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Journal of Resources and Ecology >

2023 , Vol. 14 >Issue 4: 744 - 756

DOI: https://doi.org/10.5814/j.issn.1674-764x.2023.04.007

Theory and Technology of Mine Terrain Reshaping

Layered Construction of Novel Reconstituted Soils in Coal Mining Sites

  • LI Xuefeng , 1, 2, * ,
  • YANG Jinhang 1, 2 ,
  • LI Ruijie 1, 2, 3 ,
  • MA Zhigang 1, 2
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  • 1. School of Physics and Electronic-Electrical Engineering, Ningxia University, Yinchuan 750021, China
  • 2. Solid Mechanics Institute, Ningxia University, Yinchuan 750021, China
  • 3. Ningxia Highway Administration Center, Yinchuan 750011, China
*LI Xuefeng, E-mail:

Received date: 2022-10-30

  Accepted date: 2023-03-20

  Online published: 2023-07-14

Supported by

Natural Science Foundation of China(12162028)

The Science Technology Innovation Leading Talents Program of Ningxia Hui Autonomous Region(KJT2019001)

Key Research and Development Program of China(2017YFC0504404)

The Innovation Team for Multi-scale Mechanics and Its Engineering Applications of Ningxia Hui Autonomous Region(2021)

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Abstract

This study, which considers coal mining sites, aims to solve problems pertaining to the immense permeability, poor stability, and the environmental pollution of soil. In regard to reconstituted soil, the novel “2+1” layered construction technology entails designing a water-resisting layer, a modified layer, and an evaporation inhibition layer on the surface of the gangue hill. This technology, which entails constructing a novel type of reconstituted soil, considered four mechanical and physical soil properties, namely soil particle size, humidity, unit weight, and soil structure grade. The construction of novel reconstituted soils should consider the influence of biological factors and that of an arid environment, as well as the influence of the soil permeability coefficient, water content, density, temperature, and water-soil interaction; simultaneously, it should consider the chemical composition. With respect to the water-resisting layer, we selected loess and feldspathic sandstone that exhibited high clay content, and we utilized a 20 cm design thickness. The modified layer was composed of sandy loess, weathered coal, and feldspathic sandstone. This layer was exhibited a 60 cm design thickness, and it considered the characteristics pertaining to vegetation, soil compactness, and soil chemical composition. The evaporation inhibition layer was designed using materials such as gravel and straw checkerboard. Regional tests indicated that verified the rational robustness of the novel “2+1” reconstituted soil layered construction. The results provide a reference for ensuring the ecological sustainability of coal mining sites that characterize the arid, desert areas of Northwest China.

Key words： geotechnical engineering; novel constructed-soil; mineral component; coal gangue; seepage

Cite this article

LI Xuefeng , YANG Jinhang , LI Ruijie , MA Zhigang . Layered Construction of Novel Reconstituted Soils in Coal Mining Sites[J]. Journal of Resources and Ecology, 2023 , 14(4) : 744 -756 . DOI: 10.5814/j.issn.1674-764x.2023.04.007

