2013-04-25_REPORT - C1980007 (9)

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831 - 032.794 April 2013 Fall 2012 Subsidence and Geologic Field Observations Box Canyon, Apache Rocks, and South of Divide Mining Areas 2.0 SUMMARY OF FALL 2012 FIELD STUDIES On October 11 -12, 2012, WWE performed subsidence and geologic field investigations of Box Canyon, Apache Rocks, and South of Divide (SOD) mining areas for MCC. These field observations have been made annually since 1996, and semi - annually since 2007. Based on these field observations, the overall effects of B -seam longwall mining in the Apache Rocks and Box Canyon mining areas have been less than were initially projected as reported in Exhibit 60 (Dunrud 1998 rev.) of the CDRMS permit. Seventeen years of experience (1996 to 2012 inclusive) indicate that subsidence features (cracks and bulges) are most visible on roads and trails, where the ground is more compact and free of vegetation. These areas (or traverses) were therefore selected for specific observations either on foot or using a 4 -wheel drive vehicle to efficiently obtain observable subsidence information. In addition, photographic comparisons are routinely made to evaluate and document changes between observations (using 13 regularly visited photographic observation points) and to record their condition for future reference (Maps 1 and 2). During the fall 2012 field traverses, no subsidence tension cracks or compression bulges were observed in the surficial deposits (colluvium and local alluvium) above mined B -seam Longwall Panels 14 to 25 in the Apache Rocks and Box Canyon mining areas. No mining effects on rockfalls or landslides were observed in the Apache Rocks mining area. However, sporadic rockfalls and landslides have been observed in the Box Canyon mining area since 2006 where the steep, upper reaches of Sylvester Gulch and Box Canyon abut West Flatiron. Rockfalls and landslides in this area and above mined B -seam Longwall Panels 18 to 22 and 25 are categorized as having a very high rockfall potential. As described in Section 4.0 of this report, observations from traverses in this area during the fall 2012 field visit found there to be little if any changes from observations made during the spring. Details regarding previously observed subsidence cracks, rockfalls, and landslides in these mining areas can be found in earlier reports. One of the frequently visited photographic observation points (Photographic Observation Point 9) is located on the north side of the North Fork of the Gunnison River (North Fork). This location is used to observe some of the historic landslides that have plagued the Colorado Wright Water Engineers, Inc. Page 3