**1. Introduction**

Surface lignite mines exploited using a continuous mining method are typically large-scale excavation sites of high significance for primary energy production. The lignite extracted from these mines fuels high capacity thermal power plants. Although the operational life of a lignite mine usually spans many decades, after an intensive exploitation period, the content of lignite basins is depleted. Then, the lignite mines enter the closure phase that necessitates restoration of the post-mining sites, with beneficial effects to environment and society.

Continuous surface mining projects are very complicated because of the uncertainties associated with their dynamic situation. In this type of projects, the risks may relate to geological, technical, environmental, social, economic and other factors. In this framework, strategic mine planning, aimed at the rational exploitation of the coal/lignite deposits, is crucial for the sustainability of such

projects. This planning should incorporate the restoration of mining sites, considering the optimal mine development and the related risks as well as the time and cost data of mining activities. It requires, therefore, an integrated approach and optimization based on technical, economic, environmental, safety or social parameters.

Post-mining land restoration is related to integrated and long term practices of high social, economic and environmental impact. Literature reports various restoration methods. Some are more technical, dealing with engineering solutions for mitigating environmental impacts of the post-mining sites [1,2]. Others focus on restorations mainly achieved by natural processes, known as 'spontaneous succession' [3] or suggest hybrid or 'near natural' solutions [3,4]. Most of the methods cited consider reclamation as a fundamental activity related to extended earthworks for the removal of the excavated waste material (mainly consisting of interburden, overburden and topsoil material) and subsequent use of as much volume of this material as possible for the filling of excavation pits, voids and other open trenches shaped during the mine life cycle [2,5,6]. Based on the rules of aesthetics as well as of landscape architecture, the reclaimed areas should be perfectly integrated towards landscape reformulation and upgrading [7].

The planning of mining restoration is a multidisciplinary task requiring the synergies of managers, engineers, environmentalists, risk analysts and experts from various fields of science and technology, as well as local stakeholders (i.e., independent parties/bodies with interests or concerns in the approval and licensing of restoration projects). The main planning objective is to select the techno-economically feasible and low risk sustainable restoration methods that are the most advantageous for each site-specific case in terms of economic growth, prosperity of local society and ecosystem balance. In this context, the spatial analysis of selected indicators and cartographic overlay using ArcGIS applications can provide a useful tool, either for evaluating the corresponding sustainable exploitation of a mineral deposit [8] or for selecting the optimum land uses that should be developed in the reclaimed lands after the mine closure [9–11].

The surface mine restoration projects are associated with numerous risks and uncertainties related to natural and technological parameters [12,13]. As a result, the decision making for the selection of the most advantageous alternative is a difficult and complicated process. Therefore, the early identification of restoration risks and their impacts enables decision makers to have a substantial basis of understanding the advantages and disadvantages of each alternative. In turn, they will be able to select the optimal one which combines the lower risks with the higher expected benefits for the environment, society and economy. This is still a challenging research topic in the field of optimization, restoration and sustainability of surface coal mines.

The development of MCDM methods and techniques is a growing trend worldwide. Recently, a considerable literature has grown up around this field, covering a wide spectrum of academic research and practical applications in the industry, such as project and risk management, business and finance, manufacturing, etc. [14–17] From this spectrum, the mining industry should not be an exception [18]. Taking into account that the mining projects are long term and large scale frameworks with high inherent complexity, the needs of solving problems regarding the production optimization, land management and reclamation planning are critical and require substantial, cost effective, environmentally friendly and regulatory compliance decisions.

In the mining industry, various MCDM techniques and associated research have been developed in the recent decades. According to Mahase et al. [18], those techniques are applied to support, in principle, decision making needs for mine planning and related problems. Sousa Jr. et al. [19] used the PROMETHEE and ELECTREE methods in a process for the selection of highway trucks for mining operations. Musingwini [20] and Musingwini and Minnitt [21] ranked the efficiency of certain mining methods using the analytical hierarchy process (AHP) technique. Relevant to the mining methods selection are also the works of Bitarafan and Ataei [22] and De Almedida et al. [23] Yavouz [24] proposed a method for equipment selection using a combination of Yager's method and AHP. For the selection of the most appropriate underground mining method, Karadogan et al. [25]

presented the application of fuzzy set theory in a case for the selection of an underground mining method. Kazakidis et al. [26] suggested the application of the AHP in a series of different case studies/scenarios to investigate the investment analysis of a new mining technology, the design aspects for the ground support, the design of tunneling systems, the shaft location selection and the mine planning risk assessment. Khakestar et al. [27] performed a multi-criteria evaluation based on a hybrid combination of the fuzzy AHP and the technique for order of preference by similarity to ideal solution (TOPSIS) techniques for the selection of the optimum method regarding the slope stability analysis in open pit lignite mines. Bazzazi et al. [28] assessed a combination of the AHP and TOPSIS techniques to select optimal loading-haulage equipment for the Sungun open pit mine in Iran. Bascetin [29] recommended a decision support system using the AHP technique for the optimal reclamation of an open-pit mine. Finally, Uberman and Ostrega [30] applied the AHP in a project selection for the revitalization of post-mining areas.

