Slope stability analysis and road safety evaluation : A case study on two roads located close to the Piteå river - in Sikfors and Nystrand

Abstract: This master thesis focuse on natural slope and embankment slope stability analysis with the Limit Equilibrium Method computer program Slope/W and the Finite Element Method computer program Plaxis 2D and a comparison of the analyzed result. The finite element method needs additional information regarding the potential performance of a slope; just basic parameter information is needed when using limit equilibrium methods. The results indicate that it is important to use the effective shear strength characterization of the soil when performing the slope stability analysis. A distinction should be made between drained and undrained strength of cohesive materials. Shortly, drained condition refers to the condition where drainage is allowed, while undrained condition refers to the condition where drainage is restricted. Most likely the worst case scenario occurs when the river water level is increased rapidly, and then quickly drops while the water table in the embankment is retained on an extremely high level so that the low effective stresses might lead to failure. The existence of trivial failure surfaces is a large problem in stability analysis of natural slopes, especially in the Plaxis 2D program. We tried to avoid these types of failure. In Slope/W analysis we consider critical slip surface failure. If the critical failure is trivial, then we consider the secondary failure. In Plaxis 2D analysis, additional displacements are generated during a Safety calculation. The total incremental displacements in the final step (at failure) give an indication of the likely failure mechanism.