This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
1. Introduction
Owing to the increasing contradiction between the daily reduction of coal resources and energy demand, the roof cutting and pressure relief technology in gob-side entry retaining can effectively reduce the waste of coal resources [1–4]. However, the roof deformation of the retained roadway is serious, and the roadway support difficulty has become a new problem. Therefore, many local and foreign scholars have conducted a considerable amount of research. Reference [5] established the structural model of the retained roadway roof and determined the design of the retained roadway roof in the excavation process. This design was successfully applied in the Halagou coal mine. Considering the numerical simulation method and laboratory experiment in [6], directional blasting can reduce roof damage of the retained roadway in the process of roof cutting and pressure relief. References [7, 8] discussed the stress distribution range of the retained roadway roof by establishing the mechanical structure model and effectively controlled the deformation of the retained roadway roof by using the constant resistance anchor cable. Reference [9] analyzed the stress distribution characteristics of retained roadway roof in medium-thick coal seam by the mechanical model and found that the goaf gangue has a good supporting effect on the retained roadway roof with a decrease in cantilever beam length. Reference [10] analyzed the application of the new telescopic energy absorption support system on the surrounding rock of the retained roadway by the numerical simulation method and found that the new support system is conducive to maintain the stability of the retained roadway roof compared with the traditional support method. In [11], the theoretical and analytical formulas of roof deformation of the retained roadway are obtained by energy theory and verified by field measurement. Reference [12] theoretically analyzed the broken position of the retained roadway roof and concluded through the assessment of UDEC numerical simulation software that the fracture of the retained roadway roof on one side of the goaf is conducive to the roadway stability. In [13], the influence of working face pressure in the roof cutting and pressure relief technology and the conventional mining method was examined by field monitoring and numerical simulation. This reference concluded that the roof cutting and pressure relief technology can produce a low-stress area in the roadway. Reference [14] obtained the deformation characteristics of gob-side entry retaining roadway under different stages of roof cutting and pressure relief through similar physical simulation experiments. The results show that the surrounding rock behind the working face reaches a stable state after the working face is excavated for 20 m, and the surrounding rock pressure of roadway is effectively relieved through roof presplitting and seam cutting. The paper introduces the partition control technology of the roadway retaining based on the analysis of roof rock movement characteristics of gob-side entry retaining in [15]. Through the analysis of the support modes in different areas in the gob-side entry retaining, the mining mode requires that the mining roadway should be driven in advance and the protective coal pillar should be reserved in the conventional mining process. Reference [16] adopted the combined support technology of constant resistance anchor cable and single hydraulic prop to solve the serious roadway deformation problem in the roof cutting and pressure relief technology of 2422 working face in Baijiao coal mine effectively.
The research results of many scholars revealed that the roadway roof with gob-side entry retaining can effectively solve the roadway deformation problem through the roof cutting and pressure relief technology despite the broken roof of the retained roadway or its short cantilever beam structure, which is closely related to the roof cutting angle and height. Therefore, the determination of the key parameters of roof cutting seam parameters is the key to the gob-side entry retaining technology. Reference [17] studied the key parameters of roadway roof blasting in the process of roof cutting and pressure relief by mathematical analysis and optimized the process of roof blasting design for retaining roadway. The analysis of the stress dynamic evolution law of the retained roadway roof in Reference [18] showed that the fracture position of the retained roadway roof is related to the key parameters of roof cutting. Reference [19] obtained the angle and height of roof cutting through physical experiments and successfully cut the goaf roof. The research methods of theoretical analysis, numerical simulation, and field measurement are used in Reference [20, 21]. The reasonable range of roof cutting height and angle in the roof cutting and pressure relief technology is determined and successfully applied in many coal mines, such as Chengjiao coal mine.
