Mine Safety and Health Administration
Protecting Miners' Safety and Health Since 1978
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U.S. Department of Labor Mine Safety and Health Administration Protecting Miners' Safety and Health Since 1978 |
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“Typical Christmas Tree Pillaring Model”
Please Note: Though each mining situation will be different due to the different parameters (strength of coal, pillar size etc.), we feel this information is relevant to all situations. Convergence, or roof to floor closure, is an indicator of ground stability. Generally, more convergence corresponds to a higher stress and less stable mining environment. In this representation, a plan view of a mining section near the gob of a Christmas Tree method of retreat mining is shown. The animated cut sequence shown here depicts the change in convergence that can occur during retreat mining with a typical Christmas Tree method. The convergence levels (contours) are colored according to their magnitude (red being the greatest and gray showing the minimum convergence) and the coal pillars, or remnant stumps, are outlined in black. As each cut is mined, the convergence in the work area increases as the coal supporting the overburden and shielding the entry from gob pressures is reduced. This can be easily seen by following the position of the 0.10' convergence contour (the green convergence band) and visualizing where miners would be positioned during the extraction of each cut. A very important thing happens as the next to last cut (cut 9 - just prior to the push-out) is taken. The 0.12' convergence contour (yellow convergence band) suddenly moves from over the remaining coal fender into the crosscut where mining was just completed, and into the intersection, and the 0.10' convergence contour engulfs the entire intersection. The large roof movement occurs when the remaining coal fender yields and can no longer support the weight of the roof, and the gob pressures work across the crosscut and intersection roof to solid coal. In practice, the fender would show substantial sloughing, the adjacent ribs would show increased sloughing, and the roof may experience cutters or flaking when this occurs. This action could trigger a roof fall if the affected area is not adequately supported, and is very possibly the cause of many retreat mining roof fall accidents that have occurred throughout the years. Be aware of the critical cut during pillar recovery at your mine and insure that adequate support (supplemental bolts, timbers or MRS units) is present in the critical area. This simulation was based on 36' by 69' pillars, a mining height of 5 feet, a mining depth of 400 feet and typical coal strength (900 psi) with the Christmas Tree method. If your mine is deeper, the coal is higher, or pillars are smaller, the critical cut may happen sooner than shown in the animation. If your mine is shallower, the coal is lower or larger pillars are used, it may not happen until the push-out cut is taken. If your mine uses a different mining method, such as the pocket-and-fender or outside lift method, the same sudden increase in convergence can occur when the fender is reduced to a critical size.  See Simulation
This simulation is a Powerpoint® animated presentation. Click on frame to start. |
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Mine Safety and Health Administration (MSHA) 1100 Wilson Boulevard, 21st Floor Arlington, VA 22209-3939 |
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