1. Introduction
Mining pit exploitation is accompanied by a negative ecological impact on the environment, which creates an obligation to continuously complete the planning, revitalization, reclamation and rehabilitation of a disturbed area in the period after exploitation. The post-exploitation remediation and rehabilitation of open-pit mining areas and disposal sites, i.e., space disturbed by mining operations, represent a very complex multiphase engineering problem. Based on the application of decision-making and management theory, it is possible to efficiently solve the problems of applying these phenomena to the processes of technical and biological reclamation of areas and analyzing their general impact on environmental factors.
The reclamation planning process through all phases of open-pit mining development has three levels, namely, preliminary, operational, and final, and can be applied both to new open-pit mines and to open-pit mines in operation for which this has not been done before. Numerous examples of the scenic, natural, and cultural attractiveness of post-mining land encourage the mining sector to create new environmental values and potentials, as well as new conditions for environmental protection. Post-mining land with its landscape generally becomes a natural part of the region and is no longer a place of previous exploitation [
1]. Many closed mines have become forests, agricultural lands, and nature reserves. Technical reclamation is a common approach used in post-mine sites and is even a mandatory practice in some regions due to legislation in many countries [
2].
The purpose of this research is to analyze a real case study of an open-pit coal mine that is viewed as a basis for experimental analysis to select a reclamation type and appropriate rehabilitation. This process modeling of a unified reclamation system includes a total of 20 alternative solutions for designing the reclamation content (which are modeled through 11 possible solutions in this particular case) and rehabilitation of the post-exploitation area of the Tamnava-West Field open-pit mine and disposal site.
The aim of this paper is the development of a hybrid IMF SWARA—Fuzzy ROV model, for the first time in the literature, to select the best reclamation solution from a set considered for the Tamnava-West Field open-pit mine. The defined aim reflects a twofold contribution from the aspect of the proposed methodology and from the professional aspect, which implies a sustainable relationship with the environment. The IMF SWARA method is used for the calculation of the importance of criteria based on expert analysis and the preferences of experts. Due to the fact that this method belongs to subjective methods for determining criterion weights, in this study, we consulted only experts with a minimum of 15 years of field experience, who have high performance, high skills, and enough knowledge to assess the mutual comparison of criteria. IMF SWARA represents an appropriate and precise tool for determining criterion weights due to its procedure and the fact that experts first need to sort criteria. After that, the experts only compared two criteria. In this way, the IMF SWARA method, compared to other methods for calculating criterion weights, is a very acceptable tool for experts. Also, compared to other methods, the number of total criteria is not limited and there is no need to form a multiphase hierarchical structure as it is mandatory in the AHP (Analytic Hierarchy Process) method. The Fuzzy ROV method represents a simple but precise tool to sort variants according to a multiphase normalization process.
Apart from the mentioned motivation, aims and purpose, the main research questions should be defined.
- (1)
Is the problem of implementing reclamation solutions in vast areas degraded by coal exploitation evident in Serbia today?
- (2)
How can we define all potential problems and potential solutions, and how can parameters be defined to fully represent the current state of open-pit coal mining?
- (3)
Which model can provide a comprehensive analysis and give positive results in practice?
The answers to these research questions will be supported by the findings presented in this comprehensive study model, aiming to satisfy all local requirements for the reclamation of open-pit coal mining areas and to follow sustainable development goals. In Serbia, the problem of implementing reclamation solutions in vast areas degraded by coal exploitation is currently evident. We recognized gaps in practice and, in this study, all necessary input indicators which have influence on sustainable development are addressed. Also, a large set of potential solutions was created based on experimental measurement and using an engineering process approach. Finally, we created an integrated fuzzy model which treats uncertainty in a precise way and enables optimal results. The verification of the feasibility of multiple-criteria decision-making models as demonstrated in this paper, yielded positive results in practice at the Tamnava-West Field mine and confirmed the advantages in ranking the given alternatives.
After the introductory details, this paper is structured into the following sections.
Section 2 provides a review of the literature with a focus on recent studies. In
Section 3, a decision-making algorithm and the steps of the Fuzzy ROV and IMF SWARA methods are presented.
Section 4 provides a description of potential reclamation solutions and the criteria based on which they are evaluated. In this section, the evaluation procedure and the results obtained are also presented.
Section 5 refers to verification tests and, finally,
Section 6 to concluding considerations.
