Study On Freeze-thaw Cycle Effect And Micro-mechanism Of Nanyang Lime-improved Expansive Soil Strength Characteristics

Expansive soil contains a large amount of montmorillonite and illite,which has strong swell and shrinkage properties,cracks and other characteristics,and has a bad influence on project quality.Nanyang Basin is one of the main distribution areas of expansive soils in China.It belongs to the seasonally frozen area,and the roads in the frozen areas are affected by freezing damage.It is a common method to improve expansive soil by adding lime,but the mechanical properties of expansive soil have obvious regional characteristics.Therefore,the study of the strength characteristics of lime modified Nanyang expansive soil and the evolutionary mechanism of its microstructure under the action of freeze-thaw cycles is of great significance to thedesign oftheexpansivesoilsubgradein Nanyangareaand theprevention ofdisasters.This article focuses on expansive soil in Dengzhou area of Nanyang and analyzed the change law of the shear strength of expansive soil through five kinds of ash mixing ratios(0%,4%,7%,10%,12%)with different curing ages(7d and 28d)under 0,1,3,6 and 9 freeze-thaw cycles.The relationship between the freeze-thaw cycle,the lime ratio and the failure mode was analyzed;The microstructure of some samples was analyzed by electron microscopy.The structural parameters such as abundance,particle size,and orientation angle were quantitatively analyzed using a combination of Photoshop and Image-Pro Plus,and the relationship between thetwowas discussed.Themain resultsareas follows:(1)Experimental studies on the physical properties of five kinds of lime modified expansive soils show that the swelling potential of expansive soils after the incorporation of 4%lime has been well suppressed;With the increase of lime content,the optimal moisture content of lime-modified expansive soil increases,the maximum dry density decreases,and the liquid limit,plasticityindex,and freeexpansion ratealso showa decreasingtrend.(2)Through the direct shear test,it was found that the shear strength of lime-modifiedexpansive soil with different cycles of freeze-thaw cycles increased with the increase of the ash-mixing ratio,but did not increase linearly.The maximum intensity increase phase is when the lime ratio increases from 7% to 10%.When the lime ratio increases from 10% to 12%,there is no significant increase in strength,.From the point of view of strength increase,it is suggested that10%limeshould beadded.(3)Under the influence of freezing and thawing,the shear strength of five kinds of improved expansive soils with different lime content showed a downward trend.The strength of the first three freeze-thaw cycles decreased greatly and gradually stabilized after 6 freeze-thaw cycles.Therefore,the strength index of soil after 6 cycles of freeze-thaw cycle should be considered as thedesign referencevalue.(4)Based on the scanning electron microscopy image analysis,it was found that agglomerate particles appeared in the expansive soil after lime incorporation,and the number of agglomerate particles increased with the increase of the lime content.Freeze-thaw results in the disintegration of agglomerate particles.After 6 freeze-thaw cycles,the agglomerate particles changeslowlyand thesoilstructuretends to bestable.(5)The particle size,abundance and orientation frequency of lime modified expansive soil agglomerates under freeze-thaw cycles were quantitatively analyzed by IPP software.It was found that the content of large aggregates decreased after freeze-thaw cycles.The fraction of small agglomerates increased first and then decreased,but the abundance and orientation frequencydid notchangeobviously.