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 0 The previously observed and studied cracks at this location were at one of the two narrowest portions of the West Flatiron ridge where the overburden depth to the B-seam is about 2,150 feet. In June 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 this time. In addition, the following observations were made by Richard Dunrud in the 2004 report based on the presence of numerous cracks and the fact that no mining had occurred in the area: 1) the northward convergence of West Flatiron may have caused increased lateral stresses, compared to other areas on West Flatiron; 2) this lateral movement, as exhibited by the numerous cracks, may be the initial stage of rockfall/landslide scarp development; and 3) the tension fractures on the road and on the cliff outcrops in the area look like mining-induced features, but must be caused by gravitationally-induced lateral movement towards the cliff faces on either side of the ridge. 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 show 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. CJ 831-032.790 Wright Water Engineers, Inc. Page 27 December 2008