Study On Dynamic Damage And Instability Mechanism Of Bedding Reservoir Bank Slope Under The Influence Of Rock Mass Deterioration In The Hydro-fluctuation Belt

Since the impoundment of the Three Gorges Reservoir Area,the reservoir induced earthquakes have occurred frequently,and the rock mass deterioration in the hydro-fluctuation belt caused by the periodic fluctuation of the reservoir water level has gradually intensified.The potential threat to the stability of bank slope has become a new problem for the prevention and control of geological disasters in the reservoir area.At present,there are few reports on the impact of the coupling effect of rock mass deterioration in the hydro-fluctuation belt and reservoir induced earthquakes（mainly frequent micro and small earthquakes）on the stability of bank slope,especially the evolution mechanism of cumulative damage–instability failure of bank slope under this load environment and its long-term stability influencing factors have not been clearly understood.In view of this,focusing on the above topics,the typical bedding rock slopes with relatively weak stability in the reservoir area（including the rock mass deterioration area in the hydro-fluctuation belt）were taken as the research objects.Integrating with the theories of rock mechanics,engineering geology,damage and fracture mechanics and nonlinear dynamics,the methods of field investigation,indoor cyclic shear test,shaking table model test,theoretical analysis and numerical simulation were conducted to mainly investigate the mechanical behaviours of macro-meso fatigue damage of the rock discontinuities subjected to pre-peak cyclic shear load,the calculation method of slope stability considering strength vibration degradation effect of the rock discontinuities,the dynamic cumulative damage response rules（failure modes）of typical bedding rock slopes under the influence of rock mass deterioration in the hydro-fluctuation belt and its variation characteristics of istability influencing factors（sensitivity）of dynamic stability.The main research contents and results are as follows:（1）The continuous excitation of multiple micro and small earthquakes on slope rock mass is defined as the pre-peak cyclic shear action.Based on this,The pre-peak cyclic shear test and PFC^2D numerical simulation of the rock discontinuities with first-order and second-order asperities were carried out,and the macro-meso fatigue damage evolution mechanism and influencing factors of the rock discontinuities subjected to pre-peak cyclic shear load were analyzed.The results indicate that the rock discontinuities have experienced six development stages,i.e.,initial nonlinear shear contraction deformation,approximate linear elastic shear dilation deformation,cyclic fatigue damage deformation,plastic deformation of the local compression-shear fracture,full plastic deformation of the stress brittle drop and ideal plastic flow deformation.When the normal stress,first-order asperity angle increase or the shear rate,shear amplitude decrease,the peak（residual）shear strength and cumulative shear（normal）displacement of the rock discontinuities increase and decrease,respectively,and both of them decrease and increase with the increase of cyclic shear number,respectively.The typical macro-meso fatigue damage and failure modes of the rock discontinuities can be summarized as three basic types,i.e.,compacting–climbing failure,climbing–cyclic abrading–extruding–gnawing failure and gnawing–sliding failure.Additionally,the meso fatigue damage cracks of the rock discontinuities are densely distributed in the vicinity of the shear surface in an approximately"trapezoidal manner",and the change tendency for the number and energy of fatigue damage cracks is positively correlated with shear displacement or cyclic shear number.（2）Combined with the pre-peak cyclic shear fatigue damage characteristics of the rock discontinuities,the influence factors of vibration wear and relative motion velocity were introduced,and the equation of strength vibration degradation coefficient（a dynamic variable closely related to cyclic shear number,cyclic shear amplitude and relative motion velocity）of the rock discontinuities was deduced.According to the limit equilibrium theory,an algorithm for calculating the dynamic stability coefficient of rock slope considering the strength vibration degradation effect of the rock discontinuities was proposed.The dynamic operation of the algorithm was realized by using the built-in Fish language of UDEC program,and the feasibility of the algorithm was further verified by using example analysis and case research.Based on this,the effects of initial cohesion（internal friction angle）of the rock discontinuities,slope angle,layer inclination and dynamic load amplitude（frequency）parameters on the dynamic stability of slope were analyzed.