Verifying the Stability of the Working Fronts of Lignite Open Pits Developed in Hilly Areas—A Case Study of Jilț North Open Pit (Romania)

Abstract

Regardless of the period for which the lignite open pits from Romania will be kept in function, operational safety is an objective of utmost importance. In this context, the present paper aims to analyze the stability of the working fronts of a lignite open pit from Romania (Jilț North open pit). The development of Jilț North open pit involves excavations in a hilly area, with a level difference between the base of the open pit and the top of the hill of approx. 195 m (151 m by the end of 2023). Thus, based on the technical documentation provided by the mining operator (situation plan, cross-sections, stratigraphic columns, etc.) and laboratory tests (on the physical–mechanical characteristics of the rocks), a stability analysis model was created with the help of a specialized software. Following the analyses, it was found that two of these slopes (T1 and T3 steps) do not present a sufficient stability reserve (in fact they are unstable, Fs ˂ 1), to allow continuing extractive activities under safe conditions. Considering these results and using a well-known slope dimensioning method, two technical solutions were proposed to increase the stability reserve: a simple one, for the T3 step, which involves reducing the slope angle from 52° to 45°, and the second one, for the T1 step, a bit more complex, involving the inclusion in the general continuous flux of the open pit of a discontinuous sub-flux that aims to achieve three sub-steps, and the reduction in the general slope angle.

1. Introduction

Twenty years ago, twenty open pits (of different sizes) were in operation in the mining area of Oltenia (in the five mining basins: Rovinari, Motru, Jilț, Mehedinți and Berbești); currently, only ten are functional, some of which are soon to be closed (by depleting reserves, due to lack of financial viability, or as a result of policies to eliminate fossil fuels from the national energy mix). Precisely because of the decarbonization policies of the energy sector in the EU and, implicitly, in Romania, all these open pits are to be closed in the next 10–12 years (in the absence of reassessments of the current ambitions and targets set at the EU level).

Although the share it has in the national energy mix of Romania has decreased from over 30% (10 years ago) to approx. 15%, lignite continues to play an important role, and this situation is expected to be maintained for the next few years. After this period, the fate of lignite, and implicitly of lignite open pits, is an uncertain one, if we consider, on the one hand, the obligations to eliminate fossil fuels from the energy sector assumed by Romania through the European Green Deal and the National Resilience and Recovery Plan, in accordance with the European decarbonization ambitions and policies, and on the other hand, the conflict taking place in Ukraine and its implications for the energy market.

Even under these conditions, operational safety is an aspect that cannot be overlooked, neither during the period in which extractive activities will continue, nor during the closure and post-closure phase of these mining perimeters.

Slope stability is one of the most important issues which must be solved throughout the lifetime of any open pit or quarry. In the particular case of lignite open pits, which usually operate in soft rocks, there is a high probability for slope sliding to occur [1].

The failure and sliding of slopes in an open pit can be triggered by multiple factors, among which are poorly designed geometric elements, insufficient knowledge of the geology and hydrogeology of the lignite deposit, the presence of uninvestigated failure plans (faults), external factors (overloads, seismic loads, precipitations), etc. [2,3,4,5,6,7,8,9,10,11,12,13,14].

Among the consequences of landslides produced over time in lignite open pits are the total or partial damage of machinery, mostly bucket wheel excavators (BWEs) [15,16,17], the degradation or destruction of the nearby habitats involved in the sliding process, the blocking of water courses, the destruction of communication routes, or even the death of people operating the machines or who live in nearby areas [1].

The partial or total destruction of BWEs (Figure 1) causes significant financial losses to mining operators, by interrupting the technological flux, by the penalties resulting from the inability to supply lignite to thermal power plants, but, above all, by the intrinsic value of these machines (depending on the age and degree of wear and tear from a few million euros up to approx. EUR 100 million for the largest and most modern of these, the Bagger 293 BWE).

In general, although open pit operators have in mind different measures to avoid the occurrence of slope slides, these often prove insufficient, being practically impossible to consider all the variables involved in this process.

In fact, slides that affect the slopes of lignite open pits, produced by the failure of the excavation step or the one on which the BWEs vehicles are working, are quite common phenomena, their occurrence and manifestation being signaled all over Europe (in Romania, Poland, Czech Republic, Greece, Serbia, Kosovo, Germany, and Spain), but also in other parts of the world (in USA, China, India, Brazil, etc.). These slope slides are well described in the literature, in terms of favoring/triggering factors and causes, unfolding mechanisms, damage caused, and preventative measures for the future [19,20,21,22,23,24,25,26].

Even though the triggering factors of major slope slides can often be technically detected, their actual prediction and mitigation remain a major challenge (for practicians, researchers, and academics) [1].

