2010-05-12_PERMIT FILE - C1981010

Home Browse Search

February 1, 2010 3.0 ANALYSIS RESULTS Page S While this report was intended only to address the K -Pit buttress fill, discrepancies were discovered in the modeling of the K -Pit mining. Therefore, both the revised K -Pit mining analysis and the K -Pit buttress analysis will be discussed in this section. 3.1 K -Pit Mining Analysis The original modeling of the K -:Pit mining focused on the stability of deep- seated failure plains as was the case in the original G -Pit slide. However, when the .near- surface failures are not filtered out, there are localized areas of potential instability. Figure 3 shows the state of stability prior to mining. It should be noted that this area of potential instability is the nose of a major natural (pre - mining) slump feature previously identified in a Google Earth photograph. This image is reproduced as Figure 4. The nose of this slump is a relatively small feature and does not constitute a condition of global instability. However, it is large enough to cause concern. In order to determine the presence of similar features, a 3D surface was created from the 2008 contour map provided by TMI. The contours were converted into a grid file with the orientation along the general dip of the K -Pit geologic structure. This orientation was parallel to the section used for the SLOPE/W modeling. The difference between grid points was calculated and the results were color -coded and plotted. Topographic areas sloping toward the southwest (toward the left along Section -4 in Figure 1) were assigned a negative slope and therefore is grouped with areas dipping less than 1.0 %. These results are presented in Figure 5. A series of models were run to determine the point at which the slope of the surface material resting on the I -Seam floor mudstone becomes unstable. Two generic slopes were analyzed, and in each case the mudstone slope was held to 10 degrees ( °). In the first case, the topography was flat behind the crest of the slope. In the second case, the topography behind the crest sloped upward at 10 °. These generic slopes are shown in Figure 6. The results of modeling indicated that areas with a slope of greater than 18° were potentially unstable. Figure 7 shows the extent of slopes in excess of 18° along with the footprint of the proposed K -Pit. It is quite likely that the topsoil. removal operations will grade these areas to acceptable slopes, thus remedying the situation. However, the potential for movement must be addressed. Me location of haul roads near these potentially unstable areas should be carefully considered', Cut and fill slopes above the I -Seam should be held to a maximum of 3H:1 V. Water should have positive drainage away from the roadways, and in some locations, it may be necessary to install. culverts. To put the size of these potentially unstable areas in perspective, the cut sequence shown in Figure 7 has bench widths on the order of 175 ft. These unstable areas are generally less than 2 acres, with the exception of the one area in the southeast corner bordering the first cut in L -Pit, which is approximately 3.5 acres. The results of the modeling along Section -4 (see Figure 1) indicate that once the slope reduction is performed, the FOS can be increased to 1.0 or above by reducing the surface slope to 3H:1 V or less. The minimum FOS remains at that level until the mining progresses to that area, in the case of Section -4 (see .Figure 1), the KIL -I5 cut, the FOS increases to 1.855. This progression of mining is shown in .Figures 8 through I I, and the cut sequence is as follows: Agapito Associates, Inc.