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Article

Reassessment of the Stability Conditions in the Lignite Open Pits of Oltenia (Romania) in Relation to the New Local Seismic Context as an Imperative for Sustainable Mining

by
Florin Faur
,
Izabela-Maria Apostu
* and
Maria Lazăr
Department of Environmental Engineering and Geology, Faculty of Mining, University of Petrosani, 332006 Petrosani, Romania
*
Author to whom correspondence should be addressed.
Sustainability 2024, 16(4), 1384; https://doi.org/10.3390/su16041384
Submission received: 15 December 2023 / Revised: 26 January 2024 / Accepted: 31 January 2024 / Published: 6 February 2024
Browse Figures
Versions Notes

Abstract

:
Responsible mining considers the three pillars of sustainability, namely the environment, the economy and social welfare. As a result, exploitation of deposits of useful mineral substances, as an economic activity, must be carried out taking into account several requirements, among which is the generation of a reduced impact on the environment and local communities. Sliding of open pits and waste dumps slopes represents a major risk, which endangers workers and machinery, as well as the components of the natural and built environment in the influenced area. In order to avoid such phenomena and their consequences, it is imperative to analyze the stability conditions whenever their possible triggering factors appear (such as earthquakes). Between February and March of 2023, the region of Oltenia (south-west Romania) was affected by intense seismic activity, out of normal patterns. Considering this series of seismic events, in this paper we aimed at reevaluating the stability conditions of the slopes of the working fronts and of the internal dumps in the lignite open pits located in the region in this new context. Research focused on three lignite open pits, namely Peșteana North (Rovinari mining perimeter), Jilț North (Jilț mining perimeter), Berbești–Alunu (Berbești mining perimeter). After describing the general geology and tectonics of the areas under study, the seismic episode that affected the region at the beginning of 2023 (which in fact extended until November) is highlighted, with increased attention given to the earthquakes of 13 and 14 February 2023, with a local magnitude ML ≥ 5. The most important part of the study is represented by the stability analyses, carried out for normal conditions (considering the characteristics of the rocks at natural humidity and in the absence of the influence of external factors) and under seismic conditions, characterized by a peak ground acceleration equal to the maximum acceleration considered for the location area of the mining perimeters taken into study. The results of the study showed that, for most of the analyzed situations, a renewal of the technical exploitation documentation is required, which, taking into account the results of this study, must adopt new geometries of the excavation and deposition fronts, so that the objectives in terms of operational and workplace safety imposed by legislation are respected.

1. Introduction

Romania’s energy system is based on a mix, which includes both the use of non-renewable resources (fossil and nuclear fuels) and renewable ones (wind, solar, hydro and biomass).
Even though coal-based energy (lignite) still contributes an average of only 15%, depending on the time and season (compared to approx. 30% some 20 years ago), it continues to play an important role, both now and in the next 8–10 years (during which it will be phased out).
Therefore, the sustainability of the energy system, directly related to national energy security, depends to an important extent on the ability of mining operations to supply coal to operating power plants.
Of course, the interruption of extractive activity which can be caused by the occurrence of landslides (the risk of occurrence of such phenomena increasing in the conditions of intense seismic activity) makes it difficult to ensure the security or even the operation (in periods when coal-based energy approaches or exceeds 20%, such as extreme negative temperatures, drought, atmospheric calm, etc.) of the national energy system.
Currently, in the mining basin of Oltenia, in the two sectors of activity (mining and energy), there are employed approx. 10,000 people. Because the phase-out of coal from the energy mix is a staggered process over 8–10 years, it is assumed that most of these employees will leave the system naturally (through retirement).
In the event of the occurrence of landslide-type events, although normally these lead to the temporary disruption of the activity, if human casualties are recorded, then the definitive suspension of the activity in the respective open pit(s) can be considered for security reasons. This translates into job losses for the rest of the workforce and a steep rise in the unemployment rate in the region, with no viable employment alternatives, which is contradictory to sustainable development goals.
Another aspect related to the economic side refers to partial or total destruction of machinery and equipment caught in landslides.
Most of the time, landslides also affect neighboring areas (whether they are natural ecosystems, inhabited areas and infrastructures, farmland, etc.), which is why the predictability of the occurrence of such events and their prevention whenever possible is a priority for mining operators and authorities, in order to achieve the goal of the sustainable development of society.
Slopes’ stability (both of the open pits and waste dumps) is a most important issue that should be monitored during the life of the open pits but also in the post-closure period. The lignite open pits mostly operate in soft rocks (sedimentary rocks and soils), such as clays, marls, sands and different combinations of them, and landslides are phenomena with a high or very high probability of occurrence.
Slope failure in any open pit may lead to serious consequences. Among the more serious consequences are damage or destruction of machinery (bucket wheel excavators—BWEs and spreaders), negative effects on nearby ecosystems, the destruction of road and railway infrastructure or even the death of operating staff or people living in neighboring areas [1,2].
As previous studies highlighted, slope failure may affect either the working steps of the open pits or the slopes of the internal or external dumps all over the world. These landslides are described by the literature in terms of favoring or triggering factors and causes, unfolding mechanisms, amount or extent of the damage caused and possible prevention and mitigation measures [3,4,5,6,7,8,9,10].
For the sustainable development of lignite mining, whenever unexpected sliding phenomena occur their causes must be identified, and appropriate measures must be taken in order to eliminate their effects [1,2,11,12,13,14,15,16]. Monitoring stability conditions and the development of early warning systems are the main ways to prevent slope failure that may result in material and/or human loses [1,17,18,19,20,21,22,23,24].
One of the most frequent triggering factors of landslides is represented by seismic activity (earthquakes).
Earthquakes, of tectonic origin or generated by the displacement of earth masses along the path of existing faults, significantly influence the stability of a slope (reducing the stability reserve), often causing it to pass from an equilibrium state into an unstable one.
During an earthquake, vertical and horizontal forces appear and act on the slopes, which may trigger landslides that usually involve large volumes of rock. Vertical forces reduce the effective normal pressure on sliding surfaces, while much stronger horizontal forces play a decisive role in slope stability. Moreover, because during an earthquake the seismic action can have any direction, as a rule, in calculations the horizontal component is considered, this being the most unfavorable case in terms of the effect on the stability of the slopes [25,26,27,28,29,30,31,32].
Such cases of earthquake-induced landslides affecting open pits are well inventoried, documented and described by the literature [33,34,35,36].
Another mechanism by which earthquakes influences the stability of a slope is related to the effect they have on the state of consistency of the rocks from its structure.
Thus, even if the rocks, clays and marls have a moisture content close to the saturation limit, during an earthquake they change their plastic state into a fluid one. This fluid state, and flow displacement, is maintained during the seismic event. The rocks tend to return to their initial plastic state immediately after the earthquake stops [37,38].
This type of behavior is called thixotropy (in the case of clays and marls) or liquefaction (in the case of sands and sandy rocks). All these types of rocks are present in the structure of the working fronts (faces) or as a mixture in the waste dumps of the Oltenia mining basin.
This type of rock movement, generated by thixotropy or liquefaction, can put in motion large volumes of rock (directly depending on the slope configuration); they are characterized by high speed, sudden occurrence and the absence of warning signs [37,38]. These characteristics make them extremely dangerous for the operating stuff but also for the machinery itself.
In order to highlight the influence that earthquakes can have on the stability conditions in the lignite open pits of Oltenia, we chose, as case studies, three mining perimeters (lignite open pits), Peșteana North, Jilț North and Berbești–Alunu, located in as many mining basins (sub-basins of the larger Oltenia mining basin), for which we performed stability analyses under normal (static) conditions but also under seismic conditions, taking into account the maximum seismic acceleration for the location area.

