1. Introduction and Problem Statement
Mineral extractive industries are an important global industry and form the foundations of our lives [1
]. World Mining Data indicates that the industry extracted over 17-billion tons of raw materials with a value of about USD 2.03 trillion or about 2% of the global GDP in 2022 [2
]. The industry is expected to grow consistent with societal needs [3
]. During minerals-related production activities, our land [4
], water [6
], and air and ecosystem resources [8
] are disturbed short-term and can also be negatively impacted long-term unless disturbed areas are appropriately reclaimed. Society supports mineral extraction activities since they have the advantages of significant economic, social, and political impacts. However, the industry must slowly transform under pressure from global competition, environmental regulations, and local communities to be sustainable [9
Coal is the main energy resource [10
] in China, with a production rate of about 4.5 b. tons in 2022 [11
]. It has contributed to the national economic development over the last few decades [12
]. Underground coal mining using longwall techniques is very common in China [13
] and accounts for 90% of coal production [14
]. Due to different geologic conditions, other major coal producing countries mainly use surface mining, such as the USA, India, Australia, and Russia [15
]. Surface subsidence impacts caused by underground coal mining vary in different regions. For instance, surface subsidence in USA is only about 0.5 to 2 m [16
], and face advance rates are about 30 m per day [14
]. A small amount of grading and water ditches development is performed to use land for agriculture. In China, the reclamation of surface subsidence poses more serious problems [14
]. There are several regions where underground coal mining is performed with shallow groundwater and large surface subsidence [17
]. For example, the deepest mining subsidence of 19.8 m was observed in Huainan city of Anhui Province [18
]. In such areas, subsidence can negatively impact on farmland [19
], the landscape topography [20
], surface and ground water resources [21
], ecosystem [23
], and socio-economic health both during and upon completion of mining activities. It can also lead to loss in agricultural productivity [24
] and production [25
] and threaten food security, if reclamation is not appropriately planned.
Currently, reclamation operations in China are mainly focused on relatively stable mining-subsided land after mining [27
], and they are known as a traditional reclamation (TR). Therefore, a large amount of agricultural land has already submerged into water, including the fertile topsoil and subsoil. Moreover, reclamation operations in waterlogged areas can also increase the reclamation cost by about 30% [28
]. Backfill materials with characteristics similar to submerged soils are not plentiful for reclaiming large subsided areas and can further increase the costs. Therefore, TR can result in the loss of farmland during the active mining period, a low percentage of reclaimed farmland in post-mining land use, poor quality of farmland, and higher reclamation costs. These deficiencies in TR led to exploring alternate reclamation technologies for actively subsiding mining areas.
Through careful planning, it is possible to successfully reclaim and restore the disturbed ecosystems [29
] and close a mine with minimal or no long-term impacts. Based on the reviews of the TR plan, China University of Mining and Technology Beijing (CUMTB) researchers extended the concurrent mining and reclamation concept to subsiding lands [30
] and demonstrated its advantages through a case study in Huaibei city. While mining in areas with a shallow groundwater table and agricultural resources, dynamic subsidence reclamation (DSR) [31
] immediately ahead of and behind the longwall face evolved to maximize the utilization of land and water resources for agriculture. Zhao and Hu [32
] proposed pre-reclamation concepts and developed a generalized technical model for its implementation. Xiao [33
] analyzed reclamation time periods for actively subsiding areas to plan for concurrent mining and reclamation. Using time-based subsidence projections and reclamation operation requirements, Hu and Xiao [34
] developed an optimized concurrent mining and reclamation plan for a case study in north Anhui. Xiao et al. [35
] suggested that topsoil must be stripped before it submerges into subsiding waterlogged areas and should be used to create alternate farmland areas behind the face or around the edges of subsiding areas to minimize the loss of farmland. Xiao et al. [36
] introduced concurrent mining and reclamation technology in eastern China for prime farmland protection. Hu et al. [29
] extended DSR approaches to minimize impacts on farmland and water resources with a simple economic analysis. Zhang [37
] researched the techniques for concurrent mining and reclamation planning in multiple coal seams mining areas. Chugh [14
] contributed to the above research and developed a state-of-the-art review paper on concurrent mining and reclamation concepts and their applications in China. Chen and Hang [38
] proposed a soil reconstruction procedure for dynamic reclamation. Li et al. [10
] integrated coal mine and reclamation planning to reduce impacts of subsidence on mined lands and identified four key steps for planning and interactions among them. Through analyses of alternate mining plans, Feng et al. [39
] developed optimum mining plans for the Guqiao coal mine. Li et al. [40
] considered coal production and aboveground development or protection to optimize the layout of underground coal mining in Jining city of China. Previous research mainly focused on reclamation time, modifying mining planning, multiple coal seams, and the soil reconstruction procedure.
DSR planning discussed here advances previous DSR tools to mitigate both environmental and socio-economic negative impacts. In the DSR technology, reclamation unit operations (topsoil and subsoil removal and replacement, farming and crop harvesting, and management of water resources) are dynamically implemented ahead of and behind the current mining areas to minimize negative impacts to land and water resources and to nurture new and/or old socio-economic enterprises in reclaimed areas.
