Experimental Study On Influence Of Water And Salt On Suction And Unconfined Compressive Strength Of Unsaturated Soil

In arid and semi-arid areas,the groundwater level is usually deep,and the soil above the groundwater level has negative pore water pressure,which is unsaturated due to a large amount of evaporation and transpiration.Unsaturated soil is the most common soil in practical engineering,and the soil suction is an important stress variable to characterize the properties of unsaturated soil,which is the premise and basis for the study of the properties of unsaturated soil.The total suction of soil consists of two parts:matrix suction and osmotic suction.The matrix suction is mainly affected by the change of soil moisture,and the osmotic suction is mainly affected by the change of salt content.Located in the Songnen Plain in the northeast of China,the soil moisture and salinity in this area are constantly changing due to the evaporation caused by monsoon,rainfall and river.Thus,the suction and strength characteristics of soil are changed.Therefore,this paper takes the saline soil in Qian'an area of Western Jilin Province as the research object.Through the suction test,the change of matric suction and osmotic suction was determined under different salt content and moisture content,and the influence and mechanism of water salt content on suction were analyzed.The unconfined compression test is used to test the strength of soil,and the influence of matrix suction and osmotic suction on the mechanical properties of soil is further analyzed.The main research contents are as follows:The change characteristics of water and salt content of soil in different seasons in the study area were determined by investigation and sampling at different times.Through laboratory tests,the composition characteristics and basic physical and chemical properties of soil samples were studied.The test results show that the soil at the sampling point is mainly composed of silt and clay particles,and the proportion of primary minerals is relatively high,and the main clay minerals are illite smectite mixed layer.In the soil samples,the cations are mainly sodium,followed by calcium,and the anions are mainly chloride and bicarbonate.According to the annual moisture content and salt content changes of sampling points,samples with different salt types,moisture content and salt content are made.The total suction,matrix suction and osmotic suction are tested by filter paper method to analyze the suction characteristics of soil when water and salt change.The results show that with the increase of moisture content,the total suction of the soil sample gradually decreases,the matrix suction decreases,and the osmotic suction increases first and then decreases.With the increase of salt content,the total suction and osmotic suction of soil samples increased,but the matrix suction changed little.With the increase of salt ion concentration,the osmotic absorption first increases and then decreases.Equal suction moisture content is defined.When the moisture content is greater than this equal suction moisture content,the osmotic suction power will be greater than the matrix suction power.As the salt content increases,the value of equal suction moisture content decreases.The unconfined compression test was carried out on the samples with different moisture content and salt content,and the relationship between strength,matrix suction and penetration suction was analyzed.The results show that the shear strength is the highest when the salt content of NaCl,KCl and CaCl₂ is 1.5%.With the increase of moisture content,the shear strength of the sample increases first and then decreases.With the increase of moisture content,the failure mode of soil gradually changes from brittle failure to plastic failure,and the concrete failure mode of different ion samples is different.The strength of CaCl₂ soil sample is stronger under the same salt content.The water and salt content have influence on the strength,so the matrix suction and penetration suction determine the strength characteristics of the sample together,and two kinds of suction should be considered together when analyzing the strength.It can be found that the contribution of matrix suction and permeability suction to strength is different with the change of moisture content.