The slope angle and height of the slopes are the main geometric elements that can influence their stability/instability [2,3,4,5,6,7,8,9,10,11,12,13,14]. Therefore, the design and compliance with the geometry of the working fronts are extremely important elements for maintaining their stability both in the exploitation process and, after the open pit is closed, in the reclamation phases. In addition, the identification and control of the factors that influence the technical condition of the slopes must remain a permanent concern throughout the lifetime of an open pit.

When unforeseen sliding phenomena occur, the causes must be analyzed, and proper measures to eliminate them must be taken [27,28,29,30,31,32]. In any situation, monitoring the stability conditions is absolutely necessary, the solution to avoid sliding phenomena, that can result in material and human losses, being represented by early warning systems [33,34,35,36,37,38,39].

Sometimes, the evolution of a major slope slide can be subdivided into several sliding models, offering the possibility of analyzing and comparing it, in terms of mechanisms, space and time, to similar ones [40].

The main target of a stability study is to evaluate the safety factor (coefficient) or to verify the stability reserve of a slope and, on this basis, to mitigate the geotechnical risks that may occur by identifying and implementing the appropriate stabilization measures [3,5,13,14,41].

In the present study, the research focused on aspects related to the geometry of the working fronts designed for the development area of the Jilț North open pit (in a hilly area). More precisely, the stability of the working fronts designed and forecast to be achieved at the end of 2023 was verified, and for two of the working steps, assessed as unstable, technical solutions were proposed to allow the continuation of the extractive activity under safe conditions.

2. Description of Jilț North Mining Perimeter

2.1. Location and Short Geological Description

Extraction of lignite in the Jilț North perimeter began in 1980, through two micro-open pits located in the Cerchez I and II hills, and excavations with bucketwheel excavators (BWEs) began in 1984 (when the E07 BWE, type SRs 1400—30/7, was put into operation) [42].

Currently, the Jilț North open pit, with a licensed capacity of 4.5 million t/year, supplies lignite for the Turceni Central Power Plant, a component unit of the Oltenia Energy Complex.

From an administrative point of view, the Jilţ mining basin (consisting of two open pits: Jilţ North and South) is located in the southwestern part of Gorj county, on the territory of Matăsari, Dragoteşti, Slivileşti, and Negomir communes. It stretches between the villages of Brădețel to the north, Ştiucani, Miculesti and Slivileşti to the west, Corobai and Strâmtu to the south, and Negomir and Timișeni to the east. Figure 2 shows the layout of the Jilț North mining perimeter [43].

Access to the area is achieved through [43]:

• Filiaşi–Turceni–Dragoteşti–Mătăsari railway;
• County road DJ 673 Turceni–Dragotești–Strâmba–Vulcan, connected to DN 66 Craiova–Târgu Jiu, which ensures access from the south;
• The modernized county road Pieptani–Strâmba–Mătăsari, connected to DN 67 Târgu Jiu–Motru–Drobeta-Turnu Severin, which provides access from the north and west.

The connection between the various settlements in the area is made by communal roads, asphalted or cobbled.

From a geomorphologic point of view, the area is characterized by a massive relief, with hills oriented approximately west–east, which structurally belongs to the Getic piedmont, which makes the transition between the subcarpathian area and the plain area (Figure 3).

The structure of the Dacian and Romanian age formations in the Jilţ mining perimeter, in which the lignite layers are interposed, fits into the general tectonic context of the area between the Jiu and Motru Rivers. The layers have a NE–SW direction and a general inclination of 2–8° to the SE [42].

On this background, on a small scale, vaults appear (Strâmba–Rovinari, Miculesti, and Broşteni–Duculeşti anticlines), which give a wavy appearance to the general structure.

The northern area of the perimeter is more troubled, due to the presence of E–W oriented faults, which can be traced from north to south, as follows: Strâmba; Brădăţel; and Runcurel faults.

Among the 21 coal (lignite) layers (D, C, B, A, I–XVII) discovered in the southwestern part of Romania (Oltenia region), in the Jilț mining basin just 17 layers were highlighted (I–XVII), and only a part of them have an important extension and thickness that allow their extraction in economic conditions: V, VI, VII, VIII, IX, X, and XII. We mention that in this mining basin, the lignite layers I–IV have been little researched, as they are located under the aquifer horizon under pressure that develops in the entire region of Oltenia, which makes their exploitation unfeasible (extremely difficult hydrogeological conditions).

The lignite layers in the Jilț North mining perimeter have the following characteristics [42]:

Layer V consists of two to nine banks, grouped, in turn, into two main banks, lower V and upper V. The lower V-layer is up to 2.80 m thick, and the upper V-layer is better developed, with thicknesses up to 4.75 m.

Layer VI consists of one to six banks grouped in a single layer, with thicknesses up to 6.55 m.

Layer VII consists of one to seven banks with a maximum thickness of 3.20 m.

Layer VIII consists of several banks separated by sterile intercalations, the total thickness of the coal complex reaching up to 9.45 m.

Layer IX has the appearance of an intermediate one, and has a maximum thickness of 3.65 m, frequently around 1.90–2.00 m.

