With about 9.3 billion tons of reserve, lignite is a major source for energy production in Turkey. The Afsin-Elbistan lignite basin, containing 47% of the overall lignite reserve in Turkey, lies in the Afsin and Elbistan districts as a part of Kahramanmaras. The new Collolar opencast mine is in the Afsin-Elbistan lignite basin, and this mine will be the second mining activity in the basin after the active Kislakoy opencast mine. The new mine will meet the coal consumption of the Elbistan B power plant. Input parameters for slope stability analysis are essential, and so they must be determined accurately and precisely. Field investigations, laboratory testing and back analyses are vital instruments for the input parameters. This study presents the results of slope stability analysis via finite difference code and a limit equilibrium software for the soil slopes of the Elbistan-collolar lignite mine. The basic input parameters, cohesion and friction angle, were determined in the soil mechanics laboratory. By back analyses of a large scale slope failure, mobilized friction angles for a critical weak clay layer under the lignite seam were determined accurately by using the 2D limit equilibrium method and 3D finite difference models. Results of the friction angles were compared in order to check the effectiveness of commonly used 2D approaches in handling the slope problems. Differences in the results of the mobilized friction angles for the weak clay layer were more than 30%. The 3D models indicated that the mobilized friction angle during the major slope failure was substantially lower than the friction angle generated by the 2D limit equilibrium method. (C) 2010 Elsevier Ltd. All rights reserved.