Research On The Shear Strength Of Nong'an Remolded Saline Soil Subjected To Freeze-thaw Cycles

The west of Jilin province is a typical seasonal frozen soil area,with a total area of 1.5×10~4km~2 of saline soil.In order to promote agricultural development and the economy level,Jilin province launched the‘Land development project in Western Jilin province".A large number of construction projects are increasing,but salt expansion,frost heaving and corrosion go against the project.The problem of large deformation and poor stability appeared in the diversion irrigation engineering.The retaining wall of embankment slope was broken.The strength of saline soil was weakened after freezing and thawing cycles,resulting in insufficient of bearing capacity.Such problems have a direct impact on the stability of engineering.Therefore,it is imperative to study the strength characteristics of the saline soil in the western Jilin under the effect of freezing thawing cycle.With composition and basic properties of the soil in Nongan County,the frozen thawing cycle,unconsolidated undrained triaxial axis shear test,granularmetric analysis,mercury instrusion porosimetry and scanning electron microscope were carried out to analyze the influence of salt content and freeze-thaw cycles on the stress-strain,strength parameters,granularmetric composition,pore characteristics and microstructure of remolded saline soil and explain the mechanism.Based on the Duncan-Chang model,the influence coefficients of strength were defined and the formulas of strength attenuation were fitted.Six main conclusions were as follows.(1)The composition and properties of saline soil in Nong'an was studied.The soil is silty lean clay~silty clay and strong saline-alkali soil.The salt content is highest in 40cm depth,and sodium bicarbonate and sodium sulfate are the most.(2)The shear strength characteristics of the remolded saline soil with carbonate,sulphate and mixed salt were studied.With salt content increasing,the cohesion of carbonate and mixed saline soil decreases.During the 0~60 cycles,the breaking strength and internal friction angle first decreased and then increased,and the inflection point was 2%salt content.At the 120 cycles,the breaking strength and the internal friction angle decreased.The shear strength parameters of sulfate soil decrease with the increase of salt content.The cohesive force and breaking strength decrease rapidly during 0~10 cycles,slowly decrease during 10~60 cycles,and decrease rapidly during 60~120 cycles again.The internal friction angle increased during the 0~30 cycles,decreased slightly during 30~60 cycles,and decreased significantly during 60~120 cycles.The shear strength from high to low is carbonate saline soil,mixed salt saline soil and sulphate saline soil.(3)The granularmetric of the remolded saline soil was analysed.With the increase of salt content,the sand and silt of the carbonate saline soil decreased first and then increased,while the clay increased first and then decreased,and the inflection point was 2%salinity.The sand of sulfate soil decreased,the silt increased first and then decreased,and the clay content remained almost unchanged.The sand of mixed salt soil decreased,the silt and clay content increased.The freeze-thaw cycle made the sand and clay content decrease but the silt content increase.(4)The effect of freeze-thaw cycle on the characteristics of pore distribution was studied.As freeze-thaw cycles increasing,the porosity of soil increased.The percentage of small pores and micropores decreased,the percentage of macropores increased,the content of middle pores increased first and then decreased,and the content of super pores decreased first and then increased.(5)The microstructure features of the saline soil were studied.Freeze-thaw cycles made the structure type of soil transform from compact flocculated structure to loose aggregated structure,made the shape of fabric element unit and the pore tend to be equiaxial,and made the fabric element unit gradually tend to be disordered.With the increase of salt content,the shape of fabric element unit and pore gradually tended to be oblate and elongated,and the average morphological parameters decreased.(6)The mechanism of the shear strength changing with salt and freeze-thaw cycle was explained.The Duncan-Chang model parameters were obtained.The strength attenuation coefficient was defined,and the function fitting was carried out.