seasonal frozen region,and its temperature changes significantly in each season,with cold and long winters and hot summers,which make the saline soil in this area experience periodic freezing and thawing processes every year.The existence of this periodic freezing and thawing process has a more pronounced effect on the mechanical features of the soil,and the development of engineering construction can not do without the study on the mechanical features and constitutive models of the soil.Nowadays,with the proposal of the plan for the western ecological economic zone in Jilin Province,large-scale construction projects have been carried out and increased in the saline soil areas,such as the river and lake connection project in the western region,the comprehensive transportation system construction project and the construction of crude oil pipelines has brought many challenges to the geotechnical engineering design of saline soil areas.Thence,it is urgent to conduct study on the mechanical features and constitutive relation of saline soil in western Jilin Province.The constitutive relationship of soil is studied on the basis of the stress-strain relationship of soil.It is a mathematical relationship expression about soil stress-strain relationship established on the basis of experiments under the assumption of assumptions.Experimental studies show that the material composition and structure of soil,stress path and stress history can all have a non-negligible impact on the constitutive relationship of soil.Among the many special soil types,due to the existence of salinity in the saline soil,the mechanical properties of the saline soil in the state of high water content are more significantly weakened after undergoing freeze-thaw cycles.The western part of Jilin Province belongs to the seasonal frozen soil area,therefore,the effect of freeze-thaw cycle is a factor that cannot be ignored in the process of engineering construction.Thence,it is of great importance to research the constitutive relationship of saline soil under freeze-thaw cycle conditions.In addition,in the geotechnical engineering design of the saline soil area,the stress-strain relationship under the conventional stress path is generally used,which is hard to fit the real condition of the project,and then a series of design deviations occur,resulting in a series of engineering problems.To solve such engineering problems,the generation of stress path constitutive relations of saline soil is urgently required,to offer a reference for the geotechnical engineering design in the engineering construction of saline soil areas.With the support of several national natural science foundations,this paper selects the saline soil in the Qian'an area of western Jilin Province for research,and conducts an in-depth study on the mechanical properties and constitutive relationship of the saline soil in this area.The main study details and results are as follows:(1)Through geological data collection,field investigation,sampling and laboratory experiments,the engineering geology,material composition,engineering geological properties and compressibility of soil in Qian'an were analyzed and studied.The results show that the Qian'an area in the western part of Jilin Province belongs to the seasonal frozen soil area and belongs to the continental monsoon climate of the middle temperate zone.The study area is a typical distribution area of carbonate-type saline soil,and the salinization is continuously aggravated by the topographic climate and freeze-thaw cycles.There is a large area of gray-white-white saline-alkali land distributed in the study area,and salt spilled,resulting in sparse surface vegetation.Under the influence of strong evaporation,the soil mass is prone to surface cracks.The research soil samples are clays with low liquid limit,mainly silt group.Its mineral composition is mainly primary minerals,belonging to non-organic soil.The total amount of soluble salt is 0.51%,the anions are mainly HCO₃^-and Cl^-,no CO₃^2-,Na⁺and Ca²⁺are the chief cations,the p H value is 7.72,and it is weakly alkaline.There is an obvious turning point in the compression curve of the studied soil sample,which is also the structural failure point of the original soil body.The soil samples studied are highly compressible soils.(2)The change characteristics of the consolidated undrained mechanical properties of the remodeled saline soil under the action of freeze-thaw cycles were studied.It was found that 10 freeze-thaw cycles were the turning point for the changing trend of the undrained mechanical characteristics of the remodeled saline soil.Through the laboratory test,the saline soil in the Qian'an area of western Jilin was selected for research,and the standard triaxial sample of the remodeled saline soil was made.The freeze-thaw cycle test and the consolidated undrained triaxial experiment were executed successively.The change characteristics and mechanism of the undrained shear mechanical properties of saline soil under the action of freeze-thaw cycles were studied.The research consequents reveal that:in the case of consolidation without drainage,the stress-strain relationship curve of the soil sample continues to move downward with the increase of the number of freeze-thaw cycles,indicating that the undrained shear strength decreases continuously and shows the characteristics of strain hardening,the pore water pressure curve of the soil sample moves up continuously,and there is an extreme value of pore water pressure under low confining pressure,it increases continuously at high confining pressure,and finally tends to be stable,and its ultimate pore water pressure value increases with the consolidation confining pressure increases.The pore water pressure coefficient continued to increase with the increase of the number of freeze-thaw cycles,and the critical state line continued to decrease,indicating that the degree of consolidation of the soil samples continued to decrease.The shape change of the effective stress path curve of the soil sample shows that the consolidation degree of saline soil decreases continuously with the increase of the confining pressure and the number of freeze-thaw cycles.The relationship between the two shear strength indexes of total stress and effective stress is??_cu>?_cu,c?_cu<c_cu;under the influence of freeze-thaw cycles,both gradually decreased,and the relationship between the magnitudes of the two shear strength indexes was as follows:???