Layer X has the largest thicknesses, it consists of one to thirteen coal banks grouped in three main banks, which develop differently within the Jilț mining basin. In the Jilţ North perimeter, the lower (X_inf), middle (X_med), and upper (X_sup) layers were identified and generally grouped, with thicknesses between 5.80–and 10.30 m (Figure 4).

Layer XII consists of one to seven banks that are grouped into two main banks, has a maximum thickness of 3.45 m, and presents areas of sedimentation discontinuity in the northern part of the basin.

Layer XI shows discontinuities in sedimentation and is eroded along the main valleys in the perimeter. It has thicknesses between 0.20 and 1.50 m (generally below 1 m), being without economic importance.

Layers XIII–XVII are either poorly developed, with thicknesses below 1 m, or were intercepted only at the top of the hills, in the form of coal lenses, left after erosion, being without economic importance.

The geological balance reserves in concession for the license period (up to 2028) are estimated at 38.18 million tons, with possibilities to increase these industrial reserves up to over 100 million tons, by attracting new reserves from adjacent areas and by expanding the Jilţ North open pit towards Runcurelu area (to the NE side of the present layout) [43].

The situation of the geological reserves in the Jilț North perimeter is presented in

Table 1. The situation of the geological reserves for the Jilţ North open pit, in thousands of tons [43].

Jilţ North Open Pit	International Classification
	Measured (331)	Indicated (332)	TOTAL (331 + 332)	Proven (111)	Probable (121 + 122)	TOTAL (111 + 121 + 122)
	97,939	2,730	100,669	881	41,715	42,596

.

Table 2. Qualitative characteristics of lignite from the Jilț North open pit [42].

Crt. No.	Characteristics	MU	Determined Value
1	Total moisture	%	27.27–50.77
2	Ash relative to anhydrous coal	%	22.36–40.20
3	Volatile matter relative to combustible mass	%	54.70–60.70
4	Carbon relative to combustible mass	%	2.43–2.67

and Figure 5 show the main qualitative characteristics of the lignite from the layers extracted in the Jilț North open pit.

The geological reserves for the Jilţ North open pit were granted through exploitation license No. 2602/2001 [43].

2.2. Equipment and Working Technologies

The specific technological flux existing in Jilț North open pit (throughout 2023) is presented in Figure 6, with the specification that the A02 spreader, which deposits sterile rocks in an exterior waste dump, is used only in case of emergency (if failure occurs to the conveyor system or the spreaders operating in the internal waste dump).

Excavation is carried out exclusively with high-capacity BWEs, type SRs 1400—30.

The transport of both the waste rocks and the lignite resulting from the excavations is carried out on conveyor belts. The waste rocks are transported to the internal dump (occasionally to the external one), and the lignite to the Jilţ coal depot and from here to the Turceni Central Power Plant. Currently, the Jilţ North open pit is equipped with the following high-capacity technological equipment [43]:

• BWEs: 6 × SRs 1400—30/7 (E06; E07; E14; E17; E18; E19);
• BWEs: 3 × SRs 470—15/3.5 (E09; E11; E16)—removed from the technological flux (in conservation, near the premises of the administrative building and the coal depot);
• High capacity conveyor belts, types: B1400; B1600; B1800; B2000; B2250, over 30 km;
• Belt wagons;
• Spreaders: 3 × A2RsB 6500.90 (A01, A03, A05) and one A2RsB 6500.60 (A02);
• Depositing machines: combined machine-type KsS 5600 × 40 and stacking machine-type ASG 6000.

Added to these are a variable number (being used jointly with the Jilț South open pit) of classic excavators (with wheels and tracks), dumping trucks, and bulldozers, used for related activities (profiling, access roads, leveling, etc.).

In the Jilţ North open pit, the exploitation method used is the one with the transshipment of a part and the transport of another part of the sterile rocks to internal dumps (Figure 7) and the technology of excavation, transport, and dumping in continuous flux, through the use of excavation, transport, and dumping/deposition complexes [43]. The material deposited in the waste dump is leveled with the help of different types of bulldozers.

It is expected that the current exploitation method and technological flux will be maintained until the cessation of productive activities (2028 according to the current license, or, by its extension, 2035 at most).

3. Stability Analyses for the Initially Designed Geometry

3.1. Modeling the Analysis Section

In order to be able to model the stability analysis section of the working fronts of the Jilț North open pit, several stages were completed.

The first consisted of drawing the calculation section on the situation plan provided by the mining operator (Figure 8). Thus, through the central area of the mining perimeter, the cross section 1–1’ was drawn, which intersects both the working fronts of the open pit and the deposition fronts of waste rocks in the internal dump.

Because the object of this study is to analyze the stability of the working fronts (not the internal dump), from the initial cross section 1–1’, only the part that intersects the open pit steps was selected, and later renamed I–I, for the existing situation (at the level of May 2023), F–F, for the designed situation at the end of 2023, respectively (Figure 9a).

