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Journal of Resources and Ecology >

2023 , Vol. 14 >Issue 4: 757 - 766

DOI: https://doi.org/10.5814/j.issn.1674-764x.2023.04.008

Evaluation and Improvement of Mine Site Quality

Site Type Classification and Ecological Restoration Technology Selection of Open-pit Coal Mine Dumps in Grassland Mining Area

  • LI Ruipeng , 1 ,
  • SHI Changqing , 1, * ,
  • YANG Jianying 1 ,
  • WEI Guangkuo 1 ,
  • LIU Jiaqi 1 ,
  • KUI Guoxian 1 ,
  • AI Xianfeng 1 ,
  • XIAO Fei 2 ,
  • SU Ruidong 2
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  • 1. School of Soil and Water Conservation, Beijing Forestry University, Beijing 100083, China
  • 2. Inner Mongolia Guangna Coal Industry Co., Ltd, Wuhai, Inner Mongolia 016000, China
*SHI Changqing, E-mail:

LI Ruipeng, E-mail:

Received date: 2022-08-12

  Accepted date: 2022-12-30

  Online published: 2023-07-14

Supported by

The Inner Mongolia Autonomous Region Science and Technology Major Project(2020ZD0021-02)

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Abstract

In order to study the counterpoint configuration about the types of mining sites and ecological restoration technologies, open-pit coal mine dumps that had been ecologically restored, since many years, in the grassland mining area in western Inner Mongolia, were selected. A comprehensive evaluation of slope protection effect, vegetation restoration effect, soil improvement effect and engineering technology cost was carried out in order to classify the types of sites for open-pit coal mine dumps and to determine their counterpoint ecological restoration technology, so as to provide theoretical guidance and technical support for ecological restoration of the grassland mining area in western Inner Mongolia. The research results are as follows: (1) Soil thickness, slope gradient and slope aspect are the dominant factors affecting the site conditions of the dump site. Base on this, the dump site of the open-pit coal mine in the grassland mining area of western Inner Mongolia was divided into 9 types of sites. (2) For half-shade and half-sunny to thick soil gentle slope, we primarily recommend plant grid slope protection + planting + sowing + sprinkler irrigation, and the second recommendation is hexagonal brick slope protection + mortar masonry ditch + sowing + sprinkler irrigation; for half-shade and half-sunny to medium-thick soil gentle slope, we primarily recommend masonry skeleton slope protection + mortar masonry ditch + sowing + sprinkler irrigation, and the second recommendation is sowing + vegetation blanket coverage + sprinkler irrigation; for half-shade and half-sunny to thin soil gentle slope, it is recommended to use gravel capping + mortar masonry ditch. For sunny thick soil gentle slope, we primarily recommend eco bag blocking + planting + sowing + vegetation blanket coverage + sprinkler irrigation, and the second recommendation is eco bag blocking + sowing + grass curtain coverage grass curtain coverage + sprinkler irrigation; for sunny thick soil steep slope, it is recommended to use plant grid slope protection + mortar masonry retaining wall + sowing + sprinkler irrigation. For shade thick soil gentle slope, we primarily recommend eco bag blocking + planting + sowing + vegetation blanket coverage + sprinkler irrigation is preferred, and the second recommendation is planting bag stacking + mortar masonry retaining wall + sprinkler irrigation; for shade medium-thick soil gentle slope, it is recommended to use eco bag blocking + corrugated pipe drainage + sowing + vegetation blanket coverage + sprinkler irrigation; for the shade medium-thick soil steep slope, we primarily recommend eco bag blocking +sowing + grass curtain coverage + sprinkler irrigation, and the second recommendation is eco bag blocking + corrugated pipe drainage + sowing + vegetation blanket coverage + sprinkler irrigation. For thick soil platform, use of planting + sowing + sprinkler irrigation is recommended. The research results provide a scientific basis for selection of ecological restoration technologies in the grassland coal mining area in western Inner Mongolia. At the same time, with respect to the semi-arid grassland mining area with relatively fragile habitats, this research be of great significance to improve the greening benefits of the mining area and for improving the mining area and surrounding ecological environment.

