The Stability Factors’ Sensitivity Analysis of Key Rock B and Its Engineering Application of Gob-Side Entry Driving in Fully-Mechanized Caving Faces

Abstract

To reveal the critical factors of the main roof influencing stability of surrounding rocks of roadways driven along goaf in fully-mechanized top-coal caving faces, this paper builds a structural mechanics model for the surrounding rocks based on geological conditions of the 8105 fully-mechanized caving face of Yanjiahe Coal Mine, and the stress and equilibrium conditions of the key rock block B are analyzed, and focus is on analyzing rules of the key rock block B influencing stability of roadways driven along goaf. Then, the orthogonal experiment and the range method are used to confirm the sensitivity influencing factors in numerical simulation, which are the basic main roof height and the fracture location, the length of the key rock block B, and the main roof hardness in turn. It is revealed that the basic main roof height and its fracture location have a greater influence on stability of god-side entry driving. On the one hand, the coal wall and the roof of roadways driven along goaf are damaged, and the deformation of surrounding rocks of roadways and the vertical stress of narrow coal pillars tend to stabilize along with the increase of the basic main roof height. On the other hand, when the gob-side entry is located below the fracture line of the main roof, the damage caused by gob-side entry is the most serious. Therefore, on-site gob-side entry driving should avoid being below the fracture line of the main roof. At last, industrial tests are successfully conducted in the fully-mechanized top-coal caving faces, 8105 and 8215, of Yanjiahe Coal Mine.

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

Adoption of the narrow coal pillar gob-side entry driving technique can efficiently increase recycling efficiency of coal resources and control effect of surrounding rocks of roadways. The technique has been widely used in China, but stability of surrounding rocks of roadways driven along goaf has limited further development of the technique [1–17]. Research has shown that there are multiple factors that influence the stability. References [1–17] are the Chinese scholars’ findings. Among them, Bai et al. [1, 2] built an arc-shaped triangular block structural mechanism model for surrounding rocks based on stress conditions of the working face; Li [3, 4] analyzed influence of big and small structures of roadways driven along goaf in fully-mechanized top-coal caving face on stability of surrounding rocks; Zhang et al. [5, 6] analyzed deformation features of surrounding rocks of gob-side entry retaining in the fully-mechanized coal face with top-coal caving and influence of multiple factors on deformation and stress of gob-side entry retaining with entry-in packing in the top-coal mining face; Wang et al. [7, 8] discussed influence of three different fracture locations of the overlaying main roof of roadways driven along goaf on stability of surrounding rocks of the roadway and rational width of the narrow coal pillar based on the fracture line location of key rock B in the main roof; Zhu et al. [9] studied the main factors affecting the deformation of the filling body; Xie [10] adopted UDEC simulation to analyze rules of six factors, such as support technique and influencing stability of surrounding rocks of gob-side entry retaining in top-coal mining faces; Zhang et al. [11, 12] analyzed four different Chinese coal mining sites and evaluated the influencing factors; Meng and Li [13] studied parameters’ sensitivity of bolt support in gob-side entry driving and found that the main influence parameters were bolt pretightening force and bolt interval; Yuan et al. [14] studied the dynamic effect and control mechanism of key strata in the immediate roof; Bai et al. [15] studied the stress state and the deformation failure mechanism of the heading adjacent to the advancing working face roof structure; Li et al. [16] studied the in situ stress distribution characteristics of the rock mass near different slope angles’ hillslope surfaces; Wang et al. [17] studied the stability of the concrete artificial side introduced to stabilize a gob-side entry.

