Study On Mechanical Properties And Constitutive Model Of Cohesive Coarse-Grained Soils Under Freeze-Thaw Cycles

With the deepening of China Western Development and implementation of “the Belt and the Road” initiative,the high and cold regions,mainly in Tibetan Plateau,ushered a new boom period of economic development and infrastructure construction.Their main features are high latitudes,cold climates,the big differences in air temperature between day and night,causing strong physical weathering.Therefore,the slope surface in the high and cold regions is generally covered with thick coarse-grained soil.In recent years,geological disasters,such as slope instability of coarse-grained soil,large landslide,and other natural disasters,etc.,frequently occurs,which poses serious threats to human lives and property,engineering construction,and geological environment.The formation and evolution of these disasters and the mechanism of catastrophe are closely related to the physical and mechanical properties of coarse-grained soil under freezing and thawing.However,there is currently no systematic research on these aspects.In the thesis,the cohesive coarse-grained soils were chosen as the research object,which were taken from the surface of a typical high and cold regionin the southwest of Maizhokunggar Countyin Tibet.Furthermore,by using field surveys,laboratory tests,theoretical analysis,and other technical methods,the physical properties,such as pore structure and electrical resistivity of cohesive coarse-grained soils with different dry densities and moisture contents,the destructive failure mechanism and its constitutive model under uniaxial and triaxial loading conditions,the shear properties and constitutive model of the freeze-thaw interface of specimens,have been systematically studied under the action of freeze-thaw cycles.The major conclusions are summarized as follows:(1)The particle composition,mineral composition,basic physical properties,and compaction characteristics of cohesive coarse-grained soils are investigated.Then the pore structure and electrical resistivity characteristics of coarse-grained soils with different dry densities and water contents are analyzed under different freeze-thaw cycles.The results indicate that the porosity and fractal dimension of the samples exhibit non-linear growth,while the pore shape factor and probability entropy of the samples continue to decrease,and tend to be stable after seven cycles,as the number of freeze-thaw cycles increases.It demonstrates that freeze-thaw cycles increase pore volume and edge roughness of samples,and align particles.The electrical resistivity of the samples firstly increases rapidly and then gradually become stable after seven cycles,with the increase of the freeze-thaw cycles.The electrical resistivity of the samples decreases significantly with the increase of water content and dry density during freeze-thaw processing.Based on the modified Archie electrical resistivity model,an electrical resistivity model of unsaturated cohesive coarse-grained soils considering is established under the effect of cyclic freeze-thaw.The impact of environmental temperature,pore water,density,pores,mineral composition,and structure on the electrical resistivity of cohesive coarse-grained soils are analyzed.(2)A series of uniaxial compression tests of the cohesive coarse-grained soils with different dry densities and water contents are carried out under freeze-thaw cycles.The results indicate that the uniaxial stress-strain relationships of cohesive coarse-grained soils are mainly strain-softening,and a few samples with high water content and low density are strain-hardening.With the increasing freeze-thaw cycles and moisture contents,the stress-strain relationships change from strain-softening to strain-hardening.The pre-peak(uniaxial compressive strength and deformation modulus)and the post-peak(residual strength and softening modulus)mechanical properties of samples after freeze-thaw cycles decrease nonlinearly,and they tend to stable after seven cycles.With the increasing of dry density and moisture content,the uniaxial compressive strength of the samples increases and decreases respectively,but there is no obvious change law in residual strength.In addition,the ice-water phase transition,coarse particle breakage,and fine particle agglomeration under the action of freeze-thaw are the internal mechanism of the freeze-thaw deterioration of the cohesive coarse-grained soils,and there is a "critical value" phenomenon.Under uniaxial loading,the shear failure surface and shear band characteristics of the specimens are closely related to the dry density,water content,and the number of freeze-thaw cycles.The quantitative relationships between the electrical resistivity and both the uniaxial compressive strength and deformation modulus of cohesive coarse-grained soils are established under freeze-thaw action.Then based on the electrical resistivity,the evaluation method of freeze-thaw resistance of cohesive coarse-grained soil is proposed.Compared with the freeze-thaw damaging evolution equations based on deformation modulus and electrical resistivity,it is found that the electrical resistivity characterization of freeze-thaw damage is more accurate.The constitutive equation of freeze-thaw damage is established under the uniaxial load of the cohesive coarse-grained soils.(3)Through the triaxial compression tests,the shear strength,deformation characteristics,and constitutive model of cohesive coarse-grained soils with different dry densities and water contents are systematically studied under freeze-thaw cycles.It is found that the shear strength and strength parameters(cohesion and internal friction angle)of samples decrease with the increasing freeze-thaw cycles,and tend to be stable after seven cycles.The shear strength increases linearly with the increase of dry density and confining stress,and decreases linearly with the increase of moisture content.Furthermore,compared with other parameters,the cohesion of the samples is greater affected by freeze-thaw and dry density,and the internal friction angle is more significantly affected by the moisture content.The shear modulus of the samples generally shows a nonlinear decay as the number of freeze-thaw cycles increase,increases with the increase of dry density and confining stress,and decreases with the increase of water content.The deviator stress-strain relationship of the samples is mainly strain-hardening and gradually changes to strain-softening as the number of freeze-thaw cycles and water content increase.Taking the confining stress and limit deviator stress as normalization factors,the normalization equation of the stress-strain relationship of specimens is constructed under freeze-thaw actions.Based on the Duncan-Chang model,a constitutive model that can uniformly characterize the strain-hardening and strain-softening characteristics of cohesive coarse-grained soils are established under freeze-thaw cycles,and experimental results verify the rationality of the model.(4)Based on the large-scale temperature-controlled direct shear testing system,a series of experiments are conducted to investigate the mechanical shear properties and constitutive model of the freeze-thaw interface in cohesive coarse-grained soils.It is demonstrated that the results indicate that the relationship of the shear stress and shear displacement of the freeze-thaw interface is mainly hardening.However,it is softening for the specimen at the large dry density and high normal stress,and there is a starting shear stress when the shear displacement occurs.With the increase of the shear displacement,the freeze-thaw interface shrinks firstly and then obviously dilates.The greater the dry density and the lower the water content,the more significant the dilatation.The shear strength and strength index(composite cohesion,internal friction angle)of the freeze-thaw interface increase significantly with the increase of dry density.They gradually decrease with the increase of water content.Furthermore,a conceptual model of the mechanical behavior of the cohesive coarse-grained soils at the freeze-thaw interface is constructed during the shearing process,and then the laws of moisture migration and redistribution at the freeze-thaw interface are analyzed.The cementation coefficient of pore ice is proposed,and its relationship with shear displacement is established to evaluate the cementation effect of pore ice at the freeze-thaw interface.By analyzing the relationship between the shear displacement and shear stress and the relationship between the initial tangent modulus and the normal stress,the nonlinear elastic constitutive model of the freeze-thaw interface in cohesive coarse-grained soils is established.