# Regional Division and Its Criteria of Mining Fractures Based on Overburden Critical Failure

^{1}

^{2}

^{3}

^{*}

## Abstract

**:**

## 1. Introduction

## 2. Overburden Critical Failure and Mining Fracture Distribution

#### 2.1. Mining Degree of Overburden Failure

#### 2.2. Characteristics of Overburden Failure and Fracture Distribution

_{li}is the height of the water conduction fracture zone, m; and M is the mining height, m.

^{−6}. Meanwhile, the remaining rock strata are uniformly applied on the top of the model with a load of 0.3 MPa. The physical simulation material selects fine sand as aggregate and calcium carbonate and gypsum as cementitious materials, borax as retarder, mica powder as weak surface between rock layers, and water as solvent. By changing the different ratio of materials, the rock layers with different strength in overburden are simulated. The mechanical parameters of each rock stratum in the model are shown in Table 1.

- Only tensile or fine fractures. After the mining of the working face is completed, the setup room and stopping line of the working face are bounded by the fracture angle formed by overburden failure on one side of the coal pillar; the mining impact is tensile deformation or no impact. At this time, there are only tensile or original fractures in the overburden, i.e., the area ranges from the coal pillar side to the overburden fracture;
- Obvious overburden structure. When the overburden is broken to a certain height layer by layer under the mining influence, the broken block is hinged to form a masonry beam structure because the free space below does not meet the conditions of rock block sliding and instability. With the upward transmission of rock fracture, the sub-key layer within the fracture zone forms a hinged structure owing to the large fracture distance, and its return angle gradually decreases with the increase of the distance from the coal seam roof until it is transmitted to the top of the fracture zone, forming an obvious layer separation. Then, the mining fractures are the permanent through ones;
- Closure of mining cracks in the overburden. As the working face advances, the mining cracks located in the fracture zone are developed periodicity in form of “formation-penetration-closure-compaction”. When the overburden reaches critical mining, the mining cracks and separation layers gradually close under the periodic pressure of the main roof above the working face, reducing the water conductivity of the rock strata. Meanwhile, the equivalent subsidence coefficient reaches maximum. The caved zone and fracture zone are compacted within this range, therefore, the surface in the middle of the goaf is also the target area for effective utilization of the mining subsidence area due to the small surface residual deformation in this area.

## 3. Hard Rock Failure and Structure Formation

#### 3.1. Shear Deformation Test

_{2}of 16 interface bisected section lines is calculated according to Formula (2) [39]:

_{i}is the height coordinate of the joint surface profile; l is the number of data points; and Δx is the interval of data points.

_{i}) can be calculated by substituting each two-dimensional section line (Z

_{2i}) into Formula (3) [40]. The joint roughness coefficient of the rock sample can be obtained by taking the weighted average value of the roughness of each section line:

_{2i}is the root mean square of the slope for the ith contour line; and JRC

_{i}is the roughness coefficient of the ith contour line.

- Compaction stage. Owing to the incomplete contact between joint bulges (roughness) in the initial state, the contact area and pressure of joint bulges increase with the increase of shear displacement, and the shear stress increases nonlinearly;
- Linear stage. When the displacement increases, the undamaged contact joint bulge increases the friction between joints, and the shear stress increases linearly;
- Yield stage. The shearing of the joint bulge reduces the roughness coefficient and causes the shear stress to increase slowly until the shear bearing capacity of the joint reaches its peak;
- Softening stage. With the increase of normal stress, the roughness of the joint surface smooths gradually, which shows that the reduction of shear stress increases with the increasing normal stress.

#### 3.2. Formation and Stability of Overburden Structure

_{max}can be obtained by Formula (4) [42]:

_{t}is its tensile strength, MPa; q is the load borne by the rock, MPa; E is the elastic modulus, MPa; and b is the width of the rectangular section, m.