2. Review of the Literature
Due to specific and very rigorous legislation that prescribes procedures, conditions, competences, and responsibilities, large degraded areas have been successfully restored in many countries. In the area of the Midwest of the USA, farms, which were brought to a state of productivity as before the exploitation of coal by means of agromelioration measures, were established. In recent times, there is often a commitment to the restoration of natural wild habitats. The reclamation technique which turns post-exploitation areas into wetlands is becoming particularly popular [
3]. In Great Britain, specific laws regulate the aspect of planning and rehabilitation, separately for England and Wales, and separately for Scotland. A permit to open a new open-pit mine is issued by the National Coal Board (NCB), whereby a contract is concluded with the mining company [
4]. In the application of project management principles for reclamation at the sites of old mining pits, the case of Thoricos Bay in Lavrion, Greece, is given as an example of industrial zone rehabilitation and the reclamation of an abandoned and closed area of mining exploitation [
5,
6]. For the analysis of ecological risk, options for risk isolation and minimization were selected, utilizing a reclamation scheme, while special additional measures were applied in areas of high-sulfide waste. The application of mining technologies, which includes the removal of tailings and the methods of their disposal on waste heaps, as well as the formation of open excavations, has an impact on habitat conditions in areas that have undergone land reclamation [
7,
8,
9]. China, the USA, and India are taking a leading role in researching the restoration of coal mining sites using modern technologies. Miners in Polish mines carry out reclamation and land rehabilitation using mining skills after the period of exploitation. The works are performed according to a high European standard, so that the land can be used for agriculture or forestry, including recreational activities [
1]. An analysis of the existing 5000 contaminated sites in Germany revealed 159 closed open-pit mines, 122 tailings (with ash or lignite), 120 sedimentation facilities, and 9 thermal upgrading plants. The study showed that more than 8.2 billion euros had been spent on lignite rehabilitation by 2008 [
10]. Brownfields in France represent contaminated sites where built-up (agricultural, port, industrial, service, etc.) land has been temporarily or permanently abandoned after the cessation of mining activities and which has been put back into use. About 200,000 former industrial sites and about 200 former mines have become brownfields [
11]. Based on research conducted at five different pits of the Zhangji coal mine (China) over a period of 11 years, the concept of dynamic subsidence reclamation (DSR) was implemented to evaluate whether DSR could improve both the environmental and socio-economic conditions for post-mining land use by traditional reclamation (TR) or TR-modified (TR(MOD)) reclamation [
12].
Based on the research conducted in the High Groundwater Coal Basins (HGCBs) in the eastern China plain at seven areas in Pei county, the study proposed an integrated model for the simulation and optimization of post-mining land use structures. Different scenarios of land use structure in relation to the depth of subsidence were developed in order to determine the optimal collapse depth [
13].
The preservation of mining heritage with the aim of making the region more attractive implies the revitalization of facilities after mine exploitation. Studies in Spain, Poland, and Great Britain show that the revitalization and attractiveness of the locations are regulated through legal regulations. This research shows that land lease contracts could contain an obligation to re-stabilize or return the exploited land to its original state of protected environment. Regardless of legal conditions, revitalization measures are being implemented with positive effects at the Gold Mine in Zloty Stok, La Tortilla Mine in Linares, and King Edward Mine, an old mining site in Cornwall [
14].
Many coal mines are included in programs of sustainable development, economic support to regions and local communities, environmental restoration, etc. Mining is not related only to exploration and exploitation, but also considers environmental responsibility to protect environmental sustainability [
9,
12,
15,
16]. Legal, social, and environmental requirements for complex mining operations are fully integrated into the mining and reclamation plan, while addressing sustainable development and engineering analyses will result in the optimization of mining operations. This systemic approach provides economic and environmental sustainability, the protection and restoration of the environment, and the efficient and effective justification of mining and reclamation. A study has shown that the coal exploitation designer must align legal, environmental, and sustainability goals with traditional mining parameters [
17]. A similar approach is given by the National Coal Board (NCB), whereby a contract is concluded with a mining company [
4].
Research in Indonesia conducted to analyze the ideal implementation of coal mine reclamation found that, after mining, the land is often left dry with large voids that are prone to flooding and soil erosion. It was found that 70.59% of land reclamation programs in Indonesia aim to reforest mining areas into secondary forests. The implemented types of reclamation are aquaculture, urban forests, park playgrounds, sports parks, livestock farms, and fauna conservation ecotourism. A new reclamation approach called eco-habitat was also developed. The reclamation process should be carried out using an eco-habitat approach based on the principles of rezonation, revegetation, and revitalization (3R) [
18].
Due to the intensive coal exploitation and deterioration of the environment in Shanxi Province in China, there has been a decrease in farmland. With the concept of “green mining industry”, a basic method of reclamation in mining areas, an engineering study of mine reclamation, and a model of ecological agricultural reclamation in mining subsidence were developed. That study suggests implementing stereoscopic types of plantations, aquaculture and cropping and raising, as potential solutions [
19]. Also, research into innovative technologies and the theory of land reclamation in the past 10 years included the analysis of exploitation, reclamation, land filling (Yellow River), self-reclamation, and topsoil alternative solutions in open-pit mines. In this research, which included the eastern, western, and southern zones, it was shown that several important laws and regulations were adopted for the five-year period. These important laws and regulations were successively promulgated, greatly promoting land reclamation [
20].
