Dispersion Mechanism And Dispersion Evaluation Of Qian'an Saline Soil Under Freeze-thaw Cycles

Dispersive soil is a kind of special soil,and the researches related to dispersive soil started relatively late in the world.Dispersive soil has a low ability to resist erosion.When it meeting with water,disintegration and dispersion may occur and lead to lots of problems in practical engineering.In order to reduce the degree of salinization,many water-diversion projects have been implemented in western Jilin Province.However,during the construction of these engineering projects,large-scale gullies and collapses appeared on the surface of the channel slope under the influence of repeated rainfall and erosion,and the soil in this area showed obvious dispersity.At present,most studies mainly consider the influence of material composition and physicochemical properties on the dispersity of soil,while there are relatively few studies considering the structural characteristics of soil and the influence of external environment.The western Jilin province belongs to seasonal frozen area,and the soil distributed in this area experienced freeze-thaw cycles every year.In addition,the climate in this region varies significant in different seasons.It is dry in spring and autumn,rainy and hot in summer.In addition,the average annual evaporation is much higher than annual rainfall,and the evaporation effect is strong.Therefore,the salt content of soil will change with the variation in season and temperature.Under the combined action of freeze-thaw cycles and salinity,the material composition and structural characteristics of soil will inevitably change,which will result in more complex dispersion characteristics and dispersion mechanism of soil in this area.Therefore,the in-depth study on the dispersion characteristics and mechanism of saline soil in western Jilin province has a theoretical guidance function and important practical significance for the smooth progress of engineering construction in this area.The saline soil distributed in Qian'an area of western Jilin province presented obvious dispersity through many field investigations and preliminary studies.Therefore,Qian'an saline soil was selected as research object in this study,and the effects of freeze-thaw cycles and salt content on the dispersion characteristics,physicochemical properties,microstructure characteristics,mechanical properties and permeability characteristics of soil was investigated through laboratory experiments.In addition,the main factors affecting dispersity were determined through rough set theory,and the artificial neural network model for evaluating the dispersity of soil in Qian'an area was established.Combined with the test results,the dispersion mechanism of Qian'an saline soil was comprehensively analyzed.The main research contents and results are as follows:(1)Through field sampling and laboratory test,the material composition and physicochemical properties of saline soil in Qian'an area of western Jilin province were investigated,and the dispersion of soil samples was determined by dispersion identification test,empirical formula method and dispersion diagram.The results indicated that the saline soil in Qian'an area belongs to soda alkaline soil,the main cation and anion is Na⁺and HCO₃^-,respectively,and the soil sample is weakly alkaline.The particle size composition of the soil is mainly silt,the proportion of primary minerals is large,and the clay minerals are mainly composed of illite/smectite mixed layer and illite.In addition,the results of different dispersion identification tests showed that the soil samples belong to dispersive soil and have obvious dispersity.(2)The dispersity of soil samples with different salt contents after freeze-thaw cycles was comprehensively determined by pinhole,fragment and double hydrometer tests,and the comprehensive identification results were quantitatively characterized,and the effect of freeze-thaw cycles and salt content on the dispersion characteristics of Qian'an saline soil was obtained.The test results showed that the change of salt content and freeze-thaw effect jointly affect the dispersity of soil.The dispersity of soil samples generally increased with increasing freeze-thaw cycles.However,the dispersity of soil samples was inhibited when the salt content exceeded a certain limit.With the increase of freeze-thaw cycles,the threshold value of salt content that inhibited the dispersity of soil samples will change to a certain extent.That is,when the salt content exceeds a certain limit,the promotion effect of freeze-thaw cycles and the inhibition effect of increasing salt content jointly affect the dispersity of soil samples.(3)The disintegration characteristics of saline soil with different salt contents after freeze-thaw cycles were obtained through disintegration test,and the changes of stability,disintegration form and disintegration rate of soil samples with freeze-thaw times and salt content were analyzed.The disintegration time of soil samples gradually shortened with increasing freeze-thaw cycles.However,the change was not monotonous with increasing salt content.When the salt content exceeded a certain limit,the disintegration time tended to shorten gradually,and the threshold salt content was approximately 1.2%.In addition,the effect of the variation of salt content on the disintegration time of soil samples decreased gradually with increasing freeze-thaw cycles.The reason for the above changes is that the variation of salt content affects the thickness of the bound water film around the soil particles,and the freeze-thaw cycles leads to corresponding changes in the structure of the soil samples.