Then, based on the coordinates for the section in Figure 9a, taking into account the stratigraphy of the deposit (Figure 9b), and the information from the written documentation [43], the section from May 2023 (Figure 10a) and the calculation model (Figure 10b) were created using the Slide software, having the same order of scale horizontally and vertically.

With the help of the measuring tools within the Slide software, the geometric elements of the working fronts were determined on the created models, for the existing situation in May 2023 and the designed one at the end of 2023, presented in

Table 3. Existing and designed geometric elements (cross sections I–I and F–F).

Geometric Elements	Slopes of the Open Pit
	Existing (May 2023)	Designed (End of 2023)
Development of the steps system (m)	1024.93	874.91
Working fronts development (m)	841.62	633.27
Total height, (m)	195.00	151.00
Number of steps	6	6
General slope angle, (°)	13 (21) *	13
Height of steps, (m)	25.40–40.00	20.00–30.00
Length of steps, (m)	200.00–1550.00	160.00–1600.00
Berm widths, (m)	44.44–347.19	61.10–116.66 **
Slope angle of individual steps, (°)	32–59	42–53

* The value in parentheses represents the overall slope angle determined between T2 and T6 steps. ** Except the upper berm of T1 step.

.

3.2. Preliminary Studies and Investigations

In order to carry out stability analyses and verify the designed geometry of the steps, several types of investigations were undertaken, as follows:

1.

Studying the documentation made available by Jilț North open pit (description of the lignite deposit, the results of several geotechnical drillings, situation plan, longitudinal and cross sections, stratigraphic columns, etc.) [43], as well as previous research/service contracts, which concerned this exploitation perimeter;

2.

On site investigations related to the technical condition of the slopes of the working fronts. During these field visits, discussions took place with representatives of the Jilț North open pit, regarding working technologies and problems of slope stability encountered so far. From these discussions, it was established that, in essence, the slopes exhibit some degree of instability. There are flows of material on the slopes in both dry and wet conditions. Following these field visits, some relevant findings were made:

• There are some geometric non-uniformities of the working fronts caused by the morphology of the terrain and excavation technology;
• Rock displacements (dry flows and material rolling) in conditions of low humidity and tendencies of plastic flow in conditions of saturation especially on the slopes of T1, T2 and T3 steps. Usually they do not involve large volumes of material (can be regarded as superficial landslides);
• Displacements that affect the stability of the step over the entire height, with variable extent depending on the slope structure and the hydrometeorological conditions.

3.

Determining some physical and mechanical characteristics relevant for the rocks encountered in the slopes of the working fronts through laboratory tests, retrieving data from the specialized literature and their statistical processing.

The stability analyses were carried out with the help of Slide geotechnical software, in which the calculation section was modeled. The cross-section F–F through the working fronts in Figure 10b was taken into account (through the middle of the Jilț North open pit—considered to be the most relevant).

3.3. Selection of Physical–Mechanical Characteristics

In order to determine the physical and mechanical characteristics necessary in the stability analyses, a total of 10 samples were collected, according to the standards [45], both from the areas where landslides occurred in the last 5 years (T1–T3 steps), as well as from the rest of the working fronts (T4–T6 steps), later to be analyzed in the specialized laboratories.

The 10 samples, of approx. 30 kg each, were collected as follows:

• S1 clays from step T1;
• S2 clayey sands from step T1;
• S3 clays from step T2;
• S4 clayey sands from step T2;
• S5 clays from step T3;
• S6 coal clay from step T4;
• S7 lignite from step T4 (layer X);
• S8 lignite from step T5 (layer VIII);
• S9 clays from step T5;
• S10 clays from step T6.

The samples were then divided in two equal parts, packed in plastic bags (to maintain the natural moisture) and transported to the two laboratories (Earth Mechanics Laboratory of the University of Petroșani—approx. 1.5 h drive and the GeoLogic Laboratory in Calan—approx. 2.5 h drive).

The samples collected from the working fronts of the Jilț North open pit were subjected to laboratory tests, and a series of physical and mechanical characteristics were determined, according to specific standards [45,46,47,48]: moisture, particle size (granulometry), specific and volumetric weight, porosity, pore index, plasticity limits, consistency indexes, cohesion and angle of internal friction (shear strength), oedometric tests, etc.

Based on the statistical processing of the values determined in the two laboratories and data retrieved from the documentations [43], the values presented in

Table 4. The values of the physical–mechanical characteristics considered in the stability analyses [42,43].

Types of Rocks Encountered in the Analysis Section		γvnat [kN/m3]	cnat [kN/m2]	φnat [°]
	Sandy clays	22.00	25.00	20.00
	Clayey sands	21.00	15.00	16.00
	Coal clay (roof of layer X, intercalation between the banks of layer X, between layers X and XII and as discontinuity of layer XII)	18.00	33.25	28.00
	Lignite (layers V, VI, VII, VIII, IX, X, XII)	13.40	200.00	35.00
	Compact clay (roof of the pressurized aquifer, bed of layer V, base terrain for the internal waste dump)	19.00	52.00	30.00

γ_vnat, c_nat, φ_nat—volumetric weight, cohesion, and angle of internal friction (at natural moisture).

were selected, and further used in the stability analyses.

