Study On Cumulative Damage And Dynamic Stability Of Layered Rock Slope In Reservoir Area Under Frequent Microseisms

It has been shown that the earthquake with different energy levels was monitored after each impoundment of Three Gorges reservoir,and the seismic energy is also released in the process,which is characterized by high-frequency and low strength.The influence of this kind of earthquake on the stability of the slope in the reservoir area is becoming a new problem as for the prevention and control of geological hazards in the reservoir area.However,the existing studies is mainly focused on the critical unstable state of landslides and status of high-strength earthquake and little attention is paid to the impact of frequent microseisms on slope stability.Multi-discipline knowledge of rock mechanics,engineering geology,and engineering seismology are applied in this paper.The bedding rock slope with the poorest stability in the reservoir area is taken as the research object.On the basis of engineering project of layered rock slope in Three Gorges reservoir,using methods in combination with simulation physical experiment(shaking table model test),numerical simulation and theoretical analysis,this paper mainly researches on the cumulative damage process and long-term stability of the layered rock slope.Taking Shanshucao landslide as an example,which is destructed under frequent microseisms,the paper has also studied the failure process of the landslide and the contributing factors of its stability.The main research contents are as follows:(1)The design of shaking table test is carried out in detail in four aspects: the similarity relation of the test,the determination of the ratio of similar materials,the selection of the slope model box and the test equipment,the design and construction of the typical layered joint slope,the selection of loading wave and the design of the loading scheme.Based on the shaking table model test,the dynamic characteristic parameters(natural frequency and damping ratio)of the slope at various loading stages are obtained through the transfer function method.The results show that with the increase of earthquake action times,the basic law of the dynamic characteristic changes is the decrease of the natural frequency of the slope and the increase of the damping ratio.Besides,as the earthquake action strength increases,the cumulative damage of the slope accelerates gradually.(2)The acceleration response law of the model slope shows the "elevation effect" and "the surface effect".The elevation effect reflects the vertical enlargement of the response of the slope acceleration.The surface effect embodies the enlargement of the surface of the slope acceleration response,and the dynamic response of the slope is more with the increase of the seismic frequency and the magnitude of the earthquake load.The dynamic response of the anti-dip layered slope is the strongest,the steeply dipping slope and the mid dip slope are the least,and the dynamic response of the gently inclined bedding slope is the weakest,and the PGA magnification coefficient of the four slope models all presents a weakening trend with the cumulative damage of the slope body.(3)The forming process and instability mechanism of typical layered slope under multiple earthquakes are summarized.Under the effect of multiple earthquakes,the interlayer secondary joint is first cracked by the lower tensile strength,and the shear strength of the slow dip layer decreases gradually under cyclic shear,which leads to the gradual slipping and falling off of the rock slope,and the overall instability and breakage of the gentle slope is manifested by the sliding of the slope rock block layer by layer.At first,the upper rock mass of the slope of the middle tilted slope produces a sliding deformation trend along the plane,and then causes the secondary joint of the rear edge of the slope to be gradually pulled down from the top of the slope.Finally,the failure surface is formed by the combination of the vertical pull crack and the lower penetration level of the rear edge.The integrity of the rock mass is better before the slope is unstable,and the slide body is mostly giant after the instability.The surface of the steep slope is separated from the interior of the slope,and the secondary joint will be pulled apart in turn,and the secondary joint will be cracked because of the bending of the rock.The open layer and the secondary joint of the crack form a fissure network in the upper rock mass.The integrity of the rock mass is very poor before the slide body is unstable,and the accumulation of the sliding body after the instability is extremely high.The failure mode of the anti-dip layered slope shows that the block caving is first appeared on the slope shoulder and slope,and then the upper strata are peeled off by layer by layer,and finally the circular arc failure surface is formed.The main failure characteristics of the slope are rockfall and the collapse of the upper and middle strata at the slope shoulder.(4)Based on the prototype slope of the shaking table slope,the UDEC discrete element method is used to establish the numerical analysis model,and the characteristics of the deformation development and the regularity of the stability of the typical layered joint slope under the continuous small earthquake are analyzed.With the increase of the frequency of microseismic action,the cumulative permanent displacement of slope.increases gradually,while the slope stability coefficient decreases gradually,and the change amplitude of the two is slow.In contrast to the dynamic stability of the typical layered rock slope,the steep slope and the reverse lamellar slope,which is not exposed,are basically stable under the frequent occurrence of microseism,and the long-term stability of the surface slope is poor,and the slope is risk of overall slip,and the frequent micro earthquakes have relatively small influence on the gentle slope and the bedding slope.However,after a long period of minor seismic action,the slope surface is broken,and the risk of local falling and local instability is on the slope.(5)The UDEC discrete element software is used to analyze the characteristics of the slope dynamic stability and the failure mode of the slope of the Chinese fir trough under the frequent micro earthquake.At the same time,the influence of three factors,which is the amplitude of the earthquake load,the thickness of the rock slope and the slope angle,on the long-term stability of the slope of the Chinese fir trough slope is discussed.The results show that the failure evolution process under repeated small earthquake(0.04 g sine wave-V degree intensity)can be divided into three stages: initial damage stage(small slope deformation),damage accumulation stage(joint fracture in the middle and lower part of slope)and failure stage(surface slip body along "weak joint surface + mudstone sandstone interface" whole Body slip).With the decrease of the thickness of the rock slope,the amplitude of seismic load and the increase of slope angle,the displacement and deformation of the slope increase,the safety factor of the slope decreases,the fatigue life of the slope reduces,thus the the slope is more likely to fail.