# Stability Conditions in Lignite Open Pits from Romania, Case Study: Olteu Open Pit
## 2 Materials and Methods
### Field Observations and Laboratory Tests
#### 2.2.3 The Lignite  
The first and second lignite layers (I, II a and II b, see Figure 2) represent the main (with economic importance) lignite layers in the Olletu perimeter by quantity. They are thick and extend over almost the entire mining field. Within the Olletu mining perimeter, the first layer varies between 2.3 and 3.2 m thick. Qualitatively and qualitatively, it represents the main layer of lignite [56]. The second lignite layer is complex from a lithological point of view. It consists of two coal banks (layers II a and b) separated by a thin intercalation of sandy marl rocks. The second layer varies from 2.6 to 5 m in thickness [56]. The two lignite layers are separated in the roof and in the bed by sandy marl layers with varying thicknesses. Generally, the physical-mechanical properties of lignite are favorable in terms of slope stability, but their values also vary as a result of the partially woody structure and clay inclusions. Field research also led to the following findings:  
* The geometric non-uniformity of the excavation fronts is caused by the morphology of the land, as the excavation process is completed in one step. This way of excavation leads to variations of the geometric elements, both on the width and on the depth (expansion) of the working front. * Phenomena of fragmentation and detachment of the rocks from the slopes in conditions of low moisture and plastic flow tendencies of the clay from the upper part of the step under conditions of its saturation. * Local manifestation of instability, affecting the entire height of the step, with variable extent depending on the structure of the massif and the hydro-meteorological conditions. * Under the conditions of the initial geometry (December 2021) of the excavation fronts, the risk of massive landslides, which would involve large volumes of rocks and affect the safety of machinery and personnel, is relatively low. * Taking into account the terrain morphology, the difficulties of ensuring a relatively uniform geometry of the excavation fronts will be accentuated with their advance. * In the areas with increased amplitudes (the western part of the perimeter), taking into account the functional technological parameters of the E-02 excavator (ERC 1400-30/7 type) and possible instability phenomena, the design of appropriate exploitation technologies must be imposed so that the geometry of the slopes ensures their stability. In order to carry out the stability analyses, three sampling campaigns were carried out (22 October; 12 November, and 2 December 2021) from all the rock layers encountered, and the samples were analyzed within the Soil Mechanics Laboratory (LMP) of the University of Petrosani (Figure 5). To verify the results, some tests were demanded from the Calan GeoLogic laboratory, especially for the first layer from the surface, consisting of yellow-brown clay (Table 1). Following the analyses, it was clear that the yellow-brown clay was the least resistant rock. It has the highest affinity for water, and therefore, the deterioration of these characteristics is likely under unfavorable hydro-meteorological conditions. In this situation, the state of tension in the massif changes, and structural deformations occur. This conclusion is also confirmed by field observations. Stability analyses were performed, taking into account these aspects.