Key words： open-pit coal mine dump; site type classification; ecological restoration technology; Inner Mongolia

Cite this article

LI Ruipeng , SHI Changqing , YANG Jianying , WEI Guangkuo , LIU Jiaqi , KUI Guoxian , AI Xianfeng , XIAO Fei , SU Ruidong . Site Type Classification and Ecological Restoration Technology Selection of Open-pit Coal Mine Dumps in Grassland Mining Area[J]. Journal of Resources and Ecology, 2023 , 14(4) : 757 -766 . DOI: 10.5814/j.issn.1674-764x.2023.04.008

1 Introduction

Inner Mongolia Autonomous Region is rich in coal resources that have played an immense role in its economic and social development. However, due to disorganized mining for a long period of time, the regional ecological environmental problems have become very prominent (Huang et al., 2015). In recent years, the state and autonomous region have geared up to manage the geological environment and ecological restoration in mining areas on a large scale, but numerous investigations find that due to excessive pursuit for “green”, the principles of fortification and local conditions are being overlooked, and the restoration effect is not satisfactory (Hu et al., 2012; Shen et al., 2016; Zhang et al., 2018). Types of sites and their quality are to a great extent reflective of different geographical and environmental factors. If the sites are scientifically divided into different types, the benefits of ecological restoration can be maximized. Zhang et al. (2014) selected the western dump of Antaibao Open-pit Coal Mine for research with the aim to classify the sites into different types. The research results provide a basis for the improvement of the sites, restoration and maintenance of the vegetation, and restoration of the ecosystem. Jin (2015) classified the Huangyuan quarry on the basis of the type of microsite, which provided a reference for the selection of greening technologies in other mining areas. The abandoned land in the mining area has poor soil and is subjected to harsh conditions, and neither single plant measures nor engineering measures are often much effective. Shan et al. (2017) have provided technical support for the ecological reconstruction of typical mines through a combined research on application of plants and engineering measures. It can be seen that the combined application of various ecological restoration technologies has become a crucial factor for the ecological restoration of abandoned land in mining areas (Bai et al., 2018; Yan, 2020; Wang et al., 2021). As far as the dump site is concerned, its characteristics such as large area have led to harsher conditions at the site. Due to the confusion of application of the technology, the same restoration measures are frequently adopted for different dump sites (Hou et al., 2022). Some scholars have started to study the selection of suitable restoration techniques for different site types, and have achieved good results, but most of them focus on forestry (Ren et al., 2020). Therefore, the premise of accurate and reasonable evaluation of the ecological effect of damaged slope is to reasonably divide the site types and construct a suitable evaluation system, which is the basic work and the primary problem to improve the survival rate of vegetation and the ecological benefit of greening.
To the best of our knowledge, currently, few studies have explored the alignment of the types of dump sites and measures for their restoration, and many studies have focused on the effects of unilateral ecological reconstruction or evaluation of the site quality (Ren et al., 2016; Zhang et al., 2016; Xiong et al., 2018). For ecological restoration of the mining areas, selection of appropriate technologies according to the types of the sites can play a crucial role in the ecological reconstruction of the abandoned mining areas. Therefore, the dumps in the mining area that were typically treated were selected for the study. The methodology involves field investigation and UAV aerial survey. In pursuance of the aim of the study, the sites were divided into different types and the control measures that are being adopted in those sites were comprehensively evaluated. Furthermore, the appropriate ecological restoration technology that aims to provide a scientific basis and guidelines for implementation of ecological restoration of the dumps in the western grassland mining area of Inner Mongolia was determined.

2 Methods

2.1 Study area

The study area is located in the semi-arid grassland in the southwest of Inner Mongolia Autonomous Region, in the hinterland of the Ordos Plateau, at an elevation of 1000- 1500 m, which is a part of the semi-arid continental climate zone in the north-temperate zone. Since many years, the average temperature of the area is 6.2 ℃, the daily maximum temperature is 38 ℃, the daily minimum temperature is -31.4 ℃, and the frost-free period extends from 130 to 170 days. The average annual precipitation is 348.3 mm, and the precipitation occurs mostly during three months (i.e., July, August and September), accounting for about 70% of the annual precipitation, and the average annual evaporation is 2506.3 mm. The westerly and the north-westerly winds prevail throughout the year, with an annual average wind speed of 3.6 m s-1. Chestnut and brown zeta are the main soil types, with low soil fertility and lack nitrogen and phosphorusand. Native plants include Salix cheilophila, Juniperus sabina, Medicago sativa, Chenopodium glaucum and so on.
The coal resources in the study area are rich and widely distributed. The coal bearing area covers an area of about 60000 km2, accounting for more than 70% of the land area of the whole city. The proven reserves are 2017 billion tons, accounting for about 1/6 of China. Dumping process is one of the basic processes of open-pit mining. A large amount of soil and slag discharges have caused a series of problems such as land occupation, vegetation destruction and environmental pollution in the mining area and its surrounding areas. By the end of September 2021, 126.05 km2 of green area and 81.61 km2 of open reclamation area have been completed in the mining area. Among the 629 mining enterprises in the study area, only 170 are green mines, and the remaining mines still need ecological management.