_{max}is 0.05 h, m; L is the initial limit breaking distance of the rock stratum, m; θ is the angle for the key blocks of the overburden structure, °; H

_{m}is the height of the caved zone, m; k

_{0}is the expansion coefficient of the caved zone; and M is the mining thickness of the coal seam, m.

_{z}, l

_{z}is the length of the periodic broken block, m; tan φ is the friction coefficient between broken blocks, 0.3; P is the load on the key block, MN; and σ

_{c}is the compressive strength of the rock stratum, MPa.

_{z}near the working face is approximately 10 m, and the load on the key block is P = γ(H

_{li}− H

_{m}), approximately 1.02 MPa. The overburden structure meets the required conditions for no instability.

## 4. Regional Division Method and Discussion of Mining Fracture

#### 4.1. Division and Discrimination of Mining Fractures in Overburden

- Original fracture area. Combined with the mining subsidence theory and the maximum bending deflection of the rock stratum, the boundary with an inclination value of 3 mm/m (layer separation rate of 3‰) in the movement angle is the boundary between the original and tensile fractures, which is the position of the yellow point in the Figure 5b;
- Tensile fracture zone. The connecting line between the measuring point with an inclination value of approximately 3 mm/m in the physical simulation and the mining boundary is taken as the boundary between the original and the tensile fracture zones. The interface angle at the setup room of the working face is 71°, while at the stopping line, it is 78°. The rock block forms a masonry beam structure after the overburden is broken, therefore, the overburden shape and stress state change significantly. Therefore, the overburden fracture is taken as another boundary of the tensile fracture area. The length l
_{1}of this area at the top of the fracture zone is:$${l}_{1}={H}_{li}\left(\mathrm{cot}{\theta}_{1}+\mathrm{cot}{\theta}_{2}\right)$$_{li}is the height of the water conduction fracture zone and can be calculated in the regulation, which is 36.4 m; and θ_{1}and θ_{2}are the movement and breaking angles of the overburden, respectively, at 71° and 67°. The length of this area is 27.98 m, and the length within the goaf is 15.45 m; - Structural void zone. The structural void zone begins when the masonry beam structure is formed. With the advance of the working face, as the fracture zone reaches the maximum, the separation gap between the fracture and bending zones reaches the maximum, and the overburden reaches the critical failure. Then, the midpoint of the ultimate breaking distance of the rock stratum at the bottom of the bending zone is taken as the base point, and the rock strata breaking boundaries are created parallel to the overburden structure. The area between the adjacent boundary lines is the structural fracture area. The relationship between overburden failure height and separation void meets the following conditions:$$\{\begin{array}{l}{H}_{li}\left({k}_{0}-1\right)\ge M+\omega \\ \omega \le {\omega}_{\mathrm{max}}\end{array}$$The bending deflection of the hard rock layer at the upper part of the fracture zone is the largest when it reaches the initial limit breaking distance. According to the above formula, when the overburden failure reaches the critical height and the adjacent layer reaches the initial limit breaking distance L, the advancing distance of working face with overburden critical mining is the largest, i.e., L
_{s}= H_{li}(cot θ’_{1}+ cot θ’_{2}) + L. L_{s}= 79.6 m, and the error from the critical advancing distance (75 m) of the working face in Figure 5 is only 5.7%. Therefore, this formula can be used to determine the advancing distance of working face when the overburden failure reaches critical mining, and also to determine the range of the structural fracture area, providing a basis for overburden separation grouting and goaf stability evaluation; - Void compaction area. After overburden failure reaches supercritical mining, the separation gap of the rock stratum at the bottom of the bending zone reaches the maximum, which is equivalent to the bottom of the surface subsidence basin. This area is located in the middle of the structural fracture area. Combined with Figure 5 and the evolution of overburden fractures, it is consistent with the physical simulation.