The above research results revealed that the roof cutting and pressure relief technology can cut off the connection between the roof of the retained roadway and that on the goaf side. However, most of the previous studies focused on the influence of roof cutting seam parameters on the inner area of the solid coal side of the retained roadway, while the research on the influence of the roof cutting seam parameters on the roof support of the retained roadway is limited. Based on previous studies, this paper analyzes the fracture principle of roof strata in gob-side entry retaining and pressure relief roadway roof. The influence of different roof cutting angles and heights on the roof support resistance of retaining roadway is also studied. The stress distribution characteristics of surrounding rock under different roof cutting seam parameters are analyzed by FLAC3D numerical simulation software. The reasonable values of roof cutting height and angle of II 632 haulage drift in the Hengyuan coal mine are determined. Finally, the corresponding support scheme is proposed in accordance with the actual situation of II 632 haulage drift in the Hengyuan coal mine. The research results provide a certain reference value for the design of roof support parameters of gob-side entry retaining under roof cutting and pressure relief.
2. Roof Breaking Mechanism of Gob-Side Entry Retaining with Roof Cutting and Pressure Relief
The research results of many scholars on the structural characteristics of roof strata of mining roadway [22, 23] show the rapid collapse of the strata in the goaf behind the support with the advancement of the mining face. The overhanging area above the retained roadway becomes increasingly large, thus allowing the goaf to form an O-X fracture structure. With the continuous advancement of the coal mining face, the location with a certain area of overhanging behind the support will break again under the action of gravity, thus resulting in periodic weighting behaviors of goaf.
In the gob-side entry retaining technology of roof cutting and pressure relief, one side of the roadway is solid coal, and the other side is the rock block caving area in the goaf. The key block B structure will be formed in the rock section of the retained roadway roof. The immediate roof is the most important factor affecting the stability of retained roadways. This paper mainly studies the fracture form of the key structure of the roadway roof and the influence of the roadway support resistance on the stability of the retained roadway. The key block B is simplified as a triangular structure model to facilitate analysis. Figure 1 shows the space top view, where α is the angle between the space triangle and the collapse distance, °; l is the interval of periodic weighting, m; Ld is the horizontal distance of key block B, m.
[figure omitted; refer to PDF]
Table 1
Rock mechanical parameters.
| Lithology | Bulk density (kg/m3) | Bulk modulus (GPa) | Shear modulus (GPa) | Internal friction angle (°) | Cohesion (MPa) | Tensile strength (MPa) |
| Sandy mudstone | 2652 | 2.54 | 1.38 | 30 | 1.3 | 0.43 |
| Siltstone | 2610 | 10.9 | 8.58 | 30 | 17.2 | 5.75 |
| Mudstone | 2570 | 7.87 | 6.40 | 38 | 2.07 | 1.68 |
| Fine sandstone | 2630 | 6.68 | 1.40 | 28 | 1.66 | 1.70 |
| Mudstone | 2570 | 7.87 | 6.4 | 38 | 2.07 | 1.68 |
| Coal | 1360 | 1.9 | 0.93 | 26 | 0.84 | 0.28 |
| Mudstone | 2570 | 7.87 | 6.4 | 38 | 2.07 | 1.68 |
| Siltstone | 2610 | 10.9 | 8.58 | 30 | 17.2 | 5.75 |
| Mudstone | 2570 | 7.87 | 6.4 | 38 | 2.07 | 1.68 |
4.4. Analysis of Numerical Simulation Results of Roof Cutting Height
Figure 5 shows the influence of simulated roof cutting height of 6, 8, and 10 m on the surrounding rock stress of cut retained roadway roof. A comprehensive comparison can also be obtained. When the roof cutting height is 6 m, the stress concentration in the solid coal side of the retained roadway is 24.98 MPa, the peak value range of the stress concentration in the solid coal side of the retained roadway is large, and a certain pressure relief area exists around the retained roadway roof. When the roof cutting height is 8 m, the peak value of the stress concentration at the solid coal side of the roadway is 23.89 MPa, which is far away from the retained roadway surface, and the pressure relief range around the roadway roof is further increased. When the roof cutting height is 10 m, the peak value of the stress concentration at the solid coal side of the retained roadway is 23.95 MPa. No significant difference is observed between the stress concentration range of the retained roadway solid coal side and the roof cutting height of 6 m, and the pressure relief range of the retained roadway roof is slightly larger than that of the roof cutting height of 8 m. These findings are obtained from the above analysis. When the roof cutting height is 8 m, the peak stress concentration is smaller than that when the roof cutting height is 6 and 10 m. When the stress concentration range is less than the roof cutting height of 6 m, the pressure relief area of the retained roadway roof increases with the roof cutting height. Therefore, the pressure relief thickness of the retained roadway roof increases with the roof cutting height, and the required support resistance also rises.
