| The existing monitoring results show that there will be earthquakes at different levels occurring after each impoundment of Three Gorges reservoir,and most of the seismic energy is released in the form of high frequency and low intensity.Whether this kind of earthquake with many times and small magnitude has an impact on slope stability and how much it affects,which will become a new problem for the prevention and control of geological hazards in the reservoir area.Existing studies have focused on the condition that the sliding mass is in the critical unstable state and the earthquake intensity is great,but little attention has been paid to the influence of the frequent micro seismic on the slope stability.Multi-discipline knowledge of rock mechanics,engineering geology,and engineering seismology are applied in this paper.The bedding rock slope which is relatively unstable in the reservoir area is taken as the research object.With the methods of direct shear test,shaking table test,theoretical analysis and numerical simulation,this paper mainly studies the macro-and-micro cumulative damage characteristics and strength deterioration model of rock joint under pre-peak cyclic loading,cumulative damage evolution and damage model of slope rock mass,slope stability and its influencing factors under frequent microseisms.The main research contents and results are as follows:(1)The influence of frequent microseisms on slope rock mass structure is summarized as pre-peak cyclic loading.According to the three typical deformation modes(climbing,climbing-gnawing and gnawing),the impact of the number,rate and amplitude of cyclic loading on the shear characteristics of rock joint are analyzed with direct shear test under pre-peak cyclic loading.The result shows that the pre-peak cyclic loading has a "contact effect" and a "damage effect".The former has a beneficial effect on the joint shear strength by increasing the contact area of the joint shear plane.The latter will result in the abrasion of the second order undulation of the joint and the gnawing of the first order undulating body,which will lead to the degradation of the joint roughness.The evolution of the two effects is the fundamental reason for the change of joint shear strength.From the deformation characteristics of joint and the damage state of shear surface,it is obvious that the cumulative damage of joint shear surface is more obvious when the number of cyclic loading is higher,the loading rate is faster and the loading amplitude is higher.(2)Based on the PFC(particle flow numerical method),the characteristics of cumulative damage evolution of rock joints under pre-peak cycle loading are studied from the viewpoint of micromechanics.The results show that joint fracture cracks mainly occurs near the second-order asperities in the climbing mode.The tensile and shear cracks have also appeared,which shows the wear effect of second order under cyclic loading.In gnawed mode,in addition to some tension and shear cracks around second-order asperities,the damage crack mainly appears on first-order asperities,and the tensile crack is the main one,which reflects the gnawing effect of cyclic loading first-order asperities.The number of cracks on the joint fluctuation increases with the cyclic loading times,and the increasing rate is accelerating.The number of cracks increases sharply when cyclic loading times reaches to a certain order,indicating that that the expansion of fatigue cracks on joint fluctuation has the characteristics of evolutionary mutation.(3)The joint strength model considering the wear and tear of the fluctuating angle is established based on the experimental phenomenon of the gradual increase of the tangential displacement in the pre-peak cyclic loading.The wear and failure process of the fluctuation angle is represented by the attenuation function of tangent displacement plastic work index in this model.A program which can automatically update the strength parameters of slope structure plane in dynamic time history was compiled through Fish and then a numerical calculation flow of slope dynamic stability which considers the cumulative damage of structural plane is established.In the numerical calculation of the dynamic stability of the slope,the degradation of structural plane fluctuation angle caused by cyclic shear and the decrease of dynamic friction coefficient between two sliders caused by the relative velocity are mainly taken into consideration.(4)A set of complete shaking table test flow is designed for the dynamic cumulative damage problem of bedding rock slope.The dynamic characteristics of the slope are calculated by the transfer function method,and the damage variables are defined by the change of the first-order natural frequency.The nonlinear cumulative damage model of the typical slope rock mass in the stage of micro earthquake action and strong earthquake action.The results show that the damage accumulation of the model slope is in the form of three polynomial which is a slight decline in the initial period,a linear growth in the medium term,and a slight increase in the later period like a positive "S" type curve in the stage of micro earthquake.The damage accumulation of the model slope is in the form of the exponential that the accumulative velocity of slope rock mass is significantly accelerated in the stage of strong earthquake.The double K criterion of fatigue crack growth can be used to explain the variation trend of the cumulative damage curve of four models.(5)Based on shaking table test,the dynamic response characteristics of the slope and the change of dynamic response in each damage stage are studied,and the forming process of sliding surface and instability mode of typical slopes under repeated earthquake are obtained.The results show that the acceleration response of the slope shows "elevation effect" and " surface effect",and with the increase of seismic frequency and seismic load amplitude,the dynamic response of slope is stronger.With the accumulation of slope damage,the dynamic response of the slope shows a weakening trend.Under repeated earthquakes,the rock strata of horizontal layered slope is first shifted horizontally,and then the secondary joints in the strata are cracked.The secondary joints in each layer are connected with the staggered layers,and finally forming a stepped failure surface exposed on the slope surface.The upper rock mass of bedding slope with middle dip angle firstly has a tendency of sliding deformation along the layer,which leads to the gradual cracking of the secondary joint at the trailing edge of the slope,eventually forms a failure surface composed of a vertical tensile fracture at the trailing edge and a lower penetrating layer.The layers of bedding slope with steep dip angle are pulled from the surface to the interior of the slope,then the secondary joints will be pulled apart under the bending moment.The failure surface is mainly composed of three parts: the steep slope segment of the trailing edge,the middle step fluctuation section and the gentle cutting section of the leading edge.(6)In order to analyze the deformation characteristics and stability of slope under multiple earthquakes,UDEC discrete element method was used to establish numerical slope model and the shear strength degradation model was applied to update the strength parameters of joints during seismic action.The results show that the cumulative displacement of the slope increases with the addition of earthquake action times,and the stability of the slope decreases with the increase of the number of earthquakes.And the variation range of the above phenomenon follows the law of first being slow and then fast,that is,when the times of microseism loading increase to a certain number,the displacement increases abruptly and the slope will be destroyed.This phenomenon shows that under the action of frequent microseism,the stability of bedding rock slope will show a certain degree of abrupt change.Under frequent microseismic action,the stability of bedding slope with exposed layer is poor and the slope is likely to have the instability mode of overall slipping,and the slope has obvious signs of deformation before the overall slip instability,eventually leading to more broken the sliding body.The stability of bedding slope with steep dip angle is relatively good,and the geological disasters such as collapses and rock falls may occur.The horizontal layered slope will be slightly influenced by microseismic,but after suffering a number of microseismic,the slope surface will become broken,which will result in possible partial loss and local instability. |
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