# Refinement Study on Pressure Relief Zone of Gas Extraction Borehole Considering Roughness

^{1}

^{2}

^{3}

^{*}

## Abstract

**:**

## 1. Introduction

## 2. Theoretical Fundamentals

#### 2.1. Relationship between Roughness Coefficient and H-B Failure Criterion

_{r}is the joint roughness; J

_{n}is the joint set coefficient; J

_{a}is the joint alteration, J

_{w}is the joint water reduction coefficient, and SFR is the stress discount factor. The parameter values can be referenced in the literature [50].

_{r}for a certain number of other variables.

_{1}, σ

_{3}are the maximum and minimum compressive stresses, respectively; σ

_{ci}is the uniaxial compressive strength of the rock; m

_{b}, s, a are the corresponding rock dimensionless parameters; m

_{i}is the dimensionless parameter, which can be obtained experimentally; GSI is the geological strength parameter; D is the disturbance factor.

#### 2.2. Ontological Model

_{s}> 0, the material will suffer shear failure.

#### 2.3. Relationship between Permeability and Gas Drainage Boreholes

- (1)
- Strong permeability of the sealing material or the existence of cracks, causing air to flow into the borehole from the sealing material, as shown in Figure 2a.
- (2)
- The sealing material cannot seal the cracks developed in the surrounding rock of the borehole, causing air to leak in through these cracks, as shown in Figure 2b.
- (3)
- The sealing depth did not exceed “an area where the surrounding rock cracks are connected with the free surface of the roadway side”, resulting in the cracks of the borehole orifice being connected with the cracks in the deep part of the sealing section, and then air leakage, as shown in Figure 2c.

#### 2.4. Permeability Model

- (1)
- Permeability evolution equation of coal mass after excavation and decompression of the roadway

_{a}—permeability of coal mass in elastic deformation phase, m

^{2};

_{0}—Initial permeability of the coal mass, m

^{2}.

_{σ}—Coefficient of effect of volumetric stress on permeability, MPa

^{−1}.

_{1}+ σ

_{2}+ σ

_{3}, MPa.

_{b}—permeability of coal mass in plastic deformation state, m

^{2};

^{p}—equivalent plastic strain.

^{p}*—initial equivalent plastic strain in the residual strength phase.

_{σ}= −0.1 MPa

^{−1}, the initial equivalent plastic strain in the residual strength phase ε

^{p}* = 0.001 and the permeability surge factor ζ = 10. The equivalent plastic strain ε

^{p}can be calculated by the following equation

_{1}

^{p}, ε

_{2}

^{p}, ε

_{3}

^{p}—plastic principal strain.

_{c}—coal mass permeability during the residual strength phase, m

^{2};

_{1}—permeability after pressure relief, m

^{2}.

_{1}—porosity of the roadway after decompression.

_{0}—original porosity.

## 3. Case Study and Geometrical Model Generation

#### 3.1. Background of Case Study

^{3}/t, and the gas pressure is 0.25 MPa. The situation of the top and bottom of the coal seam is shown in Table 1.

#### 3.2. Numerical Simulation

## 4. Numerical Simulation Results and Discussion

#### 4.1. Stress Results

#### 4.2. Permeability Results

^{2}/(MPa

^{2}·d), it is difficult to extract coal seams. When the permeability coefficient of coal seams is more than 10 m

^{2}/(MPa

^{2}·d) it is easy to extract coal seams.

^{2}/(MPa

^{2}·d) is equivalent to 0.0025 md (permeability).

^{2}/(MPa

^{2}·d). According to the coal seam permeability coefficient classification standard, 0.1 < 0.18 < 10, this coal seam is an extractable coal seam. It is known that even if the coal seam permeability is much improved when the relative permeability reaches 55, the coal seam permeability has not changed qualitatively, and still qualitatively belongs to the extractable coal seam. In this case, the relative permeability is 51 at 6 m from the roadway wall, which can be approximately regarded as the critical position of CCR and NCCR.

