Study On Residual Strength Characteristics Of Loess In South Jingyang Platform

The geological conditions of China's Loess Plateau are fragile and the environment is harsh,resulting in frequent occurrence of loess geological disasters.Loess landslides are one of the most serious geological disasters in the loess area.Since the water diversion irrigation in 1976,loess landslide in south Jingyang,Shaanxi Province has occurred frequently,and it has become a typical high incidence area of loess landslide in Shaanxi.Due to the special structure and water sensitivity of loess,the strength of the loess is greatly reduced after encountering water.The damage of soil is usually shear failure,so shear strength becomes one of the most important mechanical parameters of the soil.In this paper,the Q2 loess in south Jingyang,Shaanxi Province was taken as research samples.The remolded loess was tested by high-precision ring shear instrument to analyze the change of stress on the shear zone.On the basis of experiments,quantitative analysis of soil particle breakage,orientation alignment,pore water migration and local deformation on the shear zone was carried out by SEM and particle test.The residual strength of remolded loess was discussed under different normal directions,pre-consolidation pressure,shear rate and water content and its influence on the microscopic characteristics of the shear zone.At the same time,the shearing process in the ring shear test was simulated by numerical method.The microscopic parameters of remolded loess under different water content were calibrated,and its influence on loess residual strength is further tested under different normal stress and shear rate.The main conclusions of this paper include the following aspects:(1)The stable residual strength reached by the samples at different shear rates is basically the same,the strength varies from 1% to 5%.When the water content is lower,the shear stress changes of the samples at different shear rates are basically the same.When the water content is higher,the peak strength of the samples is quite different at different shear rates.Peak strength increases with the increase of shear rate.Strain softening occurs when shear rate is higher,and when shear rate is lower,the shear stress directly reaches the stable residual strength from peak strength.(2)Under different consolidation pressures,the peak strength of the soil increases with the increase of the pre-consolidation pressure,and the strain softening characteristics are also more obvious,but the residual strength is almost unaffected.The final residual strength value is only related to the stress state at the time.(3)Under different normal stresses,the stable residual strength reached by the sample and normal stress are approximately linear.During the shearing process,the soil particles on the shear zone will be slightly broken.Microscopic parameters are mainly changed as: before and after the shearing action,the particle area ratio and the degree of orientation and abundance will increase.At the same time,as the normal stress increases,the particle area ratio also increases.As for the degree of orientation of the particles,it will increase with the increase of normal stress under lower normal stress and is substantially the same under higher normal stress.(4)Under the same normal stress,the residual strength of the sample decreases with increasing water content,but the degree of reduction under different normal stress is slightly different.As the water content increases,the residual cohesion of the sample reaching the residual strength first increases and then decreases.The residual internal friction angle decreases with increase of water content,but the variation range is much smaller than that of residual cohesion.(5)When the ring shear test was simulated by the particle flow program PFC,the residual strength under different normal stresses was basically consistent with lab test results.