Roof Fracture Characteristics and Strata Behavior Law of Super Large Mining Working Faces

College of Mining Engineering, Taiyuan University of Technology, Taiyuan, Shanxi, China Key Laboratory of In-situ Property-Improving Mining of Ministry of Education, Taiyuan University of Technology, Taiyuan, Shanxi, China State Environmental Protection Key Laboratory of Efficient Utilization Technology of Coal Waste Resources, Institute of Resources and Environmental Engineering, Shanxi University, Taiyuan, Shanxi, China


Introduction
With the rapid growth of the world economy, the market demand for coal continues to increase year by year, and highyield and highefficiency coal mines are increasingly common [1][2][3][4][5]. After years of development, full-height fully mechanized mining at one time has become an important technology for highyield and highefficiency mining in thick coal seams [6]. In addition to increasing the mining height, increasing the length of the working face has become a new direction for high yield and high efficiency working face mining [7]. It is generally considered that a working face with a length of more than 250 m is a super large face [8]. Lengthening the working face can improve the per unit yield of the working face and simplify the production system and reduce coal loss, which all enhance mine productivity. Super large working face production technology has become a crucial development direction for intensive coal mine production technology [9].
In recent years, research on super large working faces has mainly focused on medium-thick coal seams. In 2002, the average length of longwall working faces in Australia was 227 m, the average annual distance was 2,160 m, and the average yield of working faces was 2.82 million tons [10]. In 2004, the length of some fully mechanized coal mining faces in the United States was close to 300 m, the maximum advancing length was 4,580 m, and the average yield of working faces increased from 1.52 million tons to 3.26 million tons [10]. According to incomplete statistics, there are 11 working faces over 300 m long in the United States and 18 in Germany, with the longest working face of 506 m (coal seam thickness of 1.9~2.3 m) [11]. Russia, Poland, and other major coal producing countries are also developing super large working faces. In 2007, the first super large working face of 400 m (with an average thickness of 1.7 m) was put into production in China at the Yujialiang Coal Mine in Shendong Mining District of the Shenhua Group [12]. In 2012, the longest face was the 450 m fully mechanized working face (with an average thickness of 2.11 m) at the Halagou Coal Mine in Shendong Mining District [13,14].
While lengthening working faces, China is also researching very high working faces for thick coal seams. In 2005, Shangwan Coal Mine in Shendong Mining District built the world's first fully mechanized coal mining face a lengthened working face with a large mining height of 300 m and average coal seam thickness of 5.5 m [15]. In 2008, Shangwan Coal Mine completed the design of a lengthened thick working face with a large mining height of 300 m and average coal seam thickness of 5.5 m, which is the world's first fully mechanized mining working face with yield of over 10 million tons [15]. In July 2010, in high gas conditions, the fully mechanized mining technology for large mining height was applied in the Dongsi Panel of the Sihe Coal Mine of the Jincheng Anthracite Coal Mining Group, and a 300 m super large working face was successfully mined with an average thickness of 6.2 m [16].
To sum up, working faces of 300 m length are mainly concentrated in medium-thick coal seams [17], and the thick coal seam mining is difficult; so, the application is relatively less. The increase of face length and mining height will form a larger roof structure, which will exert more load on face supports. Thus, when the length of the working face increases, the roof will present new deformation and loading characteristics, which is a very important and meaningful topic. This paper systematically compares and studies the roof fracture characteristics and strata behavior of long working faces using field investigation, theoretical analysis, and numerical simulation [18,19]. The study uses the coal seam occurrence status and mining characteristics of the 3500 Working Face, which has large mining height, in the Wangzhuang Coal Mine in China. The 3500 Working Face is a typical working face with variable sizes. The working face length has experienced three stages: 242.4 m, 195 m, and 376 m, and the average thickness of coal seams is 5.02 m. It is a fully mechanized full-seam large coal mining working face. This working face provides a practical basis for the comparative analysis of the mine pressure behavior law of working faces with different lengths, and therefore, it has vital theoretical and practical value.

Case Study Overview
The 3500 Working Face of the Wangzhuang Coal Mine is located in the south of Dongzhang Village in Changzhi, Shanxi Province, China. The mined coal seam is the No.  Table 1 summarizes the distribution and the physical and mechanical characteristics of the coal seam and roof strata of the 3500 Working Face.