Reclaimed mine soils developed at sites after coal mining form very diverse lithological layers that show significant variability depending on soil texture. However, a research study [
21] showed that plant habitat is based on lithology and the genesis of sediments from parent rocks, so the predicted fertility potential of rock tailings resistant to weathering can be misinterpreted, providing an overestimated final result of habitat classification. Also, the implementation of a reclamation optimization plan is important for the construction of appropriate environmental protection projects and the regulation of life and production activities in the mining area during the completion of construction works [
22].
Based on research in China [
23], Concurrent Mining and Reclamation (CMR) technology was analyzed, and applied principles and technologies were provided for underground coal mining. That study showed that the advantages of a high percentage of land reclamation are adapted to the new technology. According to this research, CMR could increase the percentage of farmland reclamation to 37.59%, compared to the percentage of farmland reclamation post mining.
A study conducted in Loess, China, which investigated the regulation of soil nutrients, analyzed the degree of soil nutrients in different types of soil used. The study showed that nutrient levels in the soil increased significantly in the first five years after reclamation. In that study, four types of nutrients (potassium, phosphorus, nitrogen, and organic matter) in the soil were analyzed [
24]. Based on the given levels of nutrients in the soil, guidelines were given for the precise management of reclaimed mining land with optimization measures of land reclamation. Also, some laboratory research on the physico-chemical parameters of soil substitutes showed the possibility of their application for the reclamation of highly acidic coal mine disposal sites. The results of applying soil cover to the used pits gave spontaneous successions of mesic and dry meadow species after the second year of vegetation growth. The content of heavy metals after the second year of vegetation showed a concentration of toxic metals (the share of which did not exceed the permitted level according to Polish standards), but it is possible to apply these reclaimed parts for green areas, wooded, and bushy lands [
25].
Research based on linear programming techniques carried out in the area of the Banji mine in China, with an annual production of about three million tons of coal obtained from underground mining, showed that the reclamation carried out for the purpose of fishing, grain production, orchards, and the like was optimized under the limitation of fixed capital (capital, resources, workforce, etc.) [
22].
Based on previous studies on mining soils [
26,
27,
28,
29,
30], the specifics of soil properties that provide useful information for soil reclamation are particularly emphasized. The vulnerability of the mining area is caused by reclamation norms that have been studied but are difficult to apply in practice. Therefore, other comprehensive studies are also included, among others, research related to the application of the MCDM model [
31,
32,
33,
34]. Similar research has been proposed using the subjective–objective MCDM model in [
31] in which AHP and Entropy were used to determine the criterion weights, while modified VIKOR sorted possible reclamation variants. However, this study considered only 3 possible variants, while we defined 11 alternatives in our study. Also, the methodology applied to calculate the significance of criteria requires quantitative data on the one hand, while, on the other, a large number of criteria need to be used, and all criteria should be considered simultaneously. Article [
32] provided the MCDM framework for plant species for the reclamation of copper mines. While the methodologies of Fuzzy AHP, PROMETHEE (Preference Ranking Organization Method for Enrichment Evaluation), and Fuzzy TOPSIS (Technique for Order of Preference by Similarity to Ideal Solution) are old and have been extensively explored in prior studies, our research introduces a novel methodology to address the subject of research. The selection of a strategy for the sustainable transformation of a mine constitutes a complex decision-making process that presents various practical challenges [
33]. Article [
34] discusses the application of the strengths–weaknesses–opportunities–threats (SWOT) analysis and AHP for selecting a sustainable strategy, using the case study of a closed open-pit lignite mine in Greece.
A review of other studies suggests that we developed a highly comprehensive and appropriate tool for selecting a suitable sustainable reclamation variant for open-pit coal mining areas. The advantages of our proposed approach were discussed through research questions and partly in this section.
5. Verification Tests and Discussion
This part of the study is dedicated to the three-phase verification procedure of the initially obtained solutions, and includes the simulation of new criterion weights, comparative MCDM analysis, and the calculation of statistical correlation tests.
5.1. Changing the Weights of the Criteria (Sensitivity Analysis)
In each MCDM model, criteria play an important role, and by simulating their weights through new scenarios, it is possible to identify a potential change in the ranks of alternatives [
42,
43], i.e., in this case, reclamation solutions. Thus, it is possible to include all potential results and their changes and strive towards their proactive management. In this part, 12 × 10 = 120 scenarios were formed, in which the weights of all 12 criteria were modeled in the 5–95% interval. The values of the new weights across the scenarios are given in
Figure 4.
