# Stability Conditions in Lignite Open Pits from Romania, Case Study: Olteu Open Pit
## 3 Results and Discussion
### Analysis of the Stability of In Situ Slopes  
The stability analyses were carried out in seven sections, including two longitudinal (L1 and L2) and five cross (P0, P1, P2, P4 and P5), and in each section, the position of the working fronts was taken into account at the level of December 2021 and quarters I-IV of 2022, which led to the analysis of 13 geotechnical profiles, drawn on the situation plan with the projection of the open-pit evolution (Figure 6). The stability analyses for the lateral slopes were performed only in the case of the slopes located on the southwest side, as slopes on the north-eastern side have heights that do not exceed 15 m, so they do not raise stability problems. The cases of lateral slopes in sections P0, P1, and P5 were analyzed from the perspective of ascending slopes, considering that their height increases and substantial changes in the geometry of the steps are required. We mention that, at the request of the research team, the mining operator made available eight sections L1, L2, P0-P5, essential for the development of the stability study. Of the provided sections, the P3 cross section was not used, being outside the interest area. The stability calculations were carried out in three variants, respectively:  
Figure 6: Evolution of Olletu open pit and the longitudinal and cross sections [56]. * Drained slopes, unaffected by external factors (F\({}_{\text{s1}}\));
* Undrained slopes, where pore water pressure can induce landslides (F\({}_{\text{s2}}\));
* Undrained slopes and under the influence of seismic shocks, caused by the vibrations generated by the excavator and conveyor belts and/or the occurrence of an earthquake (F\({}_{\text{s3}}\)). The results of the stability analyses are presented in Table 2 for the working front slopes in sections L1 and L2, and for lateral sections P2 and P4 (it is planned that in the IV quarter of 2022, the E02 excavator will be withdrawn in the approximately same position as December 2021, and the excavation will be carried out in the lateral slope, corresponding to cross sections P2 and P4, thus becoming the working front) taking into account the advance of the open pit, in Table 27, for the lateral slopes, P0, P1, and P5 (in accordance with the semester in which they materialize) and in Figures 7-10. Figure 8: Section L2, F\({}_{\rm s1}\) = 1.22 (December 2021). Figure 7: Section L1, F\({}_{\rm s1}\) = 1.01 (December 2021). The stability analyses were performed using the SLOPE software, a software specialized in geotechnical problems, considering that a possible landslide can occur along a curved sliding surface. This software performs stability analyses based on limit equilibrium theory (LEM), the safety factor, or the stability coefficient being given by the ratio between the sum of the moments of the resistance forces and the sum of the moments of the forces generating the slide. For this purpose, based on the recommendations of the literature [17; 18; 21; 34], the methods of Fellenius, Bishop, Simplified Janbu and Morgenstern-Price were used. The values presented in Table 2 and 3 are the lower ones as determined for each section and quarter. As a result of carrying out the stability analyses, the following conclusions can be drawn:  
* The stability analyses were performed for the initial conditions of the slopes only in the L1 (December 2021) and L2 (December 2021) sections, using the stratigraphic columns provided by the mining operator, and confirm the situation observed during the field investigations. Thus, in the area of L1 section, for the configuration of the slope from  
Figure 10: Section P4, IV quarter, F\({}_{\rm s1}\) = 1.01. Figure 9: Section P2, IV quarter, F\({}_{\rm s1}\) = 0.80. December 2021, instability phenomena of the clay and marl layers were manifested. The stability analysis carried out led to a value of 1.01 for the stability factor under conditions of no external stress on the slope (limit equilibrium state) and to sub-unit values for the case where pore water pressure and/or the effect of vibrations were manifested.