Mechanical Properties And Crack Characteristics Of Composite Solidified Saline Soil Under Alternating Dry-wet-freeze-thaw Cycles

Soil salinization is a major soil degradation process that threatens ecosystems and is considered one of the most important issues globally threatening agricultural production and sustainability in arid and semi-arid regions.As one of the three major carbonate saline soil distribution areas in the world,the saline soil region in western Jilin has an arid climate,low rainfall,strong evaporation,and large temperature fluctuations,and is in the middle-thickness seasonal permafrost area,with maximum seasonal permafrost depth can reach 180cm.The above environmental factors cause saline soil to be highly susceptible to periodic rainfall evaporation and freezing and thawing,thus changing the physical properties and structure of the soil,with serious negative impacts on regional engineering and construction.Therefore,a systematic study of the effects of dry-wet-freeze-thaw alternating cycles on the mechanical properties and crack variation of saline soil can help to reveal the disaster mechanism of carbonate saline soil slope in the Qian’an area of western Jilin Province.On the other hand,it can provide a scientific basis for saline soil management and prevention of soil disasters in this area,which is of great significance to engineering protection and the prevention and control of geologic hazards.In this paper,saline soil in the canal section of the Qian’an area in western Jilin was taken as the research object,and the synergistic solidification of saline soil by a variety of green sustainable solidification materials was aimed at improving the defective engineering properties of saline soil.The mechanical properties of saline soils under long-term dry-wet-freeze-thaw alternating cycles were investigated by combining the natural environmental factors of semi-arid regions and seasonal permafrost zones.The hydrophobicity and disintegration of composite solidified saline soil were tested by combining various hydrophobicity tests,microstructural characterization tests,dispersion characterization tests and hydrostatic disintegration tests.Finally,by simulating the natural environmental effects in the study area,the crack variation and deterioration mechanism of saline soil under the action of dry-wet-freeze-thaw alternating cycles were systematically investigated.The main research content and results of this paper are as follows:（1）The material composition and physicochemical properties of saline soil in the Qian’an area of western Jilin were comprehensively analyzed through literature research,engineering geology,field investigation and laboratory tests.The results of the study show that the saline soil in this area is dominated by silt,with clay content as high as 26.84%,large specific surface area,the soluble salts in the soil are rich in exchangeable cations,of which Na⁺is dominant,anions are dominated by HCO₃^-,the total amount of soluble salt is 0.404%,the soil is weakly alkaline,and it belongs to the carbonate saline soil.The primary minerals of the studied soil samples were mainly quartz,feldspar,dolomite and calcite,and clay minerals accounted for about 12%of the total mineral content,mainly the illite/smectite mixed layer.The results of the study provide basic data and a theoretical basis for further exploration of its mechanical properties and crack evolution mechanism.（2）A novel composite solidification material of sulfur-free lignin,basalt fiber and hydrophobic polymer was proposed for the solidification of saline soil.A quantitative model of the composite solidified material content and the unconfined compressive strength of the saline soil was established by response surface methodology,and the material ratios of the composite solidified soil were optimized.Firstly,the optimal contents of sulfur-free lignin,basalt fiber and hydrophobic polymer in the single-factor case were 10%,0.2%and 8%,respectively,which corresponded to strength increases of 56.33%,45.16%and 32.54%,respectively.Based on the results of single factor,the test scheme of composite solidified saline soil was designed,and the regression model of mechanical properties of composite solidified material content and unconfined compressive strength of saline soil was established by response surface method.After optimization,the optimum ratio of composite solidified soil was 9.87%,0.24%and6.29%,and the strength increase rate was 98.87%.（3）The solidified saline soil under the optimal composite ratio obtained based on the response surface method was used as the research object.The hydrophobic properties and physical and chemical properties of the composite solidified saline soil were studied and analyzed through a series of hydrophobic tests and microstructure characteristic tests.The water stability of the composite solidified soil was investigated by the dispersion characteristic test and the hydrostatic disintegration test.Finally,these characterization results were correlated with water stability,and the solidification mechanism of the composite solidified soil was discussed.Sulfur-free lignin and hydrophobic polymers enhanced the particle agglomeration and microstructural strength of saline soils through hydrophobic matrix and cation exchange mechanisms.The hydrophilic groups（-OH）on the surface of soil particles and lignin were replaced by hydrophobic groups（-CH₃）,which improved the erosion resistance and durability of the soil.The composite solidified material formed cemented agglomerates and particle clusters that together improved the internal structure and microporosity characteristics of the soil.（4）Considering the influence of natural environmental factors on the mechanical properties of composite solidified soil,a consolidated undrained triaxial compression test based on dry-wet-freeze-thaw cycles was carried out.The stress-strain relationship,deformation modulus,failure strength and shear strength parameters of composite solidified saline soil under different dry-wet-freeze-thaw cycles were analyzed by regression analysis.The deviatoric stress ratio between the composite solidified soil and the untreated soil was proposed as the solidification effect coefficient,and the effects of the number of dry-wet-freeze-thaw cycles and the confining pressure on the solidification effect coefficient were further analyzed,and the normalized stress-strain relationship and strength prediction model for dry-wet-freeze-thaw cycles and confining pressure were established.Under the same dry-wet-freeze-thaw cycles,the failure strength of the composite solidified saline soil is 85.27%higher than that of the untreated soil,and the failure strength ratio（R_n）is increased by 50.54%.The increase rate of failure strength is positively correlated with confining pressure.The number of dry-wet-freeze-thaw cycles was positively correlated with the solidification effect coefficient R_σ,and the highest was about 1.85 times.In addition,based on the hyperbolic model parameter fitting method,the effects of dry-wet-freeze-thaw cycles and the confining pressure on the corresponding model parameters were analyzed,and the corresponding expressions were established.The ultimate deviatoric stress（σ₁-σ₃）_ult is adopted as the normalization factor,which can better predict the stress-strain relationship.（5）The effects of alternating dry-wet-freeze-thaw cycles on the crack characteristics of composite solidified saline soil and the microscopic mechanism were further investigated.Based on the dry-wet-freeze-thaw cycles,the crack tests were carried out on untreated soil,composite solidified soil and single material solidified soil,and the crack characteristics of different samples were emphatically discussed.In addition,digital image processing techniques were used to quantitatively analyze a series of macroscopic cracks and microscopic pores in the samples.The results show that the alternating dry-wet-freeze-thaw cycles lead to changes in volume,stress and moisture of the soil,resulting in the formation of microfractures due to the continuous shrinkage and frost heave of the soil,while the interaction of dry-wet-freeze-thaw cycles aggravates the development of the cracks in the soil.In addition,the solidified material has a positive effect on inhibiting the development of cracks.the sulfur-free lignin by agglomerating the clay particles and filling the pores,the basalt fiber acted as the internal skeleton of the soil and played the role of reinforcing,and the hydrophobic polymer provided hydrophobic protection for the soil particles and the sulfur-free lignin to reduce the effect of moisture erosion,and the interaction between the three could effectively reduce the crack extension.The geometric and statistical parameters of the samples are related to the number of dry-wet-freeze-thaw cycles.Changes in microstructural features such as crack rate,length and width,fractal dimension and pore orientation reflected the crack extension and failure mechanisms.The research results effectively solve the problem of cracking and destruction of saline soils in semi-arid and seasonal permafrost regions,which is of guiding significance for the prevention and mitigation of soil erosion and geologic disasters.