An investigation on the mechanisms of instabilities and safe design of the south slope at a lignite pit (SW Turkey) based on a sensitivity approach

Turkey has significant lignite reserves which are generally being extracted using open pit mining methods. The Hüsamlar pit is one of the operated lignite pits in the well-known Mugla lignite province in SW Turkey. Some local failures and one large failure, which caused the evacuation of the Hüsamlar village located next to the slope crest and interruption in coal production, occurred along the south slope of this pit. This paper outlines the results of the field and laboratory geotechnical investigations associated with the causes and mechanisms of the instabilities, and assessments on the possible modifications in the current and planned final slope geometries to improve the stability of the south slope. Since no sufficient data on groundwater conditions in the pit were available, in order to reduce the uncertainty associated with groundwater, different pore pressure ratios ( r u ) were considered and a sensitivity approach was used in the stability assessments. The back-analyses of the observed instabilities including one or more benches in the overburden indicated that the most critical modes of failure for the south slope are circular and composite sliding surfaces. Although kinematical analyses suggested that structurally controlled failures would not be expected, one local planar failure that occurred in the south slope emphasizes that the possibility of local planar sliding should be considered when the dip of bedding planes locally exceed 20° and pore pressure becomes high. In addition, the back-analyses revealed that r u was probably between 0.3 and 0.4 and the residual shear strength along the bedding planes was critical when slope instabilities occurred along the south slope. The stability assessments for the current and the final south slope, which was planned by the mining organization operating the pit, indicated that some modifications in bench and slope geometries are necessary to achieve a factor of safety of 1.3, which is a commonly used value in open pit practice. In addition, these assessments also suggested that the most critical zone in the overburden was the thinly bedded marl in terms of stability, and at the thickest part of this material (30 m), the overall slope angles satisfying F = 1.3 at r u values of 0.2, 0.3 and 0.4 should be 18°, 17° and 15°, respectively. Except those in the thinly bedded marl, bench widths in the overburden units and coal seam are reduced and steeper slopes with F ≥ 1.3 were achieved.