# Stability Conditions in Lignite Open Pits from Romania, Case Study: Olteu Open Pit
## 2 Materials and Methods  
### Problem Statement  
Berbesti Mining Basin (Figure 1) is localized within the Amaradia-Taraia interfluve, the basin also bearing this name in the specialized literature. Geographically, the basin is located in the hilly area between Gorj and Valcea counties. The high-capacity mining exploitations (open pits) operating in Berbesti Mining Basin are Oltetu, Alunu, Panga, and West Berbesti. These exploitations have produced, since the date of commissioning and up to now, a quantity of approximately 90 million tons of lignite, and approximately 550 million cubic meters of waste rocks resulted to obtain this production. The average lignite/overburden ratio, of approximately 1/6.2 t/m\({}^{3}\), is due to the location of these perimeters in a hilly area [56].  
For the Oltetu open pit (located in the Alunu exploitation perimeter along with Alunu open pit), part of Govora Electric-Heating Plant (more precisely, the mining division), which is planned to be kept in operation at least until 2025, at the end of 2021, it was  
Figure 1: Berbesti Mining Basin [57; 58].  
considered imperative to conduct a research on the stability of the actual working fronts and lateral slopes, as well as a predictive one, for the 2022 forecasted situation.  
The major structural unit of which the lignite deposits in Northern Oltenia are a part is the premontane \"Getic Depression\", which took over the function of a sedimentation area, evolving as such in the Paleogene and Neogene [59].  
The premontane depression, formed during the Laramic movements, subsequently endured a sedimentation process that began in the Paleogene and during which several discontinuities with non-general character were noted. The sedimentary rocks of the Getic Depression belong to the Paleogene-Quaternary interval, and they are kilometers thick [60,61].  
The studies carried out between 1985 and 1992 by [PERSON], [PERSON] et al. [62, 63, 64] led to the division of the Dacian-Romanian deposits into three lithost stratigraphic formations (Figure 2). This division represents a local fragmentation of the aforementioned deposits, the Jiu-Motru Formation incorporating the other two, the Berbesti Formation and the Cindesti Formation, with lithost stratigraphic differences specific to each area. It can also be noted the comprehensive nature of the formation of Jiu-Motru, which includes both Dacian and Romanian.  
Figure 2: Lithost stratigraphy of Berbesti Mining Basin [61].  
The exploitation of lignite in the Olletu open pits includes a series of activities, such as excavation (mostly carried out in continuous flow with bucket wheel excavators, in working tiers with heights of 25-30 m); the transport of the excavated material (on belt conveyors); the deposition of waste rocks with dumping machines (into internal and external dumps); and the deposition of coal using storage machines.  
In the particular case of the Olletu open pit, at the upper part of the excavation slope, there is a layer of clay that is very unstable, sliding frequently and covering the excavation front (Figure 3).  
For this reason, there are losses of time and productivity of the machines, which is reflected in the economic efficiency of the mining activity. Moreover, the continuous sliding of the clay layer can cause the lower layers to move as well, triggering a large-scale slide that can put the excavator in danger.