Research On Analysls Method For Multi-layer Flexural Toppling Of Rock Slope

Toppling is a typical mode of slope instability,and its failure mechanism is completely different from common sliding modes.With the development of large-scale engineering projects such as hydropower,open-pit mines,and transportation at home and abroad,the instability of slopes caused by rock toppling has been widely exposed,which has become a critical issue restricting the construction of related projects.At present,the current research on such slopes still lacks in-depth and conclusive results,resulting in controversies and difficulties for engineers in dealing with related problems.This study takes the toppling failure mode of rock slopes as a research object,focusing on the multi-layer flexural toppling,the deformation evolution features,mechanical mechanisms and instability laws of toppling masses are revealed by means of geological analysis,theoretical analysis and numerical simulation.In particular,stability analysis and evaluation methods for multi-layer flexural toppling slopes are systematically studied,as well as the influence of key parameters on analysis results.The main contents and conclusions are as follows-(1)Analyze and summarize 76 cases of slope toppling reported at home and abroad,the deformation evolution law and failure features of the slopes are discussed from three aspects of slope geometry,engineering geological characteristics and instability inducements.Based on the Goodman-Bray's work,a more comprehensive toppling classification is proposed,including five basic categories:basic toppling mode,combination toppling mode,creep mode,cantilever mode,and under-dip toppling.This system broadens the research scope of toppling failure,and lays a solid geological foundation for the quantitative analysis of toppling slope stability.(2)For anti-dip layered rock slopes,the failure features of multi-layer flexural toppling that are different from block toppling mechanisms are analyzed,and a corresponding analytical model is re-generalized and established.A novel stability analysis method is proposed for this model.This approach makes a slope reach limit states by applying horizontal load to rock masses,and obtains the safety factor of the slope based on horizontal limit accelerations.The applicability of the method is verified against a physical model test.Results show that the force conditions,limit accelerations and safety factors of toppling mass have achieved good consistency when the dip angle of fracture surface changes,and they can all reflect the stability of the model.(3)The feasibility of discrete element modeling in stability analysis of block toppling and multi-layer flexural toppling is demonstrated by comparison,and main points for numerical analysis of these two kinds of toppling slopes are put forward.Calibration analysis of a typical toppling model test proves that discrete elements can achieve good simulation results,and can reflect the gradual change process of internal stress fields during toppling failure.Numerical experiments by discrete elements show that:block toppling exhibits significant kinematics characteristics,while flexural toppling displays obvious structural properties of superposed cantilever beams.Sensitivity analysis of key mechanical parameters shows that:the tensile strength of rock mass has a great influence on the stability of multi-layer flexural toppling slopes.Therefore,in addition to reducing the shear strength of rocks and joints when conducting strength reduction analysis for such slopes,it is necessary to consider reducing the tensile strength of rocks(4)The effects of joint spatial morphology(dip angle and thickness),slope shape(slope angle)and rock strength on limit failure modes,toppling characteristics and safety factors for layered rock slopes are studied.It focuses on the analysis of the impact of key mechanical parameters and geometric parameters on the failure surface of anti-dip slopes.The typical fracture surface of multi-layer flexural toppling is a straight line that develops from the toe and gradually penetrates to the back edge of the slope.The dip angle is greater than the normal of joints,and the included angle is generally between 0° and 20°.The dip angle of the fracture surface increases with the enlarge of joint friction angles,while joint cohesion and rock tensile strength have little effect on the it;the larger the slope angle,the greater the fracture surface inclination,and the larger the rock inclination,the smaller the fracture surface inclination;the failure surface of anti-dip slopes with steep joints and steep toe is often deep-seated,and one or more secondary failure zones develop inside the main toppling mass.(5)Taking the left bank slope of Delsi Hydropower Station as an example,the geological and landform features and related monitoring data in construction are analyzed in detail.The typical anti-dip steeply inclined rock structure and special lithological composition(thin gneiss)are the prerequisites for flexural toppling of the slope.External factors such as excavation and heavy rainfall aggravate the deformation of rock strata.Numerical modeling results show that,the excavation of rock mass below the elevation of 1493m causes the entire slope to experience deep flexural toppling,and the fracture surface is approximately linear with a dip angle of 21°;controlling the excavated height and angle of low-elevation rocks can effectively reduce the degree of toppling deformation.Moreover,a simulation method of prestressed anchor cables is proposed.The characters of cable stress and rock deformation under different supporting strength,positions and construction time are studied.The reinforcement effects of the cables and the stability of the slope in each scheme are evaluated.Finally,for high slopes that are prone to toppling failure,prevention suggestions during excavation and reinforcement are provided.