Floor fracture evolution and relief gas drainage timeliness in deeper underground short-distance upper protective coal seam extraction

In deeper underground short-distance upper protective seam extraction,substantial amounts of relief gas ab- sorbed in the protected coal seam would influx into the working face of the protective seam through the floor mining-in- duced fractures,which might cause gas concentration exceeding the statutory limit. This paper takes the 5th Mine of Ti- anan Coal Co. ,Ltd. as a case,and studies the floor fracture evolution and relief gas drainage timeliness in deep under- ground short-distance upper protective seam extraction,using the methods of theoretical analysis,laboratory test,field survey and 3D distinct element numerical simulation. It concludes that the floor area could be divided into initial stress region,pressure relived and permeability enhanced region ( PRPER),and re-compacted region. In the PRPER,the coal permeability enhancements was accompanied with its swelling,the mining-induced floor fracture became fracture gas channel connecting the protected coal seam to the protective working face. On the time scale,the PRPER occurred during the period between the initial extraction and the collapse of main roof. The fracture gas channel re-compacted and disappeared gradually with the collapse of main roof. During the normal excavation,the area of PRPER held a pos- itive correlation with the periodic weighting,and reached its peak when the preliminary extraction proceeded. In the re- compacted region,coal and rock stratum would suffer the stress of loading-unloading-reloading,generating certain dam- age and failure,and finally contribute to the gas desorption as well as the shrinkage of coal. The PRPER is the primary space that relief gas generates and migrates,and it is thus the target area for highly efficient relief gas drainage. When proceeding gas drainage projects,it is advised to consider the effect of the time-space on the fracture evolution induced by mining.