Study On Mechanical Properties And Mechanism Of Unsaturated Saline Soil And Solidified Soil With Lime In Zhenlai Under Freeze-thaw Cycles

The western Jilin Province is not only a typical soda saline soil region,but also a seasonal frozen soil area.The winters here are long and severe cold and the summers are quite hot.The change of the climate causes water and salt migration which leads to salinization disaster.The biggest difference between unsaturated saline soil and common unsaturated soil is that there is a certain amount of soluble salt and insoluble salt.The liquid phase in saline soil is salt solution.When the water content is low,supersaturated soluble salt exists between soil particles in the form of crystal as a skeleton.So the saline soil is hard when the water content is low.There is weak cementation between soil particles which has a certain bearing capacity.When the external condition changes(rainfall,irrigation,snowmelt and so on),the soluble salt in saline soil will be completely dissolved.The soil structure will damage and the strength of saline soil will decrease which may causes thaw collapse and other disasters.When the temperature is below 0? in winter,moisture and salt in saline soil will migrate as the temperature changes.Water freezes gradually and expands which may cause frost heave disaster.There are a lot of adsorption cations in saline soil which have strong hygroscopicity and moisture retention.So saline soil can absorb water from external environment easily and dissipate with difficulty which will reduce the strength of soil greatly.Therefore,the solidification and utilization of saline soil must be considered in engineering construction and human activities.In recent years,the salinization of saline soil in western Jilin Province increases year by year and the ecological environment is becoming more and more fragile.The salinization progress threatens the human activities and engineering,and it restricts the development of the economy.Therefore,the saline soil in Zhenlai district which belongs to the western Jilin Province is studied in this paper.Impact factors which influence the properties of saline soil in Zhenlai are determined through long-term monitoring.Saline soil in Zhenlai has hydroscopicity and moisture retention properties,so lime is used to improve the properties of saline soil.The mechanical properties are studied through unconfined compressive strength test and shear test.The unsaturated properties are studied through filter paper.And the microstructure of saline soil and solidified saline soil with lime are studied through MIP and SEM.The change mechanism are studied combined the macro and micro properties.Based on damage theory,constitutive model of saline soil in Zhenlia is constructed which provides a certain reference basis to the rational use of saline soil.The main research contents in this paper include:(1)The mechanical properties of saline soil and solidified saline soil are studied in this paper.Through the unconfined compression experiment and the conventional triaxial experiment,the relationships between compressive strength,shear strength and constitutive property,water content,salt content,lime content,curing age,freeze-thaw cycles are studied.The attenuation rules of saline soil and solidified saline soil under freeze-thaw cycles are analyzed respectively.The result shows:The unconfined compressive strength and shear strength of undisturbed saline soil is lower than remoulded saline soil,because there are some wormholes,sand columns and other phenomena in disturbed saline soil in Zhenlai.The unconfined compressive strength and shear strength of saline soil decreases with the increase of moisture content.When the soluble salt can be dissolve totally in the soil,the unconfined compressive strength and shear strength decreases with salt content increases.After the saline solution is gradually saturated,the unconfined compressive strength and shear strength increases with salt content increases.The unconfined compressive strength and shear strength increases first and then decreases with lime content increases.The solidified saline soil with 6%lime content can meet the best strength.The unconfined compressive strength and shear strength of solidified saline soil increases with curing age increases.The unconfined compressive strength and shear strength of saline soil and solidified saline soil decreases to stable with freeze-thaw cycles increases.But the strength attenuation rate of solidified saline soil is smaller the saline soil which means that lime can improve the frost resistance of saline soil.(2)The matric suction of saline soil and solidified saline soil are studied in this paper.Through the filter paper method,the relationship between matric suction and water content,salt content,lime content,freeze-thaw cycles is studied.The relationship between unconfined compressive strenghth,shear strength and matrix suction respectively are discussed as well.The results shows:Salt content and lime content has little influence on matric suction.Matric suction decreases as the water content increases.As the freeze-thaw cycles increase,the matric suction decreases until it gets stable.The first three freeze-thaw cycles has much influence on matric suction,and after the tenth cycle,the matric suction has little change with the freeze-thaw cycles.The relationships between matric suction and unconfined compressive strength,shear strength of saline soil and solidified saline soil are discussed as well.When the matric suction is high(>100kPa),matric suction has more significant effect on unconfined compressive strength and shear strength.(3)Microscopic characteristics of saline soil and solidified saline soil are discussed in this paper.Mercury injection experiment is used to study the characteristics of pore distribution of saline soil.Through the scanning electron microscopy(SEM),the microstructure of saline soil is analyzed qualitatively and quantitatively.As the water content increases,the large pores increase,the little pores and micro pores decrease,circularity and shape factor increases as well.As the salt content increases,the porosity increases first and then decreases.After a series of reactions between lime and soil,the particles can be cemented together to increase the integrity of the saline soil.As the freeze-thaw cycles increase,the tension cracks caused by soil shrinkage develop gradually,and the soil becomes broken gradually.The destruction of the saline soil in Zhenlai under freeze-thaw cycles is mainly caused by the shrinkage cracks which are produced when the soil melts.(4)The mechanism of saline soil and solidified saline soil is analyzed.According to the mechanical properties of saline soil and solidified saline soil(unconfined compressive strength and shear strength),comibined with matrix suction and microscopic characteristics,the mechanism of strength changes of soil is analyzed with the combination of micro and macro methods.(5)The elastoplastic damage model considering the freeze-thaw cycles of saline soil in Zhenlai is established.Based on the damage theory,according to the results of the unconfined compressive test of saline soil,Weibull distribution function is used to construct the constitutive model of saline soil.Two constitutive models are derived in the situations of considering damage threshold or not.Based on the established constitutive model,elastoplastic damage model considering the freeze-thaw cycles is established,and the rationality of the model is verified by laboratory experiment.