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

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7.4 Angle of Draw The angle of draw defines the extent that subsidence can be detected beyond the limits of mining. The angle of draw is the angle formed by the vertical line above the outer limit of mining and the lateral limit of detectable subsidence. It has special importance to land-use planning because it indicates where the surface will be unaffected by mining- induced subsidence. Reported angles of draw are highly variable, as indicated by Table 6. Angles of Draw for Coal Mining in the United States and Europe which presents angles of draw from 19° to 45° collected from various countries and sources. The study by Abel and Lee (1984) demonstrated that the potential for error in applying the angle of draw measured in one country to another, or even within one country and(or) district, is considerable. Table 12. Angles of Draw for Mines in Flat-Bedded Sedimentary Rocks with Respect to Lithology of Overburden, from their paper, shows a wide range of angles of draw, from 0° to 40°, indicates that lithology statistically appears to plays a roll in determining the angle of draw. The various sources of data demonstrate that the NCB's 35° angle of draw is a conservative estimate. 7.5 Break Angle The historic concept of a break angle as the location of the tensile surface cracking has been discarded because it coincides with the location of maximum tensile strain (+E). In areas of thick soil or alluvium, tensile cracking at the surface may be difficult to see because the tensile strain typically produces several narrow cracks, as can be seen on Figure 18. Ribside Tension Cracks in Road Fill and Cliff Face, York Canyon Mine. Narrow cracks fill rapidly because the alluvium contains fines and has little tensile strength. When bedrock is close to the surface, the easiest tensile crack to see open is over the starter room, because it initially increases in width and doesn't close as the longwall face advances. Cracks on the surface over a starter room are usually the first to open and take a long time to fill by the natural processes of weathering, mass wasting, and erosion. The tensile crack accompanying the advance of the longwall face is mobile, i.e. it advances as the longwall face advances. However, the opening of bedding cross joints in the moving tensile strain zone ahead of an advancing underlying longwall face is temporary. These tensile cracks start to close after the longwall face has passed about 0.15 times the depth (approximately 8°) and the horizontal compressive strain starts. Closure in the compressive strain zone reaches a maximum when the longwall face is approximately 0.3 times the depth past the tensile fracture. Figure 15. Cross Panel Compression Ridge in Alluvium, York Canyon Mine shows a compression mound that was pushed up when the soil that fell into the initial tension crack was compressed by the trailing compression zone. Similarly, the tensile strain zones on the ground surface roughly over the panel ribsides, starter room and eventually the shield recovery room is relatively easy to see as it develops. As the longwall face passes a position on the surface overlying any location along either gateroad the tensile crack, or cracks, develop. After the longwall face has advanced approximately 0.7 times the depth the trough and associated tensile crack remains open, as shown on Figure 17 Ribside Tension Crack On Steep Slope, York Canyon Mine. C-36 DBMS 328