Study On Laws Of Water-salt Migration And Salt-frost Heaving Of Sulfated Soil In Ningxia Based On Freeze-thaw Cycles

Located in the central Ningxia arid zone in the northwest,due to the dry climate,strong evaporation,and high degree of groundwater mineralization in the region,the soil salinization in this area is serious,and sulfated soil is distributed in varying degrees.Under the influence of the external environment,it is prone to repeated salt swelling,melting,and dissolution causing the destruction of the soil structure,resulting in the destruction of a large number of industrial and agricultural infrastructure.To this end,comprehensively understanding and exploring the impact of the external environment on the salt and frost heave of sulfated soils has a positive guiding significance for scientifically and reasonably solving the engineering problems of sulfated soils in this area.In this paper,the sulfated soil in Hongsipu area of Ningxia was taken as the research object.The in situ soil samples were subjected to decontamination and salt washing to conduct the indoor soil column freeze-thaw cycle test.The water content,electrical conductivity,temperature and displacement at different locations were sorted and analyzed using MATLAB data to discuss the water-salt transport law and the effect of salt-frost expansion on soil deformation during the freeze-thaw cycle.The results are as follows:(1)During the top-down freezing process,the water at the bottom of the soil moves toward the freezing front,which causes the salt to accumulate upwards.At the same time,the relationship between the volumetric water content and conductivity of the soil with temperature changes is obviously "hysteretic".(2)During the one-way freezing process from top to bottom of the soil,the vertical deformation of the soil in the freezing stage undergoes three changes,the first is the adjustment zone,which is caused by the formation of Glauber's salt;the second is the rapid deformation zone,It is caused by the combined action of the formation of thenardite and ice crystals;the third is the slow deformation zone,which is caused by the formation of a small amount of ice crystals.The melting of the soil is carried out bidirectionally from the surface layer and the bottom layer,that is,bidirectional melting.At this stage,a rapid dissolution zone appears.(3)During the freeze-thaw process of sodium sulfate-containing saline soil,the relationship between the amount of vertical deformation and time shows a "peach-shaped" trend,and the rate of vertical deformation of each freeze-thaw soil body is consistent.As the number of freeze-thaw cycles increases,the salt-freeze expansion rate also increases.The higher the salt content of the soil,the more significant the effect of the vertical deformation of the soil.(4)Under the same freezing time,with the increase of freezing and thawing times,the structure of the soil body is gradually destroyed,the soil body tends to loosen,and the minimum temperature of the soil body gradually decreases.(5)The salt-free frost heaving volume of salt-free soil is obviously smaller than the amount of collapse,and it is necessary to pay attention to prevent collapse in the project.The salt-frost-heavy volume of the soil with 1%and 3%salt content is significantly higher than the collapse volume.(6)Regardless of whether it contains salt or not,for the case of the same layer of soil and different layer of soil,it shows that the conductivity and volumetric water content reach the minimum value at-10?,and the maximum value at 5?,so sulfate-containing soil in NingxiaThe body freezing temperature is-10? and the melting temperature is 5 ?.