Analyses of Roof Fracture Characteristics of Super Large Working Faces
With the advance from open-off cuts of the working face, the suspended roof length of the goaf continues to increase. Due to the action of mine pressure, every rock stratum on the roof will bend, subside, and become distorted, and when the rock stratum reaches its ultimate strength, it will fracture and cave in [20][21][22][23][24]. Based on the level, thickness, strength, and load of rock strata, the influence of different working face lengths on the roof caving step length is compared and analyzed.
3.1. Calculation of Roof Strata Load. The initial caving step length is the main sign to measure the stability of strata behaviors of the working face. Before calculating the caving step length, it is necessary to calculate the load of each rock stratum. Due to the different thickness and lithology of each stratum, the load on the i stratum is calculated according to the following formula: where ðq n Þ i is the load on the i stratum when the n stratum above the i stratum is considered: E i ⋯ E i+n is the elastic modulus of each stratum.

Geofluids
h i ⋯ h i+n is the thickness of each stratum. γ i ⋯ γ i+n is the density of each stratum; i = 1, 2, 3 ⋯ When ðq i+n+1 Þ i < ðq i+n Þ i , that is, ðq i+n Þ i as the load applied to the i stratum, the result of equation (1) is used as the load on the i stratum to calculate its caving step length.
The data in Table 1 is put into equation (1), and the load on each stratum is calculated. The calculation results are shown in Table 2.

Calculation of Caving
Step Length of Each Roof Stratum. For super large working face mining, it is difficult to fully  3 Geofluids define the problem by solving a single plane problem. Therefore, the roof of the coal seam is regarded as a plate structure, and the initial caving step length and the periodic caving step length of each roof stratum are calculated according to plate theories [25], as shown in Figure 2.
The maximum tensile stress intensity theory is used as the fracture criterion of slope roof fracture: where σ 1 is the maximum tensile stress in the roof, MPa; σ b is the tensile strength of roof strata, MPa. The maximum tensile stress of the roof σ 1 is calculated by the following formula: where M max is the maximum bending moment acting on the roof strata, MPa·m; I is the area moment of inertia of roof strata, if the section thickness is h, I = h 3 /12, the unit is 1; then, where q is the uniform load of the roof, MPa; μ is Poisson's ratio of the rock stratum.
For the island mining face of the three-sided goaf, if the cut length is 242.4 m, all three sides of the mining face are goafs. According to the theoretical analysis of the plate, the initial caving step length is where b is the length of the working face, m.
For the working face with two adjacent sides mined out and two adjacent sides fixed, if the cut length is 195 m and 376 m, the two adjacent sides of the mining working face are goafs, and the others are unexploited solid coal. According to the theoretical analysis of the plate, the initial caving step length is where b is the length of the working face, m. When the working face length of the No. 3 coal seam is 242.4 m, equation (5) is introduced; when the length of No. 3    (6) is introduced, and the initial caving step length of each roof stratum is calculated according to plate theories, as shown in Table 2. The periodic caving step length of the old roof is often determined by the cantilever fracture of the old roof. According to the calculation of mechanics of materials, the initial caving step length is 2.45 times the periodic caving step length. The calculation results of the periodic caving step length are shown in Table 2.

