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February 1, 2010 Page 2 <br />be trucked to the Horse Gulch Fill, and to a buttress at the toe of the ultimate K -Pit. It was <br />thought that a buttress should enhance the stability of the down -dip portions of the K -Pit. The <br />first buttress design extended well to the north of the old G- Strike Pit, and computer modeling <br />indicated the buttress would have a FOS on the order of 1.8. During subsequent slope stability <br />modeling of the K -Pit, it was discovered that the area of the planned buttress had been previously <br />mined and reclaimed. The buttress was remodeled with this information and it was determined <br />the FOS for this condition was 1.2. This is well below the 1.5 required for a long -term spoil <br />placement. TMI staff were informed of the condition and they designed a smaller buttress <br />centered more closely to the reclaimed G- Strike Pit. <br />Subsequent computer slope modeling of the K -Pit mining sequence indicated that the <br />buttress was not required for long term stability of the reclaimed K -Pit; however, indications <br />were that the buttress enhanced the stability of the slope downdip from the K1L -10 cut and that <br />the buttress should be in place before completing the KIL -10 cut. This modeling was performed <br />using selected parameters that focused on deep seated failure surfaces, such as the initial K -Pit <br />slide in the L -Seam mudstones. <br />The latest modeling is based on the best available information with respect to material <br />properties, pit geometry and mining sequence. <br />2.0 SLOPE STABILITY ANALYSIS <br />T.M:I has indicated that the landslide- affected area in K -Pit will be mined using the truck - <br />and- shovel method of mining. The mining plan in the K -Pit area calls for the spoils to be placed <br />in three separate areas: (I) the downdip of the K -Pit (as a toe buttress), (2) Horse Gulch, and (3) <br />previously mined K -Pit cuts. Within the K -Pit, mining will progress up to the R -Seam and the <br />spoils will be set on the R -Seam floor. A simplified stability analysis of the backfill spoil and <br />downdip toe buttress was performed to assess their stability after reclamation, and this report was <br />submitted to TMI (Agapito Associates, Inc. 2008a). Based on results obtained during this <br />analysis and operational changes, a new configuration for the K -Pit toe buttress was developed <br />by TMI. The revised toe buttress configuration was given to AAI, to investigate the long -term <br />global stability of the buttress structure. <br />Prior to this report, the slope stability modeling was done using FLAC /Slope and <br />FLAC /313 (Itasca Consulting Group, Inc. 2005). For this phase of analysis, Geo- Studio's <br />SLOPE /W (GEO -SLOPE International Ltd. 2008) was used for its ability to analyze the potential <br />of deep and shallow failure surfaces separately. For a given model, a range of failure surfaces <br />can be specified or the entire model can be analyzed for the lowest FOS for the section. <br />Additionally, multiple failure surfaces can be displayed for a range of factors of safety. The <br />model inputs for SLOPE /W are the same as for FLAC, and the resulting minimum factors of <br />safety were essentially equal. for the same modeled conditions, thus giving AAI the confidence to <br />use SLOPE /W for this phase of modeling. Both analyses used calibrated rock mass properties <br />obtained from the G -Pit landslide FLAC31) stability analysis and spoil properties obtained <br />during field investigations and literature review (Agapito 2007, Douglass and Bailey 1.985, and <br />Capstone 2008). The stability of the toe buttress was restricted to one cross section oriented <br />along the dip of the Q -Seam and through the heart of the proposed buttress. <br />Agapito Associates, Inc. <br />