(4)The change rules of the physicochemical properties of saline soil samples with salt contents and freeze-thaw cycles were obtained through laboratory tests.The results showed that the freeze-thaw cycles mainly affect the physical properties of soil.With the increase of freeze-thaw cycles,the sand content in soil samples decreased significantly and the silt content increased,while the changes of cation exchange capacity,exchangeable sodium ion percentage and p H value were relatively small and no obvious law can be observed.The variation in salt content will change the particle size composition in the soil.When the salt content was greater than 1.2%,with the increase of salt content,the bound water film between soil particles become thinner and the agglomeration of clay particles become stronger,resulting in the decrease of clay content in the soil.In addition,the variation of plasticity index versus freeze-thaw cycles and salt content showed a similar trend to the variation of clay content.(5)The microstructure of saline soil samples after freeze-thaw cycles was analyzed qualitatively and quantitatively by mercury intrusion porosimetry(MIP)and scanning electron microscope(SEM),and the thermal fractal model was adopted to analyze the fractal characteristics of pore size distribution of soil samples,the effects of freeze-thaw cycles and salt content on the microstructure characteristics of Qian'an saline soil were discussed.The results displayed that with the variations in freeze-thaw cycles and salt content,the microstructure and pore size distribution characteristics of soil will change accordingly,and the effect of freeze-thaw cycles is more obvious.Freeze-thaw cycles mainly transformed small pores and mesopores into macropores in the soil samples.With the increase of freeze-thaw cycles,the content of large pores and cracks increased,and the fragmentation degree of soil particles improved,the microstructure type of soil gradually transited from flocculation to agglomeration,and finally changed to loose agglomeration structure.However,with the variation of freeze-thaw cycles and salt content,the shape and direction of particles and pores changed relatively little.(6)The mechanical and permeability characteristics of saline soil with different salt contents after freeze-thaw cycles were investigated through direct shear test,unconfined compressive strength test and variable head permeability test,and the correlation between the above properties of soil samples and their dispersity characteristics was analyzed.The results showed that the dispersity of Qian'an soil showed a certain positive correlation with its permeability coefficient and negative correlation with the cohesion and unconfined compressive strength overall,and there was no obvious correlation with the dispersity and internal friction angle.However,the correlation was poor and the coefficient obtained by fitting was relatively small on the whole.(7)According to the parameters of material composition,physicochemical properties and microstructure of saline soil with different salt contents after freeze-thaw cycles obtained from the above-mentioned experiments,the main factors affecting the dispersity of Qian'an soil were determined by rough set theory,and the influence degree of each factor was quantified by calculating its weight value.According to the test data and analysis results,the dispersion mechanism of Qian'an soil were discussed comprehensively.The results showed that the main factors affecting the dispersity of Qian'an saline soil included exchangeable sodium ion percentage,plasticity index,p H value,thermal fractal dimension of pore and porosity.Among these factors,the main factors affecting the dispersity of Qian'an saline soil are the physicochemical properties and material composition of soil.In addition,freeze-thaw cycle effect is an important external cause of soil dispersion in this area.The change of pore structure characteristics caused by freeze-thaw cycles affects the dispersity of soil to a certain extent.(8)Using the five parameters obtained by rough set theory as input variables,the BP neural network model and BP neural network model based on principal component analysis were established to evaluate the dispersity of soil in Qian'an area,and different evaluation indexes were adopted to compare and analyze the evaluation effects of the two models.The two models established in this paper can satisfy the evaluation of the dispersity Qian'an soil,and the latter model has higher accuracy.The dispersity of Qian'an soil can be evaluated by combining its physicochemical properties and microstructure parameters through the models established in this paper.which provides a new way for the dispersity evaluation of Qian'an soil in Western Jilin Province.