2. General Geology and Local Tectonics

2.1. Geology and Stratigraphy of the Region

The geological formations in the area of the Oltenia mining basin were formed within a process of sedimentogenesis that began in the Paleogene and ended in the Upper Pliocene, within the Getic Depression tectono-genetic unit. Functioning as a sedimentation basin, the deposits in the region are mainly made up of sandstones, conglomerates, sands and gravels, which host the lignite layers. The foundation of the Getic Depression consists of crystalline schists with granitic inclusions and a cover of Mesozoic sedimentary formations [1,39,40].
The stratigraphy of the region, highlighted by exploratory research, presents a complete sequence of sedimentary formations starting from the Paleogene and ending with the Upper Pliocene, over which the Quaternary deposits are disposed (Table 1).
Geologically, the deposits in Oltenia belong to the Pliocene formations (Dacian and Romanian) and are made up of 21 layers of lignite, with variable extents, separated by sterile rocks (soft, cohesive and non-cohesive rocks, predominantly clays and sandy rocks) [2,39,40].
The thickness of the lignite layers varies from a few centimeters to several meters, appearing in a compact form or in the form of several coal banks, which make up the complex of a layer [2].

2.2. Tectonics

2.2.1. Tectonics of the Rovinari Mining Sub-Basin

Being the most extensive sub-basin in Oltenia, it is also characterized by the most complex tectonics.
In the Rovinari mining sub-basin, the movements that affected the Getic Depression did not cause significant deformations, the Sarmato-Pliocene deposits being almost horizontal. In the Late Pliocene, simultaneously with the more active uplift of the Carpathians, there was a narrowing of the sedimentation basin, and in the Quaternary, the Wallachian movements led to the definition of the current morphostructure.
To the west of the Motru River and in the SW part of the Motru-Jiu interfluve, the Pliocene deposits are arranged in a homocline structure. In the NE part of this interfluve, two wide anticlinal folds are outlined, with slopes of 3–7° on the flanks, namely the Strâmba–Rovinari and Negomir–Peșteana anticlines, separated by the Bâlteni syncline. In the north of the Strâmba–Rovinari anticline, the Câlnic–Târgu Jiu syncline is outlined (Figure 1) [39,40].
Besides these major creases, small undulations appear in the area. They are attributed to a sedimentogenesis process that did not occur during the deposition and formation of the lignite layers.
The flanks of the anticlines were compartmentalized by normal faults (the compartment in the roof moved down the slope of the fault), thus resulting in a gradual fall of the deposit towards the submerged areas of the synclines [39,40].

2.2.2. Tectonics of the Jilț Mining Sub-Basin

In the case of the Jilț mining sub-basin, the analysis carried out highlights that the exploitation area of the lignite deposit is part of the Getic Depression. Its constitutive formations are of Cretaceous, Paleogene, Neogene (Pliocene) and Quaternary age, within which the Dacian, Romanian and Pleistocene type deposits are the formations bearing lignite layers [43].
The tectonics of Neogene deposits is represented by both folds and faults, which correspond to deep structures and usually channel the river valleys. In the investigated area, namely the area with mining activity, the presence of the Baia de Aramă–Runcurelu–Plopşoru fault can be noted, which broadly follows the routes of the Jilţul Mare River and the Runcurelu stream. This fault separates the sector Dragoteşti–Mătăsari from the sector Tehomir–Cojmăneşti–Dealul Arşiţei [44].

2.2.3. Tectonics of the Berbeşti Mining Sub-Basin

The lignite deposit in the Berbeşti mining sub-basin is not affected by major tectonic disturbances, compared to the deposits in other mining sub-basins.
As a result of the action of external forces on the deposit, phenomena of deformation, translation and rotation appeared, which led to the formation of wide, straight and small-amplitude folds, longitudinal cracking framed in centimeter-sized macrocracks, as well as the appearance of faults whose horizontal and vertical jumps do not exceed the order of meters, as is the case of the Gilort fault [45].
With the exception of the Dacian deposits in the northern part of the Gilort–Amaradia sector, which are contained in the Pădurea Bolovanului syncline, oriented NW–SE, the other deposits form a wide W–E oriented monocline, inclined to the south 2–6° in the Albeni area, increasing to 5–10° towards the eastern limit [46].

3. Seismic Framing of the Area

3.1. Current Seismic Framing

According to [47,48], the area of the Rovinari and Jilț sub-basins are characterized by a 71 (MSK scale) seismic intensity, and a return period of about 100 years. The other sub-basin, namely Berbești, is characterized by an 8 (MSK scale) seismic intensity, and a return period of about 50 years.
According to [48], the peak ground acceleration is 0.15 g in the Rovinari and Jilț sub-basins, respectively, and ag = 0.20 g in the Berbești sub-basin. The average recurrence interval (IMR) is 225 years, with an overtaking probability of 20%. The control (corner) period Tc of the response spectrum is 0.7 s (Figure 2).
The NW region of Târgu Jiu, Gorj County, was known until recently as a seismic zone that produces, as a rule, small and moderate surface earthquakes. The short seismic history of the area (1900–2022, according to the ROMPLUS Catalogue) shows that during this entire period there were only four earthquakes with a magnitude ≥4 Mw (4.5 Mw—9 July 1912, 5.2 Mw—20 June 1943, 4 Mw—26 July 1962, 4.5 Mw—4 May 1963) [49].
The significant earthquakes of 1912 and 1963, although they occurred in the same seismogenic zone, were located in the Hațeg Basin, i.e., north of the Southern Carpathians.