The results of the above investigation indicate several interesting findings:


In this context, the aim of this paper is to propose a new approach to the research field of planning the land reclamation of surface mines by (a) demonstrating the decision-making problems and risks related to the selection of an appropriate post-mining restoration technology/method and (b) suggesting a low risk methodology based on multi-criteria decision making (MCDM) techniques, aiming to better understand and control the risks in surface mining restoration projects. Within this framework, the paper introduces a hybrid MCDM methodology [31–35], which is a combination of AHP (analytical hierarchy process) [36–38] and TOPSIS (technique for order of preference by similarity to ideal solution) [34,35], and fills a gap in the literature regarding the quantitative risk assessment in the field of mine land reclamation. A case study approach was used for the implementation of the methodology considering the selection of the lower risk restoration/reclamation method in a continuous surface lignite mining project.

The paper is organized as follows: the second section provides background information on restoration planning and the related risks and impacts. The third section presents practical decision-making problems and questions. The fourth section describes the suggested MCDM methodology. The fifth section demonstrates the implementation of the methodology in a case study. The final section provides conclusions, and directions for future research.

#### **2. Background**

In many countries, such as Germany [1], the Czech Republic [2], India [5], UK [39], the US [40] and Greece [41], the restoration of mining sites and industrial areas is a regulatory requirement. In this frame, the restoration of continuous surface lignite mines requires a multidisciplinary approach and properly planned activities of returning the mined-out lands to a form of acceptable environmental condition and productivity [5], along with beneficial returns to society and the economy. Nevertheless, the analysis of mining industry practices reveals that there are several restoration approaches, since the restoration does not follow a standard conceptual model, because of the significant differences in mining conditions. The exploitation models depend on the specific physical and operational characteristics of each mine. However, the post exploitation situation depends, mainly, on the geo-environmental features of the mined-out sites, as well as on the environmental legislation and regulatory constraints. Empirical

evidence and literature deals with various restoration approaches, however, three representative technologies can be, in principle, figured out: (a) technical restoration [2,42], (b) natural restoration driven by interactions between various bio-ecological and non-biotic factors [1] and (c) combined restoration consisting of partial human intervention developed in parallel to the natural restoration processes [2,4,6].

The basis for a techno-economically effective and environmentally friendly restoration plan is the successful reclamation of mined-out lands [1,2,5,6]. Reclamation is a large scale reinstatement activity aimed at returning the mined-out site to its original form, productivity and land use. Thus, reclamation enables landscape remodeling and, being conducted with other human activities and/or natural restoration processes, it contributes to the recovery of areas affected by the intensive operations of mine exploitation. The reclamation works during mining operations are mainly related to the extended backfilling of open pits, benches, open trenches and voids shaped by the mining activities.

From the project management viewpoint, restoration may be seen as a sub-project in a wider environmental management program, which maintains its own budget, execution schedule, organization, human resources, equipment and materials. Once a restoration project is under planning, managers, engineers and restoration experts are working in synergy to outline, in an appropriate level of integration, a multidisciplinary project definition report. For this purpose, they decompose and analyze thoroughly the geo-environmental conditions of the mining landform characteristics, as well as the techno-economic profile of the restoration project. In this analysis, critical parameters for the identification of restoration methods are assessed, i.e., quality of disposed soils, lakes and rivers, air pollution, topsoil fertility, infrastructure (roads, railways and welfare facilities), abandoned mining machinery, toxicity of discarded wastes, reforestation, recreation, geo-hazards, land use parameters, etc. The data required for the restoration analysis are collected through field investigations across the mined-out sites, water and soil laboratory analyses, satellite imagery and cartographic/GIS analysis products [2,43], as well as through review of statistical records, scientific literature and technical documents of the mine facility. The synthesis of a comprehensive restoration plan, recommending several technically adequate restoration methods and suggesting the most advantageous one for implementation, is a common practice in the evaluation phase of such projects. The plan is also delivered to the involved parties and stakeholders (environmental agencies, investors, municipalities, NGOs, etc.) for consideration and decision making.

On the other hand, the long term mine restoration projects, with time frames varying from few years to decades, are related to various risks. These risks have to be identified as early as possible, and an assessment of the impacts should be conducted. This work is critical for the determination of the overall risk associated with each alternative restoration method, in order to evaluate and finally select the lower risk alternative as appropriate and technically adequate for implementation.

A thorough investigation of the fundamental restoration risks has been reported in the work of Roumpos et al. [12], however, a further analysis on this topic exceeds the scope of this paper. The main risks are grouped in five categories, while each category is further divided into sub-categories as depicted in Tables 1–5: technical risks, geological and geotechnical risks, permitting risks, socio-economic risks and environmental risks.