#### 4.2. Engineering Practice

## 5. Conclusions

- (1)
- Based on the overburden critical failure, the characteristics of overburden fracture were analyzed. The fracture zone was divided horizontally into original rock fracture, tensile fracture, structural void, and void compaction areas, and the description of each area was proposed;
- (2)
- The formation mechanism for the shape of the water conduction fracture zone was determined to be the same as the surface mining subsidence. The relationship between the maximum subsidence deflection of the hard rock layer and the thickness of the broken rock layer was clarified with the deduced theoretical formula, and the masonry beam structure was found to have long-term stability;
- (3)
- Based on field monitoring of surface deformation, the subsidence characteristics of measurement points at different positions above the working face were analyzed. The overburden structure on both sides of the working face and the void in the uncompact caved zone was suggested to be the main factor causing different subsidence of the measuring points, and the rationality of overburden regional division was verified, which is vital for safe mining under water, accurate restoration of the eco-environment in mining subsidence areas, sustainable development of the mining industry, and economic growth.

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

## References

- Bell, F.G.; Stacey, T.R.; Genske, D.D. Mining subsidence and its effect on the environment: Some differing examples. Environ Geol.
**2020**, 40, 135–152. [Google Scholar] [CrossRef] - Fan, L.M.; Ma, X.D. A review on investigation of water-preserved coal mining in western China. Int. J. Coal Sci. Technol.
**2018**, 5, 411–416. [Google Scholar] [CrossRef] [Green Version] - Karakus, M.; Tutmez, B. Fuzzy and Multiple Regression Modelling for Evaluation of Intact Rock Strength Based on Point Load, Schmidt Hammer and Sonic Velocity. Rock Mech. Rock Eng.
**2006**, 39, 45–57. [Google Scholar] [CrossRef] - Bai, E.H.; Guo, W.B.; Tan, Y. Negative externalities of high-intensity mining and disaster prevention technology in China. Bull. Eng. Geol. Environ.
**2019**, 78, 5219–5235. [Google Scholar] [CrossRef] - Sun, Q.; Zhang, J.X.; Li, M.; Zhou, N. Experimental evaluation of physical, mechanical, and permeability parameters of key aquiclude strata in a typical mining area of China. J. Clean. Prod.
**2020**, 267, 122109. [Google Scholar] [CrossRef] - Sasaoka, T.; Takamoto, H.; Shimada, H.; Oya, J.; Hamanaka, A.; Matsui, K. Surface subsidence due to underground mining operation under weak geological condition in Indonesia. J. Rock Mech. Geotech. Eng.
**2015**, 7, 337–344. [Google Scholar] [CrossRef] [Green Version] - Yu, S.C.; Xu, J.M.; Zhu, W.B.; Wang, S.H.; Liu, W.B. Development of a combined mining technique to protect the underground workspace above confined aquifer from water inrush disaster. Bull. Eng. Geol. Environ.
**2020**, 79, 3649–3666. [Google Scholar] [CrossRef] - Altun, A.O.; Yilmaz, I.; Yildirim, M. A short review on the surficial impacts of underground mining. Sci. Res. Essay
**2010**, 21, 3206–3212. [Google Scholar] - Yang, B.B.; Yuan, S.C.; Liang, Y.K.; Liu, J.W. Investigation of overburden failure characteristics due to combined mining: Case study, Henan Province, China. Environ. Earth Sci.
**2021**, 80, 143. [Google Scholar] [CrossRef] - Kim, J.M.; Parizek, R.R.; Elsworth, D. Evaluation of fully-coupled strata deformation and groundwater flow in response to longwall mining. Int. J. Rock Mech. Min. Sci.
