2008-01-16_APPLICATION CORRESPONDENCE - C2008086 (3)

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DRAFT 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 Maximum Strain and Slope Prediction Graph in Affected Environment/Subsidence). The final graph provides various proportions of the maximum tensile strain (+E) and maximum compressive strain (-E) along a profiles from the center of a panel, across the side of the panel to the limit of subsidence outside the panel (Figure 12. NCB Horizontal Strain Profile Graph in Affected Environment/Subsidence). The method has been modified by others to extend its application to room-and-pillar panel mining and to consider the impact of varying proportions of sandstone, limestone and shale or mudstone in the overburden. The NCB subsidence method is used directly for longwall mines and has been modified for room-and-pillar mines. Reported subsidence predicted and measured over room-and-pillar workings at the Roadside Mine east of Palisade Colorado predicted 1.61 feet, measured 1.02 feet in the Cameo Seam; the Eagle No. 5 Mine southwest of Craig, Colorado reported having measured 10% more subsidence than predicted and the Southland Mine near Canon City, Colorado predicted 1.51 feet, measured 0.89 feet. The NCB method has been applied for subsidence prediction over a longwall panel in the Mid Continent Mine west of Redstone, Colorado predicted 4.99 feet and measured 1.71 feet ;the York Canyon Mine west of Raton, New Mexico predicted 8.1 feet and measured 7.09 feet. Subsidence was predicted using a modified NCB method at the Chimney Rock coal augering mine east of Pagosa Springs, Colorado predicted 2.59 feet, measured 0.49 feet. 6.1 Subsidence Zones Page 24 of 57