| 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.The earthquake frequency and intensity have been increasing in recent years,while which still keep its original state of the weak earthquake activity.Whether this kind of earthquake has an impact on slope stability or not,and how much it affects,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.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 constitutive model of muddy soft rock 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 bedding soft and hard interbedded slope which is relatively unstable in the reservoir area is taken as the research object.The main research contents and results are as follows:(1)Based on the direct shear test,the deformation characteristics and shear strength of the muddy soft rock are studied under different water content,normal pressure,cyclic loading times,shear upper limit values and loading amplitudes from the viewpoint of macroscopic.The results show that cyclic-loading may finally lead to the accumulative deformation of muddy soft rock,and the accumulative increase of the displacement in the shearing direction may exert certain effect to the shearing strength for muddy soft rock.With the increase of the cycle-index,the peak shear strength of muddy soft rock increases firstly,and then decreases.The higher value of water content,the larger value of the shear upper limit value,as well as the shearing amplitude.Under the same circulation times of the shearing test,the more obvious of the cumulative deformation effect,the larger amount of the intensity attenuation.The shear failure surface of the muddy soft rock that undergo 100 times and 10000 times cyclic loading and then static lapse tests shows that shearing phenomenon in different levels are caused due to multiple cyclic shear effect,and shearing structure in hilly form can be found in the shearing fracture face.The influence of the many factors on the shearing face are mainly reflected by the shearing damage range and extent in the shearing surface.(2)Based on the PFC(particle flow numerical method),the characteristics of crack initiation,expansion and penetration of muddy soft rock under pre-peak cycle loading are studied from the viewpoint of micromechanics.The results show that the number of cracks increases with the growth of cyclic loading times,and the speed has an accelerating trend.Under different normal pressures,water contents,shear upper limit values and shear amplitudes,the degree of crack propagation and the number of cracks in the muddy soft rock reflect the law consistent with the laboratory direct shear tests.Moreover,the high water content,large shear upper limit value,great shear amplitude and low normal pressure are the key factors determining the cumulative damage velocity of muddy soft rock.(3)Based on the deformation characteristics of the muddy soft rock under cyclic loading.A circular fractional order western original model which can describe three kinds of deformation laws of soil stability,critical type and failure type is established.TOPT(Levenberg-Marquardt,LM)algorithm is used to calculate the parameter values of multi parameter nonlinear equations and to fit the nonlinear curves according to the experimental data of muddy soft rock.The applicability of the constitutive model is verified.(4)According to the characteristics of the slope in Three Gorges reservoir,four typical soft and hard interbedded slope model tests are designed.Based on the transfer function theory,the dynamic characteristic parameters of the slope in each stage of the damage(first order self vibration frequency and damping ratio)are obtained.The variation of the slope body damage is defined by the change of the frequency of the vibration,and the solution expression of the damage degree D is given.Type.After microseismic action,the natural frequency of the slope decreases and the damping ratio increases,which indicates that the microseismic action will cause damage to the slope.When the strength of the seismic action increse,the variation of the dynamic characteristic parameters of the slope is obviously increased,which shows that the cumulative damage of the slope is accelerated with the increase of the earthquake intensity.(5)Based on the change of vibration frequency,the cumulative damage development curves of four model slopes are obtained.The double K criterion of fatigue crack propagation can explain the variation trend of cumulative damage curve well.With the increase of seismic load amplitude and the number of action,the cumulative damage evolution curves of the four model slopes show obvious characteristics of three stages: the initial damage stage,the meso crack propagation stage and the macro crack expansion stage,and the curve shape is roughly inverted "S" shape.The cumulative damage curve of rock mass based on the vibration frequency test is regressed and fitted,and the nonlinear fatigue cumulative damage evolution model of the slope rock mass in two stages of small earthquake action and strong earthquake action is established.Three polynomial forms and power exponents are used to regression analysis on the cumulative damage evolution data of rock mass under the action of small earthquakes and strong earthquakes,and the fitting results of the four slope bodies are all relatively satisfying.