Case Study on Pre-Splitting Blasting Reasonable Parameters of Goaf-Side Entry Retained by Roof Cutting for Hard Main Roof

Abstract

The key parameters of pressure relief pre-splitting blasting technology (PRPBT) contribute to the implementation effect of goaf-side entry by roof cutting (GSERC). This study is an attempt to design the key parameters including the roof cutting height, the roof cutting angle, the spacing between blasting boreholes, the charge structure and the sealing length by using theoretical analysis, numerical simulation and field implementation. The basic quality index of the main roof is 666.27, and combined with the peeping observation result, the main roof belongs to the category of a hard main roof. Different from a weakened roof and a compound roof, the unreasonable parameters of the PRPBT for the hard main roof lead to three problems including an insufficient pre-splitting blasting effect, roof suspension in the goaf and serious damage to the rock mass for the roof cutting rib. The PRPBT effect is closely related to key parameters, including the roof cutting parameters and the pre-splitting blasting parameters. Hence, to solve those three problems, a new design strategy was proposed based on the optimized directions of increasing the roof cutting height, decreasing the spacing between the blasting boreholes, changing the charge structure and adjusting the sealing length. According to the results of the theoretical calculation, numerical simulation and in situ measurement, the roof cutting height, the roof cutting angle, the spaced distance between the blasting boreholes, the charge structure and the sealing length were determined as 10 m, 15°, 500 mm, 4 + 4 + 3 + 3 + 2 and 2.4 m, respectively. The fissure rate was proposed to assess the developed effect of different pre-splitting blasting schemes. The proposed design strategy achieved a better effect of pre-splitting blasting for the entry, and eliminated the roof suspension of the hard main roof in the goaf. The design strategy in this study could contribute to other similar coal mines under the condition of a hard main roof.