Study on roof control with blasting from both ground and underground for shallow buried caving coal working face

During the initial mining stage of fully mechanized top-coal caving faces with shallowly buried hard roofs, problems such as delayed first weighting, long suspended roof and difficult coal caving may occur. A variety of technical schemes for roof control have been proposed for safe initial mining under such conditions, but the evaluation of technical effects is obviously insufficient, especially the in-depth analysis of low-efficiency schemes. In order to comprehensively evaluate the technical effect of roof control during initial mining and optimize the technical scheme, based on the 15206 working face of a mine in Shuozhou, Shanxi Province, methods including case study, theoretical analysis and numerical simulation were adopted to investigate the technical scheme of roof control by underground pre-splitting blasting and surface deep-hole blasting applied in its initial mining stage. The results show that although the first weighting was realized after one round of underground pre-splitting blasting and two rounds of surface deep-hole blasting, the whole treatment lasted more than 2 months, with a total production shutdown of 24 days during initial mining, and the recovery rate decreased by 11% due to the failure of coal caving, leading to reduced production efficiency and economic benefit, and the implemented underground-surface combined treatment presented an unsatisfactory effect. From multi-perspective analysis based on plate theory, fracture toughness and blasting energy efficiency, the unsatisfactory performance is attributed to inappropriate parameters including explosive charge, borehole spacing, operation position and coupling mode, which failed to effectively relieve the boundary constraints. The constructed blasting works could hardly overcome the fracture toughness of hard rock mass to form macroscopic fracture surfaces, resulting in poor control effect. An optimized scheme of “blasting first, then water injection” is proposed, which achieves controllable roof weakening through combined blasting and water injection, and can significantly reduce the blasting engineering quantity and realize multi-purpose utilization of single borehole. Numerical simulation indicates that the technical scheme after optimized can effectively shorten the first weighting span, improve the roof caving morphology and facilitate top-coal caving.