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

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DRAFT 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 surtace 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 surtace, 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 surtace 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. 7.6 Rate and Duration of Subsidence The first potentially detectable subsidence at a given point on the ground surface ahead of an approaching longwall face begins when the longwall face is something less than approximately 0.75 times the overburden depth of the seam, has subsided about 15% of Smax when the longwall face passes under the point, is approximately 50 percent complete when the longwall face is 0.2 to 0.3 times the overburden depth beyond the point, and appears to have stopped subsiding when the face is between 0.5 and 0.6 times the overburden depth beyond the point. However, there is still 5% to 9% of residual subsidence to take place after the longwall face has either mined beyond the influence distance or the panel has been completed. Residual subsidence is probably the result of consolidation of the gob and closure of some overlying bed separations in the overburden. Measuring the time until residual subsidence is complete requires extremely precise leveling to measure subsidence. Collins (1977) reported the results of an eight year program in the South Wales Coalfield. He reported the results from six panels at depths from 207 feet to 2330 feet. Longwall mining of the six panels was completed over periods from 0.5 to 1.5 years and measurable residual subsidence continued for 2.0 to 4.5 years afterwards. Complete stability is not significant because the potentially damaging strains and tilt are directly dependent on the magnitude of the subsidence and the magnitude of residual subsidence is small in relation to the subsidence that takes place during the active period. Page 40 of 57