## 5. Field Testing

#### 5.1. Coal Roadway Test

#### 5.1.1. Test Site and Protocol Design

#### 5.1.2. Experimental Equipment

#### 5.1.3. Operation Steps

- (1)
- Drill the hole with 42 mm air-coal drill and blow the hole clean.
- (2)
- Connect the capsule to the pipeline and feed it into the designated position of the borehole.
- (3)
- Water is injected to expand the capsule, and the capsule pressure is approximately 3 MPa.
- (4)
- Inject the mucus with surfactant and a small amount of bentonite to stabilize the mucus pressure at 2 MPa.
- (5)
- Inject SF
_{6}gas through the gas injection tube to make the pressure gauge reading reach 0.2 MPa. - (6)
- Use XP-1A qualitative leak detector in the borehole or downwind of the roadway to test for SF
_{6}escape. - (7)
- Observe and record the change of gas pressure in the borehole.

#### 5.1.4. Test Results

#### 5.2. Rock Roadway Test

#### 5.2.1. Test Site and Protocol Design

#### 5.2.2. Test Results

## 6. Conclusions

- (1)
- Roughness can measure the degree of stress concentration when the rock body is stressed. Through the relationship between roughness and geological strength GSI in H-B criterion, the value of geological strength GSI is optimized, and the theoretical model for the analysis of the stability of the borehole surrounding rock is optimized. Based on the optimized theoretical model for numerical simulation, the boundary of the pressure relief zone (loose circle) of the roadway is 7 m from the roadway wall, the peak point of the stress is 9.5 m from the roadway wall, and the original rock stress area is 20 m from the roadway wall.
- (2)
- The pressure relief zone or so-called loose circle in the surrounding rock of the borehole can be distinguished into CCR and NCCR. However, it is difficult to judge whether whether the crack is connected to the free surface of the roadway through current research results. At the same time, the permeability of the surrounding rock exists in practice, to a certain extent. Due to the objective existence of permeability of the surrounding rock, the gas in the borehole will definitely penetrate into the roadway through the surrounding rock or the sealing material. Therefore, theoretically, the problem of “gas leakage” in boreholes is inevitable. As the research on permeability is more mature, we propose that the quantitative standard of whether the borehole is “air leakage” or whether the surrounding rock cracks around the borehole are connected to the free surface can be characterized by permeability.
- (3)
- Since there is no quantitative classification standard for “whether the crack is connected to the free surface of the roadway”, we can refer to the standard for the degree of difficulty of coal seam gas drainage; that is, the difficulty degree of gas drainage can be measured by the permeability coefficient. In the engineering context of this paper, it is known that even if the coal seam permeability is much improved when the relative permeability reaches 55, the coal seam permeability has not changed qualitatively, and still qualitatively belongs to the extractable coal seam. In this case, the relative permeability is 51 at 6 m from the roadway wall, which can be approximately regarded as the critical position of CCR and NCCR.
- (4)
- In the direction from the permeability minimum point to the free surface of the roadway wall, the trend of relative permeability is monotonically increasing, while the variation rate of relative permeability has an inflection point at 2–4 m from the free surface of the roadway wall. Specifically, in the direction from the permeability minimum point to the free surface of the roadway wall, the variation rate of relative permeability of the surrounding rock within the range of 2 m from the free surface of the roadway wall is much larger than that within 4–9.5 m from the roadway wall. Therefore, it can be confirmed that the area represented by the inflection point of the variation rate of relative permeability is the area where the variation rate of relative permeability changes from quantitative to qualitative. At the distance of 2–4 m from the free surface of the roadway wall, the cracks begin to interconnect and connect with the free surface of the roadway wall. Therefore, the area within 2–4 m from the free surface of the roadway wall is the boundary area between CCR and NCCR.
- (5)
- In order to verify the existence of NCCR in the pressure relief zone, the qualitative detection of sulfur hexafluoride gas was carried out by tracer gas. When the sealing depth of the coal roadway is 6 m and the sealing depth of the rock roadway is 5 m, there is no gas leakage in the borehole; that is, there is an area where the fissure is not connected with the free surface of the roadway in the pressure relief zone.