Considering the positive influence that lignite can exert on the stability reserve, due to its superior resistance characteristics compared to the surrounding (waste) rocks,

Table 5. The position of the lignite layers relative to the working fronts.

Lignite Layer	Existing Profile (May 2023)	Designed Profile (End of 2023)	Observations
V	Intersects step T6	Intersects step T6	Formed of 2 banks
VI	Intersects step T6	Intersects step T6	-
VII	Intersects step T6	Intersects step T6	Presents a discontinuity, then forms a complex with layer VI
VIII	Intersects step T5	Intersects steps T6 and T5	-
IX	Intersects step T5	Intersects step T5	It splits into 2 banks
X	Intersects step T4	Intersects steps T5 and T4	Formed of 3–4 banks
XI	-	-	Not present in the advancement area of the open pit
XII	Intersects step T4	Intersects step T4	Without economic importance in the advancement area of the open pit. It splits into 2 banks

presents a situation regarding the position occupied by the lignite layers in relation to the slopes (existing, as of May 2023, and designed at the end of 2023).

Next, in the model made with the help of the specialized software Slide (Figure 10), the values of the physical and mechanical characteristics (Table 4) were entered, after which the calculation program was run.

3.4. Results of the Stability Analyses

Over time, the literature has been enriched with numerous methods for stability analyses, developed by academics and researchers in the field, among which we mention: methods based on the limit equilibrium concept [1,2,3,4,5,7,8,9,10]; methods based on finite element/difference method [3,4,7,8,9,10,12,13,49] (which is a modern and accurate one, able to be applied to complex structures); methods based on 3D analyses [29,40,41,50,51,52,53,54] (for slopes with complex geometries, partially loaded, with variable structure, having a well-defined failure mechanism and assuming the addition of the third dimension in 2D models); probabilistic methods [55,56,57,58] (implies probabilistic analysis and processing of the physical–mechanical properties of the rocks and probabilistic computation by analytical or numerical methods); methods using Fuzzy logic [59,60,61,62,63] (analyses of this type being, in general, of qualitative type); and different combinations of the mentioned methods.

For the present study, the stability analyses were performed using methods based on the limit equilibrium theory (Fellenius, Bishop, and simplified Janbu), as they offer credible and satisfactory results [1], without taking into account any potential influence of external factors, such as precipitations or seismic loads.

These methods, based on limit equilibrium theory, are the commonly used methods (in engineering practice) for determining the safety factor of a slope, as they have proved their reliability and are easy to understand by mining operators [1,64]. Most of them assume that the sliding surface is a circular one, and the applied computing algorithms are based on this assumption [1,2,3,4,5,7,8,9,10,49,65].

Depending on the considered hypothesis, these methods assume the solution of a system of equations, of static mechanics, that will satisfy the equilibrium of moments and/or forces for each vertical strip [1,2,3,4,5,7,8,9,10,49]. The vertical strips in which the sliding mass is divided represents the discretization element of a potential sliding surface.

Table 6. Methods and computing hypotheses [64].

Method	Equilibrium of Moments	Equilibrium of Forces
Fellenius	YES	NO
Bishop	YES	NO
Janbu simplified	NO	YES

presents the satisfied assumptions within the static equilibrium by these methods [64].

The conditions imposed by the static equilibrium are applied for an individual strip as well as for the entire sliding mass, and involve solving the following situations [64]:

• The static equilibrium of each strip is ensured by the equilibrium of the projections in two orthogonal directions and the equilibrium of the moments of the forces with respect to any point;
• Total vertical equilibrium is ensured when the vertical component of the resistance forces on the sliding surface balances the entire weight of the body (including external forces) and the vertical forces on the contour;
• Total horizontal equilibrium is ensured when the horizontal component of the resistance forces acting on the sliding surface is in equilibrium with the horizontal forces acting on the contour;
• The equilibrium of the total moments is ensured when the sum of the moments of the differential forces between the slices is in equilibrium with the moments of the forces acting on the contour.

The stability analyses were performed using Slide, a specialized software for geotechnical studies. This software analyses the stability of natural and artificial slopes of any geometry, both under static and seismic conditions, as well as in the presence of water (hydrostatic and/or hydrodynamic conditions, submerged slopes) [65].

The problem of slope stability is considered bidimensional and is associated with plain strain conditions along the sliding surface. Also, the hypothesis that the braking is instantaneous along the sliding surface is accepted. The transmission of the sliding can be analyzed in two situations: progressive sliding, when failure occurs in the middle of the slope, due to compressive stress; regressive sliding, when failure occurs at the base of the slope and is transmitted to the top [66].