3.2. Description of Seismic Activity in 2023

The unusually intense seismic activity for this area in 2023 began on 13 February, with the occurrence of the 5.2 ML earthquake, turning the Oltenia area into a real epicenter of research related to the reassessment of the seismic hazard and risk.
By the time this study was carried out (mid-November 2023), over 2500 seismic movements were recorded in the area of interest, of which over 40 had magnitude ≥ 3 ML, updated after [50] (Figure 3):
  • over 30 earthquakes with a magnitude between 3–3.9 ML;
  • nine earthquakes with a magnitude between 4–4.9 ML: 4.1 ML (3.2 Mw)—14 February 2023, 4.2 ML (3.3 Mw)—16 February 2023, 4.3 ML (3.3 Mw)—17 February 2023, 4 ML (3.2 Mw)—22 February 2023, 4.9 ML (4.4 Mw) 20 March 2023, 4.2 ML (3.3 Mw)—19 June 2023, 4.2 ML (3.3 Mw)—18 July 2023; 4.5 ML (3.3 MW)—21 August 2023; 4.3 ML (3.3 Mw)—10 November 2023;
  • two earthquakes with magnitude ≥5 ML: 5.2 ML (4.8 Mw)—13 February 2023 and 5.7 ML (5.4 Mw)—14 February 2023.
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Figure 3. Location of epicenters and hypocenters (up right corner) of earthquakes (up to 15 March 2023), modified after [51].
Figure 3. Location of epicenters and hypocenters (up right corner) of earthquakes (up to 15 March 2023), modified after [51].
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Local magnitude (ML) and moment magnitude (Mw) are two different ways of determining the size of an earthquake. The ML magnitude scale (Richter magnitude) is calculated from the maximum amplitude of ground motion recorded at different seismic stations [52], and Mw represents the energy released in the hypocenter at the time of the earthquake. Mw is different from ML, usually having a lower value than ML (between these two measuring scales there is a proportional relationship) [53].
According to the Seismological Society of America [54], for earthquakes with 4.3 < ML ≤ 6.5, the following relationship (1) can be applied:
Mw = b ∙ ML + CII,
where b = 1.28, CII = −1.50; b depends on the combined effects of source scaling and crustal attenuation; CII on regional attenuation, focal depth and rigidity at source.
The energy released by earthquakes is converted into seismic waves that propagate on the Earth’s surface. The amount of energy released is proportional to the magnitude of the earthquake (Mw)—larger earthquakes release much more energy than smaller ones. A one-unit increase in magnitude (ML) corresponds to an increase in energy released of about 32 times [55].
The seismic doublet type system recorded during this period represents the most intense activation of the area since there has been seismic monitoring in this region. The two earthquakes with magnitudes of 5.2 and 5.7 ML (as well as the one with the magnitude of 4.9 ML) were felt with intensities V–VI on the Mercalli scale in the epicentral area and produced minor and moderate damage [56].
It should be stated that the high number of aftershocks recorded after the two earthquakes with magnitude ≥5 ML is explained by the development of the National Seismic Network. This development has enabled both more accurate detection and recording of earthquakes and microearthquakes. It is appreciated that even the moderate earthquakes of the past were followed by a multitude of aftershocks, unrecorded due to the absence of measuring instruments as well as the lower level of understanding of seismic phenomena.
The Gorj earthquakes have a tectonic nature and are generated by the stresses accumulated in the contact zone between the Moesic Platform and the Southern Carpathian Orogene. Most fault planes are normal, generally oriented NE–SW.
The southern sector of the Southern Carpathians is made up of several small tectonic blocks. At the level of the crust, several systems of longitudinal, transverse and oblique faults are known, with the orientation determined by the mountain range. These led to the formation of depression areas that during the Neogene period were affected by extensional and transtensional forces, i.e., forces that cause both widening and lateral movement of rocks [56] (Figure 4).
Data related to the two earthquakes with magnitude ≥ 5 ML, recorded on 13 and 14 February 2023, are presented below.
The earthquakes were recorded by the seismic stations operated by the National Research and Development Institute for Earth Physics (Romanian Seismic Network—RSN) and, in this paper, data from the five closest strong motion stations being presented (Figure 5):
  • GZR—Gura Zlata;
  • HERR—Băile Herculane;
  • MHISU—Drobeta-Turnu Severin;
  • RMGR—Halânga;
  • SRE—Strehaia.
The peak (maximum) ground acceleration recorded (PGA) for the strongest earthquake was 106.7 cm/s2, while the maximum computed spectral acceleration (SA) was 307.1 cm/s2, corresponding to a period of 0.23 s, on the North-South (NS) component of GZR station [56].
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Figure 5. Map of the two earthquake epicenters and the nearby RSN stations, modified after [56].
Figure 5. Map of the two earthquake epicenters and the nearby RSN stations, modified after [56].
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Table 2 shows peak ground motion parameters for the two earthquakes with magnitude ≥5 ML.
Since, as we have shown in paragraph 1, in the framework of stability analysis we are particularly interested in the horizontal component of the ground acceleration, Figure 6 shows the graph of this parameter for the earthquake with magnitude ML = 5.7 from 14 February 2023, recorded at the GZR station (the maximum recorded peak ground acceleration PGA = 106.7 cm/s2).
As can be observed from Table 1 and Figure 6, in the case of the earthquake with magnitude ML = 5.7 from 14 February 2023, the maximum acceleration was recorded at the GZR station, and the duration was of 23 s in the N–S direction. The longest duration was recorded at the MHISU station, of 28 s, both in the E–W and vertical directions, but for much lower ground accelerations of 24.7 and 11.8 cm/s2, respectively.
For the SRE station, located south of two of the areas of interest of the present study (the Rovinari and Jilț mining sub-basins), the maximum acceleration was 37.6 cm/s2, in the E–W direction, and the maximum duration, of 23 s, was recorded in the vertical direction.
However, looking at the map in Figure 5, we notice that the two mining sub-basins are approximately halfway between the epicentral areas and the SRE station (50.8 km), which is why we can appreciate that the maximum acceleration in the area of the lignite open pits under study was higher than that the one recorded at this station.
Unfortunately, for the third area studied, the Berbești–Alunu mining sub-basin, located to the ESE of the epicentral area, there is no seismic station in the vicinity, which prevents us from assessing what was the maximum acceleration recorded and which was the duration of the earthquake.
Regarding the seismic future of the Oltenia region (Gorj County in particular), according to specialists, it is premature to make assessments in this regard. However, according to statements made by the general director of National Research and Development Institute for Earth Physics [58], we can expect in the future that earthquakes with a local magnitude (ML) of 6 or even 6.5 will be recorded.

