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January 15, 2008 <br />4.0 SPOIL STABILITY ANALYSIS <br />Page 23 <br />The mining plan in the K-Pit area calls for spoils to be placed in three areas: (1) downdip <br />of the landslide, (2) in Horse Gulch, and (3) in previously mined K-Pit cuts. In K-Pit area, the <br />spoils will be set on relatively steep inclined bedding of the weak Q-Floor mudstone where spoil <br />stability is a concern. A stability analysis of the spoil pile was performed to assess the maximum <br />height of the spoil pile before spoils become unstable on the bedding. <br />The analysis was performed using the two dimensional (213) numerical modeling <br />program, FLAC.14 The analysis used calibrated rock mass properties obtained from the G-Pit <br />landslide FLAC313 15 stability analysis. 16 The strength properties of the spoils were calibrated <br />from recent minor spoil failure events in the Z-Dip Cut #2. The spoil stability analysis was <br />performed to determine the maximum stable height of spoils for expected spoils and Q-Floor <br />mudstone bedding conditions. Various spoil slope angles, fill heights, and groundwater levels <br />were simulated for assessing overall spoil stability. <br />4.1 Calibration of Spoil Strengths <br />A simple 3D model was developed of Z-Dip Pit Cut 42. The purpose of the model was to <br />calibrate the strength of the spoils to observe spoil slump behavior. While mining ZD-02 on the <br />Q-Seam cut, a large volume of spoils began to creep from the spoil pile to the west into the open <br />seam area. Although the dragline was sitting on the spoils at the time, the spoil movement was <br />downdip. <br />Figure 14 shows the surveyed cross section of the ZD-02 pit spoil failure showing pre- <br />slide profile (green line) and post-slide profile (magenta) of the spoil slide that started on <br />August 18, 2007. The east-west cross section is shown at N 404,700. The extent of the cracking <br />to the north on August 19, 2007, had also been mapped. The slide extended further to the north, <br />but no one was allowed in the area after August 20, 2007. The spoil pile was about 120 ft high <br />and had a slope face angle of about 42°. <br />This information was used to calibrate the strength properties of the spoil since it is <br />known that 120-ft-high spoil pile placed on the 8° northward sloping Q-Floor mudstones with a <br />42° slope face is known to exceed its limit of equilibrium and fail. <br />A 3D analysis was performed using FLAC313 numerical modeling program. Figure 15 <br />illustrates the 3D conceptual model of the ZD-03 spoil pile with the open Cut #2 to the east and <br />the 120 ft of spoils to the west. The bedding was assumed to dip 8° to north. The spoil face was <br />assumed to dip 42° to the N83.7°E to allow for thickening downdip. The thickness of the <br />Q-Floor mudstone was assumed 14 ft thick to include carbonaceous mudstone, thin sandstone, <br />14 FLAC (2005), "Fast Lagrangian Analysis of Continua - User's Guide," Itasca Consulting Group, Inc. <br />Minneapolis, MN, Version 5.0. <br />15 FLAC313 (2005), Fast Lagrangian Analysis of Continua in 3 Dimensions - User's Guide," Itasca Consulting <br />Group, Inc., Minneapolis, MN, Version 3.0. <br />16 Agapito Associates, Inc. (2007), "G-Pit Landslide Stability Report - Task 2," prepared for Trapper Mining, Inc., <br />March 19. <br />Agapito Associates, Inc.