2011-04-11_REPORT - C1980007 (9)

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Fall 2010 Subsidence and Geologic Field Observations Box Canyon, Apache Rocks, and South of Divide Mining Areas 2.0 SUMMARY OF FALL 2010 FIELD STUDIES On October 6-7, 2010, 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.). Fifteen years of experience (1996 to 2010 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 were made to evaluate changes since the last observation (using thirteen routinely visited photographic observation points) and to record their condition for future reference (Maps 1-4). During the fall 2010 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. As previously reported, observed cracks along the Box Canyon access road beneath Longwall Panel 18 have healed and are no longer visible. 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 2010 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 the Spring 2010 Subsidence and Geologic Field Observations Report (Spring 2010 Subsidence Report). 831-032.792 Wright Water Engineers, Inc. April 2011 Page 3