2.2 Data collection

In the study area, 12 typical coal mine dumps were selected (Fig. 1), and 3-5 years was the restoration and treatment period. Dump slopes and platforms were explored in June 2021. Using the typical sample plot method and a total of 45 sample plots was selected on the slope and platform of the dump in the study area, and the sample plots were investigated one at a time. RTK, and the aerial survey which was carried out with the help of drones was used to locate the sample plot and the site factors such as aspects of the slope like slope length and soil thickness was investigated. The characteristics of the ecological restoration technology in the sample plot, in addition to the size and number of eroded ditches, were determined(Tian, 2011; Yang et al., 2019). According to the state of the dump site, the herb sample square (1 m×1 m), the shrub sample square (5 m×5 m) and the tree sample square (10 m×10 m) were set up, along the slope surface, its middle and lower parts, respectively and the existing plant species, height of the plot, coverage area, etc, were recorded. From each quadrangle, 0-30 cm of mixed soil samples (fresh and undisturbed) was obtained using a 100 cm3 ring knife, and were taken to the laboratory for assay (Li et al., 2022). In combination with the relevant technical solutions for the mining area, information pertaining to the economic costs of various technical measures, including material costs, labor costs and maintenance costs was obtained from the on-site technicians or the maintenance workers (Bai et al., 2018).
Fig. 1 Location of study area
The vegetation coverage of each quadrant was calculated which was based on the extracted and visible light images obtained through DJI Phantom 4 UAV and in reference to the plant diversity index. The organic matter in the soil sample was determined by the potassium dichromate dilution heat method, the alkali-hydrolyzed nitrogen was determined by the alkali-hydrolyzed nitrogen diffusion method, the available phosphorus was determined by the molybdenum antimony anti-colorimetric method, and the available potassium was determined by the flame photometer method. Physical parameters such as soil bulk density and total porosity were determined by the ring knife method (Bao, 2008).

2.3 Classification of sites and selection of technology

The study area as a whole is located in the Ordos Plateau, which is a high-elevation area. Each site had similar climatic conditions, and the elevations of each sample point were also similar, so elevation and precipitation were not considered as factors influencing site classification. On the basis of the field investigation results of the dump site in the mining area and relevant standards and specifications, after comprehensive consideration, the follow factors which including different aspects of the slope, slope length, soil texture, slope, thickness of soil covering and statistical analysis was performed were selected as main influencing factors. Referring to the previous experience pertaining to the classification of the abandoned land in mining areas and relevant standards and specifications (Tian, 2011; Liu, 2021), and then considering the actual situation of the mining area, the standards for classification of the main influencing factors were established (Table 1). Principal Component Analysis (PCA) was used to screen the dominant factors, and the types of sites were determined on its basis.
Table 1 Main impact factors grading standards
Parameters Level
1 2 3
Slope aspect (°) Sunny slope Half shade and half sunny slope Shade slope
135-225 45-135, 225-315 0-45, 315-360
Slope length (m) Long slope
>30		 Medium slope
20-30		 Short slope
<20
Soil texture Rock Rocky soil Soil
Soil thickness (cm) Thin soil
1-20		 Medium-thick soil
20-40		 Thick soil
40-60
Slope gradient (°) Steep slope Gentle slope Platform
35-45 25-35 0-5

Note: There are no slopes with slope gradient between 5 and 25° in all surveyed plots.

