Failure Mechanism And Stability Assessment Of Deep-Seated Toppling In Interlayered Rock Slopes At Upstream Of Yalong River

Interbedded toppling is the main failure mode of rock slopes at upstream of the Yalong River.Because of the complicated formation mechanism and huge development scale,it has become the major engineering geological problem that restricts the hydropower engineering construction in research area.In view of the inapplicability of the limit equilibrium method to analysis the stability of deep-seated toppling,a research on the geological characteristics and stability assessment method of interbedded toppling was carried out using engineering geological method,which include the prototype research,theoretical model establishment,mechanism analysis and evolutionary process simulation,and quantitative evaluation of stability.The essence of this method is to evaluate the stability status of the slope by retrieving the whole process of slope deformation evolution and the degree of slope stability accompanying this process.Firstly,on the basis of the study of the geological conditions of the research area,the deep-seated topplings developed at upstream of the Yalong River are taken as typical examples to analyze the distritubtion law,development characteristics and formation conditions of the inbedded toppling.Also,a phenomenon model was established with the deformation characteristics and failure pattern of interbedded toppling.Then,based on the construction of the theory model of interbedded toppling,UDEC-DM numerical method was used to simulate the whole evolution process of toppled slope,so as to reveal the intrinsic mechanical mechanism and spatiotemporal evolution of interbedded toppling.Finally,for the stable behavior of different evolution stages of the slope,the engineering geological evolution method and the numerical simulation reduction method were used to qualitatively and quantitatively evaluate the overall stability of the interbedded toppling from the view of deformation process and the evolution stage.With the system study of the deformation characteristics,failure mechanism and evolution process,and stability evaluation method of interbedded toppling,the following preliminary conclusion are reached.(1)Interbedded toppling is the main failure mode of rock slopes at upstream of the Yalong River.Its development scale is huge and the degree of deformation is severe.On the topography,the interbedded toppling is mainly developed at the end of the ridged bank slope of the V-shaped valley.The exposed lithology is composed of interbedded metasandstone and slate of the Triassic Lianghekou group(T3lh1).Slopes with welldeveloped discontinuities dipping steeply into the slope and trending parallel or subparallel to the slope crest are generally considered susceptible.The slope angle is generally between 40 ° and 70 °,and the dip angle of stratum is about 65 ~ 85 °.(2)The formation of deep-seated toppling in research area is governed by the combination of inner and outer factors,and is mainly affected by the mechanical properties of rock mass and the structure characteristics of slope.The structure of soft and hard rock interbedded with well-developed cross-joints perpendicular to the bedding surface had played a key role in the deformation and evolution of the slope.In addition,external factors such as river valley evolution,weathering,rainfall and earthquake have also assisted and triggered the formation of deep-seated toppling.(3)The interbedded toppling has obvious zoning.In space,a well-developed interbedded toppling can be divided into sliding zone,strong toppling zone,weak toppling zone and original rock zone with distinctive geological and mechanical characteristics.A quantitative description system,which is established based on the measured data,can be used as the basis for interbedded toppling indentification and classification.(4)Under the gravitational field whith residual tectonic stress,the interbedded toppling is a compound mode characterized by the block overturning and cantilever beam bending,which is block-flexure toppling in hard rock and flexural toppling in soft rock,respectively.In time,with the evolution of valley,the deep-seated toppling underwent four major evolution stages,unloading and rebound stage,initial deformation stage,breaking in flexure,and failure as a landslide.Unlike the existing conclusions,the failure surface is developed along the cross-joints in strongly deformed rock,rather than the maximum bending zone.(5)The transformation of failure mode was a significant indicator of the acceleration failure of interbedded toppling.The occurrence of the sliding zone represented a limiting condition of the toppling process to be reached,and its stability factor is between 1.0 and 1.05.Reasonable division of deformation zones and accurate judgment of the evolution stage are the key to evaluating the stability of interbedded toppling.The stability evaluation method based on the evolution process can effectively evaluate the stability of interbedded toppling.(6)In view of the long-term and staged evolution characteristics,the stabilization of interbedded toppling should be based on the evolutionary thinking.And different treatment meaures can be adopte for the slopes of different evolution stages.