1 Introduction

Soil matrix improvement, which can be applied to mining areas, is a crucial mechanism for land reclamation and ecological reconstruction. The discussion that entails the comprehensive utilization of the local, natural resources that characterize the mining area (i.e., coal mining sites), which can facilitate soil matrix improvement, is quite prominent. The application of novel reconstituted soil exhibits regional limitations, and researchers should develop methods of facilitating the effective utilization of different soil properties, which is a prerequisite for the construction of novel recon stituted soil. The coal mining site that is located in northwest China comprises sandy loess, grey calcite, feldspathic sandstone, and weathered coal; thus, this study is concerned with the effective utilization of the various geotechnical materials, which facilitates the construction of novel reconstituted soils under the influence of complex and arid environments. In regard to the utilization of feldspathic sandstone, Yang (2014), who considered the feldspathic sandstone area, analyzed the spatial distribution characteristics of geomorphic features and their combination types, and they considered ASTER GDEM data. Wang et al. (2022) added different volume ratios (0, 30%, 50%, 80%, and 100%) of feldspathic sandstone into the aeolian sand of the Kubuqi Desert, and they mixed the samples thoroughly; thus, they analyzed the improvement effect that feldspathic sandstone exerts on the physical structure and nutrient characteristics under different blending ratio conditions. The results indicated that the 50% incorporation rate could be utilized as the cut-off value for enhancement, and that the vertical variation characteristics, namely soil compactness, capillary porosity, and field moisture capacity, were small.
In regard to the research on novel reconstituted soils that cover soil layers, Shao et al. (2017) utilized the local topsoil, subsoil, and Yellow River sediments as cover material; thus, they increased the thickness of the covered soil, and they enhanced the texture of the local soil. The results indicated that the cover material that exhibited a 1:1:0.86 to 1:1:2 ratio was the most optimal soil type. The soil covering material that exhibits a 1:1:0.86 to 1:1:2 ratio is characterized by a 1.38 to 1.41 (g cm-3) bulk density, which are optimal for crop growth. For gravel area, Zhang et al. (2018) and Li et al. (2023), who considered bare rock gravel, analyzed the effects of covering soil thickness and the effect of different proportions of mixed soil on crop growth; the aforementioned researchers set up six test areas that exhibited different mulch thicknesses, namely 30 cm, 40 cm, 50 cm, 60 cm, 80 cm, and 100 cm. The comprehensive results indicated that when the mulch thickness was 50 cm, the bare rock gravel land could achieve the crop growth requirements, and all crop growth indexes that were treated under this condition were significantly better than the other mulch thicknesses. Tian et al. (2020) collected 140 surface soil samples from the 0-20 cm thickness level, and they analyzed the effects of soil physicochemical properties and plant configurations on soil. To determine soil physicochemical properties as well as to distinguish between gravel, coarse sand, fine sand, silt, and clay content, soil samples that reflected five different configuration types were set. The results of the current study provide a reference for exploring soil organic carbon content, and it enables researchers to determine the suitable soil for vegetation growth.
Ren et al. (2015) collected the sandy loess that characterize the Jin-Shan-Meng contiguous area, and they collected the novel reconstituted soil that was mixed with sandy loess, which was the research object; furthermore, they set up the novel reconstituted soil that was mixed with sand loess, weathered coal, and feldspathic sandstone in different proportions. The results indicated that weathered coal promoted respiration, which increased the rate of carbon release and altered soil respiration. For effective treatment, Fan et al. (2019) designed five types of soil, including aeolian soil, feldspathic sandstone, loess, feldspathic sandstone covered with soil, and feldspathic sandstone-enhanced aeolian soil; furthermore, the researchers planted Lupinus micranthus Guss and Zea mays, which was introduced from Mexico, and they studied the growth difference of the two plants in feldspathic sandstone-enhanced aeolian soil and the original soil that characterizes the Jin-Shan-Meng contiguous area. The results indicated that feldspathic sandstone physically and chemically enhanced aeolian soil, which led to high soil productivity and an enhanced plant-growth ability. To enhance the efficiency of land utilization, numerous scholars have performed a considerable number of studies on the feldspathic sandstone-based enhanced soil. Wang et al. (2018) selected three kinds of artificially matured novel reconstituted soils. Using the weighted synthesis method, the nutrient quality of several soil types was comprehensively assessed. The results indicated that the soil nutrient content of different enhancement modes was the highest in the aeolian soil mixed with weathered coal, followed by the aeolian soil mixed with feldspathic sandstone, and that the aeolian soil mixed and loess exhibited the lowest enhancement mode. Cao et al. (2017) collected four distinct soil types using four different land reclamation methods: coal gangue filling (CGF), mixed flat filling (MFR), mud pump filling (MPF), and fly ash filling (FAF). By measuring the amount of organic and black carbon contained in the soil, it was observed that the reclamation method influenced the amount of organic and black carbon contained in the soil. Thus, they verified the plant growth properties and suitability of the feldspathic sandstone enhanced soil.
