# Stability Conditions in Lignite Open Pits from Romania, Case Study: Olteu Open Pit
## 4 Conclusions  
As a result of this research, the following observations were made regarding the characteristics and behavior of rocks:  
* The lignite layers in the perimeter of the Olletu quarry are buried in sedimentary formations, being intercalated by clayey and marly rock layers;
* According to the grain size analyses, these rocks contain variable amounts of sand (generally fine sand), dust, and gravel;
* The geometry of the working fronts is uneven, being conditioned by the geomorphology of the region;
* The variation of the physical and mechanical properties of rocks is accentuated and highly influenced by the water content (moisture);
* The mechanical characteristics (cohesion and internal friction angle) decrease with increasing moisture, being dependent on hydro-meteorological conditions;
* The great variety of physical-mechanical characteristics required statistical processing and the selection of values considered representative for the stability analyzes;
* Considering the reduced resistance characteristics and the plastic behavior of the yellow-brown clay, the first layer from the land surface, it can be stated that it determines the occurrence of instability phenomena, such as sliding, disaggregation, dry flow, and plastic failure, regardless of the rock moisture;
* The stratigraphy is variable and not known precisely until the time of excavation (relatively small number of prospecting drillings on the advancement direction). The stability analyses were carried out for the areas of interest of the mining operator, respectively, for the forecasted slopes in 2022 according to the preliminary production plan. The results of the analyses show that the most unfavorable stability conditions occur in the case of front structures where the clay layer at the top of the slopes exceeds 10 m in thickness. In the presence of other disturbing factors (precipitation, vibrations, and seismic shocks), in most cases, large-scale instability phenomena can be triggered. It is recommended to provide slope angles of maximum 52\({}^{\circ}\) and step heights of 25-26 m for front slopes. In the case of the lateral slopes, in order to ensure the reserve of stability of the step configured in the clay layer, it is necessary to divide it into sub-steps of 10 m each, and to reduce the slope angle to the value of 45\({}^{\circ}\), resulting in a general slope angle of 25\({}^{\circ}\). By studying the behavior of the rocks that appear in the geometry of the working and final slopes depending on the thickness of the layers, their location in the lithological structure, and the existing meteorological conditions allowed the authors to draw some general conclusions regarding the necessary measures that must be taken in order to safely continue the lignite extraction activity in Olletu open pit. However, as the general geology (in particular the lithological structure), as well as other natural and anthropogenic factors, is similar throughout the entire Berbesti Mining Basin, these results can provide valuable information to mining developers and engineers operating the other three open pits on how to deal with the top layer of clay so as to avoid unwanted events (large landslides capable of producing significant economic or even human losses). The present paper has the merit of addressing a geotechnical phenomenon in the mining perimeters of Olletnia that was not particularly taken into account in the stage of designing the geometry of the open-pit slopes, namely the dry or wet flow of the yellow clay from the upper part of the open pit. Such flows generate changes in the state of stresses and strains and can lead to the triggering of slope failure along the entire height. Thus, detailed laboratory studies were carried out and, following stability analyses, the main failure mechanisms were defined.