（3）Aiming at the problem about the dynamic cumulative damage mechanism and stability of bedding rock slope in reservoir bank considering the deterioration of rock mass in the hydro-fluctuation belt,a complete shaking table similar model test system and process were designed.Based on this,the response rules of acceleration,rock pressure,cumulative displacement,pore water pressure in the hydro-fluctuation belt,dynamic characteristic parameters and cumulative damage degree of slope were discussed,and the nonlinear dynamic cumulative damage mathematical model of slope rock mass was established.The results show that under the continuous action of multiple seismic loads,the acceleration response of slope presents significant"surface effect"and"elevation effect",and its PGA amplification coefficient is weakening.The rock pressure,cumulative displacement and pore water pressure in the hydro-fluctuation belt of slope gradually decrease,increase and increase,respectively,and the damping ratio and natural vibration frequency of slope increase and decrease,respectively.Especially,the"S-type"cubic function（slight decrease in the initial stage,approximate linear increase in the middle stage and gentle increase in the later stage）and the"steep rise"exponential function（rapidly increasing）can be used to describe the cumulative damage degree of slope rock mass sbujected to the micro and small earthquake excitation and strong earthquake excitation,respectively.（4）Based on the shaking table model test,the whole process of dynamic cumulative damage evolution and instability failure mode,which for the gently inclined（Type I slope）,medium inclined（Type II slope）and steep inclined（Type III slope）bedding rock slope with straight rock discontinuities and the medium inclined（Type IV slope）bedding rock slope with saw-tooth rock discontinuities（including the rock mass deterioration area in the hydro-fluctuation belt）,were revealed and generalized.The results indicate that for Type I slope and Type II slope,multiple potential sliding surfaces are formed due to crack initiation,propagation and penetration of the trailing edge of slope top,secondary joints and rock layers→the rock mass overall slides layer by layer along the composite sliding surfaces aggregated by the tensile cracks at the rear edge and the through rock layers in the slope→the residual sliding trace is in a steep ladder shape after complete instability and failure of slope.For Type III slope,the dense damage fracture networks are formed due to crack initiation,propagation and penetration of the trailing edge of slope top,secondary joints and rock layers→the rock mass peels off and slides layer by layer along the failure surfaces aggregated by the steep inclined bedding plane section at the rear edge,the step undulating section in the middle part and the gentle shear out section at the front edge of slope→the residual sliding trace is in a steep straight shape after complete instability and failure of slope.For Type IV slope,the upper sliding body slides with climbing mode along the low undulation layer,and a small amount of tension cracks run through the layers→the middle sliding body slides with climbing–gnawing mode along the medium undulation layer and the lower sliding body locally slides with gnawing mode along the high undulation layer,and the number of tension cracks increases sharply→the residual sliding trace is in a steep saw-tooth shape after complete instability and failure of slope.The rock masses in the hydro-fluctuation belt of four typical slopes are seriously broken after being subjected to the combined action of vibration,dissolution and scouring,resulting in overall sliding instability failure and forming a significant fracture cavity（the order of deterioration degree is Type II slope>Type I slope>Type III slope>Type IV slope）（5）Considering the strength vibration degradation effect of the rock discontinuities,the cumulative permanent displacement,dynamic stability coefficient and cumulative damage–instability failure evolution process of typical bedding rock slope were analyzed by UDEC discrete element method,and the influence characteristics and sensitivity of various factors on the long-term stability of slope were discussed.The results show that the numerical simulation and model test results are basically consistent,and the cumulative permanent displacement and dynamic stability coefficient of slope under various influencing factors show the form of increasing exponential function and the trend of gradual decline first and then steep decline,respectively.Meanwhile,for various influencing factors,there are significant differences in slope deformation and failure,velocity response,bedding plane（joint）damage and pore water pressure diffusion under different loading stages.In general,the sensitivity of various factors to the influence of slope dynamic stability is as follows:dynamic load amplitude>range of rock mass deterioration area in the hydro-fluctuation belt>slope angle>slope height>dynamic load frequency>water level height in the hydro-fluctuation belt>morphology of rock mass deterioration area in the hydro-fluctuation belt>inclination of bedding plane>thickness of bedding plane.