Zhang et al. (2022), who performed their experiment in a field setup, utilized 1:1, 1:2, 1:5 proportions of feldspathic sandstone and sand compound soil; thus, they performed plot experiments, and they analyzed the distribution of water-stable aggregates in three proportions of compound soil under different maize planting years, which crucially affects the exploration of wind erosion and soil erosion in aeolian soil. Ma (2016), who considered the feldspathic sandstone weathered soil that characterizes the Jin-Shan-Meng contiguous area, systematically studied the effect of the water infiltration process on ryegrass growth, and they observed that the water infiltration mechanism affects feldspathic sandstone weathered soil. Feldspathic sandstone weathering products and coal gangue, which can be utilized as a novel reconstituted soil additive, can effectively prevent surface water from penetrating to the lower layers. Jia (2019) conducted outdoor and laboratory simulation tests using feldspathic sandstone and aeolian soil as the main soil and as the modifier, respectively. The effects of different feldspathic sandstone enhancement modes on the growth adaptability of alfalfa were studied; furthermore, the soil mechanical parameters, infiltration performance, and evaporation performance were studied. The results indicated that when the ratio of feldspathic sandstone in the aeolian soil increased, the field water holding capacity and saturated water content of the soil gradually increased; the saturated hydraulic conductivity and infiltration rate gradually decreased; and the soil water content gradually increased under the same matric suction. The results indicated that feldspathic sandstone could enhance the water retention capacity of aeolian soil and hinder water movement, and that it could be utilized as a physical modifier for enhancing the water absorption and water retention capacity. Moreover, it was observed that when feldspathic sandstone was utilized to enhance aeolian soil in a layered form, the following phenomenon occurred: when the position of the feldspathic sandstone layer was high and when its thickness was immense, the hindrance to soil water infiltration became apparent.
In addition, by combining coal gangue and other soil materials, numerous scholars have performed research on novel reconstituted soil. Zhang et al. (2021) researched the enhancement effect of coal gangue on saline soil as well as its effect on soil physicochemical properties. Different proportions and different coal gangue particle sizes were applied to saline soil, and using alfalfa potting experiments, soil physicochemical properties were determined. The results indicated that the soil that was treated with 20% small particle size and mixed particle size coal gangue exhibited the highest SQI, and that the enhanced saline soil quality and plant growth were optimal. Li and Li (2023) analyzed the stability of the modified coal gangue slope. The results indicated that the modified coal gangue slope was less affected by rain. By utilizing aeolian soil, adamic earth, coal gangue, corn straw scale, and humic acid as raw materials, Rong et al. (2022) conducted an alfalfa pot experiment, and they utilized different mixing ratios. The results indicated that the 1:2 combination of aeolian sand soil and adamic earth and the addition of 15% coal gangue, 5% corn straw, and 0.05% humic acid soil yielded the most optimal reconstruction, and with respect to the aforementioned the coal mining site, these results provided the basis for soil reconstruction and land ecological restoration. Xu et al. (2018) analyzed the temporal and spatial response characteristics of soil moisture in filling reclamation reconstruction caused by temperature field changes during the weathering process of the coal gangue matrix. Wang et al. (2016) analyzed the effect of soil reconstruction of purple soil, cold sand loess, and loess that characterize Chongqing, and they also revealed the best ratio of sponge nutrient soil to promote plant growth.
With regard to the arid desert area of Northwest China, there are a large number of coal mining sites that are located in the coal base. The coal mining site are covered with geotechnical materials including sandy loess, grey calcite, feldspathic sandstone, and weathered coal. A variety of rock and soil materials are mixed in the coal mining site; thus, with respect to the ecological conditions of the soil, the natural recovery process is inhibited. A large number of scholars have explored the construction of novel reconstituted soils, and they have considered different types and proportions; thus, they have constructed novel reconstituted soils, and they have promoted the optimal growth of vegetation under complex geological, environmental, and resource conditions. If a novel reconstituted soil typology can be rationally constructed in the coal mining site, the ecological restoration and reconstruction of the damaged land can be accelerated, and the combined economic and social benefits will be significant.

