Water-preserved coal mining in water-shortage mining areas: a case study in the Yonglong mining area of China

WPCM

water-preserved coal mining

WCFZ

water-conducting fractured zone

Coal provides an important energy guarantee for the global economic development, but its mining also causes a lot of damage and waste to the precious groundwater resources in the mining area (Zhang 2005; Booth 2006; Zhang et al. 2011; Sun et al. 2016). During coal mining, water resources will be consumed and destroyed. According to statistics, about 2 tons of mine water will be produced for each ton of coal mined. Coal mine surface subsidence has a serious impact on surface water runoff and surface environment, and mine water discharge will also pollute water resources (Booth & Bertsch 1999; Liu & Elsworth 1999; Guo et al. 2019; Qiu et al. 2019). Most of the large coal bases in the world are located in areas where the contradiction between supply and demand of water resources is relatively prominent. For example, among the 96 state-owned key mining areas in China, water-shortage mining areas account for 71% (Cao 2017). The shortage of water resources seriously restricts the coal consumption and mining (Shi & Zhang 2021). Therefore, how to maximize the protection and utilization of water resources while coal mining is an inevitable requirement for sustainable development strategy.

Since the 1970s, the contradiction between the expanding scale of global coal production and the water environment has become increasingly acute, attracting the attention of governments and scholars (Booth 1986; Zipper et al. 1997). For example, in the 1970s, the United States has paid attention to the contradiction between coal mining and water consumption in the Yellowstone River watershed, and began to study the competition of water resources among industry, agriculture and other industries (Thomas & Anderson 1976). The Tongue River Basin in Montana entered the period of large-scale coal mining in the 1980s, and also began to study the phenomena of riverbed drainage, water quality deterioration and aquatic ecosystem damage induced by mining (Hickcox 1980). Some European countries mainly focus on the treatment mechanism and technology of mine water, the impact of abandoned mine water discharge on the environment and have conducted legislative and technical research on the protection of mine water resources (Gombert et al. 2018). The geological and hydrogeological conditions of mines in Australia, India, South Africa and other countries are relatively simple, and more attention is paid to the utilization of mine water and the treatment and utilization of open pit ponding lakes (Tiwari et al. 2016). In the 1990s, coal mining in the early stage of the development of Yushenfu mining area in China caused water environment problems such as the decline of phreatic water level and the drying up of springs. Some scholars initially proposed the thinking and method of water-preserved coal mining (WPCM), and thought that coal mining, water conservation, and ecological environment protection should be unified planning as a system engineering (Fan & Ma 2018; Fan et al. 2019a; Yu & Ma 2019). After decades of development, countries around the world have made great achievements in theoretical research and engineering practice of water resources and ecological environment protection in mining areas.

Aiming at the protection of groundwater resources during the high-intensity mining of Yonglong mining area of Huanglong Jurassic coalfield in western China, this paper analyzed the hydrogeological structure and engineering geological characteristics, studied the overburden failure and seepage laws during the mining of extra-thick coal seam, and put forward some protective mining technologies suitable for the study area. The research results are of great significance to the protection of water resources, ecological environment and the realization of sustainable development of water-shortage mining areas.

Yonglong mining area is located in the west of Huanglong Coal Base in Shaanxi Province, China, and belongs to the Mesozoic confined water in Ordos Basin. The water-bearing bedrock strata are composed of clastic rocks of various grain sizes in the Lower Cretaceous, Jurassic and Triassic. The groundwater is mainly sandstone confined fissure water, followed by loose layer phreatic water. The sandstone aquifer is mainly Cretaceous pore-fissure aquifer and Jurassic–Triassic fissure aquifer. It is divided into two types of water storage structures: Cretaceous confined water syncline and Jurassic–Triassic confined water monocline. The Luohe Formation of the lower Cretaceous system is sporadically exposed in the surface valleys of the study area, and its upper Neogene and Quaternary systems are widely covered.

According to the hydrogeological data during the exploration and mine construction of Yonglong mining area, the lithology, structure and water abundance of the strata are comprehensively analyzed. Then the aquifers in the overlying strata of the main mining No. 3 coal seam are divided into seven layers, and the aquicludes are divided into two layers, as shown in Table 1.

