Slope Stability Analysis for the Design of a New Lignite Open-Pit Mine

The paper presents geotechnical engineering studies for the design of a new lignite open-pit mine in central Poland. The lignite deposit occur in a deep tectonic rift formed in Mesozoic rock characterized by complex geological conditions. The rift is approximately 10 km long, 1 km wide and 50-250 m deep and filled with Neogene and Quaternary sediments. The design of the open-pit mine required geotechnical analysis to reveal possible problems that may occur during its construction and operation. The author of this paper had opportunity to perform slope stability studies and geotechnical analyses within the project conducted by Poltegor-Institute. The calculation procedure enabled determination of slope stability on the design of excavation and spoil dump. It included 21 analyses using Flac v.7 of which 14 regarded the slopes of the pit, 6 the slopes of the external spoil dump and one covered both areas. The results indicated that the factor of safety Fs ranges 0.75-1.65 for the pit and 1.12-1.60 for the dump. In risk areas slope inclination were lowered due to likelihood of developing of landslide processes. These included spoil dump area close to S-8 road. Due to relatively limited geotechnical data in same areas especially on northern slope and western part of spoil dump detailed geotechnical investigations will be necessary. The instability problems can be caused by the groundwater conditions and the presence of high compressibility organic peats in the spoil dump bedrock layers. Comprehensive identification and monitoring of geotechnical risks for the mine slopes and storage of overburden should be a continuous process. This activity should start from the beginning stages of construction and should be conducted also during exploration and continuing to mine final reclamation to reduce potential natural hazard impact on mining and the natural environment.