2011-02-14_PERMIT FILE - C1980007A (3)

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Subsidence Evaluation for the Exhibit 60E South of Divide and Dry Fork Mining Areas Page 43 • and 25 are categorized as having a very high rockfall potential. Rockfall and landslide activity were notably accelerated during the mining of B -seam longwall panels 19 to 22 and 25. A new rockfall/landslide area was observed on the east side of Sylvester Gulch during WWE's fall 2008 field visit. This high rockfall hazard area became active as a result of the mining of B- seam longwall panel 25 which underlies that portion of Sylvester Gulch. Observations during WWE's spring 2010 visit found there to be little, if any, change since the previous visit. Another active rockfall/landslide area visited was that first observed on the west and east sides of West Flatiron during the spring 2007 and fall 2008 field visits, respectively. These areas (Locations 4 and 5 of the fall 2009 subsidence report) are along one of the most narrow parts of West Flatiron ridge top at a site where cracks (most likely gravity- induced) were fast observed. Active cracking and slabbing above mined B -seam longwall panel 21 both on the west and east side of West Flatiron appears to have lessened over the past several years of observations (Map 3). Cumulative extension of the surface cracks, as observed in June 2008, was approximately 2.5 to 3 feet. In October 2008, the MDW pad on which these surface cracks were observed was reclaimed and nearly all evidence of surface cracks obscured except at the sandstone cliff face. Aside from a few remaining MDW pads near the north end of West Flatiron, mining- related activities in this area are complete. Nonetheless, the potential for more cracking and rockfall activity on the west- and east - facing cliff faces over B -seam longwall panel 21 is high. Such activity is expected to have an effect only on surface features, particularly vegetation. Approximately 1,500 feet north of the rockfall/landslide area first observed in spring 2007 is a • second rockfall area observed for the first time during the June 2008 site visit. This area is located on the west side of another narrow portion of the West Flatiron ridge (Location 6 of the fall 2009 subsidence report) between mined B -seam longwall panels 19 and 20. This rockfall area is considerably smaller than its neighbor to the south, as shown on Map 3, and in the head of the Box Canyon drainage. The Box Canyon drainage has no human inhabitants and one access road. Continued rockfall/landslide activities will have limited effect on the Box Canyon drainage or access road but will likely affect the drill road on West Flatiron and the MDWs which remain in operation there. However, access to West Flatiron and the MDW pads is expected to decrease now that mining has been completed on all the B -seam panels in the Box Canyon mining area. E -seam longwall mining began in the South of Divide mining area in December 2008. The spring and fall 2009 field visits represented the first opportunities to make seasonal post- mining observations in this area above completed E -seam longwall panel E1. Subsidence cracks were observed at the MDW drill pads closest to the longwall face and a small rock stack (remaining evidence of a sandstone outcrop) between MDW E1 -11.5 and E1 -15 was observed to have toppled as mining occurred beneath this area (see Section 5.4.1 of this report for Photographic Observation Point 12). Lastly, observations were made of a number of cracks in MDW pads above the unmined portion of E -seam longwall panel E2. As discussed in earlier subsidence reports, evidence suggests that the observed cracks are the result of differential settlement of portions of these pads where fill placement is the greatest." 831- 032.810 Wright Water Engineers, Inc.