Coal Mining Surface Damage Characteristics and Restoration Technology

Abstract

With the continuous improvement of mining technology and equipment in China’s coal mines, the number of working faces with high-intensity mining is increasing, and the area of surface damage continues to increase compared with previous years. In order to ensure safe production and protect the overall environment of the mining area, the damage characteristics of overlying rock and the transmission mechanism of surface damage in coal mining were analyzed, the forms of surface damage and their negative external influences were evaluated, the current situation of surface damage prevention and restoration technology was investigated, and the feasibility of surface damage economic utilization was considered. According to the above theoretical analysis and the current situation of China’s industry, this paper discusses the prevention and restoration of surface damage in China, and puts forward the following idea of mine governance: “utilization first, prevention and restoration later; natural restoration is the main approach, and artificial restoration is auxiliary; artificial restoration and natural restoration should be combined”, which provides a governance concept for mine surface damage management. This study on the negative external influence of rock and surface damage and its prevention and restoration technology is of great significance for mine safety and environmental protection.

1. Introduction

China is a major energy consumer, and economic development is often positively correlated with energy demand. The stable development of an economy cannot be separated from the corresponding energy development [1]. Energy development plays an important role in ensuring the sustainable development of an economy. In China’s energy structure, coal is the dominant energy. Related prediction studies believed that China’s energy consumption demands of 5.5–5.6 billion tons of standard coal in 2025, accounting for 50–52% of total energy consumption, and with coal as the main energy structure, will continue until 2050 [2].

Large-scale underground coal mining will inevitably lead to the destruction of overlying rock and changes of the stress field and fracture field [3], which will have a significant impact on stope support and the coating rock structure, and its level of influence often depends on the coal mining intensity [4]. The destruction of rock lithology often leads to changes to surface characteristics. According to previous studies [5], it was shown that the common destruction forms of the surface caused by coal mining are fissures or steps, collapse pits, and collapse basins, which not only cause different degrees of damage to land ecology [6,7,8], construction facilities [9,10], traffic lines [11,12], and embankment construction [13,14], but also cause mine geological disasters such as collapse, devolution, ground fissures, landslides, and debris flows [15]. Moreover, all of the above threaten people’s lives and property and the sustainable development of mining areas. While mine water, coal gangue, and surface subsidence pollute the surface water, the sewage formed breaks through the key strata and pollutes the groundwater system through the mining fissure channels formed in the process of surface damage and the natural tectonic channels existing in the original geological structure [16]. In high water periods, a large amount of water enters the underground through the water diversion channel, increasing the probability of accidents, such as mine water inrush, roadway support, and mined-out area collapse. In view of the failure forms of ground fissures, steps and collapse pits, an overall direction of underground prevention and surface restoration is adopted. Economic utilization and development in a certain specific environment can maximize the economy of the mining area. In Europe, post-mining areas still have a good open utilization. Studies have shown that cultural and recreational functions were found in one third of the sites, especially in significant historical mining areas close to population centers. Furthermore, nearly one third of the post-mining sites included new activities related to industry and infrastructure [17]. In Poland, research showed that residents perceive forests in post-mining areas of cities as an essential and expected recreational space. Notably, half of them do not see any threats therein. It was also expected that these areas will be better developed for recreational purposes in the future [18]. There is still a certain gap between China’s domestic research and the use of post-mining areas in Europe, and with the depletion of energy and the increase of historical mining areas, this is crucial.

Under the environment of increasing intensity of the mining of coal resources in China, the destruction of rock structure is enlarged. Even though the mining technology is mature and China’s investment in related science and technology is increasing, and safety management is strengthening, related accidents caused by the surface damage of mines still occur. Therefore, the prevention and repair of surface damage caused by mining are of great significance, to reduce mine safety accidents, continue the sustainable development of mining areas, and strengthen environmental construction.

In order to achieve the coordinated development of ecological and environmental protection in mining areas and continue the sustainable development of mining areas, it is necessary to summarize the mechanism and forms of surface damage in mining areas, as well as the prevention and restoration measures. Based on the above theoretical analysis and the current situation of China’s industry, the authors put forward the idea of a mine governance of utilization first, prevention and restoration second; and where restoration is the main principle, while prevention is auxiliary; and where natural restoration is the main concern and artificial restoration is subsequent, and the combination of artificial repair and natural repair provides a route for the treatment of coal mining surface damage.