The above analysis suggested that stability of the key rock block B could directly influence stability of the large structure of roadways driven along goaf and indirectly influence the stress environment of the small structure. The former mainly adopted theoretical analysis and quantitative experiments to analyze stability of the large structure of roadways driven along goaf, while the latter mainly analyzed influence of factors, such as support parameters, on stability of surrounding rocks of roadways based on numerical simulation. Few of them analyzed sensitivity of major factors of the main roof on stability of surrounding rocks of roadways driven along goaf in fully-mechanized top-coal caving faces. In view of the research gap, this paper built a structural mechanics structure for surrounding rocks of the key rock block B to quantitatively analyze influence of main roof factors on stability of surrounding rocks. Based on mining technical conditions of Yanjia Coal Mine in Binchang Mining Area, four factors of the main roof are confirmed. Combining the orthogonal experimental scheme, the author analyzes sensitivity of the four main roof factors and conducts a univariate analysis of the two factors with a stronger sensitivity. Research findings are successfully applied to the 8105 working face and further promoted in the 8215 working face. Therefore, research results of this paper can contribute to popularization of gob-side entry driving technique under similar geological conditions.

2. Mechanical Analysis of the Key Rock Block B in the Main Roof

2.1. Building of the Key Rock Block B Mechanical Model

Currently, the narrow coal pillar gob-side entry driving technique is prevailing in China. After the overlaying rock of the goaf in the working face of the former sector stabilizes, the roadway driven along goaf starts. Figure 1 shows the structure of the fracture line of the main roof surrounding rock structure within the coal wall during gob-side entry in a fully-mechanized top-coal caving face.

[figure omitted; refer to PDF]

Below is the stress analysis of the key rock block B [18]: the resultant force of shear force and horizontal thrust of the key rock block A on the key rock block B is $R_{AB}$ and $T_{AB}$, respectively; the resultant force of vertical shear force and horizontal thrust of the key rock block C on the key rock block B is $R_{BC}$ and $T_{BC}$, respectively; the dead load of the soft stratum above the block B is $F_R$; the dead load resultant force of the block B is $F_Z$; the support force of the goaf waste rock for the block B is $F_G$; the support force of the damaged immediate roof in the section without top-coal caving for the block B is $F_D$; the support force of the immediate roof in the narrow coal pillar for the block B is $F_S$; the support force of the immediate roof of the physical coal wall for the block B is $F_M$; the rotation angle of the block B is $\theta$. The stress situations of block B are shown in Figure 2. To put the surrounding rock structure of the block B in an equilibrium state, the block B should achieve equilibrium of stress horizontally and vertically after its stability. Besides, there is no allowance for slipping and rotation.

(1) Vertical:$$\begin{array}{c}\text{(1)}& {\displaystyle \sum F_y}=0,R_{AB}+F_M+F_D+F_G-2R_{BC}-F_R-F_Z=0.\end{array}$$

(2) Horizontal:$$\begin{array}{c}\text{(2)}& {\displaystyle \sum F_x}=0,T_{AB}-2T_{BC}\cos \alpha =0,\end{array}$$where $\alpha$ stands for the base angle of the key rock block B (°).

(3) Conditions for the key rock blocks B and A to lose stability are as follows [19]:

$$\begin{array}{c}\text{(3)}& T_{AB}\tan \phi \ge R_{AB},\frac{T_{AB}}{L_{1}a}\le \eta {\sigma }_c,\end{array}$$where $\text{tan}\phi$ stands for the friction factor between the two rock blocks, $T_{AB}/L_{1}a$ stands for the average crushing stress between the two rock blocks, MPa, $\eta$ stands for the special coefficient of the stress analysis between the two rock blocks, and ${\sigma }_C$ stands for the compressive strength of the rock block B, MPa.

[figure omitted; refer to PDF]

From Figure 3, it can be seen that the coal pillar stress before driving is larger than that after driving. Before the roadway driving, the overlaying strata of neighboring caving faces is basically stable, the key rock block B is in an equilibrium state, and the stress on the coal wall is concentrated. In Schemes 1–3, Schemes 4–6, and Schemes 7–9, the vertical stress scope in the central position of the narrow coal pillar is 7.41∼9.43 MPa. This suggests that the influence of the main roof height and the fracture location on the coal pillar stress before the roadway driving is not significant. After the roadway driving, the stress of surrounding rocks of roadways needs to be redistributed, and the stress peak value transfers to the inside of the coal. At the moment, the coal pillar is in the stress declining area. In Schemes 1–3, Schemes 4–6, and Schemes 7–9, the average vertical stress of the coal pillar is within the range of 4.36∼5.03 MPa, 1.56∼2.67 MPa, and 2.07∼2.38 MPa, respectively. This suggests that, when the main roof height is 0 m, the narrow coal pillar has a greater support stress, $F_S$, for the key rock block B. Along with the increase of the main roof height, the support stress of the main roof in the section without coal and the goaf waste rocks for the key rock block B is $F_D$ and $F_G$, respectively, thus reducing the compression of the key rock block B on the immediate roof above the coal pillar.

