Study On The Mechanism Of Bank Slope Instability Considering River Scouring

Mountain waterways are often threatened by geological hazards.River bank slope is an important part of waterway engineering system in China.The subcutaneous layer of bank slope is mostly Quaternary loose deposits,and its composition and density are heterogeneous.Running water causes bank slope erosions under the action of seasonality,which poses a serious threat to waterway safety.Taking the soil bank slope in the Three Gorges Reservoir area as the research object,this paper studies the variation rules of soil strength parameters under different water content and periodic immersion conditions by soil strength parameter test;it divides the process of bank slope erosion and failure in the Three Gorges Reservoir area by stages through field investigations;it analyses the typical case of Hongyanzi landslide using the limit equilibrium method,and studies its regularity of stability change under periodic immersion and continuous scouring of reservoir water on bank slope during the operation period.The evolution process of landslide formation under water erosion and different water levels is reproduced by FLAC3 D finite difference program,which reveals the mechanism of soil bank slope instability during reservoir operation.The major research contents and main achievements are as follows:(1)Taking Hongyanzi landslide soil in the Three Gorges Reservoir area as the research object,the present paper conducts direct shear tests on landslide soil with different water content and periodic immersion by using ZJ strain-controlled direct shear apparatus,and illuminates the variation characteristics of soil strength parameters under various water content and immersion times.According to the test results,the exponential function selected by least square method is fitted by the exponential expression contained the cohesion,internal friction angle and immersion times.At the same time,the correlation coefficients of fitting results with different parameters are given,which can provide a basis for future research to determine reliable shear strength index of soil.(2)Based on field investigations,the bank slope erosion damage in the Three Gorges Reservoir area are classified into four stages: river cutting stage,erosion stage,local erosion and landslide formation stage,according to the characteristics of bank slope deformation and failure and the operation mode of the Three Gorges Reservoir.(3)It would produce both transverse and longitudinal deformations for bank slope under the action of current scouring.Drawing on the calculation principle of scouring width,and depth and limit equilibrium of bank slope,this paper calculates the typical section of Hongyanzi landslide along Daning River bank in Wushan section of Three Gorges Reservoir.By establishing the limit equilibrium calculation model of bank slope stability with consideration of River Scouring,it calculates the overall and local stability of bank slope under River Scouring and periodic immersion,and obtains the scouring amount of bank slope within periodic variation at the water level of 175 m and 145 m in the Three Gorges Reservoir,and the variation law of bank slope stability coefficient during the process of scouring.The results show that the River Scouring and water immersion have great influence on the stability of bank slope,especially on the local stability of the foot of slope,which is consistent with the field investigations that there are obvious cracks in the front foot and the rear edge of the bank slope.(4)On the basis of the scouring model of bank slope,the numerical simulation software FLAC3 D is used to establish a numerical calculation model with certain thickness according to the typical geological profile of Hongyanzi landslide based on the variation law of physical and mechanical parameters of slope material provided by the experiment,and the scouring process is simulated.It get the variation rules of stress field,displacement field and plastic zone of bank slope during the process of River Scouring,which reveals the change law of stress field,displacement field and plastic zone of bank slope.Then,according to the actual geological conditions of Hongyanzi landslide,a solid three-dimensional numerical calculation model is established to simulate the impact of the change of water level before and after erosion on the bank slope,and the landslide scope and deformation characteristics for the large deformation areas in the simulation results are compared with that of the field investigation,which make basically coincidences.Comparing the simulation results before and after scouring,the displacement and deformation scale of the bank slope after scouring are relatively large,and the large deformation areas are concentrated in the middle and lower part of the bank slope.The same results are obtained by comparing the changes of water level,which verifies the proposed mechanism for bank slope instability.