2009-09-21_PERMIT FILE - C1981010 (13)

Home Browse Search

January 15, 2008 Page 54 Recoverability of reserves in L-Seam are less certain because the slide plane is below the L-Seam in some areas. The slide plane was identified in 90% of the characterization boreholes. Although the overburden thickness and undulation of the bedding varies, the depth of the slide plane averages 96 ft below ground. In only one borehole was the slide plane identified below the L-Seam. Therefore, it is estimated that most of the L-Seam will be recoverable. Note that there could be significant uncertainties in these estimates. Only nine boreholes have been placed through the landslide. The uncertainty is estimated Table 1 by the (±) values. The reserves least likely to be recovered are along the edges of the landslide. The ground has been bulked upward on the east side of the landslide near the G-Dip cut. This area may be mostly rubble above the L-Seam, but insufficient data are available for confirmation. Similarly, the shear mounds seen on the west side of the landslide are expected to be disturbed, but the degree of intermingling above the L-Seam is highly uncertain. Where the landslide has plowed the old G-Strike Pit spoils, the rock is expected to be very loose. Excavations in this area will likely result in comingling of the layers, including some of the coal reserves. The rock mass condition at the hill crest beyond the mapped cracks is uncertain because additional large-scale fracture openings may not be visible on surface as cracks. 5.4 Truck & Shovel Cut Face Stability and Monitoring Ground conditions in the K-Pit and G-Pit areas are not expected to be similar to normal highwall stability conditions. The rock within the landslide perimeter has been disturbed and, thus, is expected to be less stable when excavated; consequently steep cut faces should not be used in disturbed areas. The strategy of mining from west to east is favorable for geotechnical conditions. Removal of the slope crest improves stability of the remaining material. Decreasing the weight at the crest reduces driving forces on the landslide. The eastward advance is down-dip of the beds. Weak bedding planes plunge into the cut face thereby having minimal effect on face stability. The northern portion of K-Pit will be outside the landslide limits. The borehole logs in that area (Figures 12 and 13) indicate RQD values of 40-100 and RMR values of 60-75. These values are typical of other areas successfully mined. Removing interburden using dozers in this area may require more than rippers to loosen the rock. Bench blasting may be needed, especially for the massive sandstone units. The K-Pit cuts will be on steep beds for which the orientation transitions from northerly to northeasterly and back to northerly as mining moves eastward. The cross section schematic, shown in Figure 38, illustrates what mining in this area might look like. Dozers can push interburden either parallel to the face or down the cut face. The geologic structure on the cut face is expected to favorably oriented downdip where the potential for loose blocks rolling off the cut face is expected to be minimal. It is recommended that the cut face slope not exceed 1 H:1 V (45°). The first eight to ten K-Pit cuts within the landslide are expected to be in the rubble material above the L-Seam. It is anticipated that little blasting will be necessary in the rubblized Agapito Associates, Inc.