Evaluation Of Unsaturated Soil Shear Strength And Slope Stability Of Different Soil Types In Geohazard Prone Regions

The Bailong River basin is one of the four major geological hazard areas in China,with widespread landslides and serious hazards.There is an urgent need to conduct research on the mechanism of landslide formation and prevention in the area.Soil type and slope stability are less discussed in most previous studies,but the properties of different soils have an influence on the stress and shear strength of slopes.In this study,four soil types（Dark-brown earths,Brown earths,Cinnamon soils and Yellow-brown earths）in the Bailong River basin were selected to test the differences affection on the slope stability.The basic physical and chemical properties of the samples were determined by field investigations and laboratory analysis.In addition,direct shear tests,soil moisture characteristic curves and finite element numerical simulations were used to comprehensively evaluate the slope stability of different soil types.The main results are as follows:（1）There are significant differences in bulk density,water stable agglomerates and elemental content among the four soil types.The Dark-brown earths have the lowest bulk density and the highest porosity.The proportion of the particle composition of four soil types is sand>silt>clay,with the Dark-brown earths having a minimum clay content of 2.90%.The Brown earths and Yellow-brown earths have a significantly higher content of large aggregates than the Dark-brown earths and Cinnamon soils.（2）The shear strength of unsaturated soils of four types of soil increased with the increasing of matrix suction,Among them the Cinnamon soils having the highest unsaturated shear strength.The SWCC fitted by the VG model shows that the Dark-brown earths could absorb more water at the state of saturation than the other three soil types,but its water holding capacity is poor.The contribution of matrix suction to the shear strength ranged from 0.81-15.05 k Pa as it increased from 10 k Pa to 70 k Pa,with a maximum contribution of 7.1%to the shear strength of the Dark-brown earths.Linear regression shows that matrix suction mainly affects cohesion and has a relatively small effect on the internal friction angle.（3）Soil bulk density,particle composition and clay minerals are the main factors influencing the shear strength of unsaturated soils.Correlation analysis showed that soil cohesion has positively correlated with bulk density and silt content,while it was negatively correlated with sand content,and clay minerals also showed a significant positive correlation with cohesion（R²=0.79,P<0.05）.Principal component analysis（PCA）showed that bulk density,silt and Si O₂ content were the main factors influencing cohesion,and the internal friction angle was mainly related to sand content.（4）The COMSOL numerical simulation shows that the position of the critical damage surface and the FS of the soil slopes are both influenced by the soil type.Slope damage starts at the foot of the slope and the slope stability decreases with the matrix suction decreasing.When the slope reaches the critical damage state,the maximum depth of the potential sliding surface is about 2-5 m for Dark-brown earths,4-7 m for Brown earths and Cinnamon soils,and the potential sliding surface is deeply buried for Yellow-brown earths.At the same matrix suction（10 k Pa）,Dark-brown earths are more prone to occur shallow landslides,Brown earths and Cinnamon soils are more prone to occur moderate landslides,and Yellow-brown earths slopes are relatively the most stable（FS=2.76）.The study evaluate the unsaturated-shear strength and the soil-water coupling processes of different soil types in geo-hazard-prone areas,which can improve the understanding of the soil-hydrological effects on slopes.