Study Of Slip Zone Evolution Characteristics Of Loess Landslides Based On Ring Shear Test

One third of the geological disasters occurs in loess area in our country.The loess landslide is the most serious type of geological disasters in loess area,which has characteristics of wide distribution and large harm.The evolutionary characteristics of slip zone are a key part of the study of landslides,and the deformation and damage mechanism of the slip zone reflects the mechanism of landslide formation.Understanding the formation mechanism of landslide is beneficial to the prediction and prevention of landslide disasters.Therefore,it is important to carry out research on the evolutionary characteristics of the slip zone of loess landslides and to reveal the changes in the mechanical properties,material composition and structure of the slip zone for landslide prediction and forecasting.Research results have shown that the material basis,the dynamic conditions and the water environment are the main influencing factors in the formation of the slip zone.In this paper,ring shear tests of loess are carried out from grain size effect,rate effect and displacement effect to analyze the shear strength characteristics of loess slip zone under different conditions.The fine particle size and microstructure of the sheared specimens were analysed by laser particle size measurement and SEM to investigate the changes in the microstructure of the shear zone and the shear strength deterioration mechanism under different shear conditions.The following conclusions have been obtained:(1)The increase in clay content,shear displacement and normal stress all favour shear zone formation,resulting in an increase in shear zone density and thickness.Compared to the initial particle size,the particles in the shear zone have a smaller particle size distribution,the particle size of the upper and lower layers of the specimen does not change much,and the shear zone particle breakage rate is greater than that of the upper and lower layers of the specimen.High normal stress increases the degree of particle crushing and breakage,resulting in an increased rate of particle breakage in the shear zone.The upper and lower layers of the specimen are angular and loosely arranged,with more medium and large pores than shear zones,and the opposite variation in small pores.Meanwhile,the degree of particle orientation and particle roundness in the shear zone is stronger than in the upper and lower layers of the specimen.(2)Under different grain sizes,the shear strength of the loess shear zone increases with increasing clay content,and the shear strength is positively correlated with the normal stress and negatively correlated with the water content.With the increase of clay content,the particle breakage rate in the shear zone is decreased,and the degree of orientation and roundness of particles are increased.Small pores increase and medium and large pores decrease in the shear zone of specimens with high clay content,the boundary of the shear zone skeleton particles is blurred under high normal stress conditions,and the particles are mainly combined as a whole.In the specimen with low clay content,the skeleton particles in shear zone are dominated by mosaic contact.(3)The shear strength of the loess shear zone under different shear rate conditions exhibits a negative rate effect.With the increase of shear rate,the shear strength decreases logarithmically,and the rate effect leads to friction weakening.Shear zone particle breakage rate,orientation and particle roundness increase with shear rate,and the overall trend of fine particle content increases.Meanwhile,the shear rate increases so that the shear zone particles are less compact,the number of small and medium pores increases,and the fine particles gradually transition from a coarse particle coat to individual well-defined fine particles,resulting in weaker occlusal friction between particles and lower shear strength of the specimen.(4)Under different shear displacement conditions,the shear strength of the loess shear zone increases with increasing shear displacement,showing the phenomenon of shear hardening.The increase in shear displacement facilitates better orientation and full fragmentation of the particles.After continuous shearing,the medium and large pores of the shear zone decrease,the small pores increase,the roundness of the particles in the shear zone increases,and the fine particles gradually change from being scattered in the coarse particles and pores to being attached to the surface of the coarse particles in a cellular coat,which enhances the denseness of the shear zone and leads to an increase in the shear strength of the specimen.