4. Considerations on the Stability of Mining Workings under the New Seismic Conditions

In order to highlight the influence that seismic movements in the Oltenia mining region can have on the stability of the mining works in the three studied perimeters, we considered several previous studies, conducted by the authors and their collaborators [1,2,38,41,59,60,61].
Thus, we started from the results of the stability analysis carried out for the situations projected at the end of 2023, results obtained in conditions of natural moisture of the rocks from the composition of the working slopes and internal dumps and without taking into account the influence of external factors, from which we selected some sections for analysis (considering that they include the main rock types characteristic of the three exploitation perimeters and representative stratigraphies). The selected analysis sections for the present study are those for which low values of the stability factor were obtained, considering that they are the most prone to a loss of balance if a significant seismic event occurs in the area.
For this purpose, the stability analyses were performed using classic (based on limit equilibrium theory) methods (Fellenius, Bishop and Simplified Janbu). These methods are the ones used in mining engineering. because over time they have proved to be reliable and because they are easy understandable by mining operators [1,62].
The assumptions regarding the pattern of the sliding surface and the computing algorithms are widely known and well described by the literature [1,25,26,27,63,64,65,66].
All these methods are based on the solutions of an equation system of static mechanics which verifies the equilibrium between active/passive moments and/or forces [63,64,65,66].
These equations must satisfy the equilibrium conditions for the whole sliding mass as well for each vertical strip (representing the discretization element of the methods) [1,25,26,27].
In order to perform the analyses, we have used the Slide software—version 6.0.2.5 (a very well known, commonly used and reliable geotechnical software) [63,64,65,66,67].
The cross-sections considered for the three open pits under study were drawn through the central area of the working fronts and the slopes of the internal dumps. In the next step, using Slide geotechnical software—version 6.0.2.5 and stratigraphic data, the stability analysis models were generated.
It should be noted that for two of the open pits, Peșteana North and Jilț North, excavation is done with high-capacity BWEs (SRs 1400-30) while the sterile rocks are stored in the internal dump by transshipment (the first step) with transport by conveyors and deposition by spreaders (A2RsB 6500.90 and/or A2RsB 6500.60) (continuous technological flux), while for the third open pit, Berbești–Alunu, excavation is done with one high-capacity BWE (SRs 1400-30, step 14) and for the rest of the steps excavation is carried out by classic (power shovel) wheeled or tracked excavators. Sterile rocks are transported to the internal dump by conveyors and deposited by one spreader (A2RsB 6500.90) and by dumping trucks (discontinuous/combined technological flux).
The way in which excavation is carried out and waste rocks are deposited in the internal dumps influences the geometry of the working and deposition fronts (playing an important role in slope stability).

4.1. Peșteana North Open Pit

4.1.1. Analysis Sections and Rocks Characteristics

For this open pit, located in a meadow area, two cross sections were considered: 1-1 for the working fronts, and 1’-1’ for the slopes of the internal dump, whose characteristics are presented in Table 3. In the analysis models (for the working steps, see Figure 7; for the internal dump, Figure 8), the physical-mechanical characteristics of the rocks were then entered (Table 4).
These characteristics were determined by statistical processing of the data from the determinations made in the Laboratory of Soil Mechanics of the University of Petroșani, as well as those taken from the specialized studies carried out over the last five decades by the Institute of Scientific Research, Technological Engineering and Mine Designs on Lignite, Craiova, Romania [59].

4.1.2. Results of the Stability Analyses

Stability analyses were carried out for part of the working slopes of the open pit and the internal dump, for the initial situation (static conditions) and in the case of an earthquake (seismic load). For this stage of the study, an earthquake with a maximum ground acceleration ag = 0.15 g was considered, in accordance with the seismic framing of the Romanian territory (presented in Section 3.1 and Figure 2). The results of these analyses for the same considered sections are presented in Table 5, and, respectively, Figure 9, Figure 10, Figure 11 and Figure 12.
The lowest values of the stability factor, both for the initial situation and under the influence of a seismic load, were obtained by the Simplified Janbu method.
As can be seen from Table 5 and Figure 9, Figure 10, Figure 11 and Figure 12, the values determined for the stability factor, in the initial conditions, are above unit, meaning that the slopes are considered to be stable.
However, for two of the presented situations (steps T1 and T3), the minimum value of the stability factor is below that of 1.3 indicated by the specialized literature [25,26,27,63,68] for slopes with a long period of staying in place. For the slopes of the interior dump (steps TH2 and TH4), the minimum value is above this limit.
In the conditions in which we take into account the action of a seismic load, with a ground acceleration equal to 0.15 g, for the working slopes of the Peșteana North open pit (steps T1 and T3) the value of the stability factor drops below 1, which indicates that they pass in a state of imbalance (unstable slopes).
For the two steps of the internal dump (TH2 and TH4) under the same seismicity conditions, the value of the stability factor remains above unit, but the determined stability reserve is below 10%. Such a stability reserve is considered by specialized literature [25,26,27,63,68] to be non-compliant, especially for slopes with a medium or long duration of remaining in place, being at the limit in the situation of slopes with a short duration of remaining in place.

4.2. Jilț North Open Pit

4.2.1. Analysis Sections and Rocks Characteristics

For this open pit, located in a hilly area, two cross sections were considered: 2-2 for the working fronts, and 2’-2’ for the slopes of the internal dump, whose characteristics are presented in Table 6. In the analysis models (for the working steps, see Figure 13; for the internal dump, Figure 14) the physical-mechanical characteristics of the rocks were then entered (Table 7).
The physical-mechanical characteristics were determined by statistical processing of the data from the determinations made in the Laboratory of Soil Mechanics of the University of Petroșani, in the GeoLogic Laboratory of Călan, as well as those taken from specialized studies [2].
In the present study, some of the rocks present in Table 7 do not appear in the chosen analysis sections.

4.2.2. Results of the Stability Analyses

Stability analyses were carried out for part of the working slopes of the open pit and the internal dump, for the initial situation (static conditions) and in the case of an earthquake (seismic load). For this stage of the study, an earthquake with a maximum ground acceleration ag = 0.15 g was considered, in accordance with the seismic framing of the Romanian territory (presented in Section 3.1 and Figure 2). The results of these analyses for the same considered sections are presented in Table 8, and, respectively, Figure 15, Figure 16, Figure 17 and Figure 18.
The lowest values of the stability factor, both for the initial situation and under the influence of a seismic load, were obtained by the Simplified Janbu method.
As can be seen from Table 8 and Figure 15, Figure 16, Figure 17 and Figure 18, the values determined for the stability factor in the initial conditions are above unit, meaning that the slopes are considered to be stable. However, for three of the presented situations (T2, T5 and TH2), the minimum value of the stability factor is below that of 1.3, indicated by the specialized literature [25,26,27,63,68] for slopes with a long duration of remaining in place, and for the fourth (TH3), the minimum value is close to this limit. These values comply with the requirements stipulated by the regulations [69] only for working slopes with a short period of staying in place.
In the conditions in which we take into account the action of a seismic load, with a ground acceleration equal to 0.15 g, for all four analyzed cases the value of the stability factor drops below 1, which indicates that they are passing into a state of imbalance (unstable slopes).

