Abstract: In order to study evolution characteristics of coal pores and fractures under impact loadings, with outburst coal of Machang Mine in Guizhou as a research object, Separate Hopkinson Pressure Bar (SHPB) test system, NMR and Scanning Electron Microscope(SEM) system were utilized to investigate their evolution features under different impact loadings. Then their evolution law and influence of coal impact on coal and gas outburst were analyzed. The results show that with the increase of impact pressure, micro pores' peak value semaphore and diameter ratio increase first and then decrease and connectivity of pores and fractures is enhanced, but peak value diameter ratio of medium and large pores decreases first and then increases. And pore throat distribution of micro pores in Machang Mine accounts for 38.85%～56.14% of the total while that of medium and large ones makes up for 43.86%-61.15%. Under impact loadings, evolution of pores and fractures in coal is carried out in the form of pores expansion, expansion of fractures and formation of secondary fractures. Moreover, impact loading strengthens connectivity of pores and fractures, breaks gas adsorption and desorption balance, causes an increase of gas pressure, and produces stress set and energy accumulation in coal body, thus increasing possibilities of coal and gas outburst.

Key words: impact loading, pore structure, outburst coal, nuclear magnetic resonance (NMR), pore throat distribution