# Stability Conditions in Lignite Open Pits from Romania, Case Study: Olteu Open Pit
###### Abstract  
The problem of the slope stability of open pit mines is one of constant interest and great importance, both during the period of operation, but also post-closure. The research focused on the Olteu open pit (located in Berbesti Mining Basin, Romania) and was directed in such a way as to allow consideration in the stability analyses of natural (predisposing the investigated area to landslides) and anthropogenic (specific to open pit mining) factors and causes as well as their combined effect. The field investigations (observations on the technical condition of the slopes, discussions with the technical personnel from Olteu open pit, and sampling) were completed with analyses and laboratory tests (physical-mechanical properties of rocks in the composition of the slopes). The stability analyses took into account different hypotheses related to the actual geometry of the working slopes, and a predictive analysis was also carried out for the forecasted evolution of the working fronts and lateral slopes. Following stability analyses, it was found that for most slopes, the stability reserve is insufficient to allow continuing lignite exploitation under safe conditions. The last part of the paper presents the solutions identified by the authors (adoption of new geometries of the working front and lateral slopes) in order to increase the stability reserve to a minimum acceptable level, which would allow the safe continuation of lignite extraction, and, in the end, some practical recommendations are briefly presented. l 2022 202202020202202  
To support the research topic, we mention two large landslides which involved the displacement of important quantities of material by sliding and generated important material losses (destruction of machinery and equipment) and supplementary costs related to the elimination of the effects of these slides: the sliding of the working front of the Alunu open pit in 2019 [5,6] (total destruction of a large-capacity excavator, the loading trolley and the transport lines); and the sliding of the Alunu open pit steps system in 2020 (which led to the cessation of productive activities for more than 2 years). Additionally, several instability phenomena were recorded over time that involved the movement of material from the dumps, from the working slopes or from the natural slopes, damage to households and communication and utility networks, and geostructural changes, mainly as a result of the influence of a complex of factors (geological, hydrogeological, hydrometeorological, climatic and anthropological), among which we mention the following: the sliding occurred on the southern part of the Berbesti-Alunu mining perimeter from 2010; about 20 landslides occurred in the Panga mining perimeter (on the northern, southern, or north-eastern parts of the open-pit, and also on the slopes of the external dump) between 2008 and 2016; and the sliding of the West Berbesti external dump from 2017 [6,7]. In fact, landslides that affect either the working or final slopes of lignite open pits, or the steps of internal or external dumps, are quite common phenomena (their occurrence and manifestation being signaled all over Europe, for example, in Poland, Czech Republic, Greece, Serbia, Kosovo, Germany, and Spain, but also in other parts of the world), being described in specialized literature (in terms of favoring or triggering factors and causes, unfolding mechanisms, damage caused, and preventive measures for the future) [8, 9, 10, 11, 12, 13, 14, 15]. Although the triggers of major landslide can often be technically detected, their prediction and mitigation remain a major challenge, both for practicalians and scholars. The slope angle and height of the slopes are the main geometric elements that can influence their stability/instability [16, 17, 18, 19, 20, 21, 22, 23, 24, 25].