3.Process and management requiremen

3.Process and management requirements of coal excavation in shafts
3.1. Process of coal excavation in shafts
To ensure the continuity and safety of the entire process of coal excavation, we provide the following definition of coal mining work based on gas control methodology, namely that regional outburst prevention measures are initially adopted, and additional local prevention measures. The entire process begins at a normal distance greater than 10 m from the coal seam; this distance is 20 m in complex geological structures or rock broken zones and ends at a distance of 2 m normal to the coal seam after uncovering the coal seam. The complete process is shown in Fig. 4.
Safety measures during coal excavation in the shaft would be taken at the following four stages: at 10 m normal distance away from the coal seam (20 m for complex geological structures or rock broken zones), at 7 m normal distance, 5 m normal distance and 1.5–2 m normal distance. The safety measures contain an advanced prediction, regional prediction, regional outburst prevention measures, a regional effect test and coal excavation by remote shooting.
(1) Advanced and regional prediction. At 10 m normal distance from the coal seam, we drill three exploration boreholes to determine the layer positions, sample the coal seam and measure the gas pressure. Using these exploration boreholes, we can ascertain the position of the working face relative to the coal seam and draw plane figure and sectional drawing of geologic condition. If there are enough coal sample available, the coal samples from the boreholes are then used to measure the initial rate of gas emission DP, the coal consistency coefficient f [1], and the gas content. To minimize the cost and duration of this measure, the boreholes can also be directly sealed for gas pressure [14]. All the measured gas parameters combined with the gas dynamic phenomena during drilling can be used to predict the outburst risk of coal seam ahead of the workface.
(2) Regional outburst prevention measures. At 7 m normal distance from the coal seam, we drill several boreholes after the prediction of a non-outburst coal seam. For a coal seam at risk for an outburst, we must take regional outburst prevention measures (such as the drilling of drainage or extraction boreholes) and accelerated gas drainage measures (e.g., coal sluicing and hole cleaning). When the regional effect test is effective for measuring the residual gas pressure or content, which must be reduced below 8 m3 t