According to the results of the field investigation in the mining area, the evaluation index system of the ecological restoration technology was constructed that included four aspects: slope protection, restoration of vegetation, soil improvement and economic cost. The PCA method and the CRITIC method were used to comprehensively determine the weight of the evaluation indicators, and Grey Relational Analysis (GRA) was used to calculate the comprehensive score of each ecological restoration technology. The western grassland mining area was recommended for use.

2.4 Data processing

Excel 2019 was used for data sorting, SPSS 24.0 was used for PCA, CRITIC and GRA analysis, Agisoft PhotoScan was used for stitching of UAV images, and ArcGIS 10.6 and Origin 2018 were used for mapping.

3 Results

3.1 Classification of the dump sites

Based on the KMO value and Bartlett’s test results, it was decided that the PCA should be conducted. The rate of variance of each component revealed that the contribution of the cumulative variance rate of the first three principal components was 65.08%, indicating that the first three principal components represented 65.08% of the information of the total components, so the first three principal components were selected as important principal components for the characteristic information of the plot. The component score coefficient matrix (Table 2) obtained after PCA, reveals that with respect to the first principal component, the coefficients of slope gradient and soil thickness are the largest, indicating that the slope gradient and soil thickness make up the first principal component. With respect to the second principal component, the coefficients of total soil porosity and slope aspect were the greatest, indicating that the total soil porosity and slope aspect make up the second principal component. Regarding the third principal component, the coefficients of soil thickness, slope aspect, and slope length are the first three, indicating that these three indicators contribute to the third principal component.
The soil thickness is a key factor for the growth of vegetation and selection of appropriate engineering measures. The types of vegetation found on land having varying soil thickness are obviously different. Solar radiation is a necessary energy source for plant growth, and slope aspect is directly related to the amount of solar radiation, which has a significant impact on plant types. The slope gradient also has a significant impact on the growth and development of plants mainly because it determines the water and temperature of the soil and how water is redistributed. From this point of view, the slope aspect, slope gradient and soil thickness significantly impact the restoration effect of different techniques. Based on the above analysis, it can be concluded that the soil thickness, the slope gradient, and the slope aspect are the dominant factors affecting the classification of coal mine dump site in the grassland mining area of western Inner Mongolia. According to the screening results and the principle of site type division, and on the basis of the dominant factor combination nomenclature of “soil thickness + slope gradient + slope aspect”, division and field investigation enabled us to conclude that there are four groups of site types and 9 site types in the grassland mining area in western Inner Mongolia. The specific types and characteristics are shown in Table 2. The real scene is shown in Fig. 2.
Table 2 Site types and characteristics of dump sites
Site type group Site type Site characteristics
Half-shade and half-sunny slope Half-shade and half-sunny to thick soil gentle slope (HTGs) Located on the half-shade and half-sunny slope of the dump, the soil thickness is over 40 cm, and the slope gradient is 25°-35°, which is gentle
Half-shade and half-sunny to medium-thick soil gentle slope (HMGs) Located on the half-shade and half-sunny slope of the dump, the soil thickness is 20-40 cm, and the slope gradient is 25°-35°, which is gentle
Half-shade and half-sunny to thin soil gentle slope (HNGs) Located on the half-shade and half-sunny slope of the dump, with no soil covering or slope hardening measures taken. The slope gradient is 25°-35°, which is gentle
Sunny slope Sunny thick soil gentle slope (STGs) Located on the sunny slope of the dump, the soil thickness is over 40 cm, and the slope gradient is 25°-35°, which is gentle
Sunny thick soil steep slope (STSs) Located on the sunny slope of the dump, the soil thickness is over 40 cm, and the slope gradient is more than 35°, which is steep
Shady slope Shade thick soil gentle slope (ETGs) Located on the shady slope of the dump, the soil thickness is over 40 cm, and the slope gradient is 25°-35°, which is gentle
Shade medium-thick soil gentle slope (EMGs) Located on the shady slope of the dump, the soil thickness is 20-40 cm, and the slope gradient is 25°-35°, which is gentle
Shade medium-thick soil steep slope (EMSs) Located on the shady slope of the dump, the soil thickness is 20-40 cm, and the slope gradient is above 35°, which is steep
Platform Thick soil platform (TSPm) Located on the platform at the top of the dump, the soil thickness is over 40 cm and the slope is 0°-5°, and the terrain is flat
Fig. 2 Real scene of site types in the study area

