Research On Shear Strength And Microstructure Of Loess Under Freeze-thaw Cycle

Due to its special structure and water sensitivity,loess can maintain high strength for a long time when it is in natural moisture content and has not been affected by freeze-thaw cycles.As the moisture content increases or undergoes freeze-thaw cycles,it can maintain high strength for a long time.Its strength will be greatly reduced.Under the influence of periodic fluctuations in atmospheric temperature and moisture changes,the upper soil in the seasonally frozen soil area undergoes periodic freezing and thawing.In this process,the physical and mechanical properties of the soil are constantly changing,and these changes are directly Affect the stability of loess slopes.Moreover,for the current experimental research on the properties of loess due to freezing and thawing effects,it is relatively simple to focus only on the temperature path of extreme freezing and thawing temperature and moisture content,because the temperature is in a dynamic change during the actual freezing and thawing cycle.Within the period.Therefore,this paper studies the variation law of loess strength with temperature under the actual temperature path freeze-thaw cycle,as well as the law of loess shear strength parameter changes and microstructure characteristics under repeated freeze-thaw cycles,it provides a reference for disaster prevention and mitigation of this type of loess landslide.This paper takes the undisturbed Malan loess in the Lvliang area as the research object.Through the collection of data and laboratory experiments,the influence of different freeze-thaw cycles on the physical and mechanical properties of the loess under the freeze-thaw cycle with periodic temperature changes is studied.The work is as follows:(1)The land surface temperature data from 1960 to 2019 in the Lvliang area was collected for the past 50 years,and with five years as a time stage,12 annual minimum and maximum temperature control points were obtained.According to the characteristics of the temperature cycle,0,1,3,7,12 freeze-thaw cycles are performed.After different freezing and thawing times,the surface of the undisturbed soil deteriorated severely.The debris on the surface of the soil was more and more serious.After the freezing and thawing effect was superimposed to a certain extent,microcracks and cracks appeared on the surface of the soil.(2)The influence of temperature changes on soil strength is studied,a cycle is divided into different stages,and the changes in soil strength of soil samples with different moisture contents in each stage are analyzed.The results show that the strength of the soil has a negative correlation with the set temperature.When the temperature rises from 18℃ to the highest temperature,the strength of the soil decreases,and the high temperature has little effect on the strength of the soil.When the temperature drops from the highest temperature to the lowest temperature,the strength continues to increase.When all the water in the soil is formed The strength of the soil is greatest when it is iced.When the temperature rises from the lowest temperature to 18℃,the strength decreases accordingly.The test results show that when the temperature is above 0°C,the temperature has a small effect on the strength of the soil,and when it is below 0°C,the effect is greater.(3)According to the results of the direct shear test,when the soil samples with different moisture content undergo different cycles of freeze-thaw cycles,the soil samples with different moisture content show the largest decrease in the strength of the first freeze-thaw.It’s getting weaker.The cohesion of the soil decreases exponentially with the superimposition of freezing and thawing,and it is basically stable after 7 freezes and thawings.The cohesive force of loess with high moisture content is low,and the decline in the whole freezing and thawing process is far less than that of the soil samples with low moisture content.The intensity changes greatly.The internal friction angle of the soil has little change.(4)According to the test results of scanning electron microscopy,the freeze-thaw cycle makes the soil structure become loose,the particles are severely broken,the internal pores of the soil are developed,the particles are deflected,and the contact mode between the particles changes from surface-to-surface contact to point-to-surface contact.Surface-to-surface contact is the main method,and the arrangement of grains has changed from overhead pore formation and overhead-inlaid pore formation to overhead pore formation as the main method.Quantitative analysis of pore characteristics shows that the number of medium pores in the soil increases,and the soil The number of micropores in the body decreases,and the freezing and thawing effect makes the pores more and more irregular,that is,the circularity increases.The law of average fractal dimension of soil pores is not obvious.