2 Determination of physical and mechanical properties

We collected information on parameters such as soil distribution, water distribution, and meteorological distribution; thus, we comprehensively comprehended the key elements for the construction of novel reconstituted soils. Because the basic characteristics of the soil crucially influenced the stability of the novel reconstituted soil, the preliminary work mainly entailed outdoor sampling and laboratory tests, and the test results provided data support for the novel reconstituted soil construction.

2.1 Permeability test

To measure the loess permeability that characterizes coal mining sites, a laboratory permeability test was utilized. The test, which was time-sensitive, could measure the change in seepage under the same void ratio and different confining pressure conditions. Using the permeability test, we determined the main factors that affect the permeability coefficient of the loess, and the test results provide a reference for the construction of novel reconstituted soils and ecological restoration. Fig. 1a depicts the change of flow rate per unit area with time under different confining pressures from the permeability test. whereas Fig. 1b depicts the change pertaining to the permeability coefficient of loess, and it considers different confining pressures while maintaining the same compactness.
Fig. 1 Permeability test: (a) the flow change per unit area over time; and (b) the variation that affects the permeability coefficient under different confining pressures

2.2 Consolidation test

First, we collected the following parameters: dry bulk density, wet bulk density, dry and wet densities, water content, and the void ratio of the undisturbed soil, aeolian soil, and loess; subsequently, the shape change and void ratio of the soil, which is a function of confining pressure, were obtained using a consolidation test. The variation that affects the void ratio when it is subjected to different confining pressures and the compaction law for different types of soil, which was obtained from the consolidation tests, provide a theoretical basis for the construction of novel reconstituted soils.
Undisturbed soils were collected in the novel reconstituted soil demonstration area of the coal gangue discharge field of the Lingwu Yangchangwan (YCW) Coal Mining Site. The novel reconstituted soil was constructed in March 2019 and sampled in September. The physical parameters, such as soil compactness, were relatively stable after six months of natural weathering. The test equipment adopts the consolidation instrument of Nanjing Ningxi Soil Instrument Co., Ltd. The brief test process is depicted in Fig. 2. Furthermore, the basic physical indicators of the specimens are illustrated in Table 1, and the data that is measured after the samples were collected at different sampling points are depicted in Fig. 3. Figure 3a-c indicate that the settlement of the novel reconstituted soil, which was collected at different test points, changes with time and axial pressure. Figure 3d depicts the e-P curve. It can be observed that the void ratios of different novel reconstituted soils are completely different. The novel reconstituted soil that is characterized by a high sand content exhibits poor compressibility, whereas the soil that is characterized by a high loess content can be easily compressed, and it exhibits a high level of compactness.
Fig. 2 Consolidation test equipment and operation: (a) consolidation test instrument; (b) prepared specimens; and (c) post-test specimens
Table 1 Basic physical state indexes of undisturbed soils
Specimen number Specimen quality (g) Specimen volume (cm3) Natural density (g cm-3) Natural void ratio Natural water content (%)
1 158.4 100 1.58 0.77 5
2 158.0 100 1.58 0.77 5
3 161.8 100 1.62 0.73 5
4 155.0 100 1.55 0.81 5
Fig. 3 Consolidation test law: (a) consolidation test results of specimen No.1; (b) consolidation test results of specimen No.2; (c) consolidation test results of specimen No.3; and (d) e-P curve of consolidation test results

2.3 Mineral composition analysis

With respect to the mining area, to comprehensively understand the impact of the mineral composition that characterizes the soil on the restoration of vegetation, and to further optimize the construction scheme for the novel reconstituted soil, it is necessary to quantitatively detect the mineral content of the sandy loess, feldspathic sandstone, weathered coal, and novel reconstituted soil using different construction ratios. Herein, 11 types of novel reconstituted soil construction schemes are designed. The specific test schemes are depicted in Table 2. The total specimen mass prepared in each scheme is 200 g, and Table 2 depicts the composition of each scheme.
Table 2 Composition analysis scheme for novel reconstituted soils
Scheme number 1 2 3 4 5 6 7 8 9 10 11
Novel reconstituted soil composition A:B:C A:B:C A:B:C A:B:C A:B:C A:B:C A:B:C A:B:C A:B:C A:B:C A:B:C
Proportion 1:0:0 0:1:0 0:0:1 3:2:0 7:3:0 2:2:1 1:4:5 1:2:2 3:5:2 3:4:3 5:4:1

Note: A denotes feldspathic sandstone; B denotes sandy loess; and C denotes weathered coal.

With respect to the test equipment, we utilized a US-made X-ray fluorescence heavy metal analyzer (i.e., E-max) that can accurately measure cadmium, mercury, arsenic, lead, nickel, chromium, and 40 other kinds of heavy metal pollutants, and this information is depicted in Fig. 4a. To quickly and quantitatively detect the content of heavy metals in the soil, the experimental equipment utilized X-ray fluorescence, and the measurement requirements met the national environmental quality risk control standard for the risk control standard for soil contamination of agricultural land (GB15618-2018). However, the mineral composition detection pertaining to novel reconstituted soils requires more stringent test steps. First, feldspathic sandstone, sandy loess, and weathered coal samples were collected, and they were sealed with packaging bags as depicted in Fig. 4b. The sampling process ensured that the specimen remained pure and free form impurities; the organic matter content of the test soil specimen should not exceed 5% of the soil specimen mass. The specimen was then pulverized, and the size of the particles was maintained at less than 1 mm. The crushed powder was mixed as per the ratio that is depicted in Table 2; thus, a specific specimen with a total mass of approximately 200 g was produced, and the specimen was labelled and measured as illustrated in Fig. 4c.
Fig. 4 Hazardous mineral composition analysis: (a) test equipment; (b) specimens crushing; and (c) specimens preparation
Figure 5 depicts the radar map of harmful trace elements, and it considers 11 different schemes of novel reconstituted soils. In the figure, eight heavy metal elements, cadmium, zinc, nickel, copper, chromium, lead, arsenic and mercury, are selected as parameters; thus, the influence of pollution elements on the construction of novel reconstituted soils is analyzed. Because they are the most basic measurement elements for the investigation pertaining to the soil environmental quality of agricultural land; furthermore, to ensure the safe growth of crops and vegetation, the measured values should follow the corresponding, strict soil management system in different intervals. Figure 5 indicates that when the number of elements that are not conducive to plant growth is immense, the area of the radar map increases. To screen and optimize the ratio schemes for novel reconstituted soil, Figure 5 can be utilized, and it can be observed that schemes 4, 5, and 6 contain fewer pollution elements, and that they can be utilized as key test schemes for novel reconstituted soils.
Fig. 5 Analysis of harmful mineral composition: (a) the harmful mineral content of schemes 1-5; (b) the harmful mineral content of schemes 6-11
The data that is measured in Fig. 6 are the detailed mineral composition contents pertaining to schemes 4, 5, and 6. To explore the content of the elements that are released based on the degree of breakage that characterizes the novel reconstituted soil; thus, we compared the coarse grain size and fine grain size. As shown in Fig. 6, for example Scheme 4, Specimen 4.1 represents a coarse grain size, whereas Specimen 4.2 represents a fine grain size. As per the comparison pertaining to the results that are depicted in Fig. 6, Specimen 4.2 releases slightly more elements than Specimen 4.1. This study focuses on studying the matching schemes of schemes 4, 5, and 6 three novel reconstituted soils; thus, a reference that can enrich subsequent field tests pertaining to novel reconstituted soils is established.
Fig. 6 Quantitative analysis of mineral composition: (a) and (b) Scheme 4 mineral composition content; (c) and (d) Scheme 5 mineral composition content; (e) and (f) Scheme 6 mineral composition content

