2008-02-22_APPLICATION CORRESPONDENCE - C2008086 (43)

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shape on the exposed pillar ribsides is commonly referred to as "hour glassed". After such a cleanup, the pillars sloughed again, and repeated until the pillar ribsides were supported and restrained. 6.0 SUBSIDENCE ESTIMATION OVER CAMEO SEAM LONGWALL PANELS, RED CLIFF MINE PROJECT AREA The primarily graphical subsidence estimation method developed by the British National Coal Board (NCB, 1975) for estimating trough subsidence over longwalls was used for the Red Cliff Mine Project Area. The method was based on 177 profiles measured over named longwall panels and 10 over unnamed longwall panels. This provides a means of making aworst-case estimate of the maximum vertical subsidence (SmaX), tensile strain (+E), compressive strain (-E) and slope change or tilt (G) of the ground surface anywhere over a longwall panel, provided the mining height (m), mining depth (h) and panel dimensions are known. Graphs provide a means of constructing a subsidence profile from the center of a longwall panel across the sides or ends of the panel to the limit of subsidence. A graph also provides a method of constructing a horizontal strain profile from the center of a longwall panel across the sides or ends of the panel to the limit of subsidence. The NCB method has been routinely used to estimate the maximum potential magnitude and location of tensile and compressive strains and slope inclination changes that could be induced at the ground surface by planned longwall mining. Being able to predict worst-case subsidence effects has made it possible to take measures to mitigate damage to surface structures. Coal has been routinely mined under cities, highways, pipelines, power lines, factories, railroads, rivers, bridges, harbors, cathedrals, churches, schools, historic castles and keeps and other sensitive structures. The method provides conservative estimates so that engineering adjustments could be made to accommodate the conservatively predicted (worst case) subsidence effects before they develop. The NCB method has been used to conservatively estimate subsidence impacts in the Project Area. • The NCB method, which is a step-by-step procedure for predicting subsidence effects from mining a longwall panel based on the fundamental factors of coal extraction thickness, panel width between gateroad pillars and overburden depth. Initially the method provides a graph for estimating the maximum vertical subsidence reduction factor for the mining height based on Panel Width versus Panel Depth (Figure 8. NCB Panel Width/Depth Maximum Subsidence (Smax) Prediction in Affected Environment/Subsidence). Then the method provides a graphical plot of various proportions of the maximum subsidence along a profile based on the Panel Width/depth ratio from the center of a panel, across the side of the panel to the limit of subsidence outside the panel (Figure 9. NCB Subsidence Profile Graph in Affected Environment/Subsidence). The distance from the center of the panel is in terms of the panel depth. The next graph provides multipliers for the ratio of the maximum subsidence divided by the depth for a wide range of Panel Width/Depth ratios. The values taken from the graphical plot for the particular Panel Width/Depth ratio cross three lines, the "EXTENSION" line estimates the maximum tensile strain (+E), the "COMPRESSION" line the maximum compressive strain (-E) and the "SLOPE" line the maximum slope change or tilt (G), (Figure 11. NCB C-21 DBMS 313