Study On Influencing Factors Of Unsaturated Soil Slope Stability

Most slopes in nature are unsaturated soil（above the groundwater level）,and many high and steep loess slopes can maintain long-term stability under natural conditions.However,using classical soil mechanics to analyze the stability of these slopes,the calculation results show that the safety factor of the slope may be less than 1.In other words,the slope stability analysis method based on classical soil mechanics is not necessarily suitable for evaluating the stability of natural loess slope.In practical engineering,the conservative and safe method is usually used to design the slope,and the effect of soil suction on the strengthening of slope stability is rarely considered,which leads to too conservative in the evaluation of slope safety and stability.In addition,under the condition of rainfall,the soil suction of the slope is reduced due to rainwater infiltration,which leads to the reduction of shear strength,and finally induces shallow landslide disaster.Such problems involve the mechanical and hydraulic characteristics of unsaturated soil.Therefore,the reasonable selection of soil-water characteristic curve and permeability coefficient equation of unsaturated soil in specific areas is of great significance to evaluate the stability of unsaturated soil slope.Taking the unsaturated soil slope in a specific area as the research object,this paper systematically studies the key factors affecting the stability of unsaturated soil slope under natural and rainfall conditions by means of literature investigation,theoretical analysis,data collection,probability statistical analysis and numerical analysis.The main research contents and conclusions of this paper are as follows:（1）Carry out literature research,select 28 groups of experimental data of soil-water characteristic curve of loess from the existing literature,fit the experimental data with Fredlund and Xing^[1]equations,and obtain the fitting parameters of soil-water characteristic curve equation.Based on t statistical theory,28 groups of experimental data are statistically analyzed,and the confidence interval with 95%confidence level is calculated to obtain the upper limit soil-water characteristic curve and lower limit soil-water characteristic curve of loess.Using the equation proposed by Zhai and Rahardjo^[2],the permeability coefficient equations corresponding to the fitting soil-water characteristic curve,upper limit soil-water characteristic curve and lower limit soil-water characteristic curve are obtained respectively.（2）Taking typical unsaturated sand,silt and clay as the research objects,four working conditions are designed,different slope heights and angles are selected,and the slope stability under each working condition is analyzed by using the commercial software SLOPE/W.Considering the effect of soil suction on the shear strength and unit weight of unsaturated soil respectively,the influence of unit weight of unsaturated soil on the results of slope stability analysis（including the safety factor of slope stability and the position of the most dangerous slip surface）is discussed.It is found that the difference between the safety factor of slope stability obtained by considering the actual unit weight of unsaturated soil and the safety factor of slope stability obtained by using constant unit weight is much greater than the difference between two-dimensional calculation and three-dimensional calculation.（3）Taking loess as the research object,on the one hand,with the help of NASA POWER project,taking the 22-day actual hourly rainfall in a specific area as the model rainfall input parameters,five series of parameter analysis are designed,and the commercial software SEEP/W and SLOPE/W are used for slope seepage analysis and slope stability analysis respectively.The dynamic evolution law of slope safety factor relative to actual rainfall intensity is systematically studied,and the influence law of unsaturated soil permeability coefficient,slope height and slope angle on slope stability safety factor is clarified.On the other hand,four series of parameter analysis are designed,including soil with different unsaturated soil characteristics,rainfall intensity,initial groundwater table,slope height and slope angle,to study the influencing factors of unsaturated soil slope stability under constant rainfall.It is pointed out that under the conditions of actual rainfall and constant rainfall,for different types of soil,the larger the saturated permeability coefficient,the smaller the air-entry value,the greater the impact on the safety factor,and the overall safety factor of slope stability decreases with the increase of slope height and rainfall intensity.For different rainfall intensities,the saturated permeability coefficient has a great influence on the safety factor,and the minimum safety factor of the three types of unsaturated soil slopes changes exponentially with the rainfall intensity,which confirms the existence of rainfall threshold.