2.4 Analysis of the liquid-plastic limit of the soil

The evaluation of the boundary moisture content of the novel reconstituted soil was performed as per the national standard for soil test method (GB/T 50123-2019), and the liquid limit-plastic limit tester and the rubbing-rolling plastic limit determination method was utilized. Figure 7a indicates that the liquid limit-plastic limit test utilized the SYS digital liquid-plastic limit tester that was produced by Nanjing Ningxi Soil Instrument Co., Ltd. Figure 7b and 7c depict the boundary moisture content test results of the novel reconstituted soil, and they consider Scheme 5 and Scheme 6. Table 3 depicts the test results of 11 different schemes, and the test results indicate that the novel reconstituted soil pertaining to Scheme 6 exhibits enhanced water retention characteristics. The Scheme 6 soil ratio, which considers feldspathic sandstone, sandy loess, and weathered coal ratio is 2:2:1, respectively; furthermore, the plastic limit index of the soil fluid is 20≤Ip≤25, and the plastic index range is 0.61≤IL≤0.70. The construction soil of this scheme is in a plastic state, and with respect to the plant-growth enhancement that is facilitated by reclamation, this behavior is a basic condition.
Fig. 7 Analysis of the liquid-plastic limit: (a) Liquid-plastic limit tester; (b) and (c) the novel reconstituted soil boundary moisture content test results for Scheme 5 and Scheme 6
Table 3 Results of the combined liquid-plastic limit method for measuring the liquid-plastic limit of soil
Scheme
number		 Plasticity
index Ip		 Liquidity
index IL		 Liquidity
evaluation		 A:B:C Scheme number Plasticity
index Ip		 Liquidity
index IL		 Liquidity
evaluation		 A:B:C
1 15.65 1.30 Flow plastic 1:0:0 7 16.22 0.69 Plasticity 1:4:5
2 0.87 0.76 Soft plastic 0:1:0 8 11.13 0.72 Close to soft plastic 1:2:2
3 11.36 0.69 Plasticity 0:0:1 9 11.71 0.65 Plasticity 3:5:2
4 22.484 0.61 Plasticity 3:2:0 10 1.68 0.74 Close to soft plastic 3:4:3
5 29.86 0.62 Plasticity 7:3:0 11 20.01 0.50 Plasticity 5:4:1
6 20.87 0.68 Plasticity 2:2:1

Note: A represents feldspathic sandstone; B represents sandy loess; and C represents weathered coal.

3 Construction and demonstration

3.1 Construction ideas

Figure 8 depicts the design concepts pertaining to the novel reconstituted soil construction technology. The “2+1” three-layer soil construction technology entails the utilization of the bottom layer as the water-resisting layer and the middle layer as the modified layer. After the planting process, the surface layer is utilized as the evaporation inhibition layer, and the local soil is utilized as the raw material. The following design concept guides the water-resisting layer: to effectively maintain the soil moisture and to avoid immense water infiltration into the coal gangue, which is caused by groundwater pollution. The water-resisting layer is indirectly in contact with the coal gangue; therefore, the coal gangue particles should exhibit immense gradation and compaction characteristics. The water-resisting layer, which acts as a bearing layer, requires the soil to exhibit immense strength, and this layer maintains an optimal water and heat insulation level. Therefore, characteristics such as high clay content and high compactness are an imperative. Second, the modified layer should provide the basic soil condition for ecological restoration; the soil-sand ratio should be optimal; and the soil should exhibit an appropriate PH and high organic content characteristics. Third, by designing the evaporation inhibition layer reduces the evaporation rate of water and enhances the water retention capacity of the novel reconstituted soil.
Fig. 8 Design of the “2+1” novel reconstituted soil layered construction technology