<??_cu,?c?_cu<?c_cu;it is primarily influenced by pore water pressure.Thence,so as to more exactly depict the variety of soil deformation and strength under different freeze-thaw cycles and confining pressures,the effect of pore water pressure should be excluded,and the effective stress shear strength index should be used to express it.(3)The consolidation and drainage mechanical properties of the remodeled saline soil under the action of freeze-thaw cycles were analyzed,and the double-yield surface constitutive relation of remodeled saline soil considering the effect of freeze-thaw cycles is established.Through the indoor triaxial test,the remodeled saline soil in the Qian'an region was selected for research,and the standard triaxial sample of the remodeled saline soil was made,and then the freeze-thaw cycle test and the consolidation drainage triaxial experiment were executed in turn,to study the change characteristics of the mechanical properties of saline soil under the action of freeze-thaw cycles.According to the test results,using the ellipse-parabolic double-yield surface model,and by establishing the mathematical relationship expression of the number of freeze-thaw cycles and the model parameters,the constitutive relationship of the double-yield surface of the remodeled saline soil considering the effect of freeze-thaw cycles is established.The results revealed that as the number of freeze-thaw cycles increase,the stress-strain relationship curve of saline soil gradually moves down,showing the characteristics of strain hardening.The deformation and strength properties of the soil have a continuous weakening effect.During the test,the drainage volume in the consolidation and shearing stages continued to increase as the number of freeze-thaw cycles increase,indicating that the volume of the sample continued to decrease.Furthermore,the volumetric strain curve keeps moving upward as confining pressure increases.On the premise of using the double-yield surface constitutive relation of the associated flow law,the effect of freeze-thaw cycles was added to the constitutive relation,and the constitutive relation of the remodeled saline soil was established considering the effect of freeze-thaw cycles.After that,compare the calculated value with the measured value.The results show that they are in good agreement,indicating that the change characteristics of deformation and strength of saline soil under the action of freeze-thaw cycles can be reasonably described by the modified double-yield surface constitutive relation.(4)Considering the effect of the stress path,the mechanical characteristics of the undisturbed saline soil under different stress path conditions were studied,and it was found that the mechanical properties of the undisturbed saline soil were greatly affected by the stress path.Using the indoor triaxial test,the undisturbed saline soil samples in the Qian'an area were selected as the test object.Triaxial shear tests under 6 different stress paths including the consolidated undrained triaxial test under the conventional stress path under the isobaric consolidation condition and the consolidation drainage triaxial test under the conventional,equal p,reduced p,and increased p stress path under the isobaric consolidation condition,and the consolidation drainage triaxial test under the conventional stress path under the K₀ consolidation condition were carried out.The triaxial shear test data of undisturbed saline soil under 6 different stress paths were analyzed,and the change characteristics of mechanical characteristics under distinct stress paths were studied.The results revealed that the stress-strain relationship curves of soil samples decrease continuously in the order of increased p,conventional,equal p,and decreased p,and they all show the characteristics of strain hardening.Under the same consolidation confining pressure,the relationship of volume strain is as follows:increased p>conventional>equal p>decreased p.The curves under the same stress path have the same slope.For different confining pressures,only the relative positions of the curves are different.Under the conventional stress path,the slope is 1;under the increased p stress path,the slope is 1/3;under the equal p stress path,the slope is 3;under the decreased p stress path,the slope of the curve is-1.For the same consolidation confining pressure,the magnitude relationship of the shear strength under different stress paths is as follows:increased p>conventional>equal p>decreased p.For the c and?,the consolidation condition has a greater influence on the cohesion c,and the internal friction angle?is smaller;the drainage condition has less influence on the cohesion force c.Compared with the cohesion force c,it has a greater influence on the internal friction angle?;different stress path conditions have a greater impact on the cohesion c and the internal friction angle?.(5)According to the stress path equivalent principle and the loading and unloading criterion,the stress path constitutive relation of the undisturbed saline soil is established.Using the experimental results in the previous part,according to the micro-element equivalent principle of the stress path,the results of the consolidated drainage triaxial shear test data under the conditions of increased p and equal p linear stress paths are analyzed and sorted,and then mathematically described.Combined with the loading and unloading criteria,the loading and unloading states of the samples were determined,and the stress path constitutive relationship of the undisturbed saline soil was established.According to the total relationship of the stress-strain relationship under the increased p and equal p stress paths,curve fitting the total relationship to obtain the total expression of the stress-strain relationship;after differentiating the expression,the incremental relationship expression under two micro-element stress paths can be obtained.Then,the incremental relationship expression under the two micro-element stress paths is added to obtain the stress-strain relationship under any stress path micro-element of the undisturbed saline soil;finally,the calculated value of the model is compared with the measured value,And the results of both are basically the same,indicating that the established stress path constitutive relationship of the undisturbed saline soil can better depict the mechanics features of the saline soil under different stress paths.