It can be seen from Table 1 that the Anding Formation mudstone aquiclude in the Middle Jurassic System has large thickness, stable horizon, good continuity and good water resistance. It is a stable aquiclude between the coal measures and the overlying Cretaceous aquifer, and is the water-resisting key stratum in water-preserved mining, belonging to the high-level aquiclude structure (Miao et al. 2009). In the process of coal seam mining, the sandstone aquifer in the Middle Jurassic Zhiluo Formation, the Yan'an Formation coal seam and its roof sandstone aquifer are the direct water-filled aquifers. Because of their deep burial depth, underdeveloped fractures, poor recharge conditions, and weak water abundance, mine water filling is easy to drain, which has little effect on coal seam mining. Although the lower Cretaceous sandstone aquifer is an indirect water-filled aquifer of coal seam roof, it is thick, widely distributed, and has good water yield, and is connected with the regional aquifer with strong water abundance. Many water inrush accidents have occurred during the construction and production of the mine, resulting in the shutdown of the mine and the loss of groundwater resources. Therefore, the groundwater in the Cretaceous sandstone aquifer is not only an important regional water supply source, but also the main source of mine water inrush. To conclude, it is urgent to carry out water hazard prevention and groundwater resources protection.

The thickness of the overlying strata between the No. 3 coal seam roof and the Cretaceous sandstone aquifer is 125.43–277.80 m, with an average of 185.80 m, which is mainly composed of three sets of rock groups: Anding Formation, Zhiluo Formation and Yan'an Formation. The lithology combination of each rock group is mainly medium sandstone-coarse sandstone, siltstone -fine sandstone, and mudstone-sandy mudstone. Mudstone, sandy mudstone and siltstone of Anding Formation, Zhiluo Formation and Yan'an Formation are relatively thick, with an average cumulative thickness of 102.08 m. The test results of mudstone disintegration and expansion in the study area are shown in Table 2. From Table 2, it can be seen that the mudstone and sandy mudstone in the overlying strata have the characteristics of easy disintegration and good expansibility when meeting with water, which will lead to the closure of mining-induced fractures and reduce the development height of water-conducting fractured zone (WCFZ), thus restoring the ability to block the runoff of Cretaceous aquifer to mine goaf. To conclude, this lithology combination of overlying strata plays a positive role in water-preserved mining and mine water disaster prevention, indicating that the study area has the basic geological conditions to realize water-preserved mining.

Determining the WCFZ height is the basis for implementing the WPCM. In this paper, the WCFZ height was determined by observing the drilling fluid leakage and the water level change in the borehole. G1 borehole was a post-mining drilling fluid leakage detection hole and was also used for post-mining peeping detection. The G1 borehole was located above the No. 20301 working face of Cuimu coal mine. The mining thickness of the working face was 12 m, and the vertical depth from the coal seam floor to the orifice was 553.22 m.

Based on the comprehensive analysis of the changes of flushing fluid leakage, water level and abnormal phenomena during drilling rig construction, the borehole depth of 302.75 m was defined as the top boundary of WCFZ, which was 238.47 m away from the No. 3 coal seam.

Through calculation, the WCFZ height is 238.67 m, and the ratio of WCFZ to mining height is 19.89.

Based on the stratum structure of No. 21301 working face in Cuimu coal mine, a three-dimensional numerical model was established by using FLAC3D 6.0 software, with dimensions of 900 m long, 300 m wide and 302 m high. The design mining length of the working face was 700 m along the strike direction, 200 m along the sloping direction, and the mining height was 12 m. The boundary conditions of the numerical model were as follows: the horizontal displacement was limited on the side of the model, the vertical displacement and horizontal displacement were limited on the bottom, and the vertical load (about 7 MPa) was applied on the top to simulate the self-weight stress of the overlying rock. Elastic constitutional model was used for the initial stress simulation and the Mohr–Coulomb yielding criterion was used in the excavation simulation. The rocks are elastic–plastic media in terms of the laboratory results and the physical–mechanical parameters used in FLAC3D simulation are shown in Table 3.

The measured data of the WCFZ height under the condition of fully mechanized top-coal caving mining in the extra-thick coal seam in Yonglong mining area were collected, as shown in Table 4.