The program automatically calculates the stability coefficients, using several methods based on the limit equilibrium theory. According to this theory, the sliding mass is divided into vertical strips and the stability of the slope is analyzed in the hypotheses of a limit equilibrium between active and passive forces [65].

The results of the stability analyses performed for the individual slopes, as well as for different step systems, are presented in

Table 7. Results of the stability analyses for the designed slops (at the end of 2023).

Section	Step No.	Height H, (m)	Slope Angle α, (°)	Stability Factor (Coefficient) Fs			Observations on the Transmission Mode of the Potential Sliding Surfaces
				Fellenius	Bishop	Janbu
F–F	T1	25	48	0.890	0.911	0.884	The minimum surfaces materialize along the entire height of the slope, intersecting the upper berm, some passing through the tip of the slope, and some through the base (Figure 11)
	T2	20	42	1.300	1.408	1.274	The minimum surfaces materialize along the entire height of the slope, intersecting the upper berm, passing through the base of the slope
	T3	25	52	0.952	0.959	0.947	One of the minimum surfaces traverses the entire height of the slope, intersecting the upper berm at 5 m from its edge, and passes through the tip of the slope (Figure 12)
				1.355	1.422	1.341	Most of the minimum surfaces materialize along the entire height of the slope, intersecting the upper berm, passing through the base of the slope
	T4	25	52	2.095	2.454	2.276	The minimum surfaces materialize along the entire height of the slope, intersecting the upper berm, passing through the base of the slope
	T5	30	53	1.236	1.232	1.157	The minimum surfaces materialize on a part of the slope, intersect the upper berm, and pass through the roof of lignite layer VIII (Figure 13)
	T6	26	53	1.354	1.374	1.313	The minimum surfaces materialize on a part of the slope, intersect the upper berm, and pass through the roof of lignite layer V (Figure 14)
	Step systems			Fellenius	Bishop	Janbu	Observations on the transmission mode of the potential sliding surface
	T1–T2			2.559	2.754	2.460	Intersects the sandy clays and clayey sands layers in the overburden (Figure 15)
	T2–T3			3.516	3.665	3.430	Passes through the base of step T3, partly through the X lignite layer and coal clays (Figure 15)
	T3–T4			3.687	4.028	3.458	Passes through the base of step T4, intersecting lignite layers VIII–X and coal clays (Figure 15)
	T4–T5			3.732	3.795	3.598	Passes through the slope of step T5, intersecting lignite layers VII–X and coal clays (Figure 15)
	T4–T6			2.579	2.703	2.552	Passes through the slope of step T6, intersecting lignite layers VI–X and coal clays (Figure 15)
	T5–T6			1.995	2.059	1.947	Passes through the slope of step T6 and through the roof of the lignite layer V (Figure 15)
	General slope (all steps)			Fellenius	Bishop	Janbu	Observations on the transmission mode of the potential sliding surface
	T1–T6			2.726	2.761	2.702	Passes through the slope of step T6 and through the roof of the lignite layer V

minimum surfaces—the first 10 potential sliding surfaces with the lowest values of the stability factor.

and Figure 11, Figure 12, Figure 13, Figure 14 and Figure 15.

As a result of carrying out the stability analyses for the designed situation, the following conclusions can be drawn:

• The stability analyses in this paragraph were carried out for the designed conditions, the geometry considered for the analysis section to be achieved at the end of 2023, and took into account the stratigraphic columns received from the Jilț North open pit;
• For most situations, the lowest values of the stability factor were determined by Janbu’s method;
• For the T1 step, the stability analyses highlighted a sub-unit value of the stability factor, i.e., a slope below the equilibrium limit. The main reasons are represented by the relatively unfavorable geometrical elements (height of 25 m and inclination of 48°), but also by the physical–mechanical characteristics of the rocks in the structure of the slope. In order to solve the problem of the stability of this step, a reshaping of the initially designed geometric elements is necessary, by using a scientific method, which guarantees the continuation of extractive activities in safe conditions;
• In the case of the T3 step, following the performance of stability analyses, it was found that there is the possibility of materializing a critical sliding surface, which starts from approx. 5 m away from the edge of the upper berm and passes close to the tip of the slope. Such a slide, although superficial, can cause interruptions in the productive activity and, implicitly, economic losses. To prevent such a situation, a reshaping of the geometric elements is also recommended;
• For the rest of the situations analyzed (the rest of the individual slopes, different systems of steps, and the general slope), the values of the stability factor indicate a satisfactory reserve, in accordance with the national technical prescriptions [67] and the recommendations from the specialized literature [2,3,4,5,7,8,9,10,17,49,57] (in general, they recommend a minimum stability reserve between 10 and 15% for working slopes—with a reduced duration of staying in place);
• In the case of the steps whose structure also includes lignite layers (T4–T6), although they have more unfavorable geometrical elements (theoretically) than the steps excavated in waste rocks (T1–T3), due to the superior physical and mechanical characteristics of lignite (especially the shear strength), the values of the stability factor (coefficient) are satisfactory (stability reserves above 15%).

