Analysis On Deformation And Failure Mode And Stability Of Right Abutment Slope At The Upper Dam Site Of The Banda Hydropower Station

With the development of hydropower energy in China to the west of high mountain canyon,the stability of high rock slope under complex geological conditions is brought.The proposed Banda Hydropower Station is located in the upper reaches of the Lancang River in Tibet,and the dam body will adopt concrete gravity dam,which is up to 210 meters high.The Banda Hydropower Station is located in the southeast of the QinghaiTibet Plateau,and the slope stress on both sides of the valley is high,and the unloading of rock mass is strong and complex.The main lithology of the dam site on banda hydropower station is dacite.On the slope surface of the right bank of the dam abutment,several cataclastic loose rocks of different sizes and thickness are developed.In addition,several ductile shear zones are developed on the right bank,and the rocks in the shear zones are mylonized dacite,which makes the geological conditions of the dam site more complicated.During the construction of Banda Hydropower Station,the excavation scale of the engineering slope is large and the area is wide,and the maximum excavation height of the slope can reach more than 400 meters.Therefore,it is of great practical significance to study the excavation stability of rock high slope in Banda Hydropower Station under such complex conditions.This paper takes the right bank abutment slope at the upper dam site of banda hydropower station as the research object,analyzes the deformation failure mode and stability of the slope excavation through the combination of qualitative analysis and quantitative analysis,and obtains the following main research results:(1)On the basis of the existing geological survey data,the rock structure characteristics of the slope are found out by investigating the surface and adit of the slope in the study area.According to structural plane size classification standard,the structural plane in the study area is divided into Ⅲ ～ Ⅴ level,and the dominant occurrence of the structural plane at all levels is counted.Using the window method to calculate the connectivity of the structure plane is 35.8%,which is basically consistent with the estimation of site survey.(2)The macroscopic and microscopic characteristics of the ductile shear zone in the right bank slope of the upper dam site are summarized by the available data and field survey.There are mainly two ductile shear zones developed in the slope of the study area,and their numbers respectively are(1)-1 and(1)-2.And both shear bands are inclined into the slope.It is concluded that the ductile shear zone is right walking slip type by microscope.(3)The weathering unloading zone of the slope in the study area is divided by using quantitative indexes such as seismic wave velocity and structural plane opening width,and combined with field geological investigation.By using BQ method to evaluate the rock mass quality of dam abutment slope,the rock mass quality is divided into four categories: grade II,III,IV and V.(4)According to indoor physical and mechanical tests and field tests,and referring to similar hydropower engineering and water conservancy and hydropower engineering geological survey code,the suggested values of mechanical parameters of slope rock mass and structural plane in the study area are put forward synthetically.(5)Through qualitative analysis,it is concluded that there may be three kinds of potential deformation and failure modes before excavation of the dam abutment slope: block toppling deformation,shear slip-tensile crack and stepped slide.And by using the two-dimensional discrete element program to analyze the deformation and failure of slope before and after excavation,the results show that the slope is most likely to have two types of deformation and failure modes after excavation: shear slip-tensile crack and stepped slide.And the two types of deformation and failure modes are most likely to occur in the exposed part of the ductile shear zone(1)-1.After the slope is excavated,the displacement produced is mainly concentrated above the slope height of 3000 meters.(6)Based on the finite difference method,the stability of slope before and after excavation is systematically analyzed,and the stability of slope excavation is quantitatively evaluated by the rigid body limit equilibrium method.Through the analysis of stress field,it is found that stress relaxation occurs in ductile shear zone,while stress concentration occurs in adjacent normal rock mass,which has a great influence on slope stability.The greatest influence on the stability of the slope is the first four-stage excavation,especially the exposed part of the ductile shear zone(1)-1,where the slope is easy to cut out the damage and failured.Through the analysis of rigid body limit equilibrium method,it is concluded that the fractured loose rock mass on the slope surface and the rock mass at the exposed part of the ductile shear zone(1)-1 have poor stability under heavy rain conditions,and there is the possibility of local or even global instability failure.