4.3. Berbești–Alunu Mining Perimeter (Alunu Open Pit and West Berbești Dump)

4.3.1. Analysis Sections and Rocks Characteristics

For this open pit, located in a hilly area, two cross sections were considered: 3-3 for the working fronts, and 3’-3’ for the slopes of the internal dump, whose characteristics are presented in Table 9. In the analysis models (for the working steps, see Figure 19; for the internal dump, Figure 20), the physical-mechanical characteristics of the rocks were then entered (Table 10). The physical-mechanical characteristics were determined by statistical processing of the data from the determinations made in the Laboratory of Soil Mechanics of the University of Petroșani, in the GeoLogic Laboratory of Călan as well as those taken from specialized studies) [1,38,60].
In the present study, some of the rocks present in Table 10 do not appear in the chosen analysis sections.

4.3.2. Results of the Stability Analyses

Stability analyses were carried out for part of the working slopes of the open pit and the internal dump, for the initial situation (static conditions) and in the case of an earthquake (seismic load). For this stage of the study, an earthquake with a maximum ground acceleration ag = 0.20 g was considered, in accordance with the seismic framing of the Romanian territory (presented in Section 3.1 and Figure 2). The results of these analyses for the same considered sections are presented in Table 11, and, respectively, Figure 21, Figure 22, Figure 23 and Figure 24.
For this mining perimeter, in the case of the Alunu open pit, we focused on the individual steps and the system of steps excavated into the yellow-brown clays located at the top of the slope. The reason for this choice is represented by the results of a previous study [1] which highlighted the fact that the possible stability problems of the working steps may appear in their case.
The lowest values of the stability factor, both for the initial situation and under the influence of a seismic load, were obtained by the Simplified Janbu method.
As can be seen from Table 11 and Figure 21, Figure 22, Figure 23 and Figure 24, the values determined for the stability factor in the initial conditions are above unit, meaning that the slopes are considered to be stable. For one of the four situations presented (step TH2), the minimum value of the stability factor is below 1.3 (but very close, Fs = 1.292), indicated by the specialized literature [25,26,27,63,68] for slopes with a long duration of staying in place, the rest of the determined values being above this limit.
Considering the action of a seismic load with ground acceleration equal to 0.20 g, the situation is as follows:
  • for the individual steps excavated in the yellow-brown clay (T1–T8) the value of the stability factor remains above unit, presenting a stability reserve of over 50%, i.e., the condition Fs ≥ 1.3 being satisfied, for slopes with a long duration of remaining in place. This fact is due to the geometry adopted for these steps, more precisely due to their low height (5 m);
  • for the system of steps dug in the yellow-brown clay (T1–T8), the value of the stability factor becomes sub-unit, which indicates its transition into a state of imbalance (unstable slope).This potential situation is the most dangerous because it is observed that the slide develops on a total height of 40 m which is divided into eight steps of excavation with classic excavators (this means that there are several excavators and dumping trucks on each step working simultaneously). This results in endangering a large number of machines, but more importantly in threatening the lives of numerous workers. This is why this particular situation will be analyzed with priority in a future study (from the point of view of resizing the work fronts so that the stability conditions are met even in the case of the occurrence of an earthquake). In principle, the solution would be to increase the width of the working berms;
  • for the TH1 step of the interior dump under the same seismicity conditions, the value of the stability factor remains above unit, but the determined stability reserve is below 10%. Such a stability reserve is considered by specialized literature [25,26,27,63,68] to be non-compliant, especially for slopes with a medium or long duration of remaining in place, being at the limit in the situation of slopes with a short duration of remaining in place;
  • for step TH2 of the internal dump, the value of the stability factor drops below 1, which indicates its transition into a state of imbalance (unstable slope).

5. Final Conclusions

From the discussions held with the mining operators from the three open pits considered in the present study, it emerged that during the period with the most intense seismic activity (14 February–20 March 2023) there were no landslide-type phenomena involving large volumes of material. However, superficial landslides were recorded both in the working slopes of the open pits and in the slopes of the internal dumps, which did not endanger either people or work equipment.
We must state that the maximum ground acceleration recorded during the earthquake of 5.7 ML was below the maximum accelerations considered for the studied areas by the seismic framing of Romanian territory.
Such accelerations, comparable to those considered in the seismic framing of the Romanian territory, could be registered or even exceeded in the situation where the earthquakes were between 6 and 6.5 ML (possible according to the statements made by the general director of INCDFP [58]).
For this reason, in this study, in order to simulate the possible effects of an earthquake on the stability of the slopes of open pits and internal dumps, we considered accelerations of 0.15 g for the Peșteana North and Jilț North perimeters, respectively, and 0.20 g for the Berbești–Alunu perimeter (considered in the seismic framing of Romanian territory).
The stability analysis carried out for the initial situations (static conditions) showed that the working slopes and those of the internal dumps generally present adequate stability reserves for slopes with a short period of staying in place, or even for slopes with a medium or long period of staying in place (considering that the lignite extraction activity and implicitly deposition of waste rocks will continue at a normal pace until at least the year 2028).
The situation changes radically when we consider the action of a seismic event.
Thus, for most of the analyzed slopes, the stability factor becomes sub-unit, which translates into a loss of balance. This state of instability significantly increases the risk of landslides involving the displacement of significant volumes of material, endangering the machinery operating in the open pits or on the dumps and the safety of the personnel.
For three of the analyzed situations, the results showed that although the stability factor remains above unit the stability reserve is below 10%, i.e., below the values recommended by the specialized literature, even for slopes with a short duration of remaining in place.
In only one situation, the value of the stability factor under the conditions of an earthquake remains high (it presents a stability reserve of over 50%), but this situation is influenced by the favorable geometry adopted for the individual work steps (steps with a height of 5 m, encountered only in the Alunu open pit).
Although the analyses carried out are able to provide us with valuable data regarding the evolution of the stability reserve in the event of a significant earthquake in the studied area, the software used does not allow us to perform dynamic simulations.
The software (Slide 2D–version 6.0.2.5) only allows the introduction of a maximum value of the seismic acceleration (peak acceleration), given that an earthquake is characterized by a certain duration of action and may have several peak moments in which the acceleration has values close to the maximum.
By using methods that involve dynamic programming and which may incorporate spatial variability, time decay of the mechanical proprieties and earthquakes, both the critical sliding surface and the value of the stability factor may differ substantially from the ones obtained by classic methods (based on static mechanics) [70,71,72].
Moreover, software is currently being developed that can perform advanced dynamic simulations that can take into account a seismic sequence (a series of earthquakes), as was the case at the begining of 2023 in Oltenia.
A possible continuation of this study envisages the use of software that performs stability analysis based on finite elements or the finite differences method [73,74,75,76,77], 3D modeling [78,79,80,81,82] and probabilistic approaches [83,84,85,86].
The more important research that will continue this study should focus on resizing the geometric elements (height and slope angle) originally designed for the open pits and waste dumps in Oltenia. Such a study, which takes into account the new seismic context of the area, would be able to ensure safety at work, avoid damage or destruction of machinery or even prevent injuries or loss of human life. For resizing, there are a number of classic methods for determining the geometric elements of the slopes (grapho-analytical procedures: Hoek–Bray, Fellenius, etc.) [26,63,68], as well as modern methods based on numerical analyses [87,88].
The results of this study can, to a good extent, be extrapolated for other lignite open pits in the mining basin of Oltenia that are developed in similar conditions (in meadow or hilly regions, with similar geology and tectonics, with similar working technologies and located at comparable distances from the seismically active area): Jilț South, Roșia de Jiu, Pinoasa, Tismana, Lupoaia, Roșiuța and Panga.
We also believe that this article can be seen as useful teaching material in the training of geotechnicians and mining engineers who may face similar situations during their careers.

