Study On Slope Stability And Strength Parameters Of Malan Loess Under Freezing And Thawing Cycle In Lvliang Area

As part of the Loess Plateau,Shanxi Province's Lvliang area is a mountainous region with gullies and crisscrosses.More and more people pay attention to the related surface disasters caused by the special engineering properties of loess.Landslide is one of the typical geological disasters.In recent years,under the background of global warming trend,seasonal freeze-thaw slopes that freeze in winter and melt in spring will quickly melt during the ablation period due to the rapid rise of temperature in spring.Which will result in the rapid accumulation of water in the slope in a short period of time.At the same time,repeated freezing and thawing cycles make the soil particle arrangement and stress state in the slope also adjusted,thereby changing the original state of the slope body and affecting its stability.Therefore,studying the change law of the shear strength parameters of the loess and the stability of the slope under the repeated freeze-thaw action can fundamentally realize the influence mechanism of freeze-thaw cycle on the slope.And then provide guidance for the disaster prevention and mitigation of this type of loess landslide.In this paper,taking the undisturbed Malan Loess in Lvliang area as the research subject,by means of data collection,laboratory test and numerical simulation,the effects of freeze-thaw cycles on the shear strength parameters and slope stability of the undisturbed Malan loess were explored.The specific work of this paper is as follows:(1)Through statistical investigation and the determination of basic parameters,the depth of frozen soil in Liulin County is 1.0 m,the extreme minimum temperature is-19?,and the natural and plastic limit moisture content of soil is 12.2%and 18.2%,respectively.By comparing the water content of natural and plastic limit samples before and after five groups of freeze-thaw cycle tests,it was found that in a closed system,the water content of soil samples will slightly decrease after freeze-thaw cycle,and the higher the initial water content is,the more the water content reduction.The reduction of water content will increase logarithmically with the increase of freeze-thaw cycle times.The effect of freeze-thaw cycles in a closed system has little effect on the moisture content of the soil sample.When analyzing the relationship between the shear strength of soil and the freeze-thaw cycles,the effect of changes in water content can be ignored.(2)On the basis of 0/1/5/8/10/15 freeze-thaw cycles tests on the original Q₃Malan loess samples with two characteristics of natural and plastic limit moisture content,the unconsolidated undrained(UU)triaxial shear tests of each group of soil samples under four confining pressures were carried out,and 12groups of stress-strain characteristic curves were obtained.It was found that the deviator stress will increase with the increase of confining pressure and axial deformation.At the beginning of the deformation,the deviator stress increases sharply and shows elastic deformation.As the axial deformation increases,it will gradually change to plastic deformation,and as the confining pressure increases,the soil sample with natural water content will change from weak softening to weak hardening and finally show typical strain hardening.While the plastic moisture content soil sample shows weak softening throughout the whole process.The effect of moisture content on the increase of deviator stress is greater than that of freeze-thaw cycles.In addition,the failure stress variation rules of each group of soil samples were obtained.It was found that the freeze-thaw cycle will make the soil yield more easily when it is sheared.When the confining pressure and water content are fixed,the failure stress of the soil decreases with the superposition of the freeze-thaw cycle.(3)On the basis of triaxial shear test,the stress Mohr circle of each group of soil samples were drawn according to the Mohr-Coulomb basic theory,and the common tangent line and its envelope line expression of each group of stress Mohr circle were fitted,and then the cohesion and internal friction angle values of each group of tests were obtained,as well as the change rule of two parameters with the freeze-thaw cycle.It is found that the cohesion and internal friction angle of soil samples will deteriorate after different cycles of freeze-thaw,but the degree of cohesion degradation is greater.It decreases exponentially with the number of cycles of freeze-thaw,the internal friction angle slightly decreases but the amplitude is very small.And the shear strength parameters of soil samples with plastic limit water content are generally lower than those with natural water content.(4)With reference to the shear strength parameter values obtained from the experiment,taking a loess slope in Liulin County as an example,the slope after each freeze-thaw cycle was simulated by FLAC^3D.The analysis shows that the shear strain extends upward from the toe of the slope in the form of circular arc,and the horizontal displacement also extends gradually from the toe of the slope,there will be a prototype of the potential slip surface.With the superposition of freeze-thaw cycle,although the slope has been in a basically stable state,if the freezing and thawing effect continues and the rain,cracks and other factors are superimposed,the slope may be unstable.