**Table 1.** Surface Lignite Mines Restoration Technical Risks (R1).


**Table 2.** Surface Lignite Mines Restoration Technical Geological and Geotechnical Risks (R2).

**Table 3.** Surface Lignite Mines Restoration Permitting Risks (R3).


**Table 4.** Surface Lignite Mines Restoration Socio-Economic Risks (R4).


**Table 5.** Surface Lignite Mines Restoration Environmental Risks (R5).


#### **3. Decision Making: Problems and Questions**

The restoration of continuous surface lignite mines are projects of high technical, environmental and socioeconomic complexity, since the post-mining sites cover hundreds of square kilometers. In addition, restoration activities usually last for many years. The success of the restoration projects depends on the quality of restoration planning and decisions taken in the pre-investment phase. In fact, the improper evaluation of restoration plans might lead to unsuccessful decisions to accept inappropriate plans as feasible or, conversely, to reject restoration plans which in a deeper level of analysis are feasible and sustainable. Therefore, the decision making for the evaluation and selection of the most advantageous restoration technology for implementation is very critical, as it is directly related to whether a restoration project can be successfully implemented, delayed or may even lead to failure. Therefore, the quality of decisions is crucial and constitutes a vital element that indicates whether a restoration project can be implemented or not, particularly in the pre-investment phase, where the project feasibility and planning is subject to investigation.

The mining practice shows that the significant differences observed between restoration projects introduce various technical and managerial problems. Consequently, several considerations and screenings for the primary geo-environmental factors and reclamation technologies or methods advised by the restoration experts may be conflicting. For instance, a simple and low cost reclamation technology in sites polluted by toxic discarded materials might generate new environmental impacts of high severity. Moreover, the inappropriate restoration activity may relate to land use incompatibilities in a way that the restored land could become inappropriate for reforestation, recreation or re-cultivation. This fact might be subject to rejection by environmentalists, permitting agencies, socioeconomic analysts or stakeholders.

There are various decision making practices with respect to project management. Each practice depends on the nature of the project, the collective performance of experts and managers, environmental sustainability policies and regulations, budgetary and resource constraints and other geo-environmental factors. Some practices are very formal and move within the standards of administrative and legal protocols of public agencies and organizations involved with the regulatory compliance of mining restoration plans or wider programs. Other more empirical and subjective practices are based on mining experts' judgement and rely on comparisons with previous projects of similar scale, type and techno-economic profile. Some practices examine combinations of empirical evidence of the geo-environmental and ecological conditions of the under restoration areas, while others focus on the perspectives of restoration returns by considering the restoration project as a pure business case. In this view, project managers and restoration experts, in order to achieve an integrated and substantial approach to select the most adequate alternative technology/method of restoration, are called to solve various technical problems and to answer critical questions, such as:


As the list of relevant questions is long, it becomes obvious that there are many open points which, by their nature, generate significant and multilateral risks. Since the main decision making objective is the selection of the restoration alternative with the lower risk, it is suggested that these risks should not be simply considered as a contingency element in the project investment analysis. Instead, it is suggested that the decision makers should perform a substantial analysis of the restoration risks, where the risks are grouped and classified into specific categories. In this analysis, the risks can be quantified in a form of numerical data and used as criteria for the evaluation of the restoration alternatives. The quantification of risks can be obtained through properly structured questionnaires, where evaluators/experts can express their own perception on the significance of each risk, based on their professional experience. To this regard, the alternative technologies/methods can be evaluated, in terms of an MCDM methodological approach, considering the relative risks. Thus, the 'total risk score' for each alternative can be determined in a numerical form and the alternative with the lower total risk score can be selected as the most appropriate.

#### **4. Suggested Methodology**

The suggested methodology focuses on the application of MCDM methods and techniques [44], highlighting their utility as a tool for supporting decision making, regarding the evaluation of post mining restoration projects. The main objectives of the methodology are the following:


The methodology combines two widely known MCDM techniques: the AHP, applied for the quantitative expression of risk-based criteria and sub-criteria, and TOPSIS, applied for the calculation of the overall risk of each alternative and the ranking of the alternatives according to their total risk scores. In the AHP technique, experts use their knowledge, insight and professional experience to analyze the decision-making problem, by defining hierarchy levels and to solve it. In this framework, they follow an evaluation process based on quantified criteria. The selection criteria and sub-criteria in the restoration project of the case study are the relative weights of the identified risk and sub-risk factors. The decision makers perform pairwise comparisons, taking into account the 1–9 rating scale advised by Saaty [36–38,45], to evaluate the criteria and to structure and normalize a reciprocal pairwise comparison matrix. The computational process is verified by a mathematical consistency check and the relative weights of risk factors and sub-factors are finally identified. TOPSIS is based on the assumption that the most advantageous alternative should have the shortest distance from the ideal solution and the highest distance from the negative ideal solution (this distance is also known as 'Euclidean entity'). In this context, the distances of each alternative from the ideal and the negative alternative, respectively, enable the final ranking and classification of all alternatives [32,33,35]. Figure 1 describes the suggested methodology as a process model, based on the combination of the AHP and TOPSIS techniques.

**Figure 1.** Suggested Methodology.