**1997**, 34, 1187–1199. [Google Scholar] [CrossRef] - Wang, G.; Wu, M.M.; Wang, R.; Xu, H.; Song, X. Height of the mining-induced fractured zone above a coal face. Eng. Geol.
**2017**, 216, 140–152. [Google Scholar] [CrossRef] - Qian, M.G.; Miao, X.X.; Xu, J.L. Theoretical study of key stratum in ground control. J. China Coal Soc.
**1996**, 21, 225–230. [Google Scholar] - Xu, J.L.; Zhu, W.B.; Wang, X.Z. New method to predict the height of fractured water-conducting zone by location of key strata. J. China Coal Soc.
**2012**, 37, 762–769. [Google Scholar] - Rezaei, M.; Hossaini, M.F.; Majdi, A. A time-independent energy model to determine the height of destressed zone above the mined panel in longwall coal mining. Tunn. Undergr. Space Technol.
**2015**, 47, 81–92. [Google Scholar] [CrossRef] - Li, Z.; Xu, J.L.; Ju, J.F.; Zhu, W.B.; Xu, J.M. The effects of the rotational speed of voussoir beam structures formed by key strata on the ground pressure of stopes. Int. J. Rock Mech. Min. Sci.
**2018**, 108, 67–79. [Google Scholar] [CrossRef] - Castro, T.; Trueman, R.; Halim, A. A study of isolated draw zones in block caving mines by means of a large 3D physical model. Int. J. Rock Mech. Min. Sci.
**2007**, 44, 860–870. [Google Scholar] [CrossRef] [Green Version] - Hatherly, P. Overview on the application of geophysics in coal mining. Int. J. Coal Geol.
**2013**, 114, 74–84. [Google Scholar] [CrossRef] - Ghabraie, B.; Ren, G.; Zhang, X.; Smith, J. Physical modelling of subsidence from sequential extraction of partially overlapping longwall panels and study of substrata movement characteristics. Int. J. Coal Geol.
**2015**, 140, 71–83. [Google Scholar] [CrossRef] - Ju, M.H.; Li, X.H.; Yao, Q.L.; Liu, S.Y.; Liang, S.; Wang, X.L. Effect of sand grain size on simulated mining-induced overburden failure in physical model tests. Eng. Geol.
**2017**, 226, 93–106. [Google Scholar] [CrossRef] - Asaue, H.; Sasahara, M.; Yoshinaga, T.; Obara, Y.; Uchida, K.; Matsumoto, H. Clarifying geological structure for coal and marsh gas development using magnetotelluric method. Acta Geodyn. Geomater.
**2013**, 10, 155–162. [Google Scholar] [CrossRef] [Green Version] - Yu, B.; Zhao, J.; Kuang, T.J.; Meng, X.B. In situ investigations into overburden failures of a super-thick coal seam for longwall top coal caving. Int. J. Rock Mech. Min. Sci.
**2015**, 78, 155–162. [Google Scholar] [CrossRef] - Mills, K.W.; Garratt, O.; Blacka, B.G.; Daigle, L.C.; Rippon, A.C.; Walker, R.J. Measurement of shear movements in the overburden strata ahead of longwall mining. Int. J. Min. Sci. Technol.
**2015**, 26, 97–102. [Google Scholar] [CrossRef] - Han, P.H.; Zhang, C.; Ren, Z.P.; He, X.; Jia, S. The influence of advance speed on overburden movement characteristics in longwall coal mining: Insight from theoretical analysis and physical simulation. J. Geophys. Eng.
**2021**, 18, 163–176. [Google Scholar] [CrossRef] - Xu, D.J.; Peng, S.P.; Xiang, S.Y.; He, Y.L. A Novel Caving Model of Overburden Strata Movement Induced by Coal Mining. Energies
**2017**, 10, 476. [Google Scholar] [CrossRef] [Green Version] - Cao, Z.G.; Ju, J.F.; Xu, J.L. Distribution model of water-conducted fracture main channel and its flow characteristics. J. China Coal Soc.
**2019**, 44, 3719–3728. [Google Scholar] - Karacan, C.O.; Goodman, G. Hydraulic conductivity changes and influencing factors in longwall overburden determined by slug tests in gob gas ventholes. Int. J. Rock Mech. Min. Sci.
