# Numerical Study on the Pressure Relief Characteristics of a Large-Diameter Borehole
## 7 Conclusions  
Based on numerical simulations, this paper examined the characteristics of stress, strain energy, the plastic zone, displacement, etc. Under the various parameters of the large-diameter borehole pressure relief, we explored the influences of various parameters on the pressure relief effect, analyzed the pressure relief mechanism of the large-diameter borehole, implemented the pressure relief project of the large-diameter borehole on-site, and evaluated the pressure relief effect. This research allows us to draw the following five conclusions:  
1. After drilling, the peak value of the stress and strain energy decreases and shifts to the deep area. When the hole diameter is too small, there is basically no effect on the pressure relief and energy release. When the hole diameter increases, the stress and strain energy of the surrounding rock of the drilling decrease, and the range of the pressure relief area and energy release area around the drilling gradually increases, and the effect on the pressure relief and energy release gradually increases. \((A/L_{\rm h})\) and \((E_{\rm P}-E_{0})/E_{0}\) can be used as evaluation indexes to determine the reasonable hole diameter. 2. When the hole depth \(L_{\rm h}<L_{\rm P}\), there are no pressure relief or energy release effects, and the high energy conduction path is shortened, which can induce impact. When \(L_{\rm P}<L_{\rm h}\), the peak moves to the deep area with the increase in the hole depth, and the stress and strain energy at the peak decreases, and the pressure relief and energy release effects are further enhanced. When the peak reaches \(2.5L_{\rm P}\), the pressure relief and energy release enhancement effect reach a limit; thus, the reasonable hole depth is determined as \(L_{\rm P}<L_{\rm h}\leq 2.5L_{\rm P}\). 3. With the decrease in the hole spacing, the elastic zone between the holes gradually narrows, the plastic zone and the broken ring zone are gradually penetrated, and the stress between the holes evolves from double peaks to a saddle shape and then to single peaks. Reducing the aperture of the plastic zone, the failure zone penetration and stress form transformation efficiency decrease, and the pressure relief effect weakens. Therefore, the hole diameter is the main factor affecting the pressure relief effect of the drilling, and the hole spacing is the secondary factor. The hole diameter should be increased first, and then the reasonable hole spacing should be selected. 4. The mechanism of pressure relief works to transfer the high stress in the shallow part of the roadway to the deep part, at the cost of artificially destroying the integrity of the surrounding rock of the roadway. After drilling, cracks are generated around the borehole, the plastic zone of the surrounding rock of the borehole is closed, the plastic energy dissipation effect is enhanced, and the deep impact conduction path is weakened to protect the roadway. 5. The influences of the key parameters, such as hole diameter, hole depth, and hole spacing, on the pressure relief effect and their primary and secondary relationships were studied here, and directions for the adjustment and optimization of the pressure relief parameters of large-diameter drilling in mines were defined. Through the engineering practice recommended here, the pressure relief effect is good, and there is no impact on the safety of the working face during mining, which is of great significance in guiding engineering practice. [PERSON] conceived, designed, and analyzed the test results; [PERSON] performed the experiments and wrote the manuscript; [PERSON] provided help with the numerical simulations; [PERSON] provided engineering site assistance. All the authors revised and proofread the manuscript. All authors have read and agreed to the published version of the manuscript. This work was sponsored by the National Natural Science Foundation of China (No.