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# Three-dimensional Slope Stability Analysis by Shear Strength Reduction Finite Element Method (SSR-FEM) under Drawdown Condition

Author: WanShaoShi
Tutor: NianTingKai
School: Dalian University of Technology
Course: Geotechnical Engineering
Keywords: slope stability strength reduction finite element method(SSR-FEM) instability criteria critical slip surfaces location water drawdown conditions transient unsaturated seepage phreatic surface three-dimensional slope
CLC: TU43
Type: Master's thesis
Year: 2009
Quote: 4

### Executive Summary

 More and more attentions have been paid to shear strength reduction finite element method(SSR-FEM) for slope stability analysis.Because it has the advantages as follows:(1) No assumption to be made in advance about the shape of the failure surface;(2) Considered non-linear stress-strain relation on soil;(3) Simulation of progressive failure of slopes.But some problems need to be studied in advance.For example,mesh shape,size and order for the accuracy of the calculation;failure criteria;how to analyze slopes stability by shear strength reduction finite element method(SSR-FEM) under transient unsaturated seepages and whether it is rational that SSR-FEM is extended to 3D slope.They are worthy of further study.The main research work done in this thesis consists of the following parts.1.The effect of mesh shape,size and order on the factor of safety is systematically discussed at first.The effect of mesh on the failure criteria is then studied.When a slope arrives at the critical failure state by SSR-FEM used rational mesh shape,size and order,the four failure criteria as plastic zone extension from the toe to the top of slope,equivalent plastic strain exceeding a certain value developed along the potential slip surface,uncontrollable increase in characteristic nodal displacement on slope surface and the iteration non-convergence of nonlinear FEM numerical calculations can be found in turn.Discussed calculation results which showed the four evaluation criteria are highly consistent.Location of potential slip surface and out-slip point is associated with cohesion,internal friction angle and slope angle. The factors of safety obtained by the limit analysis method and SSR-FEM are compared for various slopes with different slope angle,slope height,cohesion and internal friction angle. Moreover,SSR-FEM is also applied to the stability of slopes under complex conditions.2.The safety factor of slope stability and failure mechanism are systematically discussed under various drawdown condtions.Water drawdown conditions can be divided into three modes as rapid drawdown,slow drawdown,simple transient unsaturated seepage or transient unsaturated seepage.Shear strength reduction finite element method(SSR-FEM) has been applied to analyze the slope stability and failure mechanism under water rapid and slow drawdown,the factors of safety achived are compared with those by the limit analysis method(LAM).The transient unsaturated seepage in soil slope is systematically investigated and the stability of a slope is analyzed by SSR-FEM.Emphases are given to discuss the influence of cohesion,internal friction angle,slope angle,location of phreatic surface,soil permeability,drawdown ratio,drawdown rate and matfic suction subjected to transient unsaturated seepage on the slope stability.And the effect of dilatancy on factors of safety and location of potential slip surfaces is also discussed.3.Shear strength reduction finite element method(SSR-FEM) has been applied to the stability of three-dimensional slopes.For further verification of the present method,a series of 3D analysis has been presented for comparison with results obtained by various investigators using other 3D slope stability techniques.Their results are in good agreement.The factors of safety and location of slip surfaces are associated with slope length/width,boundary conditions, material parameters,geometry dimension,retaining structure.Furthermore,it is compared with the factors of safety of 2D slopes.The 3D SSR-FEM is finally used to analyze the stability of slope under water drawdown con&ion.