**2009**, 46, 1162–1174. [Google Scholar] [CrossRef] - Li, S.; Fan, C.J.; Luo, M.K.; Yang, Z.H.; Lan, T.W.; Zhang, H.F. Structure and deformation measurements of shallow overburden during top coal caving longwall mining. Int. J. Min. Sci. Technol.
**2017**, 27, 1081–1085. [Google Scholar] [CrossRef] - Hu, C.S.; Apel, D.; Sudak, L.J.; Liu, W.V.; Chu, Z.Y. Physical investigation on the behaviours of voussoir beams. J. Rock Mech. Geotech.
**2020**, 12, 516–527. [Google Scholar] [CrossRef] - Shi, X.C.; Zhang, J.X. Characteristics of overburden failure and fracture evolution in shallow buried working face with large mining height. Sustainability
**2021**, 13, 13775. [Google Scholar] [CrossRef] - Majdi, A.; Hassani, F.P.; Nasiri, M.Y. Prediction of the height of destressed zone above the mined panel roof in longwall coal mining. Int. J. Coal Geol.
**2012**, 98, 62–72. [Google Scholar] [CrossRef] - He, C.C.; Lu, W.Y.; Zha, W.H.; Wang, F. A geomechanical method for predicting the height of a water-flowing fractured zone in a layered overburden of longwall coal mining. Int. J. Rock Mech. Min. Sci.
**2021**, 143, 104798. [Google Scholar] [CrossRef] - Guo, W.B.; Zhao, G.B.; Lou, G.Z.; Wang, S.R. A New Method of Predicting the Height of the Fractured Water-Conducting Zone Due to High-Intensity Longwall Coal Mining in China. Rock Mech. Rock Eng.
**2018**, 52, 2789–2802. [Google Scholar] [CrossRef] - Palchik, V. Formation of fractured zones in overburden due to longwall mining. Environ. Geol.
**2003**, 44, 28–38. [Google Scholar] [CrossRef] - Zhou, D.W.; Wu, K.; Miao, X.X. Combined prediction model for mining subsidence in coal mining areas covered with thick alluvial soil layer. Bull. Eng. Geol. Environ.
**2018**, 77, 283–304. [Google Scholar] [CrossRef] - Bai, E.H.; Guo, W.B.; Zhang, D.S.; Tan, Y.; Guo, M.J.; Zhao, G.B. Using the Magnetotelluric Method for Detecting Aquifer Failure Characteristics under High-Intensity Mining of Thick Coal Seams. Energies
**2019**, 12, 4397. [Google Scholar] [CrossRef] [Green Version] - Guo, W.B.; Lou, G.Z. Definition and distinguishing method of critical mining degree of overburden failure. J. China Coal Soc.
**2019**, 44, 755–766. [Google Scholar] - Guo, W.B.; Zhao, G.B.; Bai, E.H. Critical failure of overlying rock strata and its criteria induced by high-intensity longwall mining. J. China Coal Soc.
**2020**, 45, 3657–3666. [Google Scholar] - Yu, Q.G.; Zhang, H.X.; Deng, W.N.; Zou, Y.P. Analysis of influencing factors of surface skewed subsidence based on key strata theory. J. China Coal Soc.
**2018**, 43, 1322–1327. [Google Scholar] - Tian, Y.C.; Liu, Q.S.; Liu, D.F.; Kang, Y.S.; Deng, P.H.; He, F. Updates to Grasselli’s Peak Shear Strength Model. Rock Mech. Rock Eng.
**2018**, 51, 2115–2133. [Google Scholar] [CrossRef] - Li, Y.R.; Zhang, Y.B. Quantitative estimation of joint roughness coefficient using statistical parameters. Int. J. Rock Mech. Min. Sci.
**2015**, 77, 27–35. [Google Scholar] [CrossRef] [Green Version] - Barton, N.; Choubey, V. The shear strength of rock joints in theory and practice. Rock Mech.
**1977**, 10, 1–54. [Google Scholar] [CrossRef] - Chen, L.; Wu, B.; Xu, X.K.; Shang, Y.R. Determination of overburden failure height in alternate strata of mudstone and sandstone with fully mechanized caving method. J. Min. Saf. Eng.
**2017**, 34, 431–436. [Google Scholar] - Qian, M.G.; Zhang, D.L.; Li, L.J.; Kang, L.X.; Xu, J.L. “S-R” stability for the voussoir beam and its application. Coal Technol. Northeast China
**1994**, 3, 6–10. [Google Scholar]