2. Transmission Mechanism of Surface Damage Caused by Coal Mining

2.1. Characteristics of Overlying Rock Damage Caused by Coal Mining

Coal mining will inevitably lead to the destruction of the overlying rock, the primary geological structure and surrounding rock stress changes, leading to the destruction of rock and damage that is then transmitted to the surface. In the following, the author will analyze the treatment methods of underground mined-out areas, the destruction height of overlying rock, the full mining of overlying rock destruction, and the destructive effects of key strata on overlying rock.

2.1.1. The Treatment Methods of Underground Mined-Out Areas

There are four mainstream treatment methods for a mined-out area formed after coal mining: coal pillar method, roof bending subsidence method, full filling or partial filling method, and full caving method [19], as shown in Figure 1.

The coal pillar method retains enough coal pillar support that the mining face does not collapse, this method will not lead to a change of strata, but this mining will lead to inconvenience and waste of coal resources, so in actual mining, its application is rare. According to relevant research [20], there is no obvious strata behavior in filling a fully-mechanized mining face. The main treatment methods of a mined-out area causing surface damage are the roof bending subsidence method and all caving method.

2.1.2. The Destruction Height of Overlying Rock

According to the “masonry wall” theory [21], the rock morphology of a coal mining face after using the caving method is divided into “three zones”, that is, the working face is divided into a caving zone, fracture zone, and bending subsidence zone, as shown in Figure 2.

The caving zone and fracture zone are called “water flowing fractured zone”, which are the main channels for surface water to enter the mining area. They are important indicators for evaluating the degree of rock damage and can reflect the strength of surface damage to a certain extent. The height of the water flowing fractured zone is affected by geological conditions, the roof erosion rock structure, coal mining technology, and other factors [22,23,24]. Chinese scholars have conducted a large number of studies on the prediction of the height of the water flowing fractured zone, which effectively solved the prediction and calculation of the height of the water flowing fractured zone under different working conditions. Compared with the empirical formula, they are more simple, convenient, and practical [25,26,27,28,29].

2.1.3. The Full Mining of Overlying Rock Destruction

Full mining of coated rock damage means that the destruction height of the coated rock caused by working face mining reaches the maximum value under the mining geological conditions, and no longer increases with the increase of mining size, otherwise there is insufficient mining. Judging whether the mining is fully carried out can effectively determine whether the damage height of the overburdened rock will continue to increase with mining. The formula for judging whether rock damage reaches full mining using the critical working face size is as follows [30]:

$$\mathrm{L}=\sqrt[4]{\frac{384E_j{I}_j\left[M-{{\displaystyle \sum }}_{i=1}^{j=1}{h}_i\left(k_i-1\right)\right]}{5q}}+{\displaystyle \sum }_{i=1}^{j=1}{h}_{i}cot{\phi }_1+{\displaystyle \sum }_{i=1}^{j=1}{h}_{i}cot{\phi }_2$$

(1)

In the formula, L is the critical working face size, m;$E_j$ is the elastic modulus of layer j, pa; $I_j$ is the section moment of layer j, m⁴; $h_i$ is the strata thickness of layer i, m; M is the thickness of coal seam mining, m; $k_i$ is the residual expansion coefficient of layer I; ${\phi }_1,{\phi }_2$ are the fracture angles in front and back of the rock (°), and q is the load concentration of rock itself (kN/m).

When the working face size is greater than the critical size, the rock damage is fully exploited, and vice versa. In order to further analyze the criterion of full mining caused by erosion rock damage, the erosion rock damage area is further simplified as a trapezoidal area, and the mined-out area is simplified as a rectangular area. As shown in Figure 3, the surface subsidence area (W₁, W₂, W₃, W_max) increases with the increase of the opening depth (H₁, H₂, H_max). With the working face advancing distance (L), based on the two-dimensional plane conservation law, the theoretical calculation formula of erosion rock damage height (H_max) under full mining caused by erosion rock damage can be obtained [31]:

$$H_{max}=\frac{Ltan\alpha }{2}\left[1-\sqrt{\left(1-\frac{4\left(ML-S_s\right)}{\left(K-1\right)L^{2}tan\alpha }\right)}\right]$$

(2)

In the formula,$H_{max}$ is the height of rock failure, m; $\alpha$ is the mean value of rock breaking angle (°); M is the thickness of coal seam mining, m; K is the residual expansion coefficient; $S_s$ is the surface subsidence area, m².

