2009-01-15_REPORT - C1980007 (8)

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Fall 2008 Subsidence and Geologic Field Observations Box Canyon, Apache Rocks, and South of Divide Mining Areas visit. However, views of this area from the Sylvester Gulch road suggest that the rockfall may be much older than originally considered, based on a comparison with the new rockfall/landslide. The new rockfall/landslide shows a much lighter rock outcrop where the rockfall originated and considerable disturbance of vegetation in the rockfall runout and landslide area as compared to the area noted from the spring 2008 field visit (see Figure 5, 6 and 7). As previously noted, Photographic Observation Point 8 was not visited. As a result, no comparison photographs were obtained of the two active rockfall and landslide areas on the east side of Box Canyon at Location 4 and 9. Non-photographic observations from more distant locations found there to be little apparent change from observations made during our June 2008 field visit. 5.1.1 Location 1 This new rockfall/landslide area is over currently mined Longwall Panel 25 at about cross cut 12. The B-seam overburden thickness at this location is about 650 feet. This is one of two locations along the east side of Sylvester Gulch previously identified as a high rockfall hazard area due presence of over-steepened slopes and exposed sandstone outcrops. The source area for the rockfall is approximately 100-150 feet above the upper drill road (Figure 5). Although difficult to assess without direct observation and measurements, it appears that the failure area is about 100 feet in laterally extent and approximately 200 feet from top of outcrop failure area to the bottom of the landslide. Much of the failed sandstone outcrop appears to have been retained (or captured) by the drill road (Figure 6). The estimated size of the largest sandstone blocks is approximately 15 feet by 20 feet. • 831-032.790 Wright Water Engineers, Inc. Page 18 December 2008