From Figure 3, it can be seen that the relative deformation on the two ribs of the roadway in Schemes 1–9 is 151.50%, 141.45%, 136.04%, 163.12%, 143.72%, 131.24%, 168.07%, 76.96%, and 146.21% of the roof-to-floor relative deformation. Thus, it can be judged that deformation on two ribs dominates in roadways driven along goaf in fully-mechanized top-coal caving faces. The relative deformation on two ribs in Schemes 1–3, Schemes 4–6, and Schemes 7–9 is 274.95∼322.25 mm, 315.01∼368.86 mm, and 295.57 mm∼390.31 mm, respectively; the deformation is small in Schemes 1–3. The deformation in Schemes 4–6 and Schemes 7–9 is almost the same. After roadways driven along goaf, the surrounding rocks of roadways obtain timely support. The surrounding rocks’ stress is quickly redistributed. Relying on bolt and cable support, surrounding rocks of roadways form a small structure. With the rotary subsidence of the key rock block B, the support stress provided by the immediate roof and the physical coal wall right above the narrow coal pillar, $F_S$ and $F_M$, increases and the stress of the narrow coal pillar and the physical coal wall concentrates, thus resulting in expansion and deformation of surrounding rocks.

4.2.2. Influence of the Main Roof Fracture Location on Stability of Surrounding Rocks of Roadways

The deformation of surrounding rocks of roadways in Schemes 1∼3, Schemes 4∼6, and Schemes 7∼9 is shown in Figures 4(a)–4(c), respectively.

[figures omitted; refer to PDF]

From Figure 4, it can be seen that deformation of surrounding rocks of roadways driven along goaf mainly happens in the physical coal wall and the roof side. Under the same height but different basic roof fracture locations, the deformation of surrounding rocks of roadways is different. Below is a detailed analysis.

When the main roof height is 0 m, the deformation of the coal wall side, the roof side, the coal pillar side, and the floor side in roadways shown in Schemes 1–3 is 241.40 mm, 117.29 mm, 40.85 mm, 55.73 mm, 210.84 mm, 109.53 mm, 64.11 mm, 49.40 mm, 214 mm, 152.12 mm, 108.25 mm, and 48.76 mm, respectively.

At the moment, there is no immediate roof above the coal strata, thus resulting in no goaf waste rock support force, $F_G$, for the key rock block B. When the basic main roof fracture location is right above roadways, the key rock block B will rotate and sink directly on the roof side of the roadway and the side of the narrow coal pillar. Under the condition, the deformation of surrounding rocks of roadways is larger than that under the other two conditions.

When the main roof height is 5 m, the deformation of the physical coal wall side, the roof side, the coal pillar side, and the floor side is 360.9 mm, 229.73 mm, 89.68 mm, 85.23 mm, 386.14 mm, 248.3 mm, 85.87 mm, 85.87 mm, 343.56 mm, 277.26 mm, 131.39 mm, and 91.6 mm, respectively.

When the main roof height is 10 m, the deformation of the physical coal wall side, the roof side, the coal pillar side, and the floor side is 338.99 mm, 219.34 mm, 95.11 mm, 76.24 mm, 365.29 mm, 263.28 mm, 111.97 mm, 81.28 mm, 364.9 mm, 318.25 mm, 126.38 mm, and 72.06 mm, respectively. At the moment, there is an immediate roof between the coal strata and the main roof. After roadway driving and rotary subsidence of the key rock block B, the block B might receive the stress from the main roof, $F_D$, $F_S$, $F_M$, and $F_G$. Through the immediate roof, it will function on the coal pillar, the roadway roof, the physical coal wall, and the goaf to achieve an equilibrium state. When the main roof fracture location is right above roadways, the compression rate between the narrow coal pillar and the immediate roof right above roadways is relatively large, and the stress concentration is high. Consequently, the deformation of surrounding rocks of the narrow coal pillar side and the roof side becomes larger than that under the above two conditions.

