Experimental Study On Freezing Temperature And Deformation Of Sodium Sulfate Saline Soil Under Unidirectional Freezing Condition

Ningxia is located in the northwest inland arid-semi-arid area.The climate in the region is dry and evaporation is strong.As a result,unique inland cold and arid areas of saline soil are formed,and most of them are sulfurous acid and sulfate saline soil.The physical and mechanical properties of saline soil are quite different from other soil types,and their properties are more complex.With the changes in seasonal temperature and day-night temperature difference in the area,the soil has repeated salt swelling and thawing,resulting in damage to the soil structure,And then cause engineering hazards such as building cracking,slope instability,pavement erosion and cracking,so it is necessary to study the freezing temperature,salt frost heave and thawing characteristics of sulfate saline soil.In this paper,samples of typical sulfated saline soil in Hongsipu area were taken,and sodium sulfate saline soil was artificially prepared,and the soil column salt frost heave and thaw settlement test under unidirectional freezing was carried out.And theoretically analyzed the influence of water content and salt content on freezing temperature,salt frost heave and thaw sedimentation.The research results show that:(1)Under unidirectional freezing conditions,the freezing temperature of the same soil at different locations is the same,but the time required for the soil to reach the freezing temperature at different locations is slightly different;The initial water content has a significant effect on the freezing temperature of sulfated saline soil.When the salt content is low(0%-3%),the freezing temperature changes with the increase of the initial water content in a "first steep and then gentle" change trend,and It reaches the maximum when it is close to the optimal moisture content;When the salt content is high(5%),the freezing temperature increases approximately linearly with the increase of the initial water content.(2)Generally speaking,the freezing temperature of sodium sulfate saline soil decreases with the increase of the salt content.Affected by the initial water content,the freezing temperature drop of the soil in different salt content ranges slightly differs.The decrease in freezing temperature when the salt content changes in the range of 0%～1%and 3%～5%is greater than the decrease in the freezing temperature when the salt content changes in the range of 1%～2%and 2%～3%Amplitude.When the salt content is 1%～2%,the freezing temperature of soil with different water content and the time required to reach the freezing temperature show a "normalized" change trend with the salt content.(3)It is advisable to regard the sodium sulfate solution as a non-ideal state in the soil,and the freezing temperature calculated according to the non-ideal dilute solution is closer to the test value.Under the same initial water content,the effect of fitting the salt content of sodium sulfate with the freezing temperature according to the cubic polynomial is very good,and it has certain reference value for predicting the change of freezing temperature.(4)Under the conditions of the same salt content and different initial water content,the salt frost heave rate increases with the decrease of temperature.The temperature range at which the salt frost heave rate significantly increases is related to the initial salt content.The greater the salt content,the higher the initial temperature at which the salt frost heave rate of the soil changes significantly,and the longer the time for the sample to undergo salt heave deformation Early,the salt frost heave lasted longer.(5)Under different salt content conditions,with the gradual increase of water content,the salt frost heave rate of sodium sulfate saline soil first increases and then decreases,and reaches a maximum near the optimal water content.And the lower the salt content,the higher the water content to the maximum,and the higher the salt content,the closer to the optimal water content.(6)When the water content is low(less than the optimal water content),the maximum thawing sedimentation of the test soil samples increases first,then decreases and then increases with the increase of the salt content,and the 2%salt content is the peak point.The trough is at 3%,and the maximum is at 5%.When the water content is high(greater than the optimal water content),the maximum thawing sedimentation of the test soil samples continues to increase with the increase of the salt content,and the increase will slow down after 2%.(7)At the same water content,the difference between the maximum salt frost heave and the maximum thawed sedimentation gradually increases with the increase of the salt content,and the low water content(less than the optimal water content)and the higher water content(more than the optimal water content)is more significant.Under the conditions of high-water content and low salt content,the maximum thawing settlement of the soil is slightly larger than the maximum salt frost heave.The research results can not only provide theoretical support for the engineering design and construction of the same type of saline soil area,but also play a positive role in the improvement of the saline soil foundation theory.