2011-02-17_REPORT - C1980001 (4)

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4.0 MOFFAT STABILITY MONUMENTS A system of three control points and six monuments were established in the final Moffat pit area during the fourth quarter of 1997. The purpose of the monuments is to monitor the hillslope for long-term stability. The three control points were placed to form a large triangle. The reference control point was established along the top of the hillside in undisturbed ground above the final Moffat pit. The two additional control points (back-sites) were established in undisturbed ground northwest and south of the reference point. The back-sites were established to verify the location of the reference point. The monuments were installed in pairs with the first pair, SM-1 and SM- 2, established in the lower third of the final pit area. SM-1 was placed approximately 130 feet south of the final pit highwall and SM-2 was placed approximately 340 feet south of the final pit highwall. The second pair, SM-3 and SM-4, were installed approximately mid-way along the pit. SM-3 was placed approximately 110 feet south of the highwall and SM-4 was placed approximately 325 feet south of the highwall. The last pair, SM-5 and SM-6 was installed in the upper third of final pit area. SM-5 was placed approximately 150 south of the highwall and SM- 6 was placed approximately 350 feet south of the highwall. The monuments consist of 7' to 8' sections of 2-1/2" diameter drill steel driven 5-1/2' to 6-1/2' into the pit backfill material. Locations of the stability monuments are displayed on Plate 3. The monuments were surveyed quarterly the first year and annually thereafter. In 2003, a level loop was surveyed providing elevation information. However, due to an equipment malfunction, x and y coordinates were lost. The 2010 stability monument survey was performed August 31, 2010. The coordinates of the initial monument survey and subsequent surveys are provided in Table 4. ^^'WWCENGINEERING Page 4