3.2 Construction schemes

Figure 9 illustrates the overall construction diagram of the novel reconstituted soil. The construction of the flat water-resisting layer that is depicted Fig. 9a is easy, and with respect to Fig. 9b, the construction of the slope water-resisting layer is difficult. Figure 9c represents a layered diagram of the novel reconstituted soil. The thickness of the soil layer should be evaluated based on the soil’s permeability and the mineral composition. With respect to vegetation restoration and reconstruction, the modified layer is the core component, and its design is based on the ecological environment characteristics of coal mining sites. Herein, by mixing sandy loess, weathered coal, and feldspathic sandstone, we develop various novel reconstituted soils schemes that are devised for area demonstrations. The compactness and thickness of the modified layer was designed as per the vegetation restoration typology, and we observed that the final thickness of the modified layer was 60 cm. The design pertaining to the evaporation inhibition layer was based on the following observation: with respect to northwest China, the rainfall was immensely less than the evaporation capacity. This evaporation inhibition layer, which mainly comprised gravel, grass grid, and other materials, inhibited water evaporation. The thickness of the designed evaporation inhibition layer was 8 cm. After the plantation of trees and shrubs, gravel was utilized as an evaporation inhibition layer, and with respect to low-height vegetation, grass grids could be utilized as an evaporation inhibition layer. The evaporation inhibition layer could not only effectively reduce the evaporation of soil moisture, but it could also effectively reduce the spread of urban dust. Figure 9b indicates that with regard to slopes, the effect of utilizing an evaporation inhibition layer was highly pronounced. Herein, geotechnical materials such as loess and feldspathic sandstone are selected, and the thickness of the water-resisting layer is finally set at 20 cm, and this value varies according to the compactness, mineral composition, and permeability of the materials.
Fig. 9 Schematic diagram that illustrates the layered construction of the “2+1” novel reconstituted soil: (a) flat ground profile map; (b) slope profile map; and (c) composition diagram

