2.2. The outburst risks of coal excavation in the shaft
Based on the stress distribution characteristics around the working face of the shaft, as shown in Fig. 3, it is found that the outburst risk in front of the heading face is relatively low because the front pressure relief zone reduced the upward outburst energy of the coal-rock mass that lies ahead. Even so, the retention of a reasonable section of rock pillar is essential to impede the occurrence of an outburst under the heading face in the shaft, because when the rock pillar is not strong enough to stop the combined effects of the gas pressure and ground stress, it may also cause an outburst accident. When the heading face penetrates into the outburst-prone coal seams, the exposed shaft wall is under the combined effect of the gas pressure and ground stress. If the range of the protection barrier surrounding the shaft was not sufficient to resist the combined effect without high strength support in time, the coal mass in the concentrated zone would lose stabilization and break, thereby resulting in the occurrence of a coal and gas outburst. Therefore, outbursts readily occur at the corner, the intersection of the base and wall of the shaft that is affected by the concentration of stress when the heading face advances into the coal seams, especially at the upper coal seam along the trend surrounding the shaft due to the stress concentration of gravity stress, concentrated stress and gas pressure.
Based on the analysis presented above, we consider that the enough protection barrier, which is formed by the coal-rock masses surrounding the shaft whose outburst risk has been eliminated, is essential for safety excavation. Therefore, the key technology of coal uncovering in shafts is to release the outburst energy within a certain range surrounding the shaft.