2010-05-03_REPORT - C1980007 (9)

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Fall 2009 Subsidence and Geologic Field Observations Box Canyon, Apache Rocks, and South of Divide Mining Areas • 2008, the cracks had transected the entire ridge with the most obvious representation of their presence being the rockfall/landslide originating in the thick, massive sandstone outcrop on the west side of West Flatiron located immediately west of the MDW drill pad. Cracks caused by lateral spreading in the area were first observed by Dunrud in 2002, before mining had occurred beneath the site. The cracks, which began near the south edge of the MDW pad area were 150 feet long, as much as 3.5 inches wide, and trended southward along an old drill road. The cracks were located in the approximate centerline of the ridge, where the West Flatiron ridge narrows northward from about 2,000 feet wide to 200 feet wide in a lateral distance of about 2,500 feet. Large extension cracks were also observed on the west-facing cliffs at that time. In 2004, Dunrud reported that the tension fractures on the road and on the cliff outcrops in the area looked like mining-induced features, but must have been caused by gravitationally-induced lateral movement towards the cliff faces on either side of the ridge because no mining had occurred in the area. • The most recently observed cracks at this location (June 2008), which greatly increased in size and number when longwall mining occurred beneath the area, were individually as much as 12 inches wide and generally trended N30-40°E in an en echelon, or stair-stepping, pattern northward from west to east. The depth of some of these cracks, prior to sloughing, was thought to be as deep as the massive sandstone cliff, which is estimated to be about 50 feet. Previous observations of the two westernmost cracks showed an offset downward to the west along fractures in the massive sandstone of the Ohio Creek Member of the Mesaverde Formation that underlies the now reclaimed drill pad. As noted above, surface expressions of the five previously-noted cracks are no longer visible in the MDW pad. However, observations in the native vegetation south of the MDW pad reveal that the crack extensions into that area are still present. Figure 16 is a view of the westernmost crack looking south as it extends southward from the pad. There is a noted musty odor associated with air escaping from portions of this crack. The location of this crack is approximately 15-25 feet east of the cliff face. There is evidence that the area to the west of this 831-032.791 Wright Water Engineers, Inc. Page 29 April 2010