2.1.4. The Destructive Effect of Key Strata on Overlying Rock

According to the control of the upper strata, the local control of the upper strata is the sub-critical layer, and the overall control of the upper strata is the main key strata. From the theoretical point of view of the key strata, the lower surface is the result of the coupling of the main key strata and the surface soil layer. The breaking of the main key stratum will lead to a synchronous breaking of the upper strata and the rapid lower layer of the surface. The velocity of the lower layer of the surface changes abruptly with the periodic breaking of the main key stratum. Although this feature can be shown at any point of the surface of the mined-out area, the result is not the same everywhere. When the fault block of the key stratum is large or the thickness of the topsoil is small, the key stratum has a great influence on the surface subsidence [32,33].

2.2. Transmission Mechanism of Surface Damage

From the above analysis, it can be seen that the erosion rock structure caused by mining activities destroys the original geological structure and there is corresponding damage to the surface primary structure. After rock damage, the damage to the upper strata and surface is in a progressive relationship [34]; the specific progressive relationship is shown in Figure 4. With the advancement of the working face, the overlying strata in the mined-out area experiences the processes of complete suspension, suspension fracture, suspension collapse, suspension stability, suspension fracture, and suspension breaking in turn. Based on the fragmentation and expansion of the rock itself, during the upward transmission of the destruction height of the overlying rock, the separation height of the strata in the bending subsidence zone will gradually decrease [31].

3. Forms of Surface Damage and Its Negative External Effects

3.1. Forms of Surface Damage

(1) Ground fissures. After the mining effect is transmitted to the surface, this will promote the deformation and movement of the surface. Due to an uneven movement or deformation between surfaces, deformation occurs within the surface. When the tensile deformation exceeds the tensile deformation capacity, the ground will be deformed [12]. According to the different forms of ground fissures, ground fissures are usually divided into tensile ground fissures, collapse ground fissures, and sliding ground fissures, as shown in Figure 5. The formation of ground fissures is often accompanied by a horizontal movement of the surface, and the formation of ground fissures is the main factor affecting the water resources of the mining area.

(2) Collapse pits. When the roof caving area of a mined-out area is large, the rock damage is transmitted upward, and the surface damage is often manifested as a collapse pit, as shown in Figure 6. The formation of collapse pits is often relatively rapid, accompanied by a relatively large energy release. In some engineering examples of surface damage forms, the main damage form is collapse pits.

(3) Steps. When the upper and lower settlement is large in the mined-out area and the integrity of the overlying strata is good, dislocation deformation of the upper and lower strata easily forms; that is, the bench damage mode, as shown in Figure 7. The formation of steps causes great damage to the surface, which greatly changes the original geology and landform of the surface, and has a strong influence on surface buildings (structures) [35].

3.2. Negative External Impact of Surface Damage

Negative external impact refers to the kind of consumption or production of other adverse effects due to consumption or production [36]. Combined with the characteristics and forms of surface damage in underground mining, the negative external impact of surface damage is mainly manifested in the following aspects:

(1) Land and water resource destruction, and surface ecological environment destruction. The surface ground fissures, steps, and collapse pits caused by mining activities, not only change the original topography and change the slope and elevation of the land, but also reduce the quality and fertility of forest land and agricultural and pastoral areas, change the soil properties, decrease crop productivity, reduce surface plants, increase soil desertification, and deteriorate the surface ecosystem. Surface fissures, steps, and collapses can also cause soil erosion, land desertification, soil pollution, and microbial replacement, causing further damage to land resources. Mining activities cause surface and underground rock structure damage (Figure 8). Mine water, coal gangue, and surface subsidence areas produced by mine production can pollute surface water. As surface water supplies groundwater through soil water, soil water and groundwater are indirectly polluted [37].