[figures omitted; refer to PDF]
Figure 6 shows the following results. The roof subsidence of the retained roadway is 1000 mm when the roof cutting height is 6 m; thus, meeting the use requirements of the retained roadway is difficult. The subsidence of the retained roadway roof is 400 mm when the cutting height is 8 m, and the roof subsidence of the retained roadway is small. The roof subsidence of the retained roadway is 1000 mm when the roof cutting height is 10 m.
[figures omitted; refer to PDF]
The comprehensive comparison shows the following: when the roof cutting height is 8 m, the roof strata in goaf can be completely cut off, which weakens the influence of stress transmission on the roadway roof. Moreover, the stress in solid coal is small, and the stress concentration value is far away from the roadway. The roof stress is in a controllable range, which is conducive to the stability of the roadway roof plate. When the roof cutting height is 6 m, the influence of the basic roof on the retained roadway roof cannot be cut off. The rotation of the basic roof has a considerable influence on the roof. The subsidence of the retained roadway roof is 1000 mm, and the peak value of the stress concentration coefficient of the solid coal side is large. When the roof cutting height is 8 m, the influence of the basic roof on the roadway roof can be completely cut off, the influence of the stress transfer on the retained roadway roof is weakened, and the influence of the basic roof rotation on the retained roadway roof is reduced. Moreover, the peak stress concentration range is reduced, and the support resistance of the retained roadway roof is less than 10 m of the roof cutting height. When the roof cutting height is 10 m, the seam line can cut off the connection between the immediate and retained roadway roofs. However, the pressure relief area of the retained roadway roof is large, the support resistance required by the retained roadway roof increases, and the peak value of the stress concentration at the solid coal side is large. This condition is not conducive to the integrity of the surrounding rock of the retained roadway. Considering the workload of drilling depth and economic benefits, maintaining the roof cutting height of 8 m is reasonable.
4.5. Analysis of Numerical Simulation Results of Roof Cutting Angle
Three kinds of roof cutting models with roof cutting angles of 10°, 15°, and 20° are constructed on the basis of the above analysis, and the stress and displacement changes of gob-side entry retaining under different roof cutting angles are analyzed. The numerical simulation results are shown in Figures 7 and 8.
[figures omitted; refer to PDF]
[figures omitted; refer to PDF]
Figure 7 shows the following results. When the roof cutting angle is 10°, the peak value of the concentrated stress at the solid coal side of the roadway is 21.26 MPa, the stress concentration area of the solid coal side of the roadway is relatively small, and the pressure relief area appears on the retained roadway roof. When the roof cutting angle is 15°, the peak value of the concentrated stress at the solid coal side of the retained roadway is 19.54 MPa, and the range of the pressure relief area of the retained roadway roof is relatively increased. When the roof cutting angle is 20°, the peak value of stress concentration at the solid coal side of the roadway is 21.54 MPa, and the pressure relief area of the retained roadway roof is further increased. Based on the above analysis, the following results are presented. When the roof cutting angle is 10°, the sliding down of the basic roof rock block along the cutting seam surface is difficult, and the influence of the goaf roof on the retained roadway roof cannot be cut off. When the roof cutting angle is 15°, the basic roof can slide down along the cutting surface, cut off the connection between the goaf and retained roadway roofs, and increase the pressure relief range of the retained roadway roof. When the roof cutting angle is 20°, although the connection between the goaf and retained roadway roofs is cut off, the increase in the roof cutting angle raises the cantilever beam length of the retained roadway roof and the stress concentration value of the retained roadway solid coal side.
As shown in Figure 8, the subsidence value of the retained roadway roof is 400 mm when the roof cutting angle is 10°. Therefore, the connection between the goaf and retained roadway roofs cannot be cut off. When the roof cutting angle is 15°, the subsidence value of the retained roadway roof is 100 mm, which can cut off the connection between the goaf and retained roadway roofs and reduce the bending subsidence of the retained roadway roof. When the roof cutting angle is 20°, the cantilever beam length is increased with the roof cutting angle; therefore, the subsidence value of the roof is 400 mm.