Analysis of Fracture Characteristics of Roof Strata.
According to the calculation results in Table 2, the fracture characteristics of roof strata are as follows: (1) The roof caving mode of the goaf of the 3500 Working Face is successive layered caving. When the working face begins to advance from the cut, the first layer of carbonaceous mudstone caves in first, with a caving height of 0.3 m. When the working face continues to advance to 18.4 m, the second mudstone layer caves in, with the roof caving height of 1.7 m.
When it advances to 41.9 m, the roof is covered with fine sandstone, and the mudstone on it is fractured. The fractured height of the roof is 7.3 m. After that, the working face advances for three cycles; that is, when the working face advances to 45 m, the fifth layer of fine sandstone is fractured, and the overlying mudstone, fine sandstone, and argillaceous sand-   Figure 3-9.  6 Geofluids the roof extends to the basic roof. At this time, the subsidence curve of the 195 m working face presents an asymmetric Gaussian distribution, in which the central axis deviates slightly to one end of the working face, with roof subsidence of 400~600 mm. The roof subsidence of the 376 m working face is different. The roof of the 376 m working face shows integral subsidence. The initial fracture of the main roof occurs, and the masonry structure [28,29] is formed, with roof subsidence of about 500~700 mm. Figure 5 shows that when the working face advances to 32.5 m, the moving range of the roof of the 195 m working face continues to gradually expand, and the basic initial fracture of the main roof occurs, and the masonry structure is formed. The subsidence amount of the roof of the 376 m working face increases in local areas along the lengthwise direction of the working face. Figure 6 shows that when the working face advances to 35 m, both working faces show obvious subsidence. The difference is that the 195 m working face follows the subsidence rule of the Gaussian distribution curve, while the roof of the 376 m working face shows intermittent subsidence. Because the length of the 376 m working face is about twice that of the 195 m working face, the figure shows intermittent subsidence in two to three intervals. Therefore, it can be inferred that the roof subsidence of a super large working face can be regarded as the combined subsidence of several common working faces. Figure 7 shows that when the working face advances to 37.5 m, the damage range of the roof of both working faces continues to expand, and the subsidence amount of the roof continues to increase. Figure 8 shows that the damage range of the roof of both working faces continues to extend to the upper part of the roof. The difference is that the deformation of each part in the lengthwise direction of the 195 m working face is quite different, and the 376 m working face is again characterized by uniform subsidence. Figure 9 shows that when both working faces advance to 45 m, the displacement and deformation of the roof of the 376 m working face is larger than that of the 195 m working face, and the roof is pressed periodically.
Through the above analysis, it can be concluded that the roof displacement of the 195 m working face follows the subsidence rule of the Gaussian distribution curve, while the roof of the 376 m working face shows intermittent subsidence. Moreover, it can be inferred that the roof subsidence of the super large working face is the combined subsidence of several common working faces.

Comparison and Analysis of Roof Periodic Weighting
Law. Comparison and analysis of the periodic weighting intensity law of both working faces are shown in Figures 10 and 11. Figure 10 shows that the periodic weighting of the 195 m working face is generally stable, which is high and even in the middle and low at both ends. Within 20 m of both ends, weighting gradually increases from 675 kPa to 800 kPa. Within 20-175 m of the middle of the working face, 9 Geofluids weighting is uniformly distributed, remaining basically stable at 800 kPa. Figure 11 shows that the periodic weighting of the 376 m working face is generally high in the middle of the working face while low at both ends, fluctuating in a small range in the middle but oscillating violently at both ends. Within 100-275 m in the middle of the working face, there is arc fluctuation which is high in the middle and low at both ends, and the average weighting is maintained at 877.80 kPa. The maximum weighting is 985.67 kPa, and the minimum is 676.83 kPa within 100 m from both ends. Weighting oscillation is obvious.
Comparing Figures 10 and 11 shows that the periodic weighting of the 376 m working face is more violent than that of the 195 m working face. The 376 m working face oscillates sharply within 100 m at both ends, while the 195 m working face gradually decreases from the middle of the working face to both ends. The average periodic weighting of the 376 m working face is about 80 kPa higher than that of the 195 m working face. Through the above analysis, it can be concluded that the working resistance of the support needed in a super large working face is greater. Thus, attention should be paid to the intensity of support within 100 m of the end of the face.

Measurements and Analysis of Working
Resistance of Working Face Support The 3500 Working Face in the Wangzhuang Coal Mine is monitored and counted in real time, and the supporting strength of hydraulic support with different working face lengths is compared and analyzed. Through theoretical and numerical calculation and analysis, ZY10000/26/55D Twocolumn Shield Hydraulic Support is selected as the hydraulic support for the 3500 Working Face. Table 3   10 Geofluids working faces is within the rated working resistance range. In the 376 m working face, field measurement exceeded the rated working resistance in 2.76% of statistical cycles, among which the maximum working resistance is 10,049 kN once and 10,098 kN twice. ZY10000/26/55D Hydraulic Support can meet the support requirements of super large working faces.

Data Availability
The data used to support the findings of the study are available from the corresponding author upon request.