Mechanism study on high efficiency gas drainage of high level strike long boreholes in gob roof

This paper is devoted to improve the gas extraction efficiency for high level strike long boreholes in gob roof and to eliminate the gas exceeding limit accidents of upper corner of mining face.A combining study of numerical simulation， theoretical analysis， and field test was conducted on No.2102 fully-mechanized mining face of Shanxi Huajin Jining Coal Industry Co.， Ltd.Firstly， the 3DEC software was used to simulate the evolution process of crack field in gob roof of No.2102 fully-mechanized mining face during mining.According to the field distributions of fissure， stress， and strain from the simulation results， the roof of gob was divided into fissure strengthening area and compaction area along the mining face.During face advance， the roof of the coal seam passed the fracture strengthening area and the re-compaction area in sequence.The section of boreholes located in fracture strengthening area was the high efficiency extraction section.The mechanism of high efficiency gas drainage of high level strike long boreholes in gob was then revealed - the gas extraction efficiency of the high level boreholes was determined by the length and the development degree of the cracks of the high efficiency extraction section of the boreholes.Based on the mechanism，a number of high-level test boreholes were arranged in the roof fracture zone of the gob to investigate the gas drainage effect.The results show that the optimum location of the high level boreholes was about 60 m to ensure the high level boreholes in the inner roof fracture zone of return air roadways.When applying the high level test boreholes， the gas concentration in the upper corner was reduced from 1.1% to 0.6%.The results indicate that the gas concentration in upper corner could be significantly reduced by utilizing strike long boreholes within roof fissure zone in inner gob of return air roadways.