Experimental Study On Mechanical Strength Of Hydrophobic Saline Soil Under Freeze-thaw Cycles In Qian’an

Qian’an County is located in the seasonal frozen area of northeast China,and it is widely distributed with saline soil.Global warming and the development of people’s lives have led to an increasingly serious problem of soil salinization in the area,which is growing at a rate of 1.4% per year.Soil salinization will destroy the stability of the ecosystem,limit agricultural production,and cause damage to engineering construction.In order to reduce the development of land salinization and adjust the stability of the ecosystem,Jilin Province has vigorously built a series of diversion projects such as Yinnenlubai and Hadashan Water Control Project.However,local engineering constructions have been damaged due to long-term rain scouring,river erosion,and frost heave and thawing settlement of the bearing soil.Therefore,in order to enhance the soil’s frost resistance and bearing capacity,and improve engineering stability,this paper uses a new type of soil superhydrophobic emulsion(CN)to treat the Qian’an saline soil,and studies the effect of CN on mechanical properties of saline soil under different conditions,mainly as follows:(1)Based on the field investigation and indoor experiments,the basic physical and chemical properties of Qian’an saline soil were obtained.It is determined that the studied soil is silty clay,with an optimal moisture content of 16.80%,a maximum dry density of up to 1.794g/cm3,and salt content of 0.25%.(2)Considering the actual engineering construction conditions and the geographical environment in which the project is located in the seasonal frozen area,four variables of moisture content,compaction degree,salt content and freeze-thaw cycles are set in this study.The change laws of soil mechanical properties of untreated soil and CN treated soil under different test conditions are investigated by conducting unconfined compressive strength test and direct shear test respectively.(3)The test results show that under high salt content condition,the unconfined compressive strength of CN soil is lower than that of the untreated soil.Except for the high salt content condition,unconfined compressive strength and shear strength of CN treated soil are improved to some degree under other conditions.When the water content varies from 16.80%～ 18.80%,there exists a critical point of water content.And when the water content reaches the critical point,it will cause a change of soil damage from soft brittle damage to hardened plastic damage.The higher the degree of compaction,the lower the improvement rate of CN treated soil for the compressive strength.The improvement rate of cohesion of CN treated soil trends to stable between30-60% and the improvement rate of internal friction angle trends to stable between 0-2.5% at all compaction levels.The cohesion and internal friction angle of untreated soil and CN treated soil decrease gradually with salt content in the range of 0.0-1.5%,and increase gradually with salt content in the range of 1.5-2.0%.The increasing rate of cohesion of CN treated soil reaches the minimum value and the increasing rate of internal friction angle reaches the maximum value near a salt content of 0.3%.(4)Scanning electron microscope test was conducted to analyse the alteration of soil microstructure and qualitatively analyse the strength improvement mechanism of CN treated soil.The SEM images of both untreated and CN treated soil were quantitatively analyzed by using IPP image processing software to study the effect of CN on soil particle structure units and pore units.(5)The strength improvement mechanism and the strength change law of untreated and CN untreated soil with different moisture contents,compaction degrees and salt contents under different freeze-thaw cycles were analysed.(6)Based on the principle of equivalent effect variation,a damage constitutive model obeying the Weibull distribution and varying with the number of freeze-thaw cycles was established,and the construction and derivation of the model were explained in detail.In addition,the validity of the model simulation was verified by mechanical test results.