In the initial scenario, which includes criterion weights obtained with the IMF SWARA method, the lowest value is for the tenth criterion (0.021, 0.029, 0.042), and the highest is for the sixth criterion (0.133, 0.143, 0.193). In the hundredth scenario, the tenth criterion has the lowest value (0.001, 0.001, 0.002), which means that it tends to zero and that the value of this criterion is reduced to negligible importance. On the other hand, an extreme value is in the 110th scenario for the sixth criterion (0.153, 0.167, 0.231). It is important to note here that, in this scenario, the minimum value is of the second most significant criterion, and that is C11.
Figure 5 shows the new ranks of the reclamation variants in accordance with the previously simulated new weights of the criteria and their mutual relationship.
The results of the sensitivity analysis (
Figure 5) show a great influence of changes in weighting coefficients, that is, the sensitivity of initial results in relation to changes in the values of the criteria. The only reclamation alternative, which is ultimately the most important, is A1—Forestry, which remains in first place regardless of the value of any criterion. Other alternatives change their positions by even several places depending on a scenario. The greatest deviation is for A5 (1.789), which, in certain scenarios, changes its position even by five places, and this is a consequence of reducing the importance of the second and eighth criteria. It is important to emphasize that the standard deviation (in addition to the first alternative that does not change) is the least for the second best reclamation alternative (A3 = 0.396) and for the last-ranked alternative (A5 = 0.416). Other reclamation alternatives change their ranks in most scenarios depending on their performance according to the best criterion.
5.2. Comparative Analysis
The results obtained with IMF SWARA—Fuzzy ROV were compared with three other fuzzy MCDM methods, F-MARCOS [
41], F-SAW [
44], and F-WASPAS [
45], as shown in
Figure 6.
The results obtained after comparison with three fuzzy MCDM methods show that there are certain minor deviations that are primarily related to the reclamation solutions that are ranked lower. Five out of eleven alternatives in total show changes in positions by only one or two places. For example: A4 is ranked fourth with Fuzzy ROV, while according to other methods, it is placed eighth. The remaining changes are as follows: A8 (8 → 9), A9 (5 → 7), A10 (6 → 5), and A11 (7 → 6). This confirms the initial results obtained with the IMF SWARA—Fuzzy ROV model.
5.3. Calculation of Correlation for Newly Obtained Ranks in Verification Tests
Since it was identified that there are deviations in the sensitivity analysis and small deviations in the comparative analysis, the statistical correlation of all ranks was calculated, as shown in
Figure 7. The calculated correlation refers to the WS [
46] and SSC [
47] coefficients.
As previously determined and noted in the sensitivity analysis, the model is sensitive to changes in the weights of the criteria, so some reclamation solutions have fallen by even five positions. In general, the average values are extremely high, SCC = 0.901 and WS = 0.966, considering that there are as many as 120 formed scenarios. The lowest statistical correlation is SCC = 0.564, WS = 0.838 in scenario 110, as previously explained. Of course, in a certain number of scenarios, there is a total correlation of the results.
Figure 8 shows the calculated correlation values for the comparative analysis.
Since there are relatively small deviations in the ranks of reclamation solutions, statistical correlation tests tend towards a total correlation, i.e., a value of 1.00. The average correlations are SCC = 0.973 and WS = 0.988, and Fuzzy ROV has a correlation with other methods of 0.964 and 0.984, respectively.
6. Conclusions
The issue of environmental protection and an adequate approach to achieving sustainable development is a priority in all areas of action. In this paper, the problem of reclamation of an open-pit mining area after its exploitation is considered. A total of 20 potential reclamation variants were selected using a process approach, and for the specific case of the Tamnava-West Field open-pit mine, 11 alternatives were defined and they mostly represent a combination of 2 or more alternatives. For the purpose of evaluating these reclamation variants, a total of 12 criteria were defined and a hybrid IMF SWARA—Fuzzy ROV model was created for the first time in the literature; this model is applicable in all areas of decision-making and evaluation of variant solutions. The results show that the reclamation variant related to forestry is optimal under the considered conditions of multi-criteria evaluation. This is confirmed through verification systems. In accordance with the potential of the considered open-pit mine, it is necessary to apply the selected solution and start its implementation.
The benefits of the conducted research can be viewed through the prism of engineering and social and mathematical aspects. Advantages include considering almost all possible variants for the reclamation of open-pit coal mining areas, and their integration for specific cases, defining all sustainable inputs for their evaluation, and finally developing a proper integrated mathematical tool for precisely determining criterion weights and sorting alternatives. The selection of the right and most suitable sustainable solution for the reclamation of open-pit coal mining areas is one of the key social and environmental tasks. Thus, this study can contribute to achieving sustainable development goals. The limitations of this study can be manifested by the fact that it only considers coal mining, and the number of experts included in the evaluation process could also be larger.
Future research is related to the consideration of other open-pit mines, their evaluation, and the selection of an optimal variant, certainly depending on the specific characteristics of those mines. Additionally, the development and application of new models in the theory of uncertainty can constitute future tasks.