PERMFILE123352 - Laserfiche WebLink

Home Browse Search

Subsidence Evaluation For Exhibit 60 The Apache Rocks And The Box Canyon Mining Areas Page 3 • 4.0 SUBSIDENCE PREDICTION BASED ON LONGWALL MINING AT THE WEST ELK MINE Subsidence, as it relates to mining, is defined herein as the local downward displacement of the surface and overburden rock in response to mining under the influence of gravity. For purposes of describing subsidence effects on overburden material and the ground surface, subsidence can be divided into four zones (Figure 1): (1) caved zone, (2) fractured zone, (3) continuous deformation zone, and (4) neaz surface zone. 4.1 Caved Zone As coal is extracted and a void is produced, the roof rocks break along bedding planes, joints, and fractures and fall to the mine floor (Figure 1). Rotation of the caved debris occurs during the fall so that the caved fragments tend to pile up in a random fashion. This caved zone, according to Peng (1992, p. 1-2) occurs for the first 2 to 8 mining (or extraction) thicknesses (t) in the roof rocks (for example, if t=12 feet, the caved zone would range from 24 to 96 feet [2t to 8t]). In the current longwall panels of the West Elk Mine, according to mine personnel, this zone is estimated to be 2.5 mining thicknesses (2.~t). The B- and E-Seam roof rocks commonly consist of thinly bedded carbonaceous shales, sandy shales, claystones, and sandstones. A sofr shale that is susceptible to air slaking forms the immediate roof of the B-Seam in most areas. Thick sandstones locally form the immediate roof of the E-Seam, in addition to the shales and sandstones. The ratios of shale to sandstone are quite similar in the first 20 feet of roof in the B- and E- Seams. The shale to sandstone ratio of the first 20 feet of B-Seam roof averages about 2:3; the shale to sandstone ratio of the first 5 feet averages 3:2. The shale to sandstone ratio of the first 20 feet of the E-Seam roof is 3:2; the shale to sandstone ratio of the first 5 feet is also 3:2. Sandstones locally form the immediate roof of the E-Seam. • The B- and E-Seam roof rocks above the first 20 feet consist of shale, siltstone, lenticular sandstones, and thin coal beds. A Marine Sandstone, locally consisting of a lower and an upper tongue, and ranging from about 30 to 125 feet thick, underlies the D- and E-Seams; the D-Seam is 1 foot to as much as ~0 feet below the E-Seam. Based on information obtained from drill holes in the Apache Rocks and the Box Canyon mining areas, it is estimated that the caved zone in these areas may range from 2 to 4 extraction thicknesses depending on water conditions encountered. [n a dry environment and where thick sandstones comprise the E-Seam roof, the caved zone may be closer to 2t. This zone may be closer to 4t in a wet environment where soft shales and claystones form [he mine roof. Any water present in this caved zone will drain into the mine almost immediately afrer caving occurs. 831-032.181 Wright Water Engineers, Inc.