# Study on the Pressure Relief Mechanism and Engineering Application of Segmented Enlarged-Diameter Boreholes

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## Abstract

**:**

## 1. Introduction

## 2. Destressing Mechanism of Segmented Enlarged-Diameter Borehole

#### 2.1. Stress Distribution Characteristics around the Segmented Enlarged-Diameter Borehole

_{r}—Radial stress at a point of coal around the borehole, MPa;

_{θ}—Tangential stress at a point of coal around the borehole, MPa;

_{rθ}, τ

_{θr}—Shear stress at a point of coal around the borehole, MPa;

_{i}—Radius (radius of unenlarged-diameter section is R

_{1}and radius of enlarged-diameter section is R

_{2}), m;

_{1}, σ

_{3}—the maximum and minimum principal stresses, MPa.

_{i}, cohesion c of the coal, and internal friction angle φ.

#### 2.2. Determination of Segmented Enlarged-Diameter Position of Large Diameter Pressure Relief Boreholes

_{a}, the radius of crushing zone is R

_{b}, the radius of plastic zone is R

_{p}, the effective anchoring length of bolt is l

_{0}, and the annular distance of bolt is f

_{1}. There is assumed to be no slip between the bolt and the surrounding rock. The stress of the original rock is P

_{0}, and the uniform support resistance provided by the roadway support is P

_{i}. Based on the Mohr-Coulomb criterion, the radius of the plastic zone was calculated to provide a basis for the calculation of the segmented enlarged-diameter position [33,34].

`→`∞, both radial stress ${\sigma}_{r}^{p}$ and tangential stress ${\sigma}_{\theta}^{p}$ tend to be stable and close to the original rock stress P

_{0}.

_{p}. When r = R

_{p}:

_{p}:

_{p}) is:

_{p}into Equation (10) gives the relationship between the plastic zone radius R

_{p}and p

_{i}:

_{i}. In order to reduce the extent of roadway damage, the plastic zone radius (R

_{p}) of surrounding rock under the influence of bolt support is the minimum length of the unenlarged-diameter section of borehole.

#### 2.3. Mechanism of Segmented Enlarged-Diameter Borehole Destressing

## 3. Simulation Schemes

#### 3.1. Numerical Model and Simulation Schemes

^{3D}numerical model was constructed based on the 6307 working face. The size of the model is 340 m × 200 m × 46 m (Figure 7). Considering the burial depth of the working face, a uniform distribution load of 23 MPa was applied at the top to simulate the weight of overlying strata. Rock type and mechanical parameters were selected based on mine geology reports and laboratory test results (Table 1). The large-diameter borehole was placed to the side of the coal side of the roadway. The mesh around the borehole is shown in Figure 8.

- (1).
- Different diameters of enlarged-diameter section

- (2).
- Different lengths of the enlarged-diameter section

- (3).
- Different borehole spaces

#### 3.2. Effect of Diameter of the Enlarged-Diameter Section on Pressure Relief Effect and Roadway Deformation

^{5}J (diameter of enlarged-diameter section was 90 mm) to 4.2 × 10

^{5}J (diameter of enlarged-diameter section was 240 mm).

#### 3.3. Effect of Lengths of the Enlarged-Diameter Section on Pressure Relief and Roadway Deformation

^{5}J (the length of the enlarged-diameter section was 11 m) to 4.2 × 10

^{5}J (Figure 16). It can be concluded that increasing the length of the enlarged-diameter section can effectively reduce the accumulated energy.

#### 3.4. Effect of Borehole Space on Pressure Relief Effect and Roadway Deformation

## 4. Field Test

#### 4.1. Key Parameters for Segmented Enlarged-Diameter Borehole

- (1).
- Total borehole lengthReferring to the borehole pressure relief parameters of the adjacent working face of the 6307 working face, the length of the pressure relief borehole of the 6307 working face was determined to be 20 m.
- (2).
- Diameter of the unenlarged-diameter sectionAccording to the simulation results in Section 3.2, when the diameter of the hole in the unenlarged-diameter section was 90 mm, the roadway deformation was relatively small. Thus, the diameter of the borehole in unenlarged-diameter section was determined to be 90 mm.
- (3).
- Diameter of enlarged-diameter sectionAccording to the simulation results in Section 3.2, with increasing diameter of the enlarged-diameter section, the peak elastic strain energy density gradually transfered to the deep surrounding rock of the roadway. The displacement on both sides of the roadway was less affected by the enlarged diameter section. Therefore, the diameter of the enlarged-diameter section was determined to be 240 mm.
- (4).
- Length of enlarged-diameter sectionBased on the roadway support design in the 6307 working face, the length of unenlarged-diameter was calculated to be no less than 3.7 m (Equation (14)). According to the simulation results (Section 3.3), it has little influence on the support system when the position of enlarged-diameter section was 5 m away from the roadway side. The length of enlarged-diameter section in roadway of 6307 working face was determined to be 15 m.
- (5).
- Borehole spaceAccording to the simulation results in Section 3.4 (Figure 18), when the space between pressure relief boreholes was less than 1.6 m, the decrease in space will not improve the pressure relief effect. However, it will significantly increase roadway deformation. Considering the construction and safety factors, the space of pressure relief borehole was determined to be 1.6 m. Table 5 and Figure 23 present the key parameters of the segmented enlarged-diameter borehole in the 6307 working face roadway.

#### 4.2. Pressure Relief Effect Monitoring

#### 4.2.1. Coal Stress Monitoring

#### 4.2.2. Roadway Deformation Monitoring

## 5. Conclusions

- (1).
- According to the theory of elastic-plastic mechanics, the distribution range and influencing factors of the coal pressure relief zone around the borehole are obtained. Furthermore, the minimum length of the unenlarged-diameter section of the borehole was determined.
- (2).
- The effects of diameter of the enlarged-diameter section, length of the enlarged-diameter section, and borehole space on pressure relief and roadway deformation were investigated. The larger the diameter of the enlarged-diameter section, the better the pressure relief effect. With increasing diameter of the enlarged-diameter section, the displacement on both sides of the roadway changed slightly. Increasing the length of the enlarged-diameter section can effectively reduce the energy accumulated around the boreholes and transfer the energy peak to the deep surrounding rock. However, the longer the length of the enlarged-diameter section, the larger the deformation of the surrounding rock. With decreasing borehole space, the peak energy around the boreholes decreased, and the deformation of both sides of the roadway increased significantly.
- (3).
- The key parameters of segmented enlarged-diameter pressure relief borehole were determined for 6307 working face. Field monitoring results showed that the accumulated energy can be effectively reduced using segmented enlarged-diameter pressure relief boreholes, effectively controlling the roadway deformation. Segmented enlarged-diameter pressure relief technology can effectively mitigate the problems of excessive pressure relief (the strength of the surrounding rock was reduced after pressure relief) and insufficient pressure relief (rock burst still occurred after pressure relief) as well as provide a reference for the rock burst prevention and roadway stability control.

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

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**Figure 6.**Diagram of the 6307 working face. ((

**a**) 6307 working face layout plan; (

**b**) 6307 working face top and bottom rock; (

**c**) Diagram of roadway support at 6307 working face).

**Figure 12.**Distribution maps of elastic strain energy density around boreholes under different diameters of the enlarged-diameter section.

**Figure 13.**Distribution curves of elastic strain energy density around boreholes under different diameters of the enlarged-diameter section.

**Figure 14.**Displacement curves of surrounding rock under different diameters of the enlarged-diameter section.

**Figure 15.**Distribution maps of elastic strain energy density around boreholes under different lengths of the enlarged-diameter section.

**Figure 16.**Distribution curves of elastic strain energy density around boreholes under different lengths of the enlarged-diameter section.

**Figure 17.**Displacement curves of surrounding rock under different lengths of the enlarged-diameter section.

**Figure 18.**Distribution curves of elastic strain energy density around boreholes under different borehole space.

Rock Type | Thickness /m | Density kg/m ^{3} | Bulk Modulus/GPa | Shear Modulus/GPa | Tension Strength/MPa | Cohesion /MPa | Friction Angle /(°) |
---|---|---|---|---|---|---|---|

Medium sandstone | 4.9 | 2570 | 7.95 | 6.87 | 4.52 | 6.12 | 31 |

Siltstone | 5.4 | 2700 | 16 | 10 | 2.3 | 2.4 | 32 |

Mudstone | 2.4 | 2550 | 3 | 1.3 | 1.5 | 1.7 | 29 |

Medium sandstone | 5.4 | 2570 | 7.95 | 6.87 | 4.52 | 6.12 | 31 |

Mudstone | 4.4 | 2550 | 3 | 1.3 | 1.5 | 1.7 | 29 |

Coal | 9.4 | 1400 | 1.5 | 0.7 | 1.1 | 1.2 | 23 |

Siltstone | 3.2 | 2700 | 16 | 10 | 2.3 | 2.4 | 32 |

Fine sandstone | 10.9 | 2620 | 8.5 | 5.6 | 4.7 | 5.52 | 35 |

No. | Total Borehole Length l_{1}/m | Length of the Enlarged-Diameter Section l _{2}/m | Diameter of the Unenlarged-Diameter Section d_{1}/mm | Borehole Space s_{1}/m | Diameter of the Enlarged-Diameter Section d_{2}/mm |
---|---|---|---|---|---|

1 | 20 | 15 | 90 | 1.6 | 90 |

2 | 20 | 15 | 90 | 1.6 | 140 |

3 | 20 | 15 | 90 | 1.6 | 190 |

4 | 20 | 15 | 90 | 1.6 | 240 |

No. | Total Borehole Length l_{1}/m | Diameter of the Unenlarged-Diameter Section d_{1}/mm | Diameter of the Enlarged-Diameter Section d_{2}/mm | Borehole Space s_{1}/m | Length of the Enlarged-Diameter Section l _{2}/m |
---|---|---|---|---|---|

1 | 20 | 90 | 240 | 1.6 | 11 |

2 | 20 | 90 | 240 | 1.6 | 13 |

3 | 20 | 90 | 240 | 1.6 | 15 |

4 | 20 | 90 | 240 | 1.6 | 17 |

5 | 20 | 90 | 240 | 1.6 | 19 |

No. | Total Borehole Length l_{1}/m | Length of the Enlarged-Diameter Section l _{2}/m | Diameter of the Unenlarged-Diameter Section d_{1}/mm | Diameter of the Enlarged-Diameter Section d_{2}/mm | Borehole Space s _{1}/m |
---|---|---|---|---|---|

1 | 20 | 15 | 90 | 240 | 0.8 |

2 | 20 | 15 | 90 | 240 | 1.6 |

3 | 20 | 15 | 90 | 240 | 2.4 |

4 | 20 | 15 | 90 | 240 | 3.2 |

Total Length of Borehole /m | Diameter of Enlarged-Diameter Section /mm | Diameter of Unenlarged-Diameter Section /mm | Length of Enlarged-Diameter Section /m | Borehole Space /m |
---|---|---|---|---|

20 | 240 | 90 | 15 | 1.6 |

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**MDPI and ACS Style**

Gu, S.; Chen, C.; Jiang, B.; Ding, K.; Xiao, H.
Study on the Pressure Relief Mechanism and Engineering Application of Segmented Enlarged-Diameter Boreholes. *Sustainability* **2022**, *14*, 5234.
https://doi.org/10.3390/su14095234

**AMA Style**

Gu S, Chen C, Jiang B, Ding K, Xiao H.
Study on the Pressure Relief Mechanism and Engineering Application of Segmented Enlarged-Diameter Boreholes. *Sustainability*. 2022; 14(9):5234.
https://doi.org/10.3390/su14095234

**Chicago/Turabian Style**

Gu, Shitan, Changpeng Chen, Bangyou Jiang, Ke Ding, and Huajian Xiao.
2022. "Study on the Pressure Relief Mechanism and Engineering Application of Segmented Enlarged-Diameter Boreholes" *Sustainability* 14, no. 9: 5234.
https://doi.org/10.3390/su14095234