Study On The Change Law Freeze-thaw Cycles Of Particle Size Composition Of Saline Soil

The west of Jilin Province is located in the southwest of the Songnen Plain,as a typical seasonally frozen soil region in China,it is located in the transition zone of semi-humid and semi-arid climates.The precipitation here is rare which is mainly concentrated in the hot summer;meanwhile,the winter is both cold and dry,all of these climate factors make the evaporation in this region is greater than the precipitation in the whole year,which further makes the region form a typical seasonal meteorological drought,and leads to soil salinization finally.The type of saline soil in the region is mainly carbonate saline soil,with a total area of 1.5×10⁴ km².With the continuous development of society,large-scale land development and improvement projects are in the proposal.However,due to the special engineering geological properties of saline soil,unfavorable geological phenomena such as erosion,suffosion,collapse,frost heaving and lixiviation collapse occur frequently in this region,posing a serious adverse impact on people’s daily life and various engineering activities.The particle size composition of soil is closely related to the engineering geological properties of soil,therefore,the research on the influence of freeze-thaw cycle on the particle size composition of saline soil is of great significance to grasp the engineering geological properties of saline soil and better serve the social production activities.Relying on the water diversion channel project in Qian’an area of Jilin,this paper takes the saline soil in Qian’an area as the main research object.Through field survey,soil samples at depths of 40,80 and 180 cm in the study area were selected for research.The three soil samples were divided into low-salinity soil samples（at 180 cm,with Changchun soil samples for comparison）and high-salinity soil samples（at 40,80 cm）based on the salt content.Considering that the study area is a typical seasonally frozen region,the soil water and salt contents could be different from time to time,therefore,different water contents and salt contents（only high-salinity conditions）were prepared in the experiment,and the prepared soil samples were subjected to freeze-thaw conditions,then the soil samples after different freeze-thaw cycles were tested for obtaining particle size composition.The main findings include（1）With the change of depth,the salt content and particle size composition of each soil layer in the study area are also quite different.In terms of salt content,it shows a low-high-low change law with soil depth.The cations are mainly sodium ions（Na⁺）,and the anions are mainly carbonate(CO₃^2-)and bicarbonate（HCO₃^-）.The soil samples at 40,80 and 180 cm and Changchun soil samples were named as lean clay with sand,lean clay with sand,lean clay,lean clay.（2）The freeze-thaw cycle had a certain influence on the change of the particle size composition of the soil.With the progress of the freeze-thaw cycle,the content of clay particles and silt particles with a smaller particle size continuously reduced,and that of silt particles with a larger particle size and a small amount of sand particles increased.The particle size distribution of the same soil type after different freeze-thaw cycles had certain similarity,the particle size corresponding to the peak particle content remained stable,and the change of the particle size composition of different type of soil samples after freeze-thaw cycles was different.（3）Under low-salinity conditions,20μm was the boundary particle size between air-dried and the non-freeze-thaw soil samples under all planned water contents.The content of particles smaller than this boundary particle size increases,while the content of particles larger than this boundary particle size decreases.With the increase of water content,the dispersion of large particles becomes more significant;with the increase of freeze-thaw cycles,the boundary particle size turned to be 20μm,the content of clay and silt particles smaller than this boundary particle size continuously reduced,and the content of silt and sand particles larger than this boundary particle size increased.（4）Under high-salinity conditions,the change of the particle size composition of the soil sample was affected by the salt content and water content.With the increase of the salt content,the phenomenon of large particle dispersion of the non-freeze-thaw soil samples was more significant than that of air-dried soil.Under each salt content and water content condition,with 30μm as the boundary,the content of particles smaller than this boundary particle size increased,and the content of particles larger than this boundary particle size decreased.When the salt contents ware 1%and 1.5%,with the increase of water content,the phenomenon of large particles dispersion became more and more significant;when the salt content was 2.5%,the increase of water content inhibited abovementioned phenomenon;as the number of freeze-thaw cycles increased,the change law remained the same as a whole,but due to the addition of salt,the particle agglomeration effect got further enhanced,with 30μm as the boundary,the content of clay and silt particles smaller than this boundary particle size kept decreasing,and the content of silt and sand particles larger than this boundary particle size kept increasing.（5）Due to the colloidal properties and selective adsorption properties of clay particles,during freeze-thaw,some clay particles agglomerated with each other or connected smaller silt particles to transform them into particles with a larger size,while the other part directly adhered on the surface of larger silt and sand particles,resulting in changes of the soil particle size composition.Due to the addition of salt,such effect generally got enhanced,the boundary of transformative particle size also increased.（6）During the sample preparation process,the large particles in the air-dried soil were continuously dispersed due to the change of the bound water film as a result of water addition;affected by the plastic limit,under the same water content,the effect of water on the change of bound water film was also different.Under the high-salinity condition,the addition of salt caused an overall increase in thickness of the bound water film,and the dispersion effect was further strengthened;when the salt content reached2%～2.5%,a large amount of Na+was adsorbed on the surface of the particles,which had a certain compression effect on the bound water film,and further restricted the dispersion phenomenon to a certain extent.