*Article* **Water Preservation and Conservation above Coal Mines Using an Innovative Approach: A Case Study**

**Yujun Xu 1, Liqiang Ma 1,2,\* and Yihe Yu <sup>1</sup>**


Received: 9 May 2020; Accepted: 28 May 2020; Published: 2 June 2020

**Abstract:** To better protect the ecological environment during large scale underground coal mining operations in the northwest of China, the authors have proposed a water-conservation coal mining (WCCM) method. This case study demonstrated the successful application of WCCM in the Yu-Shen mining area. Firstly, by using the analytic hierarchy process (AHP), the influencing factors of WCCM were identified and the identification model with a multilevel structure was developed, to determine the weight of each influencing factor. Based on this, the five maps: overburden thickness contour, stratigraphic structure map, water-rich zoning map of aquifers, aquiclude thickness contour and coal seam thickness contour, were analyzed and determined. This formed the basis for studying WCCM in the mining area. Using the geological conditions of the Yu-Shen mining area, the features of caved zone, water conductive fractured zone (WCFZ) and protective zone were studied. The equations for calculating the height of the "three zones" were proposed. Considering the hydrogeological condition of Yu-Shen mining area, the criteria were put forward to evaluate the impact of coal mining on groundwater, which were then used to determine the distribution of different impact levels. Using strata control theory, the mechanism and applicability of WCCM methods, including height-restricted mining, (partial) backfill mining and narrow strip mining, together with the applicable zone of these methods, were analyzed and identified. Under the guidance of "two zoning" (zoning based on coal mining's impact level on groundwater and zoning based on applicability of WCCM methods), the WCCM practice was carried out in Yu-Shen mining area. The research findings will provide theoretical and practical instruction for the WCCM in the northwest mining area of China, which is important to reduce the impact of mining on surface and groundwater.

**Keywords:** water protection; water-conservation coal mining (WCCM); influencing factors; "five maps; three zones; and two zoning plans"; water conductive fractured zone (WCFZ)

#### **1. Introduction**

In China, coal resources are mainly distributed in the north and west, especially in the provinces of Shanxi, Shaanxi, Inner Mongolia, and Xinjiang, which account for about 70% of all Chinese coal reserves. As the major coal production region, five large coal production bases have been developed in Northwest China, i.e., Shaanbei, Huanglong, Shendong, Ningdong, and Xinjiang (Figure 1). These bases contribute to over one third of China's annual coal production, providing strong support to the country's economic growth [1].

Northwest China has a dry climate and sparse vegetation, hence water resources are scarce, accounting for only 3.9% of the country's total [2,3]. Shallow groundwater, a key water source for local vegetation [4,5], is likely to be affected by mining, making the fragile ecosystems even more vulnerable [6–13]. In the Yu-Shen mining area, in the major production area of the Shaanbei base, the groundwater table has declined by more than 15 m over an area of 306.8 km<sup>2</sup> and by 8–15 m in a

further 352.1 km<sup>2</sup> area. High intensity mining is found to be the direct cause of significant groundwater table decline in 71.5% of these areas [11]. It is clear that there are severe conflicts between water scarcity, ecological vulnerability, and coal resources exploitation.

**Figure 1.** Location and landform of Yu-Shen mining area and five large coal production bases.

Researchers have proposed the concept of water-conservation coal mining (WCCM) and domestic and foreign researchers have conducted in-depth research on mining-induced groundwater loss and WCCM [14–17]. Karaman et al. [18] used a one-dimensional flow equation to examine the effects of subsidence on water-level by introducing a sink that moves with the mining face and tested the validity of this method by estimating parameters of aquifer over a longwall coal mine in the Illinois. Shultz [19] studied the effects of longwall coal mining on the groundwater hydrology of Marshall County (West Virginia, USA). Booth et al. [20] conducted a seven-year study of a sandstone aquifer overlying an active longwall mine in Illinois (USA) and developed a comprehensive model of its influencing factors. They drew the conclusion that the changes of permeability and storativity over the longwall panel caused the decline and recover of the water levels in the sandstone. Hill et al. [21] conducted a field measurement by placing groundwater monitoring networks over the longwall panel. They collected, compiled and analyzed the data to provide documentation of the groundwater fluctuations caused by mining to local officers. Robertson [22] used Ostrom's design principles for common pool resources and revealed several management challenges of the key aquifers' water-level continued declining in the Great Artesian Basin (Australia). Howladar [23] studied the impact of underground coal mining on water environment around the coal mining area in Dinajpur (India). By analyzing the water level data and ground water major parameters collected from 2001 to 2011, he concluded that without a sustainable and long-term coal mining plan in the area, the water level will deplete and the water crisis will occur in the coming day. Gandhe et al. [24] optimized the mining methods and investigated hydrogeology conditions to protect the surface water bodies from being disturbed or destroyed in Godavari (India).