3.3 Constructing the demonstration process

The demonstration area, which is located in the YCW coal mining site demonstration area (i.e., an arid coal base area), was selected. Figure 10 illustrates the first and second ecological management areas of the gangue management area. The red area that is illustrated in Fig. 10 represents the demonstration area for the construction of novel reconstituted soils. This area constituted the first phase of the green management area, and it contained a total area of about 400 m2 (including slope land of approximately 50 m2). The main types of soil that characterize the YCW Coal Mining Site are grey calcite and aeolian soil; these soils exhibit low clay content and weak cementation, and they are susceptible to soil corrosion. Therefore, to conduct a demonstration area experiment in a small area, this study adopts the “2+1” novel soil construction technology.
Fig. 10 Orientation map of the YCW Coal Mining Site demonstration area
The schemes that were constructed by the novel reconstituted soil exhibit the following approximate dimensions: an 80 m2 area, a 50 m2 slope, and a 100 m2 reserved area. Figure 11 depicts the slope demonstration area that was constructed by the novel reconstituted soil. Two novel types of reconstituted soil construction were designed in the demonstration area, namely slope and flat construction types. Thus, the effect of the natural differentiation and fusion of soil which occurs after the construction of novel reconstituted soils was tested, and the slope type was designed on the sunny side. The first designed slope was the soil’s natural angle of repose (approximately 32 degrees), and this slope is indicated in Fig. 11a. Because the natural slope becomes unstable soil when it is subjected to rainfall, irrigation, and other conditions, it was later flattened, and the final design slope angle was 18 degrees, as depicted in Fig. 11b. The novel reconstituted earth type is a flat area, which is depicted in Fig. 11c.
Fig. 11 The novel reconstituted soil construction demonstration area: (a) the natural angle of the slope; (b) slope demonstration diagrams; and (c) flat demonstration diagrams
Based on the study that is presented in the second chapter, we utilized a series of tests pertaining to the physical and mechanical properties of undisturbed soils, and we identified three optimal schemes; thus, we constructed the novel soils, which can optimize the construction effects of novel reconstituted soils. Scheme 1 utilized a 6:4 feldspathic sandstone: sandy loess mixture; Scheme 2 utilized a 7:3 feldspathic sandstone: sandy loess mixture; and Scheme 3 utilized a 4:4:2 feldspathic sandstone: sandy loess: weathered coal mixture. The design concept of Scheme 3 entailed considering the effect of weathered coal on increasing soil porosity. Using artificial mixing and natural differentiation, the three demonstration schemes were tested. As depicted in Fig. 12a-c, the undisturbed soil sampling, feldspathic sandstone collection, and soil construction process were performed in the demonstration area. With respect to the feldspathic sandstone enrichment area, different types of feldspathic sandstone, weathered coal, and local sandy loess were collected as raw materials. Figure 12d-f depict the implementation areas pertaining to the modified layer of three different ratio schemes. The water-resisting layer mostly consisted of loess, and to increase the soil layer’s anti-seepage effect, it was mixed with feldspathic sandstone. The thickness of the soil layer was set at 20 cm, and the compactness of the soil layer exceeded 68%. After the completion of the modified layer, the gravel that exhibited an 8 cm thickness was uniformly covered; thus, the evaporation inhibition layer was formed. With regard to the severe drought environment that characterizes northwest China, to enhance synergistic water curing ability and immensely enhance the water retention effect, the reticulated grass grid was placed on the evaporation inhibition layer.
Fig. 12 Realization of the novel reconstituted soil construction scheme in the YCW coal mining site demonstration area: (a) undisturbed soil collection; (b) feldspathic sandstone collection; (c) implementation process; (d) Scheme 1; (e) Scheme 2; and (f) Scheme 3
The ecological vegetation restoration effect of the novel reconstituted soil demonstration area is depicted in Fig. 13. After collecting the test data pertaining to the undisturbed soils, the novel reconstituted soil was constructed based on three optimized schemes, and the results exhibited immense utility. The study established that the novel reconstituted soil construction can positively enhance the coal mining site environment, strengthen the soil and water conservation ability, and promote ecological reconstruction. The novel reconstituted soil construction method and its practical effect can facilitate the ecological restoration of the coal mining sites that are located in arid northwest China.
Fig. 13 Effect of ecological vegetation restoration in the novel reconstituted soil demonstration area

4 Conclusions

Soil is one of the most important carrier of biological survival and plays a decisive role in ecologically sustainable development. In particular, the development of novel reconstituted soils construction techniques in coal mining sites is of great importance for the ecological protection of coal mining sites in arid areas of Northwest China. In this paper, a new “2+1” reconstructed soil construction technique is proposed which is based on the analysis of the stability of coal gangue piles for the disturbance and failure in coal mining sites.
Soil is a crucial biological survival vector that immensely influences ecologically sustainable development. In particular, with respect to coal mining sites, the development of novel reconstituted soils construction techniques is immensely crucial; thus, the ecological protection of the coal mining sites, which characterize the arid areas of Northwest China, can be effected. This study, which aims to solve the problem pertaining to soil disturbance and failure, proposes a “2+1” novel reconstituted soil construction technology that is based on the stability analysis of gangue piles, which are utilized in coal mining sites.
(1) With respect to the consolidation test results of the undisturbed soils, we observed the following: the void ratio of the different novel reconstituted soil mixtures was fairly different. The novel reconstituted soil that exhibits a high sand content was less compressible, whereas the soil that exhibited a high loess content was more compressible, and it exhibited immense compactness.
(2) Through heavy metal analysis tests, we completed the quantitative analysis pertaining to the mineral composition of 11 types of novel reconstituted soils, and we selected three optimal types of schemes that exhibited the following scheme ratios, namely (6:4:0), (7:3:0), and (4:4:2), which indicate the feldspathic sandstone: sandy loess: weathered coal ratio. The novel reconstituted soil scheme releases less heavy metals when the grain size is coarse, which is more suitable for plant growth. According to the liquid-plastic limit test, the novel reconstituted soil has better water retention characteristics, which provides basic conditions to promote plant growth.
(3) Herein, the “2+1” novel reconstituted soil layer construction technology is proposed and suitable for arid desert areas with water shortage in northwest China. According to the characteristics of vegetation, soil compactness and soil chemical composition of the practice area (YCW), the design thickness of the layer is as follows: the water-resisting layer is 20 cm, the modified layer is 60 cm, and the evaporation inhibition layer is 8 cm. To enhance the efficiency of the soil layer, we construct an evaporation inhibition layer that is composed of gravel, and we cover it with a reticulated grass grid. The results demonstrated the feasibility and effectiveness of the novel reconstituted soil mass construction.
By conducting a demonstration test that considers several schemes pertaining to the novel reconstituted soils that characterize the YCW Coal Mining Site, we established that the construction of novel reconstituted soils enriches the soil, and that it enhances water conservation and the ecological restoration of coal mining sites. The “2+1” novel reconstituted soil construction technology exhibits experimental robustness; thus, it can provide a reference for ensuring the ecological sustainability of the coal mining sites that characterize the northwest China arid desert area. The novel construction technology realizes the sustainable development of ecological protection in coal mining sites.