Overall, the goaf roof cannot slide along the cutting seam line when the roof cutting angle is 10° despite the small pressure relief range of the retained roadway roof. Therefore, cutting off the connection between the goaf and retained roadway roofs is difficult. The pressure relief range of the retained roadway roof is relatively large when the roof cutting angle is 15°, which can cut off the stress transmission path of the roof plate. The pressure relief range of the retained roadway roof is large when the roof cutting angle is 20°, and the support required by the retaining roof increases. The increase in the roof cutting angle raises the length of the cantilever beam and the subsidence of the retained roadway roof. Considering the damaging effect of blast hole explosion on the retained roadway roof, the support resistance required for the retained roadway roof should be small and the roof cutting angle should be 15° to reduce the overall damage of blasting to the retained roadway roof and simultaneously facilitate sliding down of the goaf slide roof along the cutting seam line.
5. Industrial Practice
5.1. Project Overview
The length distance along the coal seam strike of II 632 working face in the Hengyuan coal mine is 1725.3 m, and the distance between haulage drift and ground is −637 to −778 m. The retaining roadway technology along the goaf to roof cutting and pressure releasing is adopted in the haulage drift. Therefore, the Yunshun roadway can be used as the return air roadway of the next mining face. The direct roof of the coal mining face is dark gray mudstone with a thickness of 2.25–4 m, dark gray to gray-black, slightly silty, and has an average of 3.13 m. Two coal lines with a thickness of approximately 0.1 m are often developed in the middle of the coal seam. The composite roof is formed when the lower coal line is close to the No. 6 coal seam, and the roof easily falls off. The immediate roof is gray to light gray fine sandstone with a thickness of 2.5–4.8 m. This roof is gray with developed fractures, with siltstone and mudstone local inclusions, and calcite filling.
The cross-sectional shape of II 632 haulage drift is distance shaped, as shown in Figure 9. The length × height is 5.2 m × 3 m, the roof is mainly mudstone and fine sandstone, and the floor is mainly mudstone, which belongs to the combination of the soft top and hard bottom. According to the principle of roof and floor coordinated support, the bolt mesh-cable combined support method is adopted for retaining roadway roof to ensure that the retained roadway can be used safely in the primary and secondary mining processes.
[figure omitted; refer to PDF]
The borehole peeping equipment is used to test the crack development of the roadway roof at the middle roof of roadway retaining to meet the requirements of the next mining face in the process of the roadway retaining, as shown in Figure 11.
[figure omitted; refer to PDF]
Figure 12 shows that the distance from the hole on the roof surface is 2.2 m during the observation process. The roof rock is relatively broken, and the fracture extends longitudinally. The length of the hole from the roof surface of the reserved roadway is 4.1 m, and the aperture is relatively complete. Only a small part of the cracks is developed, and the integrity of the roadway is good. The anchor cable at the roof effectively controls the roof deformation.
[figure omitted; refer to PDF]
Five observation stations are arranged in the haulage drift according to the deformation of the 500 m long gob-side entry retaining roadway of the II 632 haulage drift to observe the roadway deformation continuously through the summary and processing of the monitoring data, as shown in Figure 13.
[figure omitted; refer to PDF]
The deformation monitoring data indicate that the surrounding rock deformation of roadway within 40 m ahead of the working face is less than 200 m; when the deformation exceeds 40 m of the working face, the deformation rate of the roadway surrounding rock approaches 0 mm. The deformation increment of roadway within 0–40 m of the lagging working face is large after retaining the roadway. The deformation amount of surrounding rock of the retained roadway does not exceed 100 mm after 40 m of lagging working face. Therefore, the roadway surrounding rock is effectively controlled by the roadway retaining support mode, which provides a safe basis for the next working face.