With significant expected coal production from underground mining in China over the next few decades and global pressures for sustainable development, there has been significant advances in research on reclaiming unstable subsided lands. Underground mining environments similar to China are not common globally and therefore most research on the topic has been in China, and it has been slow and evolutionary. As discussed earlier, Zhao [32
] and Xiao [33
] analyzed the initiation of reclamation time without considering the economic benefits. Zhang [37
] extended the concepts to mining multiple coal seams. Li et al. [10
] and Feng et al. [39
] focused on underground mine plans to minimize subsidence impacts without considering original surface topography, and Chen and Hang [38
] focused on soil reconstruction procedures. Hu et al. [31
] developed DSR concepts for mining areas with shallow groundwater resources without considering topsoil and subsoil separation, socio-economic development concepts of the super-farmland, large water storage, and associated small- and large-scale business development. So, the authors here have significantly advanced DSR planning concepts to include: (1) creating super-farmland areas of high agricultural productivity to offset the loss of farmland submerged under water, and planting fruits and vegetables to diversify and enhance revenue streams; (2) creating large areas of planned quality water resources to serve existing and new or improved socio-economic ventures to support long-term community needs at the local and regional levels, and creating new water sports businesses and fish breeding areas; (3) perform simple cost–benefit analyses for modified DSR concepts; (4) develop a TR(MOD) plan to analyze the importance of stripping soils before they submerge into water and separating topsoil and subsoil; and (5) consider pre-mining surface topography to predict subsidence and develop the DSR, TR, and TR (MOD) reclamation plans based on dynamic post-mining topography. To the best of the authors’ knowledge, such research is not being conducted elsewhere in the world, since such mining conditions do not exist. An invited oral presentation was made by Dr. Chugh to World Mining Congress professionals in India in 2019 (no publication) and received significant interest.
Previous limited DSR concept field implementations in China [30
] led to the following observations: (1) the soundness of the technical concepts was well received; (2) the farmers were hesitant to strip topsoil and subsoil prior to the soil being submerged in water due to reduced agricultural production; (3) mining companies allocated more resources to production than to DSR planning with less than optimum efficiencies in the reclamation processes; and (4) socio-economic considerations and community involvement were not considered. The above was to be expected considering the newness of the concepts.
The authors propose to widely disseminate these research findings to mining professionals and to government agencies responsible for regional development to identify project opportunities for implementing these concepts in single seam and multiple-seam mining areas. Such projects should also consider optimizing both mine and DSR planning to minimize the production cost to the consumer. The authors hope that this study will encourage mine operators to implement DSR concepts in planning while considering new and ongoing mineral development projects. DSR is a powerful engineering and planning concept that can minimize the land, water [14
], and air impacts of mining and enhance socio-economic conditions under a variety of mining conditions. Furthermore, it should not be limited to only reclamation planning. Mining and DSR planning should be integrated to develop both optimal mining and reclamation plans [10
] to enhance the profitability of the mining venture and its sustainability. That should further develop new ideas for both mining and reclamation for the improved sustainability of mineral projects. Toward this goal the authors recommend that every mining project, irrespective of the stage of its development, should consider having a steering committee consisting of mining, business, and regional development professionals to review alternate opportunities for mining and reclamation and to improve project sustainability while maximizing the profit potential. That would also ensure the development of sound mine closure plans [42
] and the slow implementation of the idea that “mine closure must begin the day mining starts” while maximizing profit potential.
This paper has attempted a conceptual implementation of DSR planning concepts at a large coal mine involving five single seam longwall faces in a mining area with a shallow groundwater table about 1.5 m below the ground surface and 4.4 m of maximum surface subsidence due to mining. The study involved mining and post-mining reclamation to assess if DSR could have improved both the environment and socio-economic conditions for post-mining land use as compared to using TR approaches in China. In DSR topsoil, subsoil, farming, and water resources management were dynamically synergized concurrently with mining. Within TR, two approaches were considered: (1) designated as TR, all reclamation activities are initiated only after all mining had been completed in the area, and land was allowed to submerge into water in the subsiding areas; and (2) TR (MOD), where soils (topsoil and subsoil) ahead of mining, likely to be submerged, were stripped and stored without separation into topsoil and subsoil for reclamation after all mining was completed in the area. The authors undertook the current analyses for this hypothetical case as soon as the data became available.
A comparison of the three analyzed plans shows that: (1) upon final reclamation, farmland area and water resources in the DSR and TR (MOD) plans are increased by 5.6% and 30.2%, respectively, as compared to the TR plan. Stripping soils before they submerge into water is important for farmland reclamation and long-term economic development; (2) due to topsoil and subsoil separation in DSR plan, reclaimed farmland productivity should recover quickly and agriculture production would be larger than in the TR and TR(MOD) plans; 3) the total estimated net revenue in the DSR plan should be 2.8 times more than in the TR and 1.2 times more than that in the TR (MOD) plan; and (4) the total net revenue of the TR(MOD) plan should be increase by 8.1% as compared with the TR plan. The above net revenue benefits should be even greater for analyses over longer periods. Furthermore, in DSR, the availability of farmland and water resources will be for much longer throughout the active mining period. The mining and reclaimed areas in DSR can therefore provide an improved socio-economic environment through community business development and the settlement of labor markets both during mining and after mining ceases in the area. Although solutions and sample calculations presented are simple, the paper clearly demonstrates the usefulness of the DSR concepts to minimize negative impacts to the environment while enhancing the long-term socio-economic environment.