## 3 Stability Analyses for the Initially Designed Geometry
### Selection of Physical-Mechanical Characteristics  
In order to determine the physical and mechanical characteristics necessary in the stability analyses, a total of 10 samples were collected, according to the standards [45], both from the areas where landslides occurred in the last 5 years (T1-T3 steps), as well as from the rest of the working fronts (T4-T6 steps), later to be analyzed in the specialized laboratories. The 10 samples, of approx. 30 kg each, were collected as follows:  
* S1 clays from step T1;
* S2 clayy sands from step T1;
* S3 clays from step T2;
* S4 clayy sands from step T2;
* S5 clays from step T3;
* S6 coal clay from step T4;
* S7 lignite from step T4 (layer X);
* S8 lignite from step T5 (layer VIII);
* S9 clays from step T5;
* S10 clays from step T6. The samples were then divided in two equal parts, packed in plastic bags (to maintain the natural moisture) and transported to the two laboratories (Earth Mechanics Laboratory of the University of Petrosani--approx. 1.5 h drive and the GeoLogic Laboratory in Calan--approx. 2.5 h drive). The samples collected from the working fronts of the Jilt North open pit were subjected to laboratory tests, and a series of physical and mechanical characteristics were determined,according to specific standards [45; 46; 47; 48]: moisture, particle size (granulometry), specific and volumetric weight, porosity, pore index, plasticity limits, consistency indexes, cohesion and angle of internal friction (shear strength), oedometric tests, etc. Based on the statistical processing of the values determined in the two laboratories and data retrieved from the documentations [43], the values presented in Table 4 were selected, and further used in the stability analyses. Considering the positive influence that lignite can exert on the stability reserve, due to its superior resistance characteristics compared to the surrounding (waste) rocks, Table 5 presents a situation regarding the position occupied by the lignite layers in relation to the slopes (existing, as of May 2023, and designed at the end of 2023). Next, in the model made with the help of the specialized software Slide (Figure 10), the values of the physical and mechanical characteristics (Table 4) were entered, after which the calculation program was run. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Types of Rocks Enountered in the Analysis Section** & \(\mathbf{Y_{\text{waste}}}\) & \(\mathbf{\epsilon_{\text{nate}}}\) & \(\mathbf{\varphi_{\text{nate}}}\) \\  & [**kN/m\({}^{3}\)**]** & [**kN/m\({}^{3}\)**]** & [\({}^{\circ}\)**] \\ \hline  & Sandy clays & 22.00 & 25.00 & 20.00 \\ \hline  & Clayey sands & 21.00 & 15.00 & 16.00 \\ \hline  & Coal clay (roof of layer X, intercalation & & \\  & between the banks of layer X, between & & \\  & layers X and XII and as discontinuity of & 18.00 & 33.25 & 28.00 \\  & layer XII) & & \\ \hline  & Lignite (layers V, VI, VII, VII, IX, X, XII) & 13.40 & 200.00 & 35.00 \\ \hline  & Compact clay (roof of the pressurized & & \\  & aquifer, bed of layer V, base terrain for & 19.00 & 52.00 & 30.00 \\  & the internal waste dump) & & & \\ \hline  & \(\mathbf{Y_{\text{waste}}}\), \(\mathbf{\epsilon_{\text{nate}}}\), \(\mathbf{\varphi_{\text{nate}}}\)—volumetric weight, cohesion, and angle of internal friction (at natural moisture). & & \\ \end{tabular}
\end{table} 
Table 4: The values of the physical–mechanical characteristics considered in the stability analyses [42; 43].