The process of underground mining will also lead to the destruction of the underground rock structure and the formation of ground fissures, which will not only destroy the root system of vegetation and make plants wilt or decay, but also drain surface water and spring areas, leading to the decline of groundwater levels (Figure 9) [4]. The destruction of land resources and water resources, as well as soil erosion and other issues brought about by this, will greatly affect the surface ecological environment. In particular, in the semi-arid region of northwest China, the primary surface ecological environment system is fragile, and underground mining activities often cause the decline of the primary ecosystem [37].

(2) Surface building (structures) damage. Surface buildings are basically loaded on the surface in the form of foundations. According to the damage transmission mechanism to the surface, when the coal mining is carried out, the damage of the overlying strata is transmitted to the surface, and the original bearing stress of the surface building (structure) will change [3,38,39]. When the bearing capacity exceeds the bearing limit, the surface subsidence will lead to the destruction of the surface structure, as shown in Figure 10a. It has the characteristics of a wide range and great harm, which seriously threaten people’s lives and security.

Surface structures mainly refer to railways, highways, dams, bridges, tunnels, and high-voltage lines. For general structures such as roads (as shown in Figure 10) and highways, the damage mechanism is equivalent to that of surface buildings. However, differently from general buildings, the influence and sensitivity of high-rise buildings, such as bridges and high-voltage lines, on surface movement and deformation are relatively special [40].

(3) Mine geological disasters. In addition to direct surface geological disasters, such as collapsed pits, steps, and ground fissures caused by surface damage, secondary geological disasters such as landslides and collapse debris flows are also prone to occur, due to the chain reaction caused by the change of surface mechanical properties [41], as shown in Figure 11. In the process of surface damage, the original geological structure of the rock and surface change, and the original stress and original landform change. When the landslide instability occurs, the slope collapses and collapse accidents occur. Affected by rainfall, the surface is more prone to instability, namely the formation of debris flow. In the process of underground mining, serious rock damage can be caused, and rock damage will lead to deformation of roadway engineering and wellbores. Surface water enters the mining area through ground fissures and mining fissure channels, especially in the rainy season, which greatly increases the accident rate of roof water inrush and affects the safety of mine production [42,43,44].

4. Surface Damage Prevention and Restoration Technology

Based on the above research, it has been shown that underground mining activities are bound to produce different forms of damage to the surface and bring corresponding negative external effects. However, for underground mining, especially some specific mining environment, such as “three next one up”, it is necessary to implement pre-mining preventive measures. The implementation of restoration after surface damage can effectively avoid the negative external influences and improve the comprehensive utilization of land. Under certain circumstances, the comprehensive utilization of surface damaged areas, with economic development, can turn problems into opportunities. At present, with the relatively mature prevention and repair measures in China, prevention is often aimed at underground mining, and repair is completed on the surface. There are different preventive measures, according to the different forms of damage.

4.1. Underground Prevention of Surface Damage Technology

4.1.1. Prevention of Steps and Fissures

Combined with the characteristics of erosion rock damage and the forms of surface damage, the prevention of crack steps in China mainly involves filling the underground working face or the separation zone under the main key strata.

(1) Filling mining. The essence of filling mining involves replacing the mined coal seam with filling materials. The existing mainstream replacement materials include solid, paste, and (ultra) high water filling. Among them, the most commonly used filling methods are solid filling and paste filling, accounting for 58% and 30% of the total number of filled mines, respectively. High water and (ultra) high water filling only account for 8% and 4% of the total number of filled mines, respectively. According to the relevant research, a mining face filled with filling materials will not have obvious strata behavior, which can effectively support the overlying strata and control surface subsidence. This is an ideal mining method [20,45,46]. A filling process diagram of a mined-out area is shown in Figure 12. When a mined-out area is not filled, there are obvious “three zones” characteristics of fissures and steps, while there are almost no fissures and steps in the upper strata and surface of the working face. The filling working face mining can effectively inhibit the development of fissures and steps.

The most widely used method of solid filling is to use coal gangue to fill the mined-out area directly, which can, not only reduce the emission of solid waste in coal mines, but also reduce the problem of mining subsidence and improve the recovery rate of mine resources, and this has been applied in many mining areas [47].