6. Conclusion
(1) According to the deformation and movement characteristics of roof structures of gob-side entry retaining, the mechanical key block model of retaining roadway roof is established, and the calculation formula of support resistance under stable roadway roof state is derived. The sensitivity analysis of the related parameters affecting the support resistance is also conducted. The results show that with the increase in roof cutting angle and height, the value of support resistance needed to maintain the stability of retained roadway roofs should also increase correspondingly. The conclusion provides a theoretical basis for the design of roof support parameters of automatic roadway formation by roof cutting and pressure relief.
(2) After theoretical analysis, the numerical analysis models of different roof cutting schemes are established to obtain reasonable roof cutting parameters, and the evolution law of surrounding rock stress and deformation under different roof cutting angles and heights are analyzed. The comprehensive analysis of theoretical calculation and numerical simulation results show that when the roof cutting angle is 15° and the roof cutting height is 8 m, the goaf side roof can be cut off smoothly by the cutting line. The rotation of key blocks has minimal influence on the retained roadway roof.
(3) Taking the II 632 working face of the Hengyuan coal mine as the engineering background, the support resistance required to maintain the roadway roof stability is calculated on the basis of theoretical analysis and numerical simulation results. Considering the factors such as roof deformation mechanism and supporting material characteristics of cut and pressure relief roadway along gob-side entry retaining, this paper proposed roof support countermeasures for the II 632 haulage drift, adopted the technology of constant resistance and common anchor cables and bolt-combined support, and conducted industrial tests.
(4) Based on the field test data of II 632 haulage roadway in the Hengyuan coal mine, the data show that the roadway retained deformation of the surrounding rock is less than 100 mm after 40 m lagging behind the mining face, and the roadway forming effect is good. Through the borehole peeping, the integrity of the retained roadway roof is good. Moreover, the support scheme can effectively control the subsidence of the retained roadway roof and ensure the safety of the next working face.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (nos. 51574005, 51774010, and 51874002) which are gratefully acknowledged.
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Abstract
The mechanical model of the basic roof fracture structure is established on the basis of key block theory to study the roof breaking mechanism of gob-side entry retaining under roof cutting and pressure relief, and the analytical formula of roof support resistance is derived when the key block of the basic roof is stable. The influence of roof cutting angle and cutting height on roof support resistance is also analyzed. Determining the cutting seam parameters of the retained roadway roof is necessary to identify the support resistance of the roadway roof due to the correlation between the roof cutting parameters and the support resistance. Taking the II 632 haulage drift of the Hengyuan coal mine as the engineering background, FLAC3D numerical simulation is used in this paper to analyze the influence of different roof cutting angles and cutting heights on the surrounding rock structure evolution of retained roadways. Results show that the roof cutting angle and cutting height respond to the support resistance of the retained roadway roof, and the support resistance required by the roof increases with the roof cutting angle and cutting height. This condition ensures that the side roof of the gob can be cut off smoothly, and the support resistance required by the roof of retained roadways is within a reasonable range. Through theoretical and numerical simulation analysis, the reasonable roof cutting height of II 632 haulage drift is 8 m and the roof cutting angle is 15°. The theoretical analysis and numerical simulation results reveal that the required support resistance to maintain the stability of the roadway roof is 0.38 MPa. The supporting scheme of the roof of the II 632 haulage drift in the Hengyuan coal mine is then designed. Finally, the field industrial test is used for verification. The borehole imaging results show that the overall line of the retained roadway roof is small based on the description of field monitoring results. The deformation of the surrounding rock surface of the retained roadway is less than 100 mm, and the roadway is 40 m from the lagging working face. The deformation rate of surrounding rock decreases with the increase in distance from the working face. The integrity of the retained roadway roof is good, and the deformation of the surrounding rock is effectively controlled.
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Details
; Xin-Zhu, Hua 1 ; Zhang, Yan 2 ; Yin, Jiadi 1 ; He, Kai 1 ; Zhao, Chengxing 1
; Li, Yingfu 1 1 State Key Laboratory of Deep Coal Mine Mining Response and Disaster Prevention and Control, Anhui University of Science and Technology, Huainan 232001, China; Anhui University of Science and Technology, Key Laboratory of Coal Mine Safety and Efficient Mining, Ministry of Education, Huainan 232001, China
2 State Key Laboratory of Deep Geotechnical Mechanics and Underground Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China