3.2 Evaluation of the ecological restoration technology of the dumps

Combined with the ecological restoration of mines in the study area, the evaluation pertained four aspects: slope protection effect, vegetation restoration effect, soil improvement effect and economic cost. Since the dump site in this study is still in the initial stage of ecological restoration, vegetation coverage (X1), Shannon diversity index (X2), Pielou evenness index (X3), and Margalef richness index (X4) were selected for the vegetation restoration effect layer. With respect to the physical properties of the soil, select soil hardness (X5), bulk density (X6), and total porosity (X7); the soil delayed nutrients were fixed in minerals and organic matter, and cannot be absorbed and utilized by plants without being decomposed and released (Mueller et al., 2016). Therefore, with respect to the soil improvement effect, the available nutrient indicators that can be directly absorbed and utilized by plants were selected, while included available nitrogen (X8), available phosphorus (X9), available potassium (X10) and organic matter (X11). Additionally with respect to the slope protection effect, the soil erosion status index (X12) was selected for the layer; the economic cost (X13) was calculated as (material cost + labor cost + maintenance cost), and hence the ecological restoration effect was evaluated on the basis of a total of 13 indicators belonging to four categories.
The index data was standardized by the range method to reduce the influence of dimensions. In order to reduce the error caused by the weighted indexing of a single method, the PCA method and the CRITIC method were used to comprehensively determine the weight of each evaluation index, as shown in Fig. 3 (Hou and Wang, 2021; Li et al., 2021). The GRA method was used to calculate the comprehensive score of each technology with respect to different site types (Table 3).
Fig. 3 Evaluation index weight
Table 3 Evaluation results of ecological restoration technology (a) Half-shade and half-sunny slope site type group
No. Ecological restoration technology Score Sort
HTGs
4 Eco bag blocking + corrugated pipe drainage + planting + sowing + vegetation blanket coverage + sprinkler irrigation 0.46 7
5 Eco bag blocking + corrugated pipe drainage + planting + sowing + vegetation blanket coverage + sprinkler irrigation 0.55 4
8 Eco bag blocking + sowing + grass curtain coverage + sprinkler irrigation 0.49 6
10 Eco bag blocking + sowing + grass curtain coverage + sprinkler irrigation 0.50 5
25 Hexagonal brick slope protection + mortar masonry ditch + sowing + sprinkler irrigation 0.60 3
30 Plant grid slope protection + sowing + sprinkler irrigation 0.61 2
31 Plant grid slope protection + sowing + sprinkler irrigation 0.63 1
37 Plant grid slope protection + sowing + sprinkler irrigation 0.45 8
HMGs
9 Eco bag blocking + sowing + grass curtain coverage + sprinkler irrigation 0.53 2
16 Eco bag blocking + sowing + grass curtain coverage + sprinkler irrigation 0.52 3
17 Plant grid slope protection + eco bag blocking + corrugated pipe drainage + sowing + sprinkler irrigation 0.49 5
24 Hexagonal brick slope protection + mortar masonry ditch + mortar masonry retaining wall + sowing 0.50 4
28 Sowing + vegetation blanket covering + sprinkler Irrigation 0.53 2
29 Natural recovery + dense mesh coverage 0.46 6
34 Masonry skeleton slope protection + mortar masonry ditch + sowing + sprinkler irrigation 0.57 1
HNGs
13 Gravel capping + mortar masonry ditch 0.46 1
23 Masonry slope protection + mortar masonry retaining wall 0.41 2
26 Gravel capping + mortar masonry plastering 0.41 2
Table 3(b) Sunny slope site type group
No. Ecological restoration technology Score Sort
STGs
15 Eco bag blocking + sowing + grass curtain coverage + sprinkler irrigation 0.52 2
18 Plant grid slope protection + sowing + sprinkler irrigation 0.51 3