According to the measured values of the development height of WCFZ, the average ratio of WCFZ to mining height in Yonglong mining area is determined to be 20.67, and the maximum height of WCFZ in different boreholes under the full-thickness mining conditions of the No. 3 coal seam in Cuimu coal mine is estimated using this ratio, as shown in Table 5.

It can be seen from Table 6 that when the mining height is greater than 9 m, the development height of WCFZ will generally penetrate the Cretaceous aquifer, leading to serious leakage of groundwater resources. However, when the mining height is less than 9 m, the development height of WCFZ will not penetrate the Cretaceous aquifer. According to the hydrogeological background of the study area, the hydrogeological structure can be divided into high-level aquiclude structure, and the aquiclude is the mudstone of Anding Formation. After the mining of No. 3 coal seam, the sandstone fracture aquifer of Zhiluo Formation and the sandstone aquifer of Yan ‘an Formation will be destroyed. Due to the existence of natural high-level aquiclude (Anding Formation mudstone), it can become a natural protective barrier of Cretaceous aquifer by changing coal mining technology or adjusting mining parameters, that is, it has the basic hydrogeological structure conditions for WPCM.

Due to the large mining height of the No. 3 coal seam in Yonglong mining area and the limited thickness of rock layers between it and the Cretaceous aquifer, the WCFZ under the condition of fully mechanized caving mining can penetrate the Cretaceous aquifer, resulting in the loss of groundwater resources. Therefore, it is necessary to limit the height of WCFZ by changing the mining technologies and protect the stability of water-resisting key stratum, so as to achieve the dual purpose of roof water disaster prevention and groundwater resources protection.

• (1)

  Restriction of mining height

The average water column height of Cretaceous aquifer measured by 11 boreholes in the first mining panel of Cuimu coal mine in Yonglong mining area is 246.29 m. According to the regulation on water prevention and control in coal mines, the critical water inrush coefficient T_s of the complete aquiclude without fracture structure damage section is calculated as 0.1 MPa/m, and the thickness of safety water-resisting layer is 24.6 m. The height of WCFZ is 20.67 times the mining height, and the restriction of mining height of each borehole can be calculated by the restricted height of WCFZ. The results show that when the mining height M ≤ 9 m, it can be ensured that the effective water-resisting layer thickness meets the requirement of the ultimate safety thickness of 24.6 m, and the WCFZ will not eventually reach the Cretaceous strata, which has no effect on the Cretaceous aquifer, as shown in Table 6.

• (2)

  Using the method of slice mining

The UDEC discrete element software was used to simulate and analyze the height of WCFZ in slice mining. The numerical model was established according to the stratigraphic configuration of G14-2 borehole. The mining conditions were as follows: near horizontal coal seam, the overlying strata of coal seam were homogeneous and medium hard, and the mining depth of No. 3 coal was 490 m. The single longwall caving mining method was adopted, and the roof was managed by all caving method. The average mining thickness of No. 3 coal was 11 m, and the mining was divided into two layers, the first layer was 3.5 m, and the second layer was 7.5 m.

The discrete element simulation results showed that during the excavation of the first layer of No. 3 coal, with the increase of the advance distance, the plastic zone area of the roof gradually increased, and the height also gradually increased. With the continuous increase of the advancing distance, the height of the plastic zone did not increase after reaching full mining, and the height of the plastic zone was basically stable. The height of WCFZ was determined to be 68 m according to the scope of the plastic zone (Figure 6(a)). During the excavation of the second layer of coal, with the increase of the advance distance, the plastic zone area of coal seam roof continued to expand upward on the basis of the plastic zone of the first layered mining, and its height gradually increased. The height of plastic zone did not increase after the second layered mining reached full mining, and the height of the plastic zone was basically stable. According to the scope of the plastic zone and the fracture penetration, the height of WCFZ was determined to be 110 m (Figure 6(b)). The No. 3 coal of G14-2 borehole was 189.39 m from the Cretaceous strata, and the bottom interface of the Cretaceous strata was calculated to be 79.39 m from the WCFZ. It can be seen that the slice mining method can reduce the height of WCFZ so that it does not affect the Cretaceous aquifer, thus protecting the Cretaceous groundwater resources.