Among the common filling mining methods, paste filling mining is also utilized. The main application method is to fill the mined-out area, after mixing cement, fly ash, sand, and other materials with water. In Northwest China, there is a large amount of aeolian sand with good deformation resistance on the surface. The use of aeolian sand as the main filling material to fill the mined-out area protects the mining environment and provides a new direction for green mining in China. For example, Pengliang Liu et al., based on the actual site of Yuyang coal mine, studied an aeolian sand paste-like filling field application, and the results showed that the rock damage was slight and the aquifer was effectively protected [48].

(Ultra) high water filling mining uses fly ash or tailings as the main material, and a retarder, accelerator, curing agent, and expansive agent as auxiliary materials, and the two are mixed with water to make a mixture to fill the mined-out area, which has good fluidity, a low cost, and simple process.

(2) Separation grouting technology. Separation grouting technology is a technology that injects grouting material into the separation space by drilling from the surface to the overburden separation zone. The main technical principle is shown in Figure 13b. As shown in Figure 13a,b, the damage of ground fissures and steps is significantly reduced by grouting the separation zone below the main key strata. According to the key strata theory, the main key strata plays a control role in the upper strata, and grouting should be avoided along the water flowing fractured zone into the mining space, which affects normal production. Therefore, the position of grouting should be above the water flowing fractured zone, and the separation zone should be below the main key strata [49]. After injecting into the separation zone, the grouting material is solidified within a certain period of time, so that part of the load of the overlying strata is transferred to the caving gangue in the mined-out area, through the filling body, which can effectively receive and carry the load [50]. This technology is not only simple and easy to operate, but also can effectively support the overlying strata, reducing the transmission of the effect of mining to the ground and reducing surface subsidence.

4.1.2. Prevention of Surface Subsidence

Surface subsidence has a great influence on surface problems and is the main influencing factor on surface building (structures) damage in mining areas. Filling mined-out areas and separation areas could play a role in preventing surface subsidence, but in practice, most techniques rely on regulating mining methods to achieve the effect of preventing surface subsidence. The main approaches are coordinated mining technology and partial mining technology. Both of these technologies can effectively support the upper strata uniformly and reduce the relative movement of the rock, so as to control surface subsidence.

(1) Coordinated mining. Coordinated mining uses two or more adjacent working faces to maintain a certain advancing relationship in practice, so as to offset part or all of the surface deformation. It can be divided into coordinated mining and symmetrical mining of two coal seams (layered), or multiple working faces in the same coal seam [51]; for example, the most basic multi-coal pillar coordinated mining, as shown in Figure 14. The protective coal pillars left in the adjacent mining coal seams are staggered in their spatial relationship, so that they are not on a plane and jointly support the overlying strata erosion pressure. It can be seen from a comparison between Figure 14a,b that when the protective coal pillar is arranged on a plane, the supporting stress distribution of the protective coal pillar is uneven, and there is obvious shear stress, while the overlying strata have “three zones” and the overlying rock is seriously damaged, which is accompanied by huge collapse pits and ground fissures on the surface. When a protective coal pillar is not in a plane, the support stress distribution of the protective coal pillar is uniform, there is no obvious shear stress, and the rock is effectively supported. The surface fluctuation is small, it is not easy to form collapse pits, and there are almost no fissures.

Good results have been achieved in relevant studies, such as the upward coordinated mining of the close “thin-medium-thick” staggered distribution coal seam group [52], the “strip mining roadway backfilling” method [53], the coordinated mining method of partial mining, partial filling and partial coal pillar combined with mining-filling-retention [54], and the “mining-filling parallel” water-preserved coal mining method [55]. Field practices have been carried out with different corresponding processes, and the expected results were achieved, so that subsequent engineering processes could be based on these results. Based on the principle of maximizing economic benefits and resource exploitation, the filling method of a strip mining roadway is an advanced mining method.

(2) Partial mining. Some mining is proposed with use of “three lowers and one upper”. The “three lower and one upper” refers to linear structures such as buildings, water bodies, railways, and confined water. The main mining methods are strip mining, the Wongawilli mining method, thickness-limiting mining, room-and-pillar mining, etc.

