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

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The magnitude of the maximum predicted surface extension strains presented in Table 2 would develop roughly over the perimeters of the selected retreat mined panels, which ranged from 1780 µs to 2860 µs. Extension strains in the predicted range from approximately 1800 µs to 2900 µs on the surface would cause repairable damage, ranging from cracking of single story brick walls to cracking of reinforced concrete frames. The predicted tensile strains would result in estimated 1-inch to 2-inch wide tensile cracks at the ground surface. The predicted magnitude of the maximum surface compressive strains ranged from 1940 µs to 3000 µs, as presented in Table 2 should have developed over the centers of the selected retreat mined panels. Compressive strains in the predicted range would cause repairable damage to structures on the surface. The predicted maximum increase in surface slope resulting from subsidence over and adjacent to the five selected room-and-pillar panels, presented in Table 2, range from 0.72% (0.41° or 25 min) to 1.22% (0.70° or 42 min). Slope changes of this magnitude could adversely affect floor drainage, turbo generators and overhead crane rail operations. Railroad switching Including all facilities, that depend on rolling of rail cars could be adversely affected. Rubber-tired vehicles could be induced to roll at the grades above 1 %. None of the potential structures or land uses indicated was or is present over the McClane Canyon Mine. Increasing a short section of an already steep slope by 0.4° to 0.7° could induce downslope movement. However, the direction that a panel was mined and/or the pillars failed could also flatten a slope. Figure 9. Localized Mining Induced Slope Angle Changes indicates the normally minor effect of the direction of mining on a much steeper slope angle. 3.4 Multiple Seam Mining Longwall mining is planned as the principal mining method in the Main Cameo Seam. There are no plans to mine any other coal seams, because of the thickness and coal quality of adjacent seams and because of the local 20 to 25-foot thickness where the Cameo Seams split and/or merge. 3.5 Compression Arches and Load Transfer Compression arches commonly develop across longwall panels where the coal has been and/or is being mined, provided the panel is narrow enough and(or) deep enough for both ends of the arch to span the panel width and bear on rock. These arches are zones of tangential compressive stress where some of the weight of the overburden overlying the arch can be transferred onto abutments; ahead, behind and on either side of the longwall panel being mined (somewhat like the way stone-arched bridges transfer their weight and load to the bridge abutments). However, some or all of the downward deflected rock under the arch will bear on the collapsed rock under the arch. If the width of a longwall panel is too wide or too shallow for the arch to span the panel width, a smaller arch will form, with one side of the arch bearing on and compressing the collapsed gob. The balancing arch abutment can be on the solid barrier pillar behind the starter room, on rigid pillars in the gateroads and on the unmined coal ahead of the advancing longwall face. The arch over the longwall face will follow the advancing face. The arch abutment load following the advancing longwall face will progressively consolidate the collapsed roof rock, the gob. If the face stops moving the face arch will shorten in length and can add load the face supports. C-8 DBMS 300