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

## Abbreviations

GSI | Geological Strength Index | ||

JRC | Rock joint roughness coefficient | ||

SR | Rock structure polarity | ||

SCR | rock surface condition roughness | ||

SFR | stress discount factor | ||

RQD | rock quality designation | ||

CCR | an area where the crack of the surrounding rock of the borehole is connected with the free surface of the roadway wall | ||

NCCR | an area where the crack of the surrounding rock of the borehole is not connected with the free surface of the roadway wall | ||

Nomenclature | |||

J_{r} | joint roughness | Greek symbols | |

J_{n} | joint set coefficient | k_{a} | permeability of coal mass in elastic deformation phase (m^{2}) |

J_{a} | joint alteration | σ_{1} | the maximum compressive stresses (MPa) |

J_{w} | joint water reduction coefficient | σ_{3} | the minimum compressive stresses (MPa) |

m_{b} | the corresponding rock dimensionless parameters | σ_{ci} | the uniaxial compressive strength of the rock (MPa) |

s | the corresponding rock dimensionless parameters | b_{σ} | Coefficient of effect of volumetric stress on permeability (MPa^{−}^{1}) |

a | the corresponding rock dimensionless parameters | ΔΘ | Volume stress change (MPa) |

m_{i} | rock empirical parameters | ε^{p} | equivalent plastic strain |

D | disturbance factor | ε^{p}* | initial equivalent plastic strain in the residual strength phase |

k_{0} | Initial permeability of the coal mass (m^{2}) | ζ | permeability surge factor |

k_{b} | permeability of coal mass in plastic deformation state (m^{2}) | Φ_{1} | porosity of the roadway after decompression |

k_{c} | coal mass permeability during the residual strength phase (m^{2}) | Φ_{0} | original porosity |