Therefore, in order to reduce the stress concentration degree of the narrow coal pillar side and right above roadways, it is necessary to avoid laying gob-side entry driving right below the main roof fracture location. In this way, the bolt or the cable will control deformation of surrounding rocks of roadways more easily.

5. Engineering Application Analysis

Based on the above research, engineering application analysis is conducted on two fully-mechanized top-coal caving faces, namely, 8105 and 8215.

5.1. 8105 Fully-Mechanized Top-Coal Caving Face

8105 track transport roadway drives along the 8104 goaf. It is necessary to master the main roof fracture line location on the 8105 top-coal caving face. Therefore, the 8103 track transport roadway and the 8105 belt transport roadway lay the probing drills on the 8102 goaf and the 8106 goaf. Measurement shows that the basic main fracture line is located within the physical coal wall, which is 3.42∼3.87 m away from the goaf boundary. Therefore, the coal pillar width should not be within the range of 3.42∼3.87 m [8, 22]. (See Figure 5) The numerical simulation analysis is adopted to analyze changing rules of surrounding rock deformation and coal pillar and seam floor vertical stress under different coal pillar width. Finally, the reasonable width of the coal pillar is 6.5 m. After 20 days of roadway driving, the surrounding rocks are basically stable, and the maximum roof-to-floor and two-rib displacement is 118 mm and 65 mm, respectively. Influenced by the caving face excavation, the maximum roof-to-floor and two-rib displacement is 200∼420 mm and 380∼600 mm, respectively. The control effect of surrounding rocks of roadways is shown in Figure 6.

[figure omitted; refer to PDF]

From Figure 7, it can be seen that the stress 4 m within the coal pillar is low and changes within the range of 0.1∼0.2 MPa. The stress 5 m within the physical coal wall is large. If not influenced by excavation, it changes within the range of 0.1∼0.6 MPa, and if influenced by excavation, it changes within the range of 0.7∼0.8 MPa. The stress 10 m within the physical coal wall is relatively large. If not influenced by excavation, the change is within the range of 0.1∼0.8 MPa; if influenced by excavation, the peak stress value can reach 10.9 MPa. Results suggest that, if the coal is not influenced by excavation, the surrounding rock stress is relatively small; if influenced by excavation, the surrounding rock stress is transferred to be within the physical coal wall. However, the roadways driven along goaf far always in the stress decline area, which is beneficial to the surrounding rock control of roadways.

Changing rules of the surrounding rock deformation and the coal stress suggest that roadways, influenced by excavation, are efficiently controlled, thus meeting requirements of the working face stopping [20, 23]. In order to master the main roof fracture line of the 8215 caving face, probing drills are laid by the 8215 track transport roadway to the overlaying strata on the 8214 goaf, and the probing results are shown in Figure 8.

[figure omitted; refer to PDF]

From Figure 8, the large fractures concentrate above the coal pillar near the goaf. Their distance away from the coal pillar margin is short, namely, 1.22 m, 3.42 m, and 4.15 m, respectively. Areas with obvious fractures are gradually away from Coal 8, and they are mainly distributed near the upper right of the coal pillar in goaf and far away from the coal pillar margin. It suggests that it is reasonable to set the coal pillar to be 8.0 m, which can avoid putting the roadway right under the basic fracture line, and the surrounding rock control effects are shown in Figure 9.

[figures omitted; refer to PDF]

6. Conclusions

This paper builds the structural mechanics model of the key rock block B, points out that equilibrium of the key rock block B should meet conditions not to lose equilibrium horizontally and vertically, qualitatively analyzes the influence of the main roof factors on stress of the key rock block B, and places particular emphasis on stress-changing rules of the key rock block B vertically under three different fracture locations of the main roof.

