2010-05-03_REPORT - C1980007 (9)

Home Browse Search

Fall 2009 Subsidence and Geologic Field Observations Box Canyon, Apache Rocks, and South of Divide Mining Areas • 5.18 Traverse L1-L1' This traverse represents a drill road to MDW drill pads E1-11.5 and E1-8 as accessed from Horse Gulch road (Traverse L-L'). Mining had not occurred beneath this area during the fall 2009 field visit. E-seam overburden thickness along this traverse is between 700 and 800 feet. 5.19 Location 17 This is the site of annually observed subsidence cracks at Apache Rocks. Photographs of these cracks were obtained but not reproduced in this report. There currently are essentially five individual cracks or crack zones over a lateral distance of approximately 280 feet over previously-mined B-seam longwall Panels 12 and 13. The cracks trend primarily N65-75°E, but locally diverge to N40°E, N80°E, and E-W. The overburden depth to the B-seam is about 1,350 feet. From north to south, the maximum widths of the individual cracks or crack zones, the centers of which average 70 feet apart, were measured in • fall 2009 as follows: Crack 1 zone. 2.0 inches Crack 2 zone. 3.0 inches Crack 3 zone. 3.0 inches (total width of several branching cracks) Crack 4 zone. 5.0 inches (two cracks with widths of 3.0 and 2.0 inches) Crack 5 zone. 3.0 inches The total horizontal strain (maximum extension) over the 280-foot area during 2007 and 2008 was 16.0 inches, or 1.33 feet. In spring 2009 this value was slightly less at 15.0 inches, or 1.25 feet. Observations during the fall 2009 field visit found the total horizontal strain to be consistent with that observed in 2007 and 2008 (i.e., 1.33 feet). Assuming that extension only occurs at crack locations (a valid assumption in these brittle sandstones), the maximum . calculated cumulative strain over the area is therefore 1.33 feet divided by 280 feet, which equals 831-032.791 Wright Water Engineers, Inc. Page 63 April 2010