Study On Performance Deterioration Of Sulphate Saline Soil Under Long-term Freeze-thaw And Dry-wet Cycling In Ningxia Area

As a special kind of soil,saline soil is widely distributed throughout the world.There are a lot of sulphate saline soil in Ningxia area,due to its engineering characteristics of salt-frost heave and corrosion,with the special climate in this area,according to meteorological data,from mid-February to the end of March in Ningxia,the average temperature at night is about-10°,and the maximum temperature during the day can reach about 10°.During these 50 days,the sulphate saline soil has been experienced the freezing-thawing process repeatedly.In addition,the rainfall in Ningxia area is mainly concentrated in July and August each year,accounting for about 60%of the annual rainfall.The average temperature of the same period is also the highest of the year,up to 24.5 degrees.During this period,the soil has been undergone wetting-drying alternate process.The engineering properties of sulphate saline soils has been deteriorated in the complex environment,it has a serious impact on the construction of infrastructure and engineering in Ningxia area,at the same time it has restricted the local economic development.With the indepth implementation of the "Belt and Road" strategy and the rapid development of urbanization in Ningxia,it is crucial to carry out the research on the deterioration performance of sulphate saline soil under the conditions of long-term freezing-thawing,wetting-drying,freezing-thawing and wetting-drying interaction cycles.This thesis took the sulphate saline soil that was produced in Hongsibu area of Wuzhong City in Ningxia area as the research object.Taken the raw soil to researche basic physical testing,cleaned and washed salty for undisturbed soil.According to the maximum dry density and the optimum moisture content of soil,the different remoulded soil with different salinity were configured for the need of experiments to research the strength index,salt-frost expansion rate and microstructure of sulphate saline soil under different cyclic conditions in combination with the unconsolidated undrained triaxial shear test,salt-frost expansion test and scanning electron microscope(SEM)laboratory tests.The main results are as follows:(1)Under the condition of long-term freezing-thawing,drying-wetting,freezing-thawing and wetting-drying cycles.The stress-strain curves of sulphate saline soil are gradually changed from strain-softening type to strain-hardening type.With the increase of cycle number,salt content and confining pressure,the degree of softening gradually decreased.(2)Under the condition of long-term freezing-thawing cycle,the variation trend of the internal friction angle ? of 0%salinity soil is stabilized within 30°?34° while the soil internal friction angle values of 2%and 5%salinity are showed a decreasing trend,generally speaking,the higher the salt content,the more obvious the internal friction angle decreases in the early cycle,the internal friction angle value are decreased sharply after the 20th and 9th freeze-thaw cycles,respectively.The cohesive force c of 0%salt content increased in 1-9 cycles,decreased sharply in 9?12 times and increased again after 12 times.The cohesive force of 2%and 5%salt content showed a stepwise change in the 1-20 cycles,the overall trend decreased,at 20 times,the c value reached a minimum,freezing-thawing effect thinned the sample particles,thus the cohesion increased at 30th time,the value of 50th time is closed to the initial period of the freezing-thawing.(3)Under the condition of long-term wetting-drying cycle,salt content has a significant effect on the strength index of saline soil samples.The higher the salt content,the faster the internal friction angle decreases.At the initial stage of wetting-drying,cohesion increases with the increase of salinity,and shows a "V" change in the middle and later stages of the wetting-drying cycle.The sulphate saline soil's internal friction angle ? of 0%,2%,5%salinity decreases with the increase of wetting-drying times.The cohesion of 0%and 5%salinity soil are increased gradually,while 2%salinity soil's cohesion is increased first and then decreased and finally increased.(4)Under the condition of long-term freezing-thawing and wetting-drying cycle,the strength index of soil is not a simple superposition of the results of single freezing-thawing and wetting-drying cycles.the sulphate saline soil's internal friction angle ? of 0%salinity is increased with the cycle number,the 2%and 5%salt content are decreased at the time of 1 to 3 cycles,and increased after 3 times.,the soil cp values of 2%and 5%salinity are showed a decreasing first and then increasing trend.The cohesion of 0%salinity soil is decreased first,then increased and then decreased,2%and 5%salinity soil's cohesion are showed an "M" pattern.(5)Under the condition of long-term freezing-thawing,wetting-drying,freezing-thawing and wetting-drying cycles,the salt heaving ratio of sulphate saline soil is increased with the increase of cycle times and salt content.But salt content has a more significant effect on the salt frost heave rate of samples.The effect of freezing-thawing cycles on salt heaving rate of salty soil with higher salinity is greater than that of saline soil.In contrast,freezing-thawing cycles has a more significant effect on salt heave than dry wet cycling.