Overview of the working face
The Yingpanhao Coal Mine is located in Wushen Banner, southwestern Ordos City, Inner Mongolia Autonomous Region. The mine is divided into four mining areas: 21 mining area, 22 mining area, 23 mining area, and 24 mining area. The 22 mining area is the first mining area of the mine, located in the northern part of the industrial site of the Yingpanhao Coal Mine, with a length of 2.65 km from north to south and a width of 13.6 km from east to west, covering an area of 17.24 km2. The elevation of the ground in the 22 mining area ranges from + 1244.6 m to + 1261.4 m, and the mineable coal seams in the area are the 2–2 and 3 − 1 coal seams, with bottom elevations ranging from + 490 m to + 540 m and + 450 m to + 500 m, respectively.
The 22 mining area is the initial mining area for the 2–2 coal seam, currently the primary mining area for the 2–2 coal seam. The bottom elevation of the coal seam ranges from + 490 m to + 540 m, with an average burial depth of 722.88 m. The coal seam thickness ranges from 3.16 m to 10.24 m, with an average thickness of 6.29 m. The coal seam is predominantly dark coal, followed by bright coal, with semi-anthracite as the main type. The coal seam has a Mohs hardness of f = 1.36, and the mining method employed is full-seam mining using a comprehensive mining approach. Currently, there are six working faces arranged in the 22 mining area, with the mining sequence as follows: 2201 working face → 2202 working face → 2203 working face → 2204 working face → 2205 working face → 2206 working face. Among them, the 2201 and 2202 working faces have completed mining operations, and the 2303 working face has not completed mining operations. The layout of the working faces is shown in Fig. 1. The 2201 working face serves as the initial mining face. This study mainly focuses on the 2201 and 2202 working faces, both of which have an inclination length of 300 m and a strike length of 2709 m.
Layout of working faces and surface movement monitoring stations in the 22 mining area of the Yingpanhao coal mine.
Geological conditions of strata
The stratigraphic structure of the 22 mining area in the Yingpanhao Coal Mine consists of the Quaternary System, Cretaceous System, and Jurassic System Formations from top to bottom. The 2–2 coal seam is located in the Lower Jurassic Yan’an Formation. The lithological column of the K7-7 borehole in the 22 mining area is illustrated in Fig. 2. Specifically, the Quaternary System has a thickness ranging from 58.9 to 96.6 m, with an average of 82.6 m, predominantly composed of brownish-yellow and grayish-yellow siltstone and fine sandstone. The Lower Cretaceous Zhidan Group has a thickness ranging from 286.9 to 374.6 m, with an average of 337.3 m, mainly consisting of reddish-brown and purplish-red medium sandstone and fine sandstone. The Middle Jurassic Ziliujing Formation has a thickness ranging from 132 to 195.83 m, with an average of 166 m, primarily composed of grayish-green and grayish-white medium to coarse-grained sandstone. The Lower Jurassic Yan’an Formation mainly comprises grayish-green medium-coarse-grained sandstone and conglomeratic coarse-grained sandstone, interbedded with fine-grained sandstone and sandy mudstone.
Column diagram of borehole K7-7 in the 22 mining area.
Figure 2depicts the columnar diagram of borehole K7-7 in the 2202 working face. According to the Key layer theory25,29, it is calculated that there are five key layers above the roof of the 2202 working face.
Key Layer 1: Fine sandstone, located 51 m above the coal seam with a thickness of 34 m.
Key Layer 2: Medium-grain sandstone, located 175 m above the coal seam with a thickness of 37 m.
Key Layer 3: Fine sandstone, located 241 m above the coal seam with a thickness of 30 m.
Key Layer 4: Mainly composed of fine-grain and medium-grain sandstone, located 334 m above the coal seam with a thickness of 81 m.
Key Layer 5: Mainly composed of medium-grain and fine-grain sandstone, located 544 m above the coal seam with a thickness of 84 m.
Within the Cretaceous system, two primary key layer groups are identified, namely Key Layers 4 and 5. These layers are characterized by considerable thickness, high strength, overall integrity, and significant distance from the coal seam. The physical and mechanical properties of the overburden rock in the 22 mining area, along with the identification results of the key layers, are summarized in Table 1. These data were directly measured on-site by the coal mine staff and are both accurate and reliable.
According to the theory of strata movement in mines, during the process of coal mining, as the mining area expands, the overlying strata of the goaf continuously collapse, and the bed separation fractures gradually propagate upward, ultimately terminating at the bottom of the high-positioned thick-hard strata30. With the continuous expansion of the mining area, the space above the bed separation gradually increases, and the thick-hard strata becomes increasingly exposed. When it reaches the breaking limit, the fracture of the thick-hard strata will affect the movement of the overlying strata in the mining area, and may even lead to significant surface subsidence and high-energy mine earthquake events, posing a threat to the safe and efficient production of the mine. It can be inferred that, for the 22 mining area of the Yingpanhao coal mine, the fracture movement of the Jurassic and Cretaceous sandstones will have a significant impact on the safe and efficient mining of the lower coal seams, making targeted research necessary and urgent.