Fan et al. [25] analyzed the relationship between groundwater table decline and mining intensity in the Yu-Shen mining area based on statistical data of local groundwater table before and after large-scale coal mining. It was found that intensive mining is the primary factor causing the lowering of groundwater table in this area. Based on analysis of the spatial relationship between coal seam, aquifer and aquiclude, Wang et al. [26] divided the study area into several zones according to aquiclude stability after mining and then assessed the applicability of WCCM to different zones. Miao et al. [27] proposed the concept of water-resisting key strata based on the strata control theory. They also analyzed the mechanical behavior of compound key strata using six different composite beam models. Through stability analysis of water-proof strata overlying a shallow coal seam during mining, Huang et al. [28] investigated the fracture development characteristics above and below the water-resisting strata, and proposed the use of water-resisting strata thickness to mining height ratio as the criterion of water resisting capacity assessment. Zhang et al. [29] identified different types of eco-geological environment in the coal mining areas across Northwest China and analyzed the distribution of mining-induced fractures in aquiclude. Based on the stratigraphic structures in typical coal mines of Northwest China, a technological system of WCCM, taking into account the relationship between bedrock thickness, water-bearing of aquifer, mining methods and parameters, was established. Moreover, Ma [2,12,30–33] et al. investigated the development of water conductive fractures induced by shallow seam mining and proposed two WCCM methods. One is applicable for shortwall mining with "mining while filling", and the other is for longwall mining with partial backfilling.

After years of research and practice, WCCM theory and technology have been well-developed and successfully applied in many coal mines of North China and other parts of the world [15,34–37]. However, due to the heavy dependence of WCCM on site specific geological and hydrogeological conditions, a systematic research approach to achieving WCCM in different mining regions is still needed. In this paper, we analyzed the factors affecting WCCM in Yu-Shen mining area (part of Shaanbei coal production base), and then proposed a systematic WCCM method based on "five maps, three zones, and two zoning plans", which would systematize and standardize WCCM research and practice, providing guidance on how to achieve WCCM in Northwest China. This methodology can then be used to develop WCCM in other coal mining regions in other countries besides China.

#### **2. Identification of Factors Influencing WCCM**

Previous researchers have investigated factors influencing WCCM from various perspectives by analyzing the relationship between the decline of groundwater table and mining intensity. Adhikary et al. [6] found that the height of the water conductive fractured zone (WCFZ) depended greatly on the working face length, mining height and face advance rate. Wang et al. [26] categorized the spatial relationship between coal seam and aquifer (or aquiclude) into different types based on the thicknesses of overburden, aquifer, and aquiclude. Miao et al. [27] discussed the water-resisting property of compound key strata in overburden with different stratigraphic structures. Huang et al. [28] demonstrated that mining depth, mining height, and the thickness and properties of overburden rock strata are primary factors determining the development of upward and downward fractures in water-resisting strata. Zhang et al. [29] suggested that shallow burial depth and large mining height would result in the greater possibility of both surface water and shallow groundwater loss. In addition, they illustrated the synergistic failure characteristics of aquicludes and adjoining strata from the perspectives of overburden thickness, lithology, and stratigraphic structure. Ma et al. [30–32] categorized the thickness, permeability coefficient and groundwater recharge condition of loose aquifers and proposed an empirical equation to estimate the height of WCFZ.