Based on mining technical conditions of Yanjiahe Coal Mine in Binchang Mining Area, four factors of the main roof are confirmed, including the main roof height and hardness, the length of the key rock block B, and the main roof fracture location. The orthogonal experiment is used to set nine plans; the range method is employed to confirm the sensitivity order of the main roof, which is as follows: the main roof height ＞ the main roof fracture location ＞ the length of the key rock block B ＞ the main roof strength.

The univariate method is adopted to analyze the influence of the main roof height and the fracture location on stability of surrounding rocks of roads in fully-mechanized caving faces. It is found that surrounding rock deformation during roadways driven along goaf happens in the physical coal wall and the roof side. The deformation of surrounding rocks of roadways and the vertical stress of the narrow coal pillar tend to stabilize along with the increase of the main roof height. Meanwhile, the roadway driving right below the basic fracture line should be avoided.

The engineering application effects suggest that the narrow coal pillar layout is reasonable on the 8105 and 8215 caving face, which can avoid putting the main roof fracture location right above the roadway. The surrounding rocks of the driving roadways are in a low-stress environment, and the surrounding rocks of the roadways are efficiently controlled. All these have guaranteed safe and efficient stopping of caving faces.

Authors’ Contributions

Hong-sheng Wang conceived of the research. Hong-sheng Wang and Hai-qing Shuang analyzed the data and wrote the paper. Lei Li and Shuang-shuang Xiao participated in the design of the study and verified the results. All authors have read and approved the final manuscript.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (no. 51974231) and National Basic Research Program of China (973 Program) (no. 2015CB251600).

References

[1] J. B. Bai, W. J. Wang, C. J. Hou, "Control mechanism and support technique about gateway driven along goaf in fully mechanized top coal caving face," Journal of China Coal Society, vol. 25 no. 5, pp. 478-481, 2000.

[2] Z. Zhang, J. Bai, Y. Chen, S. Yan, "An innovative approach for gob-side entry retaining in highly gassy fully-mechanized longwall top-coal caving," International Journal of Rock Mechanics & Mining Sciences, vol. 80 no. 12,DOI: 10.1016/j.ijrmms.2015.09.001, 2015.

[3] C. J. Hou, X. H. Li, "Stability principle of big and small structures of rock surrounding roadway driven along goaf in fully mechanized top coal caving face," Journal of China Coal Society, vol. 26 no. 1, 2001.

[4] X. H. Li, "Deformation mechanism of surrounding rocks and key control technology for a roadway driven along goaf in fully mechanized top-coal caving face," Journal Coal Science Engineering, vol. 9 no. 1, pp. 28-32, 2003.

[5] D. S. Zhang, X. B. Mao, W. D. Ma, "Testing study on deformation features of surrounding rocks of gob-side entry retaining in fully-mechanized coal face with top-coal caving," Chinese Journal of Rock Mechanics and Engineering, vol. 21 no. 3, pp. 331-334, 2002.

[6] D. S. Zhang, L. Q. Ma, X. X. Miao, "Factor analysis on deformation of gob-side entry retaining with entry-in packing in top-coal caving mining face," Journal of China University of Ming Technology, vol. 35 no. 1–6, pp. 331-334, 2006.

[7] H. S. Wang, Research on the Creep Character and its Stability Control Technology of Gob-Side Entry’s Narrow Side, 2011.

[8] H. S. Wang, D. S. Zhang, S. G. Li, L. Wang, L. Z. Wang, "Rational width of narrow coal pillar based on the fracture line location of key rock B in main roof," Journal of Mining and Safety Engineering, vol. 31 no. 1, pp. 10-16, 2014.

[9] C. Q. Zhu, X. X. Miao, Z. Liu, "Mechanical analysis on deformation of surrounding rock with road-in packing of gob-side entry retaining in fully-mechanized sub-level caving face," Journal of Coal Science and Engineering, vol. 14 no. 1, pp. 24-28, DOI: 10.1007/s12404-008-0005-5, 2008.

[10] W. B. Xie, "Influence factors on stability of surrounding rocks of gob-side entry retaining in top-coal caving mining face," Chinese Journal of Rock Mechanics and Engineering, vol. 23 no. 18, pp. 3059-3065, 2004.

