Improvement of slope stability based on integrated geotechnical evaluations and hydrogeological conceptualisation at a lignite open pit

Safe design of surface coal mine slopes must be considered early in the mine planning stage and is required throughout the life of mining operation. The success and efficiency of such an operation depend primarily on the reliability and accuracy of stability assessments based on integrated geotechnical evaluations and hydrogeological assessment. The Işιkdere lignite mine is one of the most recently operated large coal pits in Turkey since late 2009 and its depth will reach up to about 170 m. A few local failures experienced in the current pit and considerations on its maximum depth forced the mine operators to consider the redesign of the final pit wall slopes. The technical objectives of this study are to evaluate the stability of the final pit wall slopes considered in the initial pit project and to investigate and demonstrate the application of possible remedial measures to achieve safe conditions based on integrated geotechnical assessments and hydrogeological conceptualisation. To study the problem, a two-year collaborative program of geotechnical and hydrogeological investigations throughout the current pit and the area in the direction of advance of the pit in accordance with laboratory tests and analyses was commenced by the authors. The analysis results indicate that stability of the final pit slopes is sensitive to multi-planar failures and confined water in the coal seam is also a factor adversely affecting the stability. Problems associated with groundwater at the site were also assessed with the analyses of piezometric level and groundwater inflow. Remedial measures investigated suggest that at some parts of the pit, the initial pit project should be modified by flattening or shifting the final pit wall slopes towards the north or stripping them to a certain elevation.