With different results, these studies focused on various influencing factors, but none of them were comprehensive and systematic. Therefore, this study divided the factors influencing WCCM into three broad categories: engineering geology, hydrogeology and mining method. We then proposed a multilevel model for the comprehensive assessment of these factors, which is summarized in Table 1. The importance of each influencing factor was assessed using analytic hierarchy process (AHP). The specific steps of calculating the weights of influencing factors are as follows:

Step 1: Influencing factors determination

$$\{II = \{\mu\_1, \mu\_2, \dots, \mu\_n\}\}\tag{1}$$

where *U* is the domain of WCCM; *u*1, *u*2, ... *u*<sup>n</sup> are the influencing factors.

Step 2: Judgment matrix construction

Taking the relative weight evaluation of sub-factors of primary factors of engineering and geological conditions by an expert for example, the judgment matrix are as follows:

$$\mathcal{W}\_{\mathbb{B}\_1 \sim \mathbb{C}} = \begin{bmatrix} 1 & 1 & 1 & 3 & 5 \\ 1 & 1 & 1 & 3 & 5 \\ 1 & 1 & 1 & 3 & 5 \\ 1/3 & 1/3 & 1/3 & 1 & 5 \\ 1/5 & 1/5 & 1/5 & 1/5 & 1 \end{bmatrix} \tag{2}$$

where *W* is the comparison discriminant matrix.

Step 3: The largest eigenvalue and corresponding eigenvector calculation

The largest eigenvalue λmax of the matrix is 5.154 and the corresponding eigenvector *W* = (0.276, 0.276, 0.276, 0.124, 0.047).

Step 4: The consistency test

The consistency was checked by *CR* = *CI*/*RI*, where *CR* is the consistency ratio, *RI* is average consistency index and *CI* is the consistency indicator, which is defined as *CI* = (λmax − *n*)/(*n* − 1). If *CR* < 0.1, the relative weights are reasonable; otherwise, the judgment matrix needs to be adjusted by redistributing the weights of influencing factors. The *CR* of the matrix is 0.035 < 0.1, indicating the weights distribution is reasonable. The weight of engineering and geological conditions is 0.347, then 0.347 × *W* = (0.096, 0.096, 0.096, 0.043, 0.016), which is the final weight of the five secondary factors among the primary factors given by the expert.

Many researchers, experts and scholars engaged in WCCM have been invited to evaluate the importance of each influencing factor in the AHP model. The mean values of calculation results of their evaluation are listed in Table 1.

As shown in Table 1, the overburden thickness and type of stratigraphic structure have greater impacts than other engineering geological factors. Among the hydrogeological factors, aquiclude thickness has the greatest impact, followed by aquifer thickness and permeability coefficient. In the category of mining method, the impact of effective mining height is an order of magnitude greater than that of other factors; it is therefore considered as the most important factor in mining method.



## **3. Overview of Yu-Shen Mining Area and Determination of "Five Maps"**

#### *3.1. Geomorphic Features*

The Yu-Shen mining area is located the north of Yulin City and is the center of the Jurassic coalfield in north Shaanxi. This area contains proven coal reserves greater than 30 billion tons. The area is located between the Mu Us Desert (southeast of Inner Mongolia) and the Loess Plateau within northern of Shaanxi Province, and characterized by eolian sand in the west and loess ridges and hills in the east as shown in Figure 1.

The overall surface elevation varies between 1200–1300 m, and is higher in the northwest and lower in the southeast. This area fits into the moderate-temperate zone with a semi-arid continental monsoon climate. Local water resource is limited and unevenly distributed both spatially and temporally. The average annual rainfall is only about 400 mm, over 70% of which occurs from July to September, while the average annual evaporation exceeds 1900 mm [38]. Moreover, the ecological environment of this area is vulnerable to mining, and the early development of coal resources has caused a series of geological and ecological problems [12,25].

#### *3.2. Geological Characteristics*

The typical stratigraphic column of Yu-Shen mining area and the lithological characteristics are shown in Figure 2. The surface is primarily covered by the Quaternary and Neogene strata, and bedrock outcrops are sporadically distributed in valleys. From oldest to youngest, the strata found in this area include: the Lower Jurassic Fuxian Formation (J1f), the Yan'an (J2y), Zhiluo (J2z), and Anding Formations (J2a) of Middle Jurassic age, and the Lower Cretaceous Luohe Formation (K1l), the Neogene System (N), and the Quaternary System (Q) [26–29,31–37].

**Figure 2.** Stratigraphic column in Yu-Shen mining area.