**Figure 4.**Shear stress displacement curves under different normal stresses. (

**a**) Stable type; (

**b**) Slow descent type; and (

**c**) Mutant.

**Figure 5.**Distribution and division of mining fracture. (

**a**) Distribution of separation rate in the fracture zone; (

**b**) Division of overburden fracture under coal mining.

**Figure 7.**Cumulative surface subsidence in the goaf. (

**a**) Measuring points A1–A12 (February 2019–October 2021). (

**b**) Measuring points A57–A72 (February 2019–October 2021).

No. | Rock Stratum | Thickness (m) | Density (kN/m^{3}) | Elastic Modulus (GPa) | Tensile Strength (MPa) | Internal Friction Angle (°) | Poisson’s Ratio |
---|---|---|---|---|---|---|---|

1 | Mudstone | 7.6 | 2560 | 10.90 | 1.68 | 30 | 0.23 |

2 | Medium sandstone | 7.4 | 2630 | 36.18 | 5.13 | 36 | 0.26 |

3 | Sandy mudstone | 4.9 | 2580 | 18.53 | 3.05 | 32 | 0.27 |

4 | Mudstone | 4.5 | 2560 | 10.90 | 1.68 | 30 | 0.23 |

5 | Siltstone | 6.8 | 2660 | 29.77 | 3.84 | 38 | 0.2 |

6 | Mudstone | 2.7 | 2560 | 10.90 | 1.68 | 30 | 0.23 |

7 | Sandy mudstone | 6.2 | 2580 | 18.53 | 3.05 | 32 | 0.27 |

8 | Fine sandstone | 7.6 | 2750 | 38.45 | 6.75 | 37 | 0.18 |

9 | Sandy mudstone | 4.7 | 2580 | 18.53 | 3.05 | 32 | 0.27 |

10 | Coal seam | 3.0 | 1400 | 2.30 | 1.03 | 24 | 0.31 |

No. | S1 | S2 | S3 | S4 | S5 | S6 | S7 | S8 | S9 |
---|---|---|---|---|---|---|---|---|---|

σ_{n/}MPa | 3 | 4 | 6 | 9 | 12 | 15 | 18 | 21 | 24 |

JRC | 6.64 | 8.54 | 7.35 | 12.37 | 7.59 | 8.13 | 10.18 | 7.87 | 8.70 |

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. |

© 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

## Share and Cite

**MDPI and ACS Style**

Bai, E.; Guo, W.; Tan, Y.; Guo, M.; Wen, P.; Liu, Z.; Ma, Z.; Yang, W.
Regional Division and Its Criteria of Mining Fractures Based on Overburden Critical Failure. *Sustainability* **2022**, *14*, 5161.
https://doi.org/10.3390/su14095161

**AMA Style**

Bai E, Guo W, Tan Y, Guo M, Wen P, Liu Z, Ma Z, Yang W.
Regional Division and Its Criteria of Mining Fractures Based on Overburden Critical Failure. *Sustainability*. 2022; 14(9):5161.
https://doi.org/10.3390/su14095161

**Chicago/Turabian Style**

Bai, Erhu, Wenbing Guo, Yi Tan, Mingjie Guo, Peng Wen, Zhiqiang Liu, Zhibao Ma, and Weiqiang Yang.
2022. "Regional Division and Its Criteria of Mining Fractures Based on Overburden Critical Failure" *Sustainability* 14, no. 9: 5161.
https://doi.org/10.3390/su14095161