[11] N. Zhang, L. Yuan, C. L. Han, J. H. Xue, J. G. Kan, "Stability and deformation of surrounding rock in pillarless gob-side entry retaining," Safety Science, vol. 50 no. 4, pp. 593-599, DOI: 10.1016/j.ssci.2011.09.010, 2012.

[12] N. Zhang, C. Liu, P. Yang, "Flow of top coal and roof rock and loss of top coal in fully mechanized top coal caving mining of extra thick coal seams," Arabian Journal of Geosciences, vol. 9 no. 6,DOI: 10.1007/s12517-016-2493-8, 2016.

[13] C. Meng, X. H. Li, "Analysis and application of parameters sensitivity of bolt support in gob-side entry driving," China Coal, vol. 39 no. 3, pp. 45-49, 2013.

[14] Y. Yuan, S. H. Tu, X. G. Zhang, B. Li, "Dynamic effect and control of key strata break of immediate roof in fully mechanized mining with large mining height," Shock & Vibration, vol. 2015 no. 4,DOI: 10.1155/2015/657818, 2015.

[15] J. B. Bai, W. L. Shen, G. L. Guo, X. Y. Wang, Y. Yu, "Roof deformation, failure characteristics, and preventive techniques of gob-side entry driving heading adjacent to the advancing working face," Rock Mechanics and Rock Engineering, vol. 48 no. 6, pp. 2447-2458, DOI: 10.1007/s00603-015-0713-2, 2015.

[16] B. X. Li, Y. Li, W. S. Zhu, C. Li, Z. X. Dong, "Impact of in situ stress distribution characteristics on jointed surrounding rock mass stability of an underground cavern near a hillslope surface," Shock and Vibration, vol. 2017 no. 1,DOI: 10.1155/2017/2490431, 2017.

[17] H. S. Wang, D. S. Zhang, L. Liu, "Stabilization of gob-side entry with an artificial side for sustaining mining work," Sustainability, vol. 8 no. 7,DOI: 10.3390/su8070627, 2016.

[18] J. B. Bai, Control Mechanism about Gateway Driven along Goaf, 2006.

[19] M. G. Qian, X. X. Miao, F. L. He, "Analysis of key block in the structure of voussoir beam in longwall mining," Journal of China Coal Society, vol. 19 no. 6, pp. 557-563, 1994.

[20] H. Q. Shuang, H. S. Wang, S. G. Li, X. Z. Zhang, L. Z. Wu, Y. J. Dong, "Reasonable width optimization of narrow coal pillar in roadway driven along goaf of fully mechanized caving face," Coal Engineering, vol. 47 no. 2, pp. 18-21, 2015.

[21] H. S. Wang, X. Z. Zhang, L. Z. Wu, S. G. Li, Y. J. Dong, "Surrounding rock control technology of roadway in “isolated island” fully mechanized coal face," Coal Technology, vol. 30 no. 8, pp. 83-84, 2011.

[22] H. S. Wang, S. G. Li, X. Z. Zhang, L. Z. Wu, Y. J. Dong, H. Q. Shuang, "Analysis on stability of narrow coal pillar influenced by main roof fracture structure of gob-side roadway," Coal Science and Technology, vol. 42 no. 2, pp. 19-22, 2014.

[23] H. Q. Shuang, Study on Stability Analysis and Control Technology about Roadway Driving along Goaf at Fully Mechanized Top Coal Caving, 2015.

[24] W. Jiang, W. J. Ju, Z. L. Wang, Z. Zhang, M. Shi, "Characteristics of overburden stress distribution and rational pillar width determination of gob-side roadway with thick and hard basic roof in fully mechanized top coal caving workface," Journal of Mining and Safety Engineering, vol. 37 no. 6, pp. 1142-1151, 2020.

[25] X. J. Meng, "Solid coal rib support technology of fully-mechanized mining along gob-side entry driving based on main roof fracture location," Coal Science and Technology, vol. 48 no. 1, pp. 61-68, 2020.