Author Contributions

Conceptualization, F.F., M.L. and I.-M.A.; methodology, F.F. and I.-M.A.; software, F.F. and I.-M.A.; validation, M.L.; formal analysis, F.F., M.L. and I.-M.A.; investigation, F.F. and I.-M.A.; resources, F.F., M.L. and I.-M.A.; data curation, F.F., M.L. and I.-M.A.; writing—original draft preparation, F.F.; writing—review and editing, F.F., M.L. and I.-M.A.; visualization, F.F., M.L. and I.-M.A.; supervision, M.L.; project administration, F.F., M.L. and I.-M.A.; funding acquisition, M.L. and I.-M.A. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Longitudinal section through the Rovinari mining sub-basin [42].
Figure 1. Longitudinal section through the Rovinari mining sub-basin [42].
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Figure 2. Seismic framing of the territory of Romania according to the peak acceleration, ag, of the land (seismically active area and location of the studied perimeters) [48].
Figure 2. Seismic framing of the territory of Romania according to the peak acceleration, ag, of the land (seismically active area and location of the studied perimeters) [48].
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Figure 4. Profile crossing the Southern Carpathians—highlighting the main faults (SDM—Danubian–Moesian System; FST—South Transylvania; FSC—South Carpathians; FTgJCa—Tg. Jiu-Călimănești, FCis—Cisnădie and the Pericarpathian Fault) [57].
Figure 4. Profile crossing the Southern Carpathians—highlighting the main faults (SDM—Danubian–Moesian System; FST—South Transylvania; FSC—South Carpathians; FTgJCa—Tg. Jiu-Călimănești, FCis—Cisnădie and the Pericarpathian Fault) [57].
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Figure 6. Ground acceleration (horizontal) recorded at GZR station [56].
Figure 6. Ground acceleration (horizontal) recorded at GZR station [56].
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Figure 7. Cross section 1-1, through the working steps of Peșteana North open pit (projected geometry for the end of 2023)—the working steps are numbered from top to bottom, from 1 to 4.
Figure 7. Cross section 1-1, through the working steps of Peșteana North open pit (projected geometry for the end of 2023)—the working steps are numbered from top to bottom, from 1 to 4.
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Sustainability 16 01384 g008
Figure 8. Cross-section 1’-1’, through the steps of Peșteana North internal dump (geometry designed for the end of 2023)—the steps of the dump are numbered from bottom to top, from 1 to 4.
Figure 8. Cross-section 1’-1’, through the steps of Peșteana North internal dump (geometry designed for the end of 2023)—the steps of the dump are numbered from bottom to top, from 1 to 4.
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Figure 9. Results for the T1 working step. (a) Initial conditions [59]. (b) With seismic load (ag = 0.15 g).
Figure 9. Results for the T1 working step. (a) Initial conditions [59]. (b) With seismic load (ag = 0.15 g).
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Figure 10. Results for the T3 working step. (a) Initial conditions [59]. (b) With seismic load (ag = 0.15 g).
Figure 10. Results for the T3 working step. (a) Initial conditions [59]. (b) With seismic load (ag = 0.15 g).
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Figure 11. Results for the TH2 step of the interior dump. (a) Initial conditions [59]. (b) With seismic load (ag = 0.15 g).
Figure 11. Results for the TH2 step of the interior dump. (a) Initial conditions [59]. (b) With seismic load (ag = 0.15 g).
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Sustainability 16 01384 g012
Figure 12. Results for the TH4 step of the interior dump. (a) Initial conditions [59]. (b) With seismic load (ag = 0.15 g).
Figure 12. Results for the TH4 step of the interior dump. (a) Initial conditions [59]. (b) With seismic load (ag = 0.15 g).
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Figure 13. Cross section 2-2, through the working steps (fronts) of the Jilț North open pit (projected geometry for the end of 2023)—the working steps are numbered from top to bottom, from 1 to 6.
Figure 13. Cross section 2-2, through the working steps (fronts) of the Jilț North open pit (projected geometry for the end of 2023)—the working steps are numbered from top to bottom, from 1 to 6.
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Figure 14. Cross-section 2’-2’, through the steps of Peșteana North internal dump (geometry designed for the end of 2023)—the steps of the dump are numbered from bottom to top, from 1 to 7.
Figure 14. Cross-section 2’-2’, through the steps of Peșteana North internal dump (geometry designed for the end of 2023)—the steps of the dump are numbered from bottom to top, from 1 to 7.
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Figure 15. Results for the T2 working step. (a) Initial conditions [2]. (b) With seismic load (ag = 0.15 g).
Figure 15. Results for the T2 working step. (a) Initial conditions [2]. (b) With seismic load (ag = 0.15 g).
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Figure 16. Results for the T5 working step. (a) Initial conditions [2]. (b) With seismic load (ag = 0.15 g).
Figure 16. Results for the T5 working step. (a) Initial conditions [2]. (b) With seismic load (ag = 0.15 g).
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Sustainability 16 01384 g017
Figure 17. Results for the TH2 step of the interior dump. (a) Initial conditions [61]. (b) With seismic load (ag = 0.15 g).
Figure 17. Results for the TH2 step of the interior dump. (a) Initial conditions [61]. (b) With seismic load (ag = 0.15 g).
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Figure 18. Results for the TH3 step of the interior dump. (a) Normal conditions [61]. (b) With seismic load (ag = 0.15 g).
Figure 18. Results for the TH3 step of the interior dump. (a) Normal conditions [61]. (b) With seismic load (ag = 0.15 g).
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Figure 19. Cross section 3-3, through the working steps (fronts) of the Alunu open pit (projected geometry for the end of 2023)—the working steps are numbered from top to bottom, from 1 to 14.