(6)By shaking table test,the dynamic response characteristics of the typical soft and hard interbed slope and the change of the dynamic response of each stage are studied,and the forming process and the instability mode of the typical slope body under repeated earthquake action are obtained.The study shows that the acceleration response of the slope shows the "elevation effect" and "the surface effect".With the increase of the seismic frequency and the amplitude of the seismic load,the dynamic response of the slope is more intense.With the continuous accumulation of the slope damage,the dynamic response of the slope presents a weakening trend.For the slope with horizontal soft and hard interbeddings,two sliding surfaces form on the slope surface and at the back end of it.The sliding surface which develops along the back end of the slope presents a failure mode of slip fracture.The sliding surface which develops along the slope surface presents creep and pressure induced fracture.The gentle and soft and hard interbed slope gradually forms the sliding-cracking failure surface of step type along the slope,and the hard rock mass is poor before the slope failure.Due to the exposure of the soft and hard interbedded slope,the shear fracture occur in all the soft layers which become the dominant area of the slope deformation and failure.The soft and hard interbed slope form piecewise slip failure,first of which is the upper soft and hard layer slip,and then the remaining sliding body is broken along the slip surface which is formed by the shear crack in the upper part of the soft layer,the secondary pull crack in the middle and the shearing crack in the lower part.For the slope with soft and hard interbeddings of steep dip angles,the whole process of instability shows that the soft and hard layer of the upper part is stripped from the slope surface to the inner layer,and the failure mode is bending-collapse.The stability degree of the four types of slope is as follows: medium dip soft-hard interbedded slope < steep soft-hard interbedded slope < gently inclined soft-hard interbedded slope < horizontal soft-hard interbedded slope.(7)The UDEC discrete element method is used to establish the numerical model of the bedding soft-hard interbedded slope,and analyze the deformation development characteristics and the stability regularity of the slope under frequent microseisms.The results show that the cumulative permanent displacement of the slope increases with the increase of loading times,and the stability decreases with the increase of earthquake action times,the change amplitude of the two shows slow first and fast then.Under the frequent microseism circumstance,the failure mode of the gently inclined soft-hard interbedded slope is overall sliding,and the slip mass is rarely disrupted.In the same case,the failure mode of the gradual inclined soft-hard interbedded slope is sliding-crack,and its surface is mainly in the limited depth range.The failure surface of the steep soft-hard interbedded slope is exfoliating and damaged along the upper soft and hard layers from the outer to the inner layer,and the hard layer is broken from the middle part,showing the geology of collapse and rockfall.While the horizontal soft-hard interbedded slope forms a step failure surface along the slope surface,and the surface slope is more broken than its inner.Under the action of microseism,the stability of the four types of slopes is the following: the gentle inclined soft-hard interbedded slope < the gradual inclined soft-hard interbedded slope < the steep soft-hard interbedded slope < horizontal soft-hard interbedded slope.(8)With the UDEC discrete element method,the numerical model is established,and seven factors affected the long-term stability of the soft-hard interbedded slope are considered,i.e.dynamic load amplitude,dynamic load frequency,slope angle,slope height,layer angle,layer spacing(thickness of soft and hard layer)and thickness of hard-soft layer.The results show that as the amplitude of the dynamic load increases,the frequency of the loading wave keeps close to the natural frequency of the slope.The greater the cumulative permanent displacement of the slope is,the lower the safety factor of the slope,and the shorter the fatigue life of the slope.With the increase of slope height and slope angle,the displacement of the X and Y direction of the slope is gradually increased,and the safety factor is gradually reduced,thus the slope failure occurs faster.In the case of layer exposure,the cumulative permanent displacement of the slope increases significantly with the increase of the layer angle,correspondingly the slope safety factor decreases obviously,which makes the loading times of the slope instability significantly reduced.Study about the thickness ratio of the hard-soft layer shows that if the thicknesses of the hard-soft layer are same,the smaller the thickness of the layer is,the more obvious the relative disturbance of the slope under the earthquake load is,the worse the long-term stability of the slope is,the more easily the slope is destroyed.For the soft-hard interbedded slope with constant hard layer and decreasing thickness of soft layer,the thicker of the soft layer is,the less times of microseism needed for the slope instability.With the increase of soft layer thickness,the less slope stability is,the larger the influence of the soft layer on the slope is.And the damage easily occurs along the inner soft layer,which brings thorough slope failure and instability. |
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