Strip mining can effectively recover part of the coal resources and has a good protective effect on the upper surface buildings in the mining area, but its recovery rate is low. As shown in Figure 15, coal seam mining is mainly arranged under surface buildings (structures). Through the comparison of Figure 15a,b, it can be seen that the support stress distribution of strip mining is uniform, and destruction of overlying strata and surface subsidence are not obvious. However, compared with other mining methods, a large part of the coal seam is left as a coal pillar, and the mining rate is not high, which has certain limitations. The coordinated mining method, combining strip mining and filling mining, namely, the “strip mining roadway backfilling” method, can effectively improve the mining rate and even realize the full mining of coal mines in the mining area [53].

The Wongawilli coal mining method is a new type of high-efficiency subsidence reduction coal mining method that has the advantages of a flexible working face layout and fast moving speed. It is of great significance to reduce the damage of stope rock erosion, control surface subsidence, and reduce the damage to surface buildings (structures). However, its frequent movements lead to its sever effects on the continuous production of the mining area. Guo Wenbing et al. proposed strip Wongawilli mining technology [56], combining the advantages of strip mining and the Wongawilli mining method, which not only has a good protective effect for surface buildings, but also effectively utilizes the advantages of Wongawilli mining technology, and this has been applied in the field. At the same time, the instability mechanism of a strip coal pillar under different roadway layouts has been studied [57]. The results showed that the stability of a strip coal pillar requires its core area rate to be greater than 17%, and the width ratio of the plastic zone is less than 0.21, which may cause instability.

Thickness limit mining is a kind of mining method that reduces the fracture space of the rock strata, reducing the height of the water flowing fractured zone and reducing the surface damage, by reasonably limiting the mining height. This mining method has the characteristic of less surface damage, but it is limited by an excessive waste of resources and is rarely used in practical engineering, so it is not mentioned in detail here.

Room-and-pillar mining is a kind of coal mining method that is normally only applied to the “three lowers”. However, with the development of mining technology and the shift of coal mine productivity center of gravity, it has occasionally been used in China. At present, it is only used for the protection of some surface buildings (structures).

4.2. Surface Restoration Technology of Surface Damage and Its Utilization

After surface damage, most mining areas will repair the damaged area, and some of the surface damage with special positions will be retained with comprehensive utilization, which effectively indicates that it is not necessary to take subsidence reduction measures to protect the surface primary structure before mining. For underground mining, the cost of preventive measures before mining is relatively high, and due to limits of the prediction and protection technology, it is not necessarily possible to achieve the expected protection effect. In China, there have been a large number of historically damaged lands, and most of the land restoration practices in mining areas have involved post-mining restoration [58]. To a certain extent, some mining areas have realized the development of economic development and utilization areas in the form of surface damage.

4.2.1. Surface Restoration Technology

Restoration after land destruction is mainly divided into natural restoration and artificial restoration. Previous studies have mostly focused on artificial restoration, but recently, the feasibility of natural restoration in mining areas after land destruction has been paid increased attention. The idea of surface damage repair in Chinese mines is considered in Figure 16. After surface damage, the feasibility of natural repair of the surface damage is judged based on the different forms of damage. If it is feasible, the implementation of natural repair can be carried out, and if it is not feasible, artificial repair is utilized, as well as some feasible implementation of a combination of the two repair methods.

(1) Artificial restoration. In the past, the repair methods of mining areas were mostly artificial repair, as shown in Figure 17. In view of the mined-out area subsidence basin, the repair methods mainly include land leveling, combing row, deep excavation, and shallow cushion and filling technology. For fissures and steps, some damage is permanent and cannot be repaired. Filling repair is generally implemented for the repairable parts, which can effectively avoid accidents caused by the inflow of surface water into the mining area. Post-mining restoration is a remedial measure after the surface has been destroyed to a certain extent. At this point, the land damage and surface subsidence have reached their maximum, the reclamation rate is low, and the reclamation cycle is long. Based on the relationship between underground mining and surface damage remediation, the mining mode of mining with reclamation can be realized by using “pre-mining analysis, mining dynamic subsidence prediction, and simulation optimization of reclamation” technology, which can effectively improve the reclamation rate of land in mining areas [59].

(2) Natural restoration. A mine ecosystem is a special ecosystem that is strongly disturbed by human activities, as shown in Figure 18. According to ecosystem theory, the mechanism and function of an ecosystem can stay relatively stable because of its own self-repair ability [60]. Natural restoration is a ubiquitous phenomenon in nature, which refers to the process or method of restoration relying on natural forces (camps). Among these processes, natural restoration includes climate changes, soil natural seed bank and natural propagation of seeds, the natural characteristics of soil and plants, and biochemical and physical effects [61]. Based on the delimitation of coal mining subsidence areas, to determine the natural restoration area [62], the implementation of different restoration methods in different regions can give full play to their advantages. The most common form of natural restoration in China is to close the damaged area, prohibiting anyone from entering the closed area or interfering, and allowing natural forces to repair the land.

Artificial restoration and natural restoration are active restoration measures for damaged ecological environment. In practice, the two should be combined, complemented, and adapted to local conditions. According to the diagnosis of coal mining damage, the natural restoration area is determined, and a natural restoration is carried out in this area. Artificial restoration is given priority in the non-natural restoration area, and the natural restoration is supplemented. Blind artificial restoration is avoided, and the restoration cost is minimized, to obtain the maximum benefit [61].

4.2.2. Economic Development and Utilization of Surface Destruction

In some regions of China, the change of surface landscape type from the surface subsidence caused by underground mining and other questions of surface damage can play a positive role in the region that the original ecological landscape cannot play. Therefore, it can be judged that it is necessary to take preventive measures against subsidence before mining in some regions of China. For example, there are many “collapse lakes” in East China due to the formation of underground mining scoring collapse pits. The original surface ecology has changed from agricultural ecology to a water–land symbiotic ecology. There are water shortages in dry season and flooding issues in rainy season, threatening the surface agricultural ecology in the area. Now, the comprehensive utilization of “collapse lakes” as water sources in the dry season and flood discharge areas in the rainy season ensures agricultural production. In recent years, with the acceleration of urbanization, some rural mines have gradually become suburban mines and even urban mines. In many places, the collapsed water area has been transformed into a recreational area [58]. There are many economically utilizated scenic spots that have been built and that are relatively mature in China, such as the Central Ecological Park of Nanhu City in Tangshan, as shown in Figure 19, the Nanhu Wetland Park in Huaibei, the Pan’an Lake Wetland Park in Xuzhou, and the Jiuli Lake Wetland Park, which have become locally and even nationally famous tourist attractions and have a high value for ecological services and recreation.

5. Discussion

China’s coal mining geology and the resulting environments are complex, and the geological and environmental problems caused by mining are still very prominent in China. In recent years, the state has paid more and more attention to this, and there have been major breakthroughs in the treatment of surface damage. However, due to local coal mines, especially small coal mines, there are still many problems regarding lack of awareness and ability. Based on China’s governance and the current situation, the author discusses the prevention and restoration of mine surfaces:

(1) Utilization first, prevention and restoration second. Prioritize the use of surface damage forms; on this basis, consider prevention and restoration. Economic development and utilization can maximize the economic benefits of mining areas, which is in line with environmental protection and is the optimal choice of the mining economy.

(2) Restoration is the best choice, while prevention is auxiliary. Economic development and utilization have many restrictions regarding the non-development and utilization of the surface damage area. In addition to some specific mining area environments, such as “three next one”, we should try to repair the main area and prevent radiation. According to the experience of surface damage control in mining areas in China, the preventive measures in mining areas are difficult to achieve, as is the corresponding control effect, and the costs are high. Many mining areas cannot achieve the corresponding preventive conditions. After underground coal mining, there will be a certain range of surface damage, taking certain measures will not affect the large area of underground mining.

(3) Natural restoration is given priority, supplemented by artificial restoration, as well as artificial restoration and natural restoration combined. According to the determination of the degree of surface damage, natural restoration of a mining area is preferred. Although the relative recovery period is long, it is expected that artificial restoration can maintain the original local ecology, as much as possible, and the cost is low. For some mining areas that are locally in line with natural restoration, artificial restoration and natural restoration should be combined, to give full play to the advantages of natural restoration and the feasibility of artificial restoration.