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2006 Subsidence and Geologic Fietd Observations <br />Box Canyon, West Flatiron, Apache Rocks, and South of Divide Mining Areas <br />1,200 to 1,350 feet. No subsidence features were observed, except in the Apache Rocks azea <br />(Location 9). <br />4.9.1 Location 9 <br />This is the site of subsidence cracks at Apache Rocks (Figures 18-22). The cracks-which are <br />' straight to branching, as much as 175 feet long, 25 to perhaps 30 feet deep, and as much as 4 <br />inches wide-are located in the tension zone above the southern edge of gate road pillars <br />' between mined longwall panels 12 and 13. The area is therefore in a double extension zone <br />produced by the mining of both panels 12 and 13. <br />The cracks trend primarily N65-75°E, but locally diverge to N40°E, N80°E, and E-W. The <br />' overburden depth to the B-Seam is about 1,350 feet. More cracks were observed soon after <br />mining than aze now present. Some of the smaller cracks in the northerly part of the crack area <br />' have closed, or healed and sealed, a year or two after they formed. There currently are essentially <br />five individual cracks or crack zones over a lateral distance of approximately 265 to 285 feet, <br />' with an average of about 280 feet. From north to south, the maximum widths of the individual <br />cracks or crack zones, which average 55 feet apart, are as follows: <br />1. 1.5 inches (Figure 18) <br />2. 2.5 inches (Figure 19) <br />3. 4.0 inches (Figure 20) <br />4. 4.5 inches (two cracks with widths of 2.5 and 2.0 inches} (Figure 21 } <br />5. 3.0 inches (two cracks with widths of 2.5 and 0.5 inches) (Figure 22) <br />The total horizontal strain (maximum extension) over the 280-foot average area is 15.5 inches or <br />1.3 feet. Maximum calculated cumulative strain over the 280-foot area is therefore 1.3 feet <br />divided by 280 feet, which equals 0.00464 (or 0.46 percent), The range of maximum horizontal <br />831-032.750 Wright Water Engineers, Inc. Page 24 <br />December 2006 <br />