# Stability Conditions in Lignite Open Pits from Romania, Case Study: Olteu Open Pit
###### Abstract
Therefore, the design and compliance with the geometry of the in situ and dump slopes are extremely important elements for maintaining their stability both in the exploitation process and in the closing and reclamation phases of the open pits. In addition, the identification and control of the factors that influence the technical condition of the slopes must represent a permanent concern throughout the life of the open pit. The stability of slopes is influenced by numerous factors, which can be grouped into several categories (geological, mechanical, meteorological, etc.) [16, 17, 18, 19, 20, 21, 22, 23, 24, 25]. It is difficult to assess the extent to which these factors contribute to triggering a landslide, which is why they must be analyzed and controlled as carefully as possible for each particular situation [25, 27, 28, 29, 30, 31]. In the conditions where unforeseen sliding phenomena occur, the causes must be analyzed, and proper measures to eliminate them must be taken [29, 32, 33, 34, 35, 36]. In any situation, monitoring the stability conditions is absolutely necessary, early warning systems being the solution to avoid sliding phenomena that can result in material and human losses [37, 38, 39, 40, 41, 42, 43]. Sometimes, the evolution of major landslides can be subdivided into several sliding models, offering the possibility to analyze the evolution, in terms of space and time, of similar ones [44]. Slope stability in mining perimeters is important for the security of the production processes, mining equipment, and especially for workers. The purpose of the stability analyzes is to evaluate the safety factor or to check the stability reserve of a slope and, based on it, to reduce the geotechnical risks that may occur by identifying and implementing the appropriate stabilization measures [17, 19, 27, 28, 45]. The specialized literature has been enriched over time with numerous stability analysis methods developed by researchers in the field, among which we mention the following:  
* The analysis methods based on the limit equilibrium concept (LEM) [17, 18, 21, 22, 23, 24, 25, 26, 27, 28, 34] considers the soil both a load and a resistance factor, being commonly used, as it gives accurate results on an acceptable level. * The finite element method [21, 25, 46] is a modern, precise method that can be applied to complex structures. It allows the consideration of hydrological and hydrogeological conditions, the elements of support and improvement of land, and drainage systems. It does not require the imposition of preliminary assumptions, and it provides indications regarding the evolution of the safety factor and deformations during the entire sliding process. * Methods based on 3D analyses [47; 48; 49; 50] are applied in the case of slopes with complex geometries, variable structure, and partial loading, with a very well-defined breaking mechanism, and assume the addition of the third dimension in the 2D procedures, which leads to an increase in the number of unknowns. Three-dimensional methods are used quite rarely due to the fact that they have limited applicability. * Probabilistic methods [21; 51; 52; 53; 25] assume, in principle, the probabilistic analysis/processing of the physical and mechanical properties of the rocks and the probabilistic calculations by numerical or analytical methods. * Different combinations of the above-mentioned methods. Dynamic programming methods can be used to determine the critical sliding surface and the stability factor and allow the incorporation of aspects such as spatial variability and time decay of the mechanical parameters. The results obtained by this method may differ substantially from those obtained by static models [54; 55]. In this paper, slope stability analysis was performed using the limit equilibrium methods (Fellenius, Bishop, Simplified Janbu and Morgenstern-Price), as they offer satisfactory and credible results.