Mechanism of dynamic phenomena of borehole for outburst prediction and their relationship during coal roadway driving

To improve the accuracy of the outburst prediction in working faces of coal roadways,the evolution mecha- nism of the dynamic phenomena such as gas and coal being ejected from the borehole ( GCEB) and drill pipe being stuck by coal mass in the borehole (DPSC)) were studied. Firstly,based on the theoretical formula of borehole wall displacement,the factors that affect the instability failure of borehole and the causes of occurrence for the GCEB and DPSC were discussed. Then the mechanical conditions and evolutionary process for the occurrence of GCEB were de- scribed,while the possible types of DPSC were analyzed. A self-designed drilling device that can simulate the in-situ outburst prediction of coal roadways was used in this study. The coal samples with different coal ranks to adsorb CO2 and N2 with different pressures in the laboratory were used to simulate the coal seams with different outburst risks. In addition,the boreholes for predicting the outburst were drilled in the simulated coal seams to study the correlation between common dynamic phenomena and outburst risks. The results show that under the same rising gradient of gas pressures,the occurrence possibility of outburst caused by coal masses adsorbing CO2 is larger than that of adsorbing N2 . When the coal seams without outburst risks change into ones with outburst risks,drilling cutting weight generally increase all of a sudden and the occurring frequency of GCEB and DPSC increase. The DPSC phenomenon indicating outburst risks is generally accompanied with GCEB,which form a linkage system and are triggered successively.