4. Redesigning the Unstable Working Fronts

Under these findings, to determine the stable geometry, we used the grapho-analytical procedure of Hoek–Bray [2], which (although appears old) has proved its viability in many cases of stability analysis and the design of the geometric elements of engineered slopes, being at the same time simple to use. The hypothesis on which this procedure is based is that the slopes slides occur following a circular pattern.

Starting from the factors influencing the stability of the slopes, Hoek and Bray graphically show (Figure 16) correlations that exist between the functions “X” (of the slope angle, α) and “Y” (of the slope height, H), depending on the geotechnical characteristics of the rocks (γ_v, c, φ) and the slope safety or stability factor (coefficient) [2].

The functions X and Y, have the expressions [2]:

X = α − 1.2 · φ,

(1)

Y = (γ_v · H)/c,

(2)

To obtain a required value for the stability coefficient, Fs > 1.15 for the T1 step and Fs > 1.30 for the T3 step of the open pit, the resizing procedure can follow either the determination of the slope angles for given heights or the determination of the maximum heights for given slope angles.

In this regard, the geometric elements were determined so as to ensure an appropriate stability reserve for the situation in which the initial designed heights are maintained. With the help of the graph (Figure 16) and calculation relations (1) and (2), the maximum allowable slope angles were determined.

Thus, for the given heights H of the slopes and by knowing the geotechnical characteristics of the rocks, the function Y is calculated, and from the points of intersection of its value with the curve of the stability coefficient Fs = 1.2, respectively, 1.4, the value of the function X, of the slope angle α, is obtained on the abscissa axis, from which the actual size of the slope angle α is determined.

Since for the step T1 the determined value of the slope angle was significantly below 40°, difficult to achieve with the help of BWEs type SRs 1400—30 (under the conditions of maintaining the designed height of 25 m), we considered a technical solution by dividing it into sub-steps, analyzing two possibilities:

• Construction of a sub-step ST1, with a height of 10 m, a berm of 25–30 m, and a slope angle of 45°, excavated using a BWE type SRs 470, and a second sub-step ST2, 15 m high, excavated using BWE E14, type SRs 1400—30 (currently operating on the T1 step). At the same time, the slope angle for the ST2 sub-step will be reduced to 40°. Since the three BWEs type SRs 470 excavators, under conservation (as specified in paragraph 2.2), are at a distance of approx. 4 km and at a difference in elevation of more than 150 m compared to the elevation of the T1 excavation step, this option would involve a relatively long period of interruption in the productive flux, with important economic consequences, which is why it is not recommended.
• Construction of two sub-steps, ST1 and ST2, with a height of 5 m, berms of 15 m, and a slope angle of 45°, excavated with wheeled classic excavators and transport of waste rocks with dumping trucks (discontinuous flux) and a third sub-step ST3, 15 m high, excavated with BWE E14, type SRs 1400—30 (currently operating on the T1 step). At the same time, the slope angle for the ST3 sub-step will be reduced to 40° (Figure 17).

As can be seen from Figure 17, the overall slope angle of the sub-step system, into which step T1 has been divided, will be 23°.

In this variant, even if the first two sub-steps are excavated in discontinuous flux, due to their position, at the top of the hill (meaning that the length of the excavation front and implicitly the volume of excavated rocks are much smaller compared to the rest of the working steps) the general technological flux of the open pit will not be interrupted (the excavations can be performed in parallel, using several wheeled classic excavators on each sub-step). The lower excavation capacity (compared to a BWE type SRs 470) is compensated for by the increased mobility of classic excavators, whose movement in the working area will not involve stopping the activity in the open pit.

In the case of the T3 step, applying the same procedure, it turned out that in order to increase the stability reserve to the desired value (Fs > 1.30) a reduction in the slope angle from the initially designed 52° to 45° is sufficient, which is easy to achieve with the E17 BWE (type SRs 1400—30) currently operating on this step.

Therefore, considering the second variant (recommended for the mining operator) presented for the T1 step and the reduction of the slope angle to 45° for the T3 step, we then proceeded to verify the results obtained from the redesigning process (through the Hoek–Bray procedure). For this purpose, a new set of analyses using the Slide software (considering the same physical–mechanical characteristics, presented in Table 4) were carried out, the results being presented in

Table 8. Results of the stability analyses for the redesigned slops (T1 and T3 steps).