• (3)

  Controlling the advancing speed of working face

The contact zone between the glutenite at the bottom of Cretaceous strata and the mudstone of the Anding Formation at the top of the Jurassic strata is easy to form a bed separation space. A large amount of water is produced when groundwater infiltrates into the bed separation space. Its inrush not only threatens the safety of mine production, but also causes the leakage of Cretaceous groundwater resources (Li et al. 2018; Fan et al. 2019b).

The applicability of WPCM technology is worth discussing. WPCM is proposed for the serious leakage of groundwater and the deterioration of ecological environment in the Salawusu Formation during the mining of Jurassic coalfields in northern Shaanxi, China. It is applicable to coal mines in the arid and semi-arid areas all over the world, and also has certain adaptability to the aquifer occurrence areas with ecological value. With the deepening of research, the scope of WPCM research is expanding, which is mainly due to the improvement of WPCM technology methods such as filling mining, slice mining, short-wall mining and grouting mining. Therefore, in the process of coal seam mining, as long as the aquifer or surface water body that needs to be protected is involved, the WPCM technology can be used to solve it. For example, the burnt rock water in the Shenfu mining area of China is one of the main groundwater resources. Coal mining experts have divided the water-preserved mining zone and put forward a new technology of burnt rock grouting curtain closure (Dong et al. 2019). The Office of Surface Mining, Reclamation and Enforcement (OSMRE) in the USA, assessed the impact of underground mining operations on streams and groundwater and proposed protective mining methods for water resources (Newman et al. 2017). The Neogene loose aquifer in the Balapukulia coal mine in Bangladesh not only threatens the mine safe production, but also has the significance of water supply. The feasibility of WPCM was discussed, and the mining mode of protecting the upper aquifer and dredging the lower aquifer based on water resources protection was proposed (Yu et al. 2019). Aquifer System in the Bogdanka Hard coal mine in the eastern part of Poland is of great ecological significance. The influence of mining on aquifer was analyzed, and the protection technology of water-resisting key stratum was applied to avoid the mining-induced fracture channel from penetrating the overlying aquifer and realize water-preserved mining (Guzy & Malinowska 2020).

To conclude, WPCM provides an idea and way to release coal resources for water–coal symbiotic mines, but groundwater must be strictly treated as a resource. Active measures should be taken in the process of coal mining to protect water resources from waste and pollution. Although this technical specification is mainly proposed for the mining conditions of arid and semi-arid mining areas in western China, its extension is relatively wide, and it is feasible to carry out WPCM practice in mining areas where water resources need to be protected in countries around the world.

In order to protect the Cretaceous groundwater resources during the high-intensity mining in Yonglong mining area, China, this study investigated the hydrogeological structure and overburden engineering geological characteristics, analyzed the WCFZ height and mining fracture seepage laws, and proposed several WPCM technologies. The following conclusions were derived:

• (1)

  Yonglong mining area is rich in coal resources and poor in water resources, and the response of groundwater to coal mining is obvious. The main aquifer threatening the water filling and safety of No. 3 coal seam mining in the study area is the Cretaceous aquifer, which is not only an important water supply source in the region, but also the main source of water inrush, so WPCM should be implemented.

• (2)

  The development height of WCFZ in the Yonglong mining area was 17.1–23.5 times of the mining height, and the WCFZ in local area could penetrate the overlying Cretaceous aquifer. The mudstone aquiclude had strong expansion and disintegration, and the permeability coefficient was small after mining. Comprehensive geological conditions and mining-induced failure characteristics showed that the Yonglong mining area had the feasibility of carrying out WPCM.

• (3)

  Combined with the geological and mining conditions of the Yonglong mining area, the technical measures for WPCM were proposed, including limiting the mining height, adopting the slice mining method and reasonably selecting the advancing speed of the working face. These technologies effectively protected the Cretaceous groundwater resources.

This research was supported by the Henan Province Scientific and Technological Plan Project (Grant Nos 232102320343 and 202102310218) and the Shaanxi Province Industrial Science and Technology Research Project (Grant No. 2016GY-172).

Data cannot be made publicly available; readers should contact the corresponding author for details.

The authors declare there is no conflict.