Figure 19. Cross section 3-3, through the working steps (fronts) of the Alunu open pit (projected geometry for the end of 2023)—the working steps are numbered from top to bottom, from 1 to 14.
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Figure 20. Cross-section 3’-3’, through the steps of West Berbești internal dump (geometry designed for the end of 2023)—the steps of the dump are numbered from bottom to top, from 1 to 6.
Figure 20. Cross-section 3’-3’, through the steps of West Berbești internal dump (geometry designed for the end of 2023)—the steps of the dump are numbered from bottom to top, from 1 to 6.
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Figure 21. Results for the T2 working step (valid for all individual steps).
Figure 21. Results for the T2 working step (valid for all individual steps).
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Figure 22. Results for the T1–T8 working steps system.
Figure 22. Results for the T1–T8 working steps system.
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Figure 23. Results for the TH1 step of the interior dump. (a) Initial conditions [60]. (b) With seismic load (ag = 0.20 g).
Figure 23. Results for the TH1 step of the interior dump. (a) Initial conditions [60]. (b) With seismic load (ag = 0.20 g).
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Figure 24. Results for the TH2 step of the interior dump. (a) Initial conditions [60]. (b) With seismic load (ag = 0.20 g).
Figure 24. Results for the TH2 step of the interior dump. (a) Initial conditions [60]. (b) With seismic load (ag = 0.20 g).
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Table 1. Sedimentary sequences in the Oltenia mining basin [41].
Table 1. Sedimentary sequences in the Oltenia mining basin [41].
PeriodEpochDescription
QUATERNARYHolocenerecent alluvium of the valleys, proluvial and colluvial deposits, gravels
Pleistocenesands and gravels with intercalations of clays, sandy clays, clayey sands, thin layers of lignite (Lower Pleistocene), sands and gravels of the high, middle and lower terraces
PLIOCENERomanianSuperiorsands and gravel in which the XIV–XVII lignite strata are intercalated
Inferiorclays and fine sands in which the VIII–XIII lignite strata are intercalated
DacianSuperioralternation of clays with gray and green sandy clays, with sands and gray clayey sands in which lignite layers V–VII are interspersed
InferiorSuperior horizonclays, sandy clays, sandy marls and the I–IV lignite strata
Inferior horizonsands with gravel intercalations and the D, C, B, A lignite strata
PontianSuperiorsands and sandy clays with micaceous sandstones intercalations
Medianclays and sandy clays
Inferiormarls and clays
MeotianSuperiormarls, clays and sandy clays
Inferiorgravel, sands, conglomerates and clays
PALEOGENE-
Table 2. Peak ground motion parameters [56].
Table 2. Peak ground motion parameters [56].
StationDepi (km)ComponentPGA
(cm/s2)		PGV
(cm/s)		PGD
(cm)		SAmax
(cm/s2)		TSAmax
(s)
ML = 5.2 earthquake (13 February 2023)
GZR35.1EW51.081.390.07168.40.14
NS43.401.660.07136.30.26
Z30.601.180.06117.20.12
HERR58.3EW29.900.580.03126.10.14
NS40.200.680.03188.60.11
Z9.900.260.0250.70.08
MHISU67.2EW12.100.450.0558.90.21
NS13.800.560.0735.60.21
Z6.700.250.0232.00.28
RMGR61.8EW19.300.660.0676.80.19
NS16.300.680.0771.00.16
Z5.400.190.0320.50.20
SRE55.7EW18.401.030.0855.00.36
NS9.900.570.0834.40.12
Z6.700.380.0431.50.20
ML = 5.7 earthquake (14 February 2023)
GZR42.5EW84.801.940.16264.30.11
NS106.703.630.19307.10.23
Z50.201.780.13197.90.09
HERR62.4EW49.600.850.07204.80.11
NS66.701.250.09299.60.11
Z28.400.470.07128.80.08
MHISU67.4EW24.701.210.2479.90.28
NS17.401.080.1763.70.3
Z11.800.520.0742.40.28
RMGR62.0EW35.402.940.3395.20.14
NS24.501.410.19101.60.23
Z12.700.460.0938.00.26
SRE50.8EW37.601.640.35144.10.13
NS25.301.420.5085.10.14
Z12.800.680.1541.00.23
Where: Depi—Distance from the epicenter; PGA—Peak Ground Acceleration; PGV—Peak Ground Velocity; PGD—Peak Ground Displacement; SAmax—Maximum spectral acceleration; TSAmax—The corresponding period; EW, NS, Z—Component (propagation direction).
Table 3. Designed geometric elements for Peșteana North perimeter (at the end of 2023) [59].
Table 3. Designed geometric elements for Peșteana North perimeter (at the end of 2023) [59].
Geometric ElementOpen Pit
Development of the working fronts (cross section) (m)395.74
Height of the general slope (m)72.88
Number of steps4
General slope angle (°)10
Height of the steps (m)14.20–21.75
Berms width (m)97.36–107.68 *
Slope angle of individual steps (°)36–46
Geometric ElementInternal Waste Dump
Height of the general slope (m)488.91
Number of steps55.34
General slope angle (°)4
Height of the steps (m)6
Berms width (m)10.52–15.10
Slope angle of individual steps (°)100.43–174.60 **
Height of the general slope (m)26–27
* Except for the upper berm of the T1 step. ** Except for the upper platform.
Table 4. Physical-mechanical characteristics used in the stability analyses for Peșteana North perimeter [59].
Table 4. Physical-mechanical characteristics used in the stability analyses for Peșteana North perimeter [59].
The Nature of Rocks from the Analysis Sectionsγvnat
[kN/m3]		cnat
[kN/m2]		φnat
[°]
Sustainability 16 01384 i001Topsoil (step T1)14.7024.0020.00
Sustainability 16 01384 i002Gravel and boulders (under topsoil—step T1)21.41035.00
Sustainability 16 01384 i003Sandy rocks: fine sand, clayey sand, dusty sand (steps T2–T4, foundation of the interior dump together with a clay layer)19.447.5827.66
Sustainability 16 01384 i004Clay—dusty rocks: clay, greasy clay, dusty clay, marly clay, dust, clay dust, sandy clay dust (generally in the roof and bed of lignite layers, the direct foundation of the interior dump)19.6441.0719.43
Sustainability 16 01384 i005Lignite layers (V–VIII)12.33213.0535.44
Sustainability 16 01384 i006Marly rocks: marl, clayey marl, sandy marl (steps T1, T3, T4, interspersed between sands and clays)19.1242.9219.83
Sustainability 16 01384 i007Mixture of waste rocks17.8713.4523.75
γvnat, cnat, φnat—Volumetric weight, cohesion and angle of internal friction (at natural moisture).
Table 5. Results for Peșteana North perimeter.
Table 5. Results for Peșteana North perimeter.
SectionIndividual StepHeight h
[m]		Slope Angle
α
[°]		Stability Factor, Fs