Step No.	Sub-Step No.	Height H, [m]	Slope Angle α, [°]	Stability Factor (Coefficient) Fs			Observations on the Transmission Mode of the Potential Sliding Surface
				Fellenius	Bishop	Janbu
T1	ST1	5	45	2.158	2.215	2.131	The critical sliding surface materializes along the entire height of the slope, intersecting the upper berm of ST1 (at 3.5 m from the edge), and passing through the tip of the slope (Figure 18)
	ST2	5	45	2.265	2.269	2.237	The critical sliding surface materializes along the entire height of the slope, intersecting the upper berm of ST2 (at 1 m from the edge), and passing through the tip of the slope (Figure 18)
	ST3	15	40	1.163	1.204	1.138	The critical sliding surface materializes along the entire height of the slope, intersecting the upper berm of ST3 (at 4 m from the edge), and passing through the tip of the slope (Figure 19)
	ST1–ST2–ST3 (sub-step system)	25	23	1.803	1.938	1.751	The critical sliding surface materializes along the entire height of the sub-step system (or T1), intersecting the upper berm of ST1 (at 5 m from the edge), and passing through the slope of ST3 (Figure 19)
T3	-	25	45	1.396	1.472	1.380	The critical slip surface materializes over the entire height of the slope, intersecting the upper berm, and passing through the base of the slope (Figure 20)

and Figure 18, Figure 19 and Figure 20.

For all analyzed situations, the minimum values of the stability factor were obtained by Janbu’s method, and the lowest value of the stability factor was obtained for the ST3 sub-step (however the value is above 1.13—a stability reserve greater than 13%).

For the sub-steps system that make up the T1 step, the value determined for the stability factor is higher (Fs = 1.751) than the one imposed (Fs = 1.2) at the time of determining the geometric elements (slope angle) by the Hoek–Bray procedure, while for the T3 step, the value determined (Fs = 1.380) is slightly below the imposed one (Fs = 1.4).

The deviation from the imposed values, for the T1 step, is due to its division into sub-steps, while for the T3 step it is due to the reading errors of the figures resulting from the interpolations on the graph in Figure 16 (these being executed classically, without the help of a computer).

Therefore, we can consider that the results obtained through the redesigning process are confirmed by the subsequent stability analyses, the values obtained for the stability factor being able to ensure the necessary conditions to continue the extractive activity safely.

5. Discussion and Conclusions

In some situations (for reasons related to ensuring the necessary lignite for the energy sector in a timely manner), the mining operator tends to adopt similar geometries of the working fronts to those of the past for the development area of the open pit, taking into account especially the technical characteristics and possibilities of the equipment (mainly of the BWEs), and to a lesser extent the morphological and stratigraphic changes (which also involve changes in the resistance characteristics of the rocks that make up the slopes). In these conditions, the risk of slope slides increases, which, in turn interrupts the productive process, but can also result in the partial or total destruction of machinery or even loss of human life.

That is why it is necessary to monitor and permanently verify the stability of the slopes in the working fronts, by applying scientific methods and using appropriate tools (modern software and calculation programs verified in practice)

This is also the case of the Jilț North open pit, where, following stability analyses, it was proven that two of the designed steps, T1 and T3, have inadequate stability reserves (in fact, based on the results, they are unstable), which implies a high risk of slope sliding, either superficial or involving larger volumes of rocks.

The stability analyses within this paper were conducted for the interest areas of the mining operator for the designed and forecasted working fronts at the end of 2023 (according to the preliminary production plan).

Considering the obtained results, it is recommended that for the working fronts (steps) that intersect the productive complex (the lignite layers), T4–T6, the maximum slope angle should be 53°, and the height should not exceed 30 m (in fact, this is also the maximum excavation height of the type SRs 1400—30 BWEs, which operate in the Jilț North open pit).

For the steps excavated in waste rocks, T2 and T3, it is recommended that their height should not exceed 25 m, and the maximum slope angle be 40–45°.

In the case of the T1 step, excavated exclusively in sandy clays, it is recommended to apply a technical solution that involves the creation of three sub-steps (two with a height of 5 m and slope angles of 45°, and one of 15 m, with a slope angle of 40°). By applying this technical solution, the general slope angle of the T1 step is reduced to 23°, and the stability factor calculated for the sub-step system allows the safe continuation of the lignite extraction activity.

The technical solution proposed for the T1 step is interesting because it involves the introduction of a discontinuous sub-flux into the general continuous technological flux of the open pit, without interrupting the productive process.

This technical solution, with small adaptations to the particularities of each individual situation, may be adopted by mining operators from other open pits located in the mining basins from Oltenia region (such as Jilț South, Roșiuța, Lupoaia, Pinoasa, Tismana, Roșia, Panga, and Olteț open pits), which operate in similar geological and geomorphological conditions (in hilly areas), using similar technologies and machinery.

Therefore, the present study makes an important contribution to current open pit lignite mining in Romania, by making available to mining operators some relatively simple but effective technical solutions to increase the stability reserve of the working fronts, so that extractive activities can continue under safe conditions.

At the same time, we appreciate that this study is also useful in the process of academic training of future mining specialists (bachelor, masters, and doctoral students), being useful material for seminars, projects, and laboratories.