## 6 Conclusions  
In this study, the bidirectional blasting technology (BBT) is introduced for composite sandstone roofs in the gob-side entry-retaining mining method. The BBT uses an energy-gathering device to achieve directional blasting of the roof of the entry and then uses the collapsed gangue to fill the goat, assisting in the implementation of temporary support technology in the roadway, and realizes the gob-side entry-retaining mining method.  
(1) The technical theory and mechanical model of BBT were established to analyze the propagation law of rock cracks. The numerical simulation models of ordinary blasting and energy-gathering blasting were established, and the distribution law of explosion stress wave under two blasting modes was analyzed under the composite sandstone roof. After ordinary blasting, a uniform stress wave of blasting stress is generated around the  
Figure 19: Crack ratio of blasthole.  
Figure 20: The effects of entry-retaining by BBT. (**a**) blasting before mining; (**b**) Gagneuxing after mining; (**c**) Gagneux support in goat side of entry; (**d**) entry-retaining effects.  
blast hole, forming circular damage and destruction. After BBT with the energy-gathering device, under the action of the blasting stress wave, bidirectional blasting of the rock can be realized in the energy-gathering direction under the composite sandstone roof.  
(2) Using the energy-gathering effect of the energy-gathering device, the directional tensile failure of the rock can be realized under the composite sandstone roof. The BBT was applied to the 3402 working face of the Shaqu No. 2 coal mine, which was divided into four groups and conducted on-site blasting tests to determine the best \"\(2+2+1\) explosives\" charge structure that can effectively produce cracks. According to the statistics of the crack ratio in the blasthole on site, the crack ratio of the blasthole reached 96%.  
(3) After the mining, the U-shaped steel is used to support the gangue on the side of the goaf, and the monomer temporary support is used in the entry. Through on-site monitoring of the surrounding rock deformation of the gob-side retained entry by BBT, after the roadway is stabilized, the deformation of the two sides of the entry and the approaching amount of the roof and floor are 75 and 300 mm, respectively. The requirements for safe production in mines have been met. The blasting plan and charge structure have a certain guiding significance for determining reasonable blasting parameters in similar projects.  
Conceptualization, H.G. and Y.G.; methodology, Y.G., J.W., H.G. and Q.F.; validation, B.Q., Y.G. and X.W.; formal analysis, B.Q. and X.Z.; resources, Y.G. and J.W.; writing--original draft preparation, H.G.; writing--review and editing, Y.G.; supervision, Y.G.; project administration, Y.G. and J.W.; funding acquisition, Y.G. All authors have read and agreed to the published version of the manuscript.  
This work is supported by the State Key Laboratory for Geomechanics and Deep Underground Engineering, China University of Mining & Technology, Beijing (No. SKLGDUEK1928) and funded by the China Postdoctoral Science Foundation (No. 2020T130702), which are gratefully acknowledged.  
Not applicable.  
The data used to support the findings of this study are available from the corresponding author upon request.  
The authors declare that there is no conflict of interest regarding the publication of this paper.