k_{1} | Permeability after pressure relief (m^{2}) |

## References

- Lee, Y.-K.; Pietruszczak, S. A New Numerical Procedure for Elasto-Plastic Analysis of a Circular Opening Excavated in a Strain-Softening Rock Mass. Tunn. Undergr. Space Technol.
**2008**, 23, 588–599. [Google Scholar] [CrossRef] - Li, S.; Feng, X.-T.; Li, Z.; Chen, B.; Zhang, C.; Zhou, H. In Situ Monitoring of Rockburst Nucleation and Evolution in the Deeply Buried Tunnels of Jinping Ii Hydropower Station. Eng. Geol.
**2012**, 137–138, 85–96. [Google Scholar] [CrossRef] - Sloan, S.W. Geotechnical Stability Analysis. Géotechnique
**2013**, 63, 531–571. [Google Scholar] [CrossRef] [Green Version] - Feng, G.-L.; Feng, X.-T.; Chen, B.-R.; Xiao, Y.-X.; Yu, Y. A Microseismic Method for Dynamic Warning of Rockburst Development Processes in Tunnels. Rock Mech. Rock Eng.
**2015**, 48, 2061–2076. [Google Scholar] [CrossRef] - Li, S.; Liu, B.; Xu, X.; Nie, L.; Liu, Z.; Song, J.; Sun, H.; Chen, L.; Fan, K. An Overview of Ahead Geological Prospecting in Tunneling. Tunn. Undergr. Space Technol.
**2017**, 63, 69–94. [Google Scholar] [CrossRef] - Yang, S.Q.; Chen, M.; Jing, H.W.; Chen, K.F.; Meng, B. A Case Study on Large Deformation Failure Mechanism of Deep Soft Rock Roadway in Xin’an Coal Mine, China. Eng. Geol.
**2017**, 217, 89–101. [Google Scholar] [CrossRef] - Bi, R.; Cheng, W.; Chen, X. Effect of Calcium Sulfate Variety and Content on Hydration Mechanism of Grout Sealants for Coal-Bed Methane Drainage Boreholes. Constr. Build. Mater.
**2019**, 203, 443–455. [Google Scholar] [CrossRef] - Chen, X.; Bi, R.; Zhang, L. Effect of Water-to-Cement Ratio on Sulfo-Aluminate Type Cementitious Grouting Materials. Mag. Concr. Res.
**2019**, 71, 298–308. [Google Scholar] [CrossRef] - Li, Y.B.; Yue, C.H.; Guo, X. Research on Drilling and Sealing Technology for Cross Bedding of Superthick Coal Seam. Adv. Civ. Eng.
**2022**, 2022, 3011856. [Google Scholar] [CrossRef] - Zhao, D.; Liu, J.; Pan, J.T. Study on Gas Seepage from Coal Seams in the Distance between Boreholes for Gas Extraction. J. Loss Prev. Process Ind.
**2018**, 54, 266–272. [Google Scholar] [CrossRef] - Xie, C.; Nguyen, H.; Choi, Y.; Armaghani, D.J. Optimized Functional Linked Neural Network for Predicting Diaphragm Wall Deflection Induced by Braced Excavations in Clays. Geosci. Front.
**2022**, 2, 101313. [Google Scholar] [CrossRef] - Ge, Z.; Cao, S.; Lu, Y.; Gao, F. Fracture Mechanism and Damage Characteristics of Coal Subjected to a Water Jet under Different Triaxial Stress Conditions. J. Pet. Sci. Eng.
**2022**, 208, 109157. [Google Scholar] [CrossRef] - Hu, S.; Hao, G.; Feng, G.; Guo, H.; Wu, D. A Method for Improving the Methane Extraction Concentrations of in-Seam Boreholes. Fuel
**2020**, 265, 117006. [Google Scholar] [CrossRef] - Huang, C.G.; Zhang, Y.B.; He, J.F.; Luo, Y.; Sun, Z.G. Permeability Improvements of an Outburst-Prone Coal Seam by Means of Presplitting and Blasting with Multiple Deep Boreholes. Energy Sci. Eng.
**2019**, 7, 2223–2236. [Google Scholar] [CrossRef] [Green Version] - Chen, X.; Shan, W.; Sun, R.; Zhang, L. Methane Displacement Characteristic of Coal and Its Pore Change in Water Injection. Energy Explor. Exploit.
**2020**, 38, 1647–1663. [Google Scholar] [CrossRef] - Lin, B.; Yan, F.; Zhu, C.; Zhou, Y.; Zou, Q.; Guo, C.; Liu, T. Cross-Borehole Hydraulic Slotting Technique for Preventing and Controlling Coal and Gas Outbursts During Coal Roadway Excavation. J. Nat. Gas Sci. Eng.
**2015**, 26, 518–525. [Google Scholar] [CrossRef] - Liu, Z.; Cheng, Y.; Jiang, J.; Li, W.; Jin, K. Interactions between Coal Seam Gas Drainage Boreholes and the Impact of Such on Borehole Patterns. J. Nat. Gas Sci. Eng.
**2017**, 38, 597–607. [Google Scholar] [CrossRef] [Green Version] - Zhang, X.; Zou, J. Research on Collaborative Control Technology of Coal Spontaneous Combustion and Gas Coupling Disaster in Goaf Based on Dynamic Isolation. Fuel
**2022**, 321, 124123. [Google Scholar] [CrossRef] - Wei, P.; Huang, C.; Li, X.; Peng, S.; Lu, Y. Numerical Simulation of Boreholes for Gas Extraction and Effective Range of Gas Extraction in Soft Coal Seams. Energy Sci. Eng.
**2019**, 7, 1632–1648. [Google Scholar] [CrossRef] - Xia, T.; Zhou, F.; Liu, J.; Gao, F. Evaluation of the Pre-Drained Coal Seam Gas Quality. Fuel
**2014**, 130, 296–305. [Google Scholar] [CrossRef] - Xu, Y.; Chen, X.; Zhao, W.; Chen, P. Effect of Water Intrusion on the Characteristics of Surface Morphology and Pore Fracture Spaces in Argillaceous Meagre Coal. J. Nat. Gas Sci. Eng.
**2020**, 81, 103404. [Google Scholar] [CrossRef] - Xue, Y.; Gao, F.; Liu, X.; Li, X. Research on Damage Distribution and Permeability Distribution of Coal Seam with Slotted Borehole. Comput. Mater. Contin.
**2015**, 47, 127–141. [Google Scholar] - Zhang, K.; Sun, K.; Yu, B.; Gamage, R.P. Determination of Sealing Depth of in-Seam Boreholes for Seam Gas Drainage Based on Drilling Process of a Drifter. Eng. Geol.
**2016**, 210, 115–123. [Google Scholar] [CrossRef] - Chen, X.; Zhang, L.; Shen, M. Experimental Research on Desorption Characteristics of Gas-Bearing Coal Subjected to Mechanical Vibration. Energy Explor. Exploit.
**2020**, 38, 1454–1466. [Google Scholar] [CrossRef] - Zheng, C.; Chen, Z.; Kizil, M.; Aminossadati, S.; Zou, Q.; Gao, P. Characterisation of Mechanics and Flow Fields around in-Seam Methane Gas Drainage Borehole for Preventing Ventilation Air Leakage: A Case Study. Int. J. Coal Geol.
**2016**, 162, 123–138. [Google Scholar] [CrossRef] [Green Version] - Zuo, S.; Zhang, L.; Deng, K. Experimental Study on Gas Adsorption and Drainage of Gas-Bearing Coal Subjected to Tree-Type Hydraulic Fracturing. Energy Rep.
**2022**, 8, 649–660. [Google Scholar] [CrossRef] - Brown, E.T.; Bray, J.W.; Ladanyi, B.; Hoek, E. Ground Response Curves for Rock Tunnels. J. Geotech. Eng.
**1983**, 109, 15–39. [Google Scholar] [CrossRef] - Hoek, E.; Brown, E.T. The Hoek–Brown Failure Criterion and Gsi—2018 Edition. J. Rock Mech. Geotech. Eng.
**2019**, 11, 445–463. [Google Scholar] [CrossRef] - Jayanthu, S.; Singh, T.N.; Singh, D.P. Stress Distribution During Extraction of Pillars in a Thick Coal Seam. Rock. Mech. Rock Eng.
**2004**, 37, 171–192. [Google Scholar] - Li, X.; Gong, F.; Tao, M.; Dong, L.; Du, K.; Ma, C.; Zhou, Z.; Yin, T. Failure Mechanism and Coupled Static-Dynamic Loading Theory in Deep Hard Rock Mining: A Review. J. Rock Mech. Geotech. Eng.
**2017**, 9, 767–782. [Google Scholar] [CrossRef] - Xiao, S.; Ren, Q.; Cheng, Y.; Zhao, H.; Cao, S.; Zhang, L.; Chen, B.; Meng, X. Damage and Fracture Characteristics of Rocks with Different Structures under High-Velocity Water Jet Impact. Eng. Fract. Mech.
**2021**, 256, 107961. [Google Scholar] [CrossRef] - Zhao, Y.; Lin, B.; Liu, T.; Kong, J.; Zheng, Y. Gas Flow in Hydraulic Slotting-Disturbed Coal Seam Considering Stress Relief Induced Damage. J. Nat. Gas Sci. Eng.
**2020**, 75, 103160. [Google Scholar] [CrossRef] - Zhao, Y.; Wang, Y.; Wang, W.; Wan, W.; Tang, J. Modeling of Non-Linear Rheological Behavior of Hard Rock Using Triaxial Rheological Experiment. Int. J. Rock Mech. Min. Sci.
**2017**, 93, 66–75. [Google Scholar] [CrossRef] - Hoek, E.; Carranza-Torres, C.; Corkum, B. Hoek-Brown Failure Criterion-2002 Edition. In Proceedings of the NARMS-Tac 1, Toronto, ON, Canada, 7–10 July 2002; pp. 267–273. [Google Scholar]
- Barton, N. The influence of joint properties in modelling jointed rock masses. In Proceedings of the 8th ISRM Congress 1995, Tokyo, Japan, 25–30 September 1995. [Google Scholar]
- Barton, N.; Bandis, S. Effects of block size on the shear behavior of jointed rock. In Proceedings of the 23rd US Symposium on Rock Mechanics (USRMS) 1982, Berkeley, CA, USA, 25–27 August 1982. [Google Scholar]
- Belem, T.; Homand-Etienne, F.; Souley, M. Quantitative Parameters for Rock Joint Surface Roughness. Rock Mech. Rock Eng.
**2000**, 33, 217–242. [Google Scholar] [CrossRef] - Fardin, N.; Stephansson, O.; Jing, L. The Scale Dependence of Rock Joint Surface Roughness. Int. J. Rock Mech. Min. Sci.
**2001**, 38, 659–669. [Google Scholar] [CrossRef] - Kulatilake, P.H.S.W.; Balasingam, P.; Park, J.; Morgan, R. Natural Rock Joint Roughness Quantification through Fractal Techniques. Geotech. Geol. Eng.
**2006**, 24, 1181–1202. [Google Scholar] [CrossRef] - Lee, Y.H.; Carr, J.R.; Barr, D.J.; Haas, C.J. The Fractal Dimension as a Measure of the Roughness of Rock Discontinuity Profiles. Int. J. Rock Mech. Min. Sci. Geomech. Abstr.
**1990**, 26, 453–464. [Google Scholar] [CrossRef] - Maerz, N.H.; Franklin, J.A.; Bennett, C.P. Joint Roughness Measurement Using Shadow Profilometry. Int. J. Rock Mech. Min. Sci. Geomech. Abstr.
**1990**, 27, 329–343. [Google Scholar] [CrossRef] - Myers, N.O. Characterization of Surface Roughness. Wear
**1962**, 5, 182–189. [Google Scholar] [CrossRef] - Yang, Z.Y.; Lo, S.C.; Di, C.C. Reassessing the Joint Roughness Coefficient (Jrc) Estimation Using Z2. Rock Mech. Rock Eng.
**2001**, 34, 243–251. [Google Scholar] [CrossRef] - Yu, X.; Vayssade, B. Joint Profiles and Their Roughness Parameters. Int. J. Rock Mech. Min. Sci. Geomech. Abstr.
**1991**, 28, 333–336. [Google Scholar] [CrossRef] - Zhang, G.; Karakus, M.; Tang, H.; Ge, Y.; Zhang, L. A New Method Estimating the 2d Joint Roughness Coefficient for Discontinuity Surfaces in Rock Masses. Int. J. Rock Mech. Min. Sci.
**2014**, 72, 191–198. [Google Scholar] [CrossRef] - Fardin, N.; Feng, Q.; Stephansson, O. Application of a New in Situ 3d Laser Scanner to Study the Scale Effect on the Rock Joint Surface Roughness. Int. J. Rock Mech. Min. Sci.
**2004**, 41, 329–335. [Google Scholar] [CrossRef] - Barton, N.; Choubey, V. The Shear Strength of Rock Joints in Theory and Practice. Rock Mech.
**1977**, 10, 1–54. [Google Scholar] [CrossRef] - Sonmez, H.; Ulusay, R. Modifications to the Geological Strength Index (Gsi) and Their Applicability to Stability of Slopes. Int. J. Rock Mech. Min. Sci.
**1999**, 36, 743–760. [Google Scholar] [CrossRef] - Marinos, P.; Hoek, E. Gsi: A geologically friendly tool for rock mass strength estimation. In Proceedings of the ISRM International Symposium 2000, Melbourne, Australia, 19–24 November 2000; pp. 1422–1442. [Google Scholar]
- Cai, M.; Kaiser, P.K.; Uno, H.; Tasaka, Y.; Minami, M. Estimation of Rock Mass Deformation Modulus and Strength of Jointed Hard Rock Masses Using the Gsi System. Int. J. Rock Mech. Min. Sci.
**2004**, 41, 3–19. [Google Scholar] [CrossRef] - Cai, M. Rock Mechanics and Engineering; Beijing Science Press: Beijing, China, 2013. [Google Scholar]
- Chang, Z. Study on New Inorganic Sealing Materials and Technology; China University of Mining and Technology: Beijing, China, 2014. [Google Scholar]
- Ruiqing, B. Study on Sealing Parameters and Sealing Material Performance of Gas Drainage Borehole; China University of Geosciences: Beijing, China, 2019. [Google Scholar]

**Figure 2.**Schematic diagram of gas leakage mechanism of gas extraction borehole. (

**a**) Air leakage of sealing material; (

**b**) air leakage from the loosened circle of the drill hole; (

**c**) Air leakage from the coal mass pressure relief zone.

**Figure 8.**Curve of relative permeability and relative permeability change rate after excavation of the roadway.

**Figure 9.**Schematic diagram of field test. 1—Fracture and free surface penetration zone. 2—Fracture and free surface non-penetration zone. 3—Coal seam. 4—Gas injection pipe. 5—Water injection pipe. 6—Liquid injection pipe. 7—Capsule. 8—Borehole. 9—Mucilage.

**Figure 10.**Schematic diagram of the structure of hole sealing device. 1—water injection thin tube. 2—35 mm O.D. capsule. 3—quarter pipe hoop. 4—quarter pipe. 5—M10 pair of wire. 6—38 mm O.D. capsule. 7—M14 pair of wire. 8—9 mm O.D. pipe line. A—piezometer pipe. B—support pipe hoop. C—grouting pipe.

**Figure 14.**Coal tunnel field test photo: (

**a**) photo of borehole sealing device; (

**b**) photo of site operation; (

**c**) photo of completed borehole sealing.

**Figure 15.**Rock tunnel field test photo: (

**a**) photo of borehole sealing device; (

**b**) photo of site operation; (

**c**) photo of completed borehole sealing.

Geological Group Color | Stratigraphic Number | Rockiness | Thickness (m) | Depth (m) |
---|---|---|---|---|

1 | Fine Sandstone | 4.5 | 782.3 | |

2 | Sandy mudstone | 13.2 | 786.8 | |

3 | E_{8} coal seam | 2.2 | 800 | |

4 | Sandy mudstone | 6 | 806 | |

5 | Siltstone | 2.4 | 808.4 |

Rockiness | Bulk Modulus (GPa) | Shear Modulus (GPa) | Modulus of Elasticity (GPa) | Poisson’s Ratio | Tensile Strength (MPa) | Density (kg/m^{3}) |
---|---|---|---|---|---|---|

Fine Sandstone | 18.49 | 14.45 | 34.4 | 0.19 | 4.16 | 2699 |

Sandy mudstone | 10.73 | 8.05 | 19.32 | 0.2 | 2.24 | 2511 |

E_{8} coal seam | 12.47 | 8.96 | 21.69 | 0.21 | 0.21 | 1421 |

Sandy mudstone | 10.18 | 7.63 | 18.32 | 0.2 | 1.23 | 2595 |

Siltstone | 13.22 | 9.92 | 23.8 | 0.2 | 2.24 | 2622 |

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**

Bi, R.; Chen, X.; Zhang, L.; Wang, S.; Liu, X.; Hua, S.
Refinement Study on Pressure Relief Zone of Gas Extraction Borehole Considering Roughness. *Sustainability* **2022**, *14*, 9541.
https://doi.org/10.3390/su14159541

**AMA Style**

Bi R, Chen X, Zhang L, Wang S, Liu X, Hua S.
Refinement Study on Pressure Relief Zone of Gas Extraction Borehole Considering Roughness. *Sustainability*. 2022; 14(15):9541.
https://doi.org/10.3390/su14159541

**Chicago/Turabian Style**

Bi, Ruiqing, Xuexi Chen, Liang Zhang, Shuangyan Wang, Xiaohui Liu, and Shuai Hua.
2022. "Refinement Study on Pressure Relief Zone of Gas Extraction Borehole Considering Roughness" *Sustainability* 14, no. 15: 9541.
https://doi.org/10.3390/su14159541