A Study On Determination Of Critical Slip Surface Of Slope

Determination of the critical slip surface and the factor of safety(FOS)is essential to the slope stability analysis.The limit equilibrium method(LEM)and the strength reduction method(SRM)based on finite element method or finite difference method are currently the most popular methods to assess slope stability.Traditional LEM is relatively simple and effective enough to make calculations.It does,however,may meet an unrealistic stability analysis of slope under complex environmental conditions as it presents an absence of considering deformation and initial stress conditions of slope and requires assumptions for side forces between slices.Although overcoming some shortcomings of LEM,being able to obtain the FOS without searching the slip surface,SRM occasionally fail to determine the critical slip surface as the calculated yield zone is spread over a wide domain.In this regard,the following research works are conducted around the determination of the critical slip surface of slope in this thesis by means of finite element analysis.(1)The stresses from finite element method are incorporated into a limit equilibrium framework to establish an analysis system based on slip surface stresses.By utilizing the generalized multiple integral mean value theorem,it is noted that the FOS defined by the ratio of the resistance forces algebraic sum to sliding forces algebraic sum has a clear physical meaning,which provides theoretical basis for the aforementioned analysis system.Therefore,the physical meaning of the FOS is the average strength reduction factor when the landslide along the slip surface achieves the overall limit equilibrium state.(2)An extended ellipsoidal slip surface which is easy to construct and flexible to search is proposed and it is certified more coincident with the actuals than spherical and ellipsoidal slip surface.The three-dimensional slip surface of slope is discretized using a series of triangular planes,and then the discrete points are located via background grid technology with the stresses of each point obtained by interpolation through stresses of nodes in element.Hence,the FOS of slope can be solved.Through the introduction of self-adaptive differential evolution algorithm combined with opposite-movement scheme,the critical slip surface search model was constructed.Subsequently,the visualization of the program was realized by utilizing Fortran and Visual Basic.NET language.(3)Several classical examples presented confirm that the proposed method provides the rationality and reliability,and at the same time the definition of FOS as well as sliding shear stress problems are analyzed.It turns out that the results obtained by the method based on stresses algebraic sum agree well with the classical methods,and the former is concisely defined and practical.It also indicates that since the point safety factor method enlarges the influence of local safety,this method may contribute to volatile calculation results,whereas vector sum analysis method(VSAM)has a clear concept and scientific definition,with close results to the method based on stresses algebraic sum.But the amount of calculation in VS AM is relatively large.Moreover,even if the slope is in the critical damage condition,the direction of the maximum shear stresses on the slip surface projected in the plane is not completely consistent with the main sliding direction.With the shear stresses projected,the calculated FOS is larger.Thus,this operation should be used carefully.(4)Slope failure is actually a process of crack initiation and propagation till transfixion,which can be described by fracture mechanics.In this thesis,the application of fracture mechanics in the analysis of slope stability is initially explored.Fracture evolution model based on phase field method is established with strength reduction approach to simulate the slip surface of slope,opening a new way for determining the critical slip surface of slope by using SRM.(5)Relying on a landslide regulation project,the works of searching for critical slip surface of complex engineering slope are launched.According to current water level condition,the FOS of each slope section calculated by the method based on stresses algebraic sum is close to that by the other common methods.The slip surface position searched through conducting the extended ellipsoidal slip surface is also consistent with the actual monitoring results,which shows that the proposed search method in this thesis is reliable and can be applied to slope stability analysis with complex lithology and environmental conditions.