References

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[1]
Cao Y H, Li L L, Liu C G. 2017. Impacts of different land reclamation methods on organic carbon and black carbon in soil in a mine subsided area. Journal of Resources and Ecology, 8(2): 191-195.

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[2]
Fan C B, Wu M M, Su R, et al. 2019. Effects of pisha sandstone-amended aeolian sandy soil on growth, nitrogen and phosphorus nutrition of two introduced plants. Journal of Soil and Water Conservation, 33(3): 228-234, 239. (in Chinese)

[3]
Jia J C. 2019. Effects of pisha sandstone addition on water movement and alfalfa growth in sandy soil. Diss., Beijing, China: University of Chinese Academy of Sciences. (in Chinese)

[4]
Li R J, Li X F. 2023. Stability analysis of improved coal gangue slope based on FEM. Journal of Ground Improvement, 5(2): 144-151. (in Chinese)

[5]
Li X F, Du C Y, Wang X, et al. 2023. Quantitative determination of high-order crack fabric in rock plane. Rock Mechanics and Rock Engineering, 2023: 1-10.

[6]
Ma W M. 2016. Water infiltration process of pisha sandstone weathering soil and the effects on ryegrass. Diss., Xianyang, China: Northwest A & F University. (in Chinese)

[7]
Ren Z S, Qi R P, Wang T T, et al. 2015. Effect of weathered coal on soil respiration of reconstructed soils on mining area’s earth disposal sites in Shanxi-Shaanxi-Inner Monglia adjacent area. Transactions of the Chinese Society of Agricultural Engineering, 31(23): 230-237. (in Chinese)

[8]
Rong Y, Wang C, Sun G L, et al. 2022. Research on effect of different ratios of reconstructed soil materials on soil improvement and alfalfa growth. Metal Mine, (6): 197-204. (in Chinese)

[9]
Shao F, Hu Z Q, Li X Y, et al. 2017. One-dimensional vertical infiltration of alternative soil covered on Yellow River sediment layer in filling reclamation. Coal Science and Technology, 45(1): 226-230. (in Chinese)

[10]
Tian Y H, Liu F H, Wang T T. 2020. Spatial distribution of surface soil organic carbon density and related factors along an urbanization gradient in Beijing. Journal of Resources and Ecology, 11(5): 508-515.

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[11]
Wang F L, Gao M, Dai W C, et al. 2016. Distribution characteristics of soil organic carbon under different land use types. Journal of Soil and Water Conservation, 30(4): 227-232. (in Chinese)

[12]
Wang L L, Zhen Q, Wang Y, et al. 2018. Affect of soil amelioration on soil nutrients at mining dumps in the Shanxi-Shaanxi-Inner Mongolia region. Acta Pedologica Sinica, 55(6): 1525-1533. (in Chinese)

[13]
Wang W W, Zhang L X, Liang Z S, et al. 2022. Physical and nutrient characteristics of sand soil improved by pisha sandstone via field experiment. Science of Soil and Water Conservation in China, 20(1): 136-142. (in Chinese)

[14]
Xu L J, Zhu X M, Liu S G, et al. 2018. Temporal and spatial response characteristics of reconstructed soil moisture under different particle size coal gangue temperature field. Journal of China Coal Society, 43(8): 2304-2310. (in Chinese)

[15]
Yang Y C. 2014. Analysis of mine water filling factors and research of mine water prevention and in Jinyuanli. Diss., Fuxin, China: Liaoning Technical University. (in Chinese)

[16]
Zhang H O, Shi C D, Li J. 2022. Changes of aggregates in soft rock improved sandy soil after years of corn plantation. Journal of Arid Land Resources and Environment, 36(2): 110-115. (in Chinese)

[17]
Zhang Y, Li Z B, Han J C, et al. 2018. Effect of covering-soil thickness on crop growth on bare rock and gravel land in an ecological restoration project. Journal of Resources and Ecology, 9(5): 484-492.

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[18]
Zhang Y H, Song Z L, Kong T, et al. 2021. Amelioration effect of coal gangue on physical and chemical properties of saline-alkaline soil. Journal of Ecology and Environment, 30(1): 195-204. (in Chinese)

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