Initial ConditionsWith Seismic Load (ag = 0.15 g)
Fellenius BishopJanbuFelleniusBishopJanbu
Peșteana North open pit slopes
1-1T1 (Figure 9)21.75461.2751.3401.2531.0261.0720.950
T3 (Figure 10)17.08401.2941.3531.2591.0101.0450.958
Peșteana North interior waste dump slopes
1′-1′TH2 (Figure 11)15.10261.5541.6831.5381.1051.1911.076
TH4 (Figure 12)14.88261.5311.6661.4921.1001.1841.071
Table 6. Designed geometric elements for Jilț North perimeter (at the end of 2023) [2].
Table 6. Designed geometric elements for Jilț North perimeter (at the end of 2023) [2].
Geometric ElementOpen Pit
Development of the working fronts (cross section) (m)633.27
Height of the general slope (m)153.41
Number of steps 6
General slope angle (°)13
Height of the steps (m) 20.00–30.00
Berms width (m)61.10–116.66 *
Slope angle of individual steps (°)42–53
Geometric ElementInternal Waste Dump
Development of the working fronts (cross section) (m)1191.23
Height of the general slope (m)107
Number of steps 7
General slope angle (°)5
Height of the steps (m) 9–20
Berms width (m)141.40–261.38 **
Slope angle of individual steps (°)28–49
* Except for the upper berm of the T1 step. ** Except for the upper platform.
Table 7. The values of the physical-mechanical characteristics considered in the stability analyses for Jilț North perimeter [2].
Table 7. The values of the physical-mechanical characteristics considered in the stability analyses for Jilț North perimeter [2].
The Nature of Rocks from the Analysis Sectionsγvnat
[kN/m3]		cnat
[kN/m2]		φnat
[°]
Sustainability 16 01384 i008Sandy clays (steps T1–T3)22.0025.0020.00
Sustainability 16 01384 i009Clayey sands (intercalation in step T2)21.0015.0016.00
Sustainability 16 01384 i010Coal clay (on top of layer X, between the banks of layer X, layers X and XII and as discontinuity of layer XII)18.0033.2528.00
Sustainability 16 01384 i011Lignite (layers V–X and XII)13.40200.0035.00
Sustainability 16 01384 i012Compact clay (on top of the pressurized aquifer, direct foundation of the internal waste dump)19.0052.0030.00
Sustainability 16 01384 i013Mixture of waste rocks18.5020.7521.00
γvnat, cnat, φnat—Volumetric weight, cohesion and angle of internal friction (at natural moisture).
Table 8. Results for Jilț North perimeter.
Table 8. Results for Jilț North perimeter.
SectionIndividual StepHeight h
[m]		Slope Angle
α
[°]		Stability Factor, Fs
Initial ConditionsWith Seismic Load (ag = 0.15 g)
Fellenius BishopJanbu Fellenius BishopJanbu
Jilț North open pit slopes
2-2T2 (Figure 15)20.00421.3001.4081.2740.9381.0000.900
T5 (Figure 16)30.00531.2361.2321.1571.0391.0170.886
Jilț North interior waste dump slopes
2′-2′TH2 (Figure 17)18.00401.1491.2241.1130.8860.9290.855
TH3 (Figure 18)20.00381.3931.5321.3821.0151.0950.982
Table 9. Designed geometric elements for Berbești–Alunu perimeter (at the end of 2023).
Table 9. Designed geometric elements for Berbești–Alunu perimeter (at the end of 2023).
Geometric ElementOpen Pit
Development of the working fronts (cross section) (m)400.03
Height of the general slope (m)90.00
Number of steps 14
General slope angle (°)13
Height of the steps (m) T1–T135.00
T1425.00
Berms width (m)T1–T1314.93
T14101.23
Slope angle of individual steps (°)T1–T1345
T1440
Geometric ElementInternal Waste Dump
Development of the working fronts (cross section) (m)1049.99
Height of the general slope (m)98.50
Number of steps 6
General slope angle (°)5
Height of the steps (m) 10.00–16.50
Berms width (m)150.00–257.50
Slope angle of individual steps (°)18–45
Table 10. The values of the physical-mechanical characteristics considered in the stability analyses for Berbești–Alunu perimeter [1,60].
Table 10. The values of the physical-mechanical characteristics considered in the stability analyses for Berbești–Alunu perimeter [1,60].
The Nature of Rocks from the Analysis Sectionsγvnat
[kN/m3]		cnat
[kN/m2]		φnat
[°]
Sustainability 16 01384 i014Yellow-brown clay (Alunu open pit)17.8222.0015.00
Sustainability 16 01384 i015Lignite (layer 4 *) (Alunu open pit)11.8030.0019.00
Sustainability 16 01384 i016Lignite layers (2 * and 3 *) (Alunu open pit)11.8070.0026.50
Sustainability 16 01384 i017Sandy marl (Alunu open pit)18.5032.0020.50
Sustainability 16 01384 i018Lignite layers (I, IIinf, IIsup, III, IV and 1 *) (Alunu open pit)11.80110.0030.00
Sustainability 16 01384 i019Compacted sandy marl (Alunu open pit)19.1145.0021.00
Sustainability 16 01384 i020Mixture of waste rocks (West Berbești internal dump)18.2721.0021.50
Sustainability 16 01384 i021Rocks from the foundation of the West Berbești dump19.3345.0021.00
* Lignite layers without economic importance; γvnat, cnat, φnat—Volumetric weight, cohesion and angle of internal friction (at natural moisture).
Table 11. Results for Berbești–Alunu perimeter.
Table 11. Results for Berbești–Alunu perimeter.
SectionIndividual Step/Steps
System		Height h
[m]		Slope Angle
α
[°]		Stability Factor, Fs
Initial ConditionsWith Seismic Load (ag = 0.20 g)
Fellenius BishopJanbu Fellenius BishopJanbu
Alunu open pit slopes
3-3T2 (Figure 21)5.00452.1872.2852.1601.5891.5931.572
T1–T8 (Figure 22)40.00151.5481.6431.5290.8200.8730.808
West Berbești interior waste dump slopes
3′-3′TH1 (Figure 23)13.50281.6461.7311.6121.1231.1811.082
TH2 (Figure 24)15.00451.3071.3711.2920.9120.9480.873
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Faur, F.; Apostu, I.-M.; Lazăr, M. Reassessment of the Stability Conditions in the Lignite Open Pits of Oltenia (Romania) in Relation to the New Local Seismic Context as an Imperative for Sustainable Mining. Sustainability 2024, 16, 1384. https://doi.org/10.3390/su16041384

AMA Style

Faur F, Apostu I-M, Lazăr M. Reassessment of the Stability Conditions in the Lignite Open Pits of Oltenia (Romania) in Relation to the New Local Seismic Context as an Imperative for Sustainable Mining. Sustainability. 2024; 16(4):1384. https://doi.org/10.3390/su16041384

Chicago/Turabian Style

Faur, Florin, Izabela-Maria Apostu, and Maria Lazăr. 2024. "Reassessment of the Stability Conditions in the Lignite Open Pits of Oltenia (Romania) in Relation to the New Local Seismic Context as an Imperative for Sustainable Mining" Sustainability 16, no. 4: 1384. https://doi.org/10.3390/su16041384

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