Threat level assessment of flying gangue in steep coal seam mining

The core technology of non pillar gob side entry retaining by roof cutting is to adopt presplitting blasting technology to cut off the mechanical connection between roadway and goaf roof.Presplitting blasting implies a coupled static dynamic loading of the main roof.Dynamic static coupling load is intended to cut the roof by the crack initiation and propagation between the blast holes along the roadway while ensuring that the roof above the roadway remains undamaged.This process,controlled by the spacing between the blast holes (SBH) and the load application length (LAL),was studied for the particular case of the tailgate at the 7135 working face in the Qidong Coal Mine,China.Based on the attenuation formula of the stress wave in the rock strata,the fracture mechanism of main roof presplitting blasting was analyzed,and the fracture criterion based on tensile strength was established to determine the smallest quantitative relationship between SBH and LAL during the crack formation.Based on the characteristics of the main roof stress in the process of presplitting blasting,the mechanical model of the dynamic static coupling action of the main roof mechanics is established.The calculation formula of the main roof stress under the coupling action is obtained.The stress distribution law of the main roof at different time under dynamic load is analyzed.The propagation law of the blasting stress wave in the main roof is revealed,which is the alternating transformation of tension and pressure,and the same position of the main roof of the roadway is continuously affected by the tension stress and pressure dynamic pulse.The relationship between the main roof tensile stress distribution and different lateral stress concentration factors under static load is analyzed.The results show that the stress concentration coefficient in the limit equilibrium zone only affects the local tensile stress distribution of the main roof of the roadway (03.0 m).The main roof stability of the roadway is analyzed and the stability criterion of the main roof is obtained,and the maximum quantitative relationship between SBH and LAL is determined.According to the VB calculation platform,the length of presplitting blasting charge at the 7135 working face of Qidong Coal Mine is 4 m,and the spacing between blastholes is 600 mm.These predictions are experimentally verified by the visual observation of the roof convergence through the blast holes.