| Rock avalanche is a common geological hazard in China.The large-scale avalanches triggered by strong earthquakes are the most prominent problems.In the middle section of the northern Qinling Mountains,there is a granite seismic rock avalanche belt with an east-west distribution for about 50 kilometers.In this thesis,Ganqiuchi(GQC)large-scale rock avalanche,the largest one in this belt,is taken as the research object.This thesis makes a detailed geological survey and analysis on the accumulation morphology characteristics of the GQC rock avalanche in Cuihua Mountain.The rock avalanche deposits is zoned and summarized by using the facies analysis method in sedimentology.The laboratory experiment is conducted to study the physics and mechanics of the collapsed stones.The pre-disaster three-dimensional topography is reconstructed by the double-cubic interpolation method based on Delaunay triangulation.The failure mode of GQC rock avalanche is summarized.The mechanism and dynamic process of the GQC rock avalanche are analyzed by the discrete element dynamic numerical simulation.And the research results are as follows:(1)According to the relative position,accumulation characteristics,development stage and formation period,GQC rock avalanche is divided into the main rock avalanche,north rock avalanche and east rock avalanche.The occurrence of the main avalanche was triggered the paleoearthquake from 11000 B.C.to 10920 B.C.,while the avalanches on the north and east sides were formed in the 1556 Huaxian M8 earthquake.The mechanical test results show that the mechanical strength of rock samples with developed joints is significantly lower.So it's considered that the occurrence of avalanche is mainly affected by three groups of joints cutting.The development of discontinuities such as joints and cracks in the same direction with slopes playing a controlling role in the formation of avalanche.(2)By using the facies analysis method in the sedimentology,on the one hand,the main avalanche is divided into central facies,marginal facies,throwing facies,clastic facies and residual facies.On the other hand,the northern avalanche is divided into central facies,throwing facies and marginal facies.The throwing facies and clastic facies show the characteristics of the high-speed initiation of the rock avalanche.The marginal facies implies the complex movement characteristics of avalanche such as collision,steering and deposition during the process.Residual facies indicates there is a second transformation process of the avalanche by gravity.According to the development of facies,it is determined that the main avalanche of GQC has entered the late stage of the development,and the north and east avalanches are in the primary stage.(3)The mechanism of the GQC rock avalanche is summarized as a "Seismic-ShearingProjectile" failure mode under the unloading action on the timeliness slope surface,fault's dilation and Seismic amplification effect.According to the numerical results,it is clear that the failure characteristics of GQC avalanches are not instantaneous large deformation such as projection and collapsing,but the first deep penetrating crack appears at the top of the slope along the unloading crack,which is deepening.In addition the latent non-penetrating discontinuities will continue to stretch and expand from the inside to the outside of the slope,and the joints will continue to yield and penetrate,eventually that will close with the tension cracks that extend downward and form deep and large penetrating cracks,and then the whole avalanche body will collapse under the huge horizontal seismic force.(4)In this thesis,a feasible method for pre-collapse topography reconstruction is proposed for the specific rock avalanche.Taking the surface morphological elements around the avalanche as the constraint,the nonlinear polynomial function is used to fit the coordinates of the interpolation points,and then the bi-cubic interpolation method based on Delaunay triangulation is used to restore the topography.Through grid reduction calculation,slope analysis and transient electromagnetic method,the plausibility test of the proposed reconstruction method is conducted,and its rationality and feasibility are verified.(5)The dynamic characteristics of GQC avalanche are very complex,and the direction and mode of motion change from the upper to lower.The movement process of GQC avalanche can be divided into four stages:(a)back edge cracking and starting stage;(b)the overall high-speed sliding stage;(c)the disintegration and collapse stage under impact;and(d)the accumulation and blockage stage.The displacement response rule shows the complex motion characteristics of whole movement process by discrete element numerical simulation such as projection,steering,collision and reverse.And the velocity response rule suggests that the avalanche body continuously oscillates and accelerates after the start-up.In the end,the velocity shows a cliff-like decline until it stabilizes after a violent collision. |
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