Research On Deformation And Failure Mechanism Of Typical Reverse Rock Bank Slope Under Rock Mass Deterioration In Drawdown Zone Of Reservoir Area

Under the influence of reservoir water dispatching of the Three Gorges Project,the reservoir water level fluctuates from 145 to 175m,forming a reservoir bank subsidence zone with a height difference of 30m.During the periodic rise and fall of reservoir water level,the rock mass in the subsidence zone is in a state of alternating wet and dry circulation for a long time,which causes a kind of"deterioration damage"to the rock mass.Under this action,the quality of rock mass and physical and mechanical parameters of rock mass weaken year by year,which is called deterioration effect.Under the deterioration,the stability of the dangerous rock mass is constantly reduced,the joint cracks at the base develop,and the water level fluctuation accelerates the disaster evolution process of the dangerous rock mass.Once the instability failure may cause surge disaster.At present,there are few researches on the disaster evolution process of tower pillar dangerous rock mass.Most research methods adopt indoor analysis or numerical simulation to deduce the evolution process of an engineering case,and few scholars carry out relevant physical model tests to explore its evolution process.Taking Jiandaodong No.3,a tower pillar dangerous rock mass located at Qingshi Pier,Wushan County,Chongqing,as an example,this paper establishes a typical geological model of the tower pillar dangerous rock mass,carries out similar material tests,physical model tests and numerical simulation,and systematically analyzes the instability process and disaster evolution law of the tower pillar dangerous rock mass.The research results are as follows:（1）According to the indoor engineering geological analysis based on the field survey,the base of the dangerous rock mass is subject to erosion throughout the year,with densely developed cracks and many dissolution caves,and the inner side of the caves collapses and is accompanied by accumulation.The integrity of the dangerous rock mass is not high because of fissure cutting.Under the action of dead weight,the foundation rock mass deteriorates obviously,gradually forming a fracture zone,and the periodic water level fluctuation accelerates the deterioration of dangerous rock mass.（2）Taking Jiandaodong No.3 dangerous rock mass as the research object,barite powder,quartz sand,gypsum,water and polyethyl acetate adhesive were selected as test materials,and mathematical models were established by fuzzy comprehensive evaluation method to analyze the rapid shear test results under different proportions.With gypsum as the base ratio of 1,the optimal ratio of similar materials in the base area is 4.2:1.8:1:0.28:0.22.Rapid shear tests and compressive tests were carried out under different soaking times.The cohesion attenuation was about 2%-5%every 20 minutes,and the internal friction Angle rapidly decreased within10 minutes after the material encountered water,with a cumulative decrease of about 19.9%.Later,with the increase of soaking time,the internal friction Angle slowly increased,but was always less than the initial value.（3）Based on the similarity criterion,a geomechanical model of the tower column dangerous rock mass is constructed to simulate the fluctuation process of reservoir water level in the Three Gorges reservoir area through four stages of"impingement-high water level operation-low water level operation".Before water storage（in the state of self-weight）,the dangerous rock mass is relatively stable.In the initial stage of the test,the surface of the base is massive and falls off,and the cracks at the rear edge are obviously developed.In the seventh cycle,the karst cave roof falls and produces a large amount of deposits.According to the deterioration rate and rock caving,the test is divided into three stages:the fracturing settlement stage of the base,the local failure stage of the rock mass near the cave and the continuous deformation stage of the dangerous rock mass.The sudden change of stress will advance and deform,and the stress will be"central-release"before and after the failure of dangerous rock mass.From the perspective of displacement,the displacement and deformation of the facing surface of the dangerous rock mass is greater than that of the trailing edge,and the displacement change rate is consistent with the change trend of the stress curve.When there is obvious deformation,the stress curve fluctuates frequently and the displacement changes quickly.（4）The Irazu^2Dsoftware based on finite element/discrete element coupling method（FDEM）is used to calculate the numerical value of the tower pillar dangerous rock mass.Numerical simulation shows the whole process of fracture development and failure of dangerous rock mass more directly.The stress in the base area is complicated,and the concentration areas of maximum principal stress,minimum principal stress,shear stress and principal strain are located at the base and around the cave.At the beginning of calculation,the base gradually breaks under the pressure of the overlying rock mass,and the blocks of the cavern and the free surface constantly break into small cells.The preset cracks continue to extend,and the free surface rock mass continues to fall off.There is a relatively stable triangle at the heel of the slope of the dangerous rock mass,which is relatively stable and not accompanied by cracks.（5）Through physical model test and numerical simulation analysis,the instability mode of Jiandaodong No.3 dangerous rock mass is base crushing type.The instability and failure of the typical tower column dangerous rock mass is mainly affected by the gravity action and the fluctuation of reservoir water level.The disaster evolution process can be divided into four stages:the fracturing settlement stage of the base,the fracturing stage of the karst cave roof,the collapse stage of the karst cave side wall rock and the continuous deformation and failure stage of the dangerous rock mass.