2010-05-12_PERMIT FILE - C1981010

Home Browse Search

February 1, 2010 Page 7 was reduced by removing the top lift of spoils and the FOS increased to 0.806. The height was further reduced by 50 ft; this resulted in an increased FOS to 1.058. These cases are presented as Figures 12 through 14. The slip surface location is indicating failure of subgrade as the cause of the instability. A typical mitigation practice for this condition would be to reduce the height as much as possible and distribute that mass at the toe of the slope. Figure 15 shows the reduced height and increased width, along with the resulting failure surface at the minimum FOS of 1.9. Based on this modeling, A.AI has determined that a buttress is not required to stabilize the K -Pit slopes for the mining operations. The modeling confirms that the reclaimed AE -Pit area is a suitable location to place spoils in order to comply with the post- mining topography (PMT) requirements. However, the spoils should not add more than 50 ft in elevation to the current topography and the reclaimed slopes should not exceed slopes of 311:1 V. 4.0 SUMMARY AND CONCLUSIONS The Trapper .Mine is somewhat different from other mines; typically, mining produces less stable slopes than the prc- mining conditions. At Trapper Mine, the presence of slumps indicates that the natural (pre- mining) surface is already in a quasi - unstable condition. However, these pre- existing areas with borderline stability are relatively small and do not indicate global instability. AAl believes these local areas can be addressed on a case -by -case basis to reduce the risk of instability. Modeling indicates that reducing the slope gradients to less than 18° will effectively stabilize these slopes. There will be limited exposure of mining operations to potential movement with the important exception of haul roads; therefore, roadway design should incorporate cuts and fills with slopes of less than 3H:1 V. Positive drainage away from the road sub -grade is essential. The mudstones at the Trapper Mine site have been problematic over the years, especially when water is allowed to infiltrate the strata and compromise their strength. These compromised mudstones heavily contributed to the October 2006 G -Pit slide and the haul road slide of 2008. The proposed site for the K -Pit buttress places the toe of fill above the I -Seam materials that contain low strength mudstones. The computer modeling indicates that these mudstones can, in general, reliably support a surcharge load of approximately 50 ft of spoils. The findings and recommendations of this phase of analysis are as follows: K -.Pit Mining • AAl has identified areas of concern with regard to slope stability, within the K -Pit and L- Pit mining footprints. These are areas with natural slopes greater than 1.8° dipping down slope (see Figure 7). Reducing the gradients of these slopes to less than 3H:1V is recommended to reduce the potential hazard. • Modeling indicates equilibrium is obtained at 18% i.e., the .FOS is equal to 1.00. To obtain higher factors of safety, the slopes must be reduced further. Agapito Associates, Inc.