19 Eco bag blocking + planting + sowing + vegetation blanket coverage + sprinkler irrigation 0.53 1
21 Eco bag blocking mortar + planting + sowing + vegetation blanket coverage + sprinkler irrigation 0.49 4
27 Mortar masonry ditch + sowing + vegetation blanket + sprinkler irrigation 0.49 4
STSs
32 Plant grid slope protection+ mortar masonry retaining wall + sowing+ sprinkler irrigation 0.48 2
36 Plant grid slope protection+ mortar masonry retaining wall + sowing+ sprinkler irrigation 0.45 3
38 Plant grid slope protection + sowing + sprinkler irrigation 0.50 1
Table 3(c) Shady slope site type group
No. Ecological restoration technology Score Sort
ETGs
1 Planting bag stacking + mortar masonry retaining wall + sprinkler irrigation 0.55 2
14 Plant grid slope protection + eco bag blocking + sowing + sprinkler irrigation 0.43 5
20 Eco bag blocking + sowing + vegetation blanket coverage + sprinkler irrigation 0.52 3
22 Eco bag blocking + sowing + vegetation blanket coverage + sprinkler irrigation 0.61 1
33 Plant grid slope protection + corrugated pipe drainage + sowing + sprinkler irrigation 0.48 4
EMGs
3 Eco bag blocking + corrugated pipe drainage + sowing + vegetation blanket coverage + sprinkler irrigation 0.61 1
35 Red brick skeleton slope protection + sowing + sprinkler Irrigation 0.47 2
EMSs
2 Eco bag blocking + corrugated pipe drainage + planting + sowing + vegetation blanket coverage + sprinkler irrigation 0.51 2
6 Eco bag blocking + sowing + grass curtain coverage + sprinkler irrigation 0.47 4
7 Eco bag blocking + sowing + grass curtain coverage + sprinkler irrigation 0.56 1
11 Eco bag blocking + sowing + grass curtain coverage + sprinkler irrigation 0.50 3
12 Eco bag blocking + sowing + grass curtain coverage + sprinkler irrigation 0.51 2
Table 3(d) Platform site type group
No. Ecological restoration technology Score Sort
TSPm
39 Planting + sowing + hole irrigation 0.66 2
40 Planting + sowing + hole irrigation 0.78 1
41 Planting + sowing + sprinkler irrigation 0.58 4
42 Border ridge + sowing + sprinkler irrigation 0.53 6
43 Planting + sowing + sprinkler irrigation 0.55 5
44 Planting + sowing + sprinkler irrigation 0.59 3
45 Apply organic fertilizer + seedling + sowing + hole irrigation 0.50 7
The results pertaining to the ecological restoration technology under different site types (Table 3), revealed four different technologies under the HTGs site type. On the whole, the technology employing plant grids has a better effect. Under the HMGs site type, the slope protection of mortar masonry skeleton slope protection + mortar masonry ditch + sowing + sprinkler irrigation, eco bag blocking + vegetation blanket/straw curtain cover + sowing + sprinkler irrigation technology evaluation score is higher, as several technologies with better recovery effect; under the type of HNGs, the gravel capping + mortar masonry ditch had the highest score (0.46); with respect to the type of STGs, the scores for different technologies differed and were not very high (0.5 approximately). For the STSs, three technologies mainly based on plant grids were reported, and indicated similar effects. With respect to the type of thick soil gentle slope in shade, the technologies based on vegetation blankets and vegetation bags obtained a higher score, and the application effect of interception and drainage measures was not obvious. With respect to the site type having medium-thick soil gentle slope, the scores of eco bag blocking + corrugated pipe drainage + sowing + vegetation blanket coverage + sprinkler irrigation technology were significantly higher than others; in the medium-thick soil steep slope site type in shade, the scores pertaining to the technologies that were mainly based on straw curtains and vegetation blankets did not differ significantly. The comprehensive score of planting + sowing + hole irrigation for TSPm site type was the highest, which indicates that the technology is better than other technologies.

