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With the application of electronic detonators, delayed blasting is considered a very effective method to improve rock fragmentation, and this technique is widely used in practical engineering. Therefore, it is necessary to study the rock-breaking mechanism of double-hole delayed blasting. In this paper, based on the finite difference method, the influence of double-hole delayed blasting on the reservoir permeability was quantitatively analyzed by adopting the adjacent hole spacing, delay time, and decoupling coefficient (K) as variables and the permeability increment as an evaluation index. Based on the parameters studied in this paper, it was found that the effect of the delay time on the reservoir permeability depends on the hole spacing. When the hole spacing is less than 20ra (ra is the borehole radius), a reasonable delay time could significantly increase the reservoir permeability, but when the hole spacing is greater than 20ra, the delay time does not affect reservoir permeability enhancement. The decoupling coefficient could control the attenuation speed of shock wave energy. For K = 2, energy attenuation occurred the slowest, and the reservoir permeability increase effect was the best. Based on site-scale numerical simulations, the mechanism of stress superposition between boreholes was revealed, and the position of stress superposition was controlled by adjusting the delay time to break hard reservoir rock so that the cracks between two holes were connected, i.e., accurate reservoir reconstruction was achieved. This work provides a certain guiding basis for actual engineering applications. [ABSTRACT FROM AUTHOR]

Aiming at the problems of hole spacing and blasting scheme selection in coal rock loosening blasting construction, the distance between the holes in the fracture zone of the double hole blasting is calculated based on rock dynamics and blasting theory, and LS-DYNA is used to analyze the influence of stress wave and crack propagation superposition effect on the spacing of blasting holes and blasting plan in double hole blasting. The research results indicate that: when two holes blasting simultaneously and the spacing between the holes is 3.4 m (twice the crack radius of a single hole blasting), the crack areas between the holes cannot be fully connected; when the hole spacing is 2.5 m, the fracture zone between the holes can be fully penetrated; and in millisecond blasting, when the spacing between the blast holes is 3.4 m, due to the obvious secondary expansion of cracks in the crack area around the first blast holes under the superposition of stress waves in the later blast holes, the crack areas between the blast holes can be basically connected. However, when the spacing between the blast holes is 3.0 m, the crack areas between the blast holes can be fully connected and cover the entire range of the drill holes. When loosening blasting is carried out on the excavation surface on site, the spacing between blast holes should generally be less than twice the crack radius of a single hole blasting (approximately 70% of the estimated spacing between blast holes for simultaneous blasting and 90% of the estimated spacing between blast holes for millisecond blasting) to ensure the effectiveness of loosening blasting. Compared with simultaneous blasting, millisecond blasting can not only guarantee the blasting effect, but also reduce the number of holes and the amount of blasting materials, so it is the preferred blasting scheme.

This paper focuses on the radius of coal failure zones under cumulative blasting with shaped charge. Based on the analysis of the mutual superposition effect of the explosion stress waves during the simultaneous detonation of two blastholes, a numerical analysis model of the double-hole cumulative blasting with linear shaped charge was established. Additionally, the propagation characteristics of the stress wave during the simultaneous detonation of two blastholes, stress state of the coal body, mechanism of coal crack propagation and coalescence, and influence of the stress wave superposition effect on crack propagation were evaluated. Results show that the stress wave superposition effect induces the formation of a pressure equalization zone in the partial region of the middle section of the two blastholes and its adjacent regions. This occurrence forces the radial cracks of the two blastholes to turn, and they cannot connect with each other, leading to the formation of a gap blank zone between the two blastholes. After the directional cracks generated under cumulative blasting load coalesce, the collision of the explosive gases produced from the two blastholes further promotes the expansion of the cracks in the directional crack coalescence zone and eventually penetrates the gap blank zone. Field test results of deep-hole cumulative blasting in coal seams show that the explosion stress waves from the blastholes in the opposite side promotes the propagation of the blasting-induced crack on the left or right side of the two blastholes. This propagation first increases and then decreases as it moves away from the blasthole. Between the two blastholes, the stress wave superposition effect from the two blastholes inhibits the propagation of the cracks in some areas, resulting in a W-like fluctuation in the degree of improvement of the gas drainage effect at different positions in the area between the two blastholes.

This study aims to investigate the fracturing characteristics of double-hole blasting under the action of coupled static stress and delayed time explosion. A total of thirteen explosive tests were carried out on rectangular concrete blocks with different constraints. The test blocks were wrapped in steel mesh in the test bed to prevent flying away of the cracked blocks after blasting. After blasting, the surface area of the crater was measured, and all pieces of the cracked blocks were collected, screened, and analyzed. The experimental results show that (1) the shape of free-surface craters was affected by static stress. The surface area and overall volume of the craters increased in the direction of loading stress. (2) The crater under simultaneous initiation was larger than that under delayed initiation when the tested block had a single free surface. (3) The fragmentation of blasting is closely related to the static confining pressure and delay initiation applied on the tested specimens. It is believed that this research will contribute to the design of multi-hole blasting in the deep geological body in the respect of the theoretical analysis.