3.3 Selection of ecological restoration technologies for dumps

Based on the technology applied in the field of the mining area, after the analysis of plant species composition, different suitable plant species underground were obtained, and it was first recommended when applied (Zou et al., 2021). The ecological restoration technology was comprehensively evaluated with respect to the following aspects: slope protection effect, vegetation restoration effect, soil improvement effect and technical and economic cost, and one or two technologies with higher scores under different site types were obtained after screening. The type having strong technical adaptability and one which was highly economical was considered and recommended for use in the dumping field of the grassland mining area in western Inner Mongolia (Table 4). To sum up, eco bag blocking + sowing + vegetation blanket or grass curtain coverage + sprinkler irrigation technology can be used in HMGs, STGs, ETGs, EMGs, and EMSs. The restoration effect of these technologies on the five sites was good, and the technologies were highly economical, hence, they can be used for rapid greening of the dump in the western grassland mining area of Inner Mongolia.
Table 4 Suitable ecological restoration techniques and plant species for different site types
Site type Ecological restoration technology Recommended plant species
HTGs Plant grid slope protection + planting + sowing + sprinkler irrigation Calamagrostis epigeios/Astragalus
membranaceus
Hexagonal brick slope protection + mortar masonry ditch + sowing + sprinkler irrigation Bassia dasyphylla/Halogetonglomeratus/
Chenopodium glaucum
HMGs Masonry skeleton slope protection + morter masonry ditch + sowing + sprinkler irrigation Medicago sativa/Artemisia sieversiana
Sowing + vegetation blanket coverage + sprinkler irrigation Astragalus laxmannii/Medicago sativa
HNGs Gravel capping + mortar masonry ditch Bassia dasyphylla
STGs Eco bag blocking + planting + sowing + vegetation blanket coverage + sprinkler irrigation Brassica campestris/Bassia dasyphylla
Eco bag blocking + sowing + grass curtain coverage + sprinkler irrigation Bassia dasyphylla/Halogeton glomeratus
STSs Plant grid slope protection + mortar masonry retaining wall + sowing + sprinkler irrigation Medicago sativa/Brassica campestris
ETGs Eco bag blocking + planting + sowing + vegetation blanket coverage + sprinkler irrigation Artemisia scoparia/Medicago sativa/Bassia dasyphylla
Planting bag stacking + mortar masonry retaining wall + sprinkler irrigation Artemisia desertorum/Astragalus membranaceus/ Achnatherum splendens
EMGs Eco bag blocking + corrugated pipe drainage + planting + sowing + vegetation blanket coverage + sprinkler irrigation Calamagrostis epigeios/Bassia dasyphylla
EMSs Eco bag blocking + sowing + grass curtain coverage + sprinkler irrigation Artemisia desertorum/Bassia dasyphylla
Eco bag blocking + corrugated pipe drainage + planting + sowing + vegetation blanket coverage + sprinkler irrigation Cymbaria mongolica/Triticum aestivum
TSPm Planting + sowing + hole irrigation Hippophae rhamnoides/Caragana intermedia/ Chenopodium glaucum

4 Discussion

Considering the principles of adapting measures to local conditions and suitable trees, scientific and rational screening of the prominent factors, and classification of sites are primarily focussed in ecological restoration of the mining areas (Wang, 2017). The slope aspect mainly adjusts the water and heat conditions of the soil as the availability of light, heat conditions, and quality of water directly determine the extent of survival of the vegetation (Madeli et al., 2018). The sunny slope has good light conditions but poor water conditions. Plants grow quickly as soon as the seeds germinate. The shady slope has better water conditions than the sunny slope, and the soil water content also differs significantly, the conditions like evapotranspiration, solar radiation, and ground temperature, are also better. These can be used as an important reference for the selection of the mine restoration plants. The growth and development of vegetation depends upon the soil, and the soil thickness impacts the survival of the vegetation. The aboveground vegetation type, coverage and biomass are likely to vary as per the the soil thickness (Nussbaumer et al., 2016; Martín et al., 2017). A vast majority of mining dumps are artificially covered with soil in the later stage. The type of soil, amount of nutrients, and the structural properties are likely to be different, and the rate of growth and development of vegetation may also vary to some extent. Slope gradient affects the growth and development of plants by affecting the water holding capacity of the soil, leaching of rainwater on the slope, runoff and scouring, etc. Most of the research areas are gentle or steep, and some slopes are greater than the safety protection standard of 33°. It is recommended that the slopes gradient larger than 33° should be decreased before treatment, so as to ensure not only their stability but also the stability of the later treatment measures (Liu, 2019). Based on this, the plant species pertaining to different types of sites can be selected considering their characteristics and aspects of the slope like, slope gradient and soil thickness, for the purpose of ecological reconstruction (Zhang, 2021).
The blanket curtain measures are easy to apply, low in cost and provide good slope protection effect, and thus are highly suitable for extension in this area. By providing a blanket of vegetation, the soil can be protected and the erosion of the topsoil can be alleviated by rainwater. The vegetation blanket facilitates retention of water and moisture by the layer of fibers, and hence has a better vegetation restoration success rate (Yan et al., 2013). For drainage purpose, both eco bags and corrugated pipes can be selected as similar effects they produce. For the same type of site, the vegetation blanket scores and those of the straw curtain slope protection technology were slightly different, which may be attributed to the difference in the construction quality or the time at which the seeds were sowed (i.e., morning or evening). The technologies based on plant grids obtained high scores that mostly used Salix twigs. Salix willow grows and reproduces in a large area in western Inner Mongolia as it is drought, wind and sand resistant, it is easy to obtain materials at low cost, so it is widely used in slope protection of drainage fields (Wang et al., 2018). The application effect of gravel capping + mortar masonry ditch is better on the side slope of the dump without covering soil, and hence is recommended for using. The gravel used in the construction can be sourced locally and stacked in sequence. The technology is simple and easy to implement, and has a better soil-fixing effect on the foundation slope of the dump site. The gravel can preserve the warmth and provide humidity and reduce evaporation (Sun et al., 2017). Compared to the hexagonal brick slope protection, it is cost-effective and easy to construct; hence, it is more suitable for ecological restoration of the local area (Zhang et al., 2013). Due to its good protective effect, skeleton protection is used for EMGs. Red bricks are likely to cause extensive pollution and can even degrade the natural resources, so it is recommended to use masonry for providing materials for the skeleton. The soil layer on the top platform of the dump is generally thick and hard, and soil erosion rarely occurs. Hence, it is easy to plant seedlings and sow grass seeds. Measures involving use of dense mesh nets and dividing the surrounding ridges are not recommended for ecological restoration of the local area.
Results of technical screening results were consistent with that of the field investigation, and were also consistent with the restoration status of each dump site. While using the technologies screened in this study for ecological restoration of soil dumps, clarification regarding the site conditions required for application of each technology should be sought, so as to avoid poor restoration effect due to obfuscation of technical applicability and mixed application of a single technology. During field construction, the construction quality should be strictly controlled, and the recommended plant species or native plants should be prioritised while selecting the plant species, so as to maximize the ecological restoration effect (Yan, 2020).

5 Conclusions

The soil thickness, slope gradient and slope aspect are the dominant factors affecting the site conditions of the dump site. Base on this, the dump site of the open-pit coal mine in the grassland mining area of western Inner Mongolia was divided into 9 types of sites.
For half-shade and half-sunny to thick soil gentle slope, we primarily recommend plant grid slope protection + planting + sowing + sprinkler irrigation, and the second recommendation is hexagonal brick slope protection + mortar masonry ditch + sowing + sprinkler irrigation; for half-shade and half-sunny to medium-thick soil gentle slope, we primarily recommend masonry skeleton slope protection + mortar masonry ditch + sowing + sprinkler irrigation, and the second recommendation is sowing + vegetation blanket coverage + sprinkler irrigation; for HNGs, it is recommended to use gravel capping + mortar masonry ditch. For sunny thick soil gentle slope, we primarily recommend eco bag blocking + planting + sowing + vegetation blanket coverage + sprinkler irrigation, and the second recommendation is eco bag blocking + sowing + grass curtain coverage grass curtain coverage + sprinkler irrigation; for sunny thick soil steep slope, it is recommended to use plant grid slope protection + mortar masonry retaining wall + sowing + sprinkler irrigation. For shade thick soil gentle slope, we primarily recommend eco bag blocking + planting + sowing + vegetation blanket coverage + sprinkler irrigation is preferred, and the second recommendation is planting bag stacking + mortar masonry retaining wall + sprinkler irrigation; for shade medium-thick soil gentle slope, it is recommended to use eco bag blocking + corrugated pipe drainage + sowing + vegetation blanket coverage + sprinkler irrigation; for the shade medium-thick soil steep slope, we primarily recommend eco bag blocking +sowing + grass curtain coverage + sprinkler irrigation, and the second recommendation is eco bag blocking + corrugated pipe drainage + sowing + vegetation blanket coverage + sprinkler